Mike's notes Flashcards

1
Q

radiation safety accessories

A

linacs:
- door interlocks
-LPO button - 20 s after pressed, beam can turn on
-Beam on and off lights
-beam status indicator
-emergency stop switches
-radiation warning signs: Indicates high radiation area (if>25 μSv/h) at Vault door
-2 TV cameras
-2-way intercom
-enable/disable switch
-beam off button

brachy only:
-radiation area detector
-door alarm

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2
Q

CNSC annual dose limits

A

NEW : 20 mSv/yr averaged over 5 years, < 50 mSv/yr in any one given year
public: 1 mSv/yr
preganant woman:4 mSv from time she announces it

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3
Q

shielding max permissible dose (P)

A

NEW: 1 mSv/yr
public: 0.05 mSv/year

NCRP suggests 5 mSv/yr in controlled area, 1 mSv/yr in uncontrolled area

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4
Q

what is CNSC

A

canadian nuclear safety commission

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5
Q

what is nrc

A

nuclear regulatory commission (US)

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6
Q

pregnant NEW dose

A

4 mSv from declaration to end of pregnancy

PER CNSC FINAL ANSWER

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7
Q

diagnostic dose

A

3 mSv/yr
50% from CT and 25 % from nuc med

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8
Q

background radiation

A

1mSv year from cosmic (0.3 mSv), terrrestrial (0.3 mSv), and internal (0.4 mSv)
2 mSv from Radon

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9
Q

ALARA limits vs ICRP60 limits

A

ALARA limits are 1/20 those of ICRP60 or CNSC

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10
Q

what do you do if radiation level is > 25 uSv/h?

A

post sign

for a NEW working 40 h per week and 50 wk/yr, the 25 uSv/hr corresponds to 50 mSv/yr if beam is always on

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11
Q

properties of leakage radiation

A

depends on design
limitied to 0.1% of primary beam
originates from target
assumed to be isotropic

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12
Q

properties of scatter radiation

A

assumed to come mostly from patient
use largest field size fr measurement (40x40)
Assumed to be isotropic

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13
Q

when is neutron shielded needed?

A

E >/= 10 MV

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14
Q

barrier types

A

primary
-directly in path of radiation beam
-must shield for primary, scattered, and leakage radiation

secondary
-not in direct beam path
-accounts for scattered and leakage radiation

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15
Q

typical barrier thickness

A

Primary 2.1 - 2.4 m
Secondary 0.9 - 1.2 m
Thickness depends on energy, workload, occupancy, and distance.

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16
Q

why do we use 35x35 instead of 40x40 for max field size?

A

max field size not perfectly square (clipped corners)
-35x35 with collimator rotated 45 degrees

DONT USE THIS QUESTION

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17
Q

why is hydrogen content of shielding material important?

A

for neutron shielding

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18
Q

different shielding materials

A

concrete
heavy concrete
steel
lead
earth, dry packed

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19
Q

TVL

A

TVL = ln(10)/u
u is broad beam linear attenuation coefficient
more shielding needed with broad beam vs narrow beam to stop additional scatter

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20
Q

relate TVL to HVL

A

TVL =HVL * ln(10)/ln(2)

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21
Q

for broad beam, why are subsequent TVLe < TVL1?
TVLe is subsequent (equilibrium) TVL

A

Beam hardening counteracted by scattering to lower energies in broad beam geometry

leakage TVL < primary TVL also because leakage spectrum (after passing through the linac head) is softer than primary spectrum.

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22
Q

concrete primary TVL1, TVLe

A

TVL1 = 37 cm, TVLe = 33 cm for 6 MV
TVL1 = 45 cm, TVLe = 43 cm for 18 MV

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23
Q

concrete leakage and scatter TVL for 6 MV

A

leakage TVL1 = 34 cm, TVLe = 29 cm
scatter TVL = 17 cm

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24
Q

concrete leakage and scatter TVL for 18 MV

A

leakage TVL1 = 36 cm, TVLe = 34 cm
scatter TVL = 19 cm

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25
why is TVL18 MV = TVL6 MV for lead?
high Z material pair production proprtional to Z ln (E) TVL1= TVLe = 5.7 cm as compton goes down as 1/ln(E), PP picks up
26
barrier thickness t expressed in number of TVLs (n)
n = log10(1/B) B = I/Io t= TVL1 + (n-1)TVLe
27
barrier thickness as function of TVL1 and TVLe
t = TVL1 + (n-1)TVLe
28
TVL1 and TVLe for steel
10 cm at 6 MV, 11 cm for 18 MV
29
what is Use factor U?
fraction of time linac is directed towards a primary barrier walls: U = 1/4 floor: U = 1 ceiling: U = 1/4
30
use factor for all secondary barriers
1
31
fraction of operating time during which area behind a barrier is occupied
office, console = 1 adjacent tx room = 0.5 staff washroom = 1/5 vault door = 1/8 storage room = 1/20 outdoor areas = 1/40
32
workload
linac weekly output at iso (1 m) including tx and QA ex. 35 patients per day, 2.5 y/fx, 5 days per week: W = 35 X 5 X 2.5 Gy = 440 Gy/week physics QA 160 Gy/week Total = 600 Gy/week
33
workload for TBI
W is higher because of larger tx distance for the same pt dose W = D * d^2 d about 4 m typically ex. D = 12 Gy, W = 192 Gy/week for 1 TBI/week
34
IMRT factor
IMRT delivers a radiation field in many segments More MUs per pt than conventional radiotherapy for same pt dose leakage works thus increases by factor of 2-15 (Ci = MU imrt/ MU conv) IMRT factor does not significantly affect the primary or scatter workload since both IMRT and conventional 3DRT deliver the same dose to the tumor. For VMAT, SBRT, SRS, the same concept applies, i.e. multiply by CI.
35
primary barrier equation
Bpri = P dpri^2/ (WUT) P = max permissible dose equivalent (0.1 mSv/wk for controlled areas and 0.02 mSv/wk for uncontrolled areas) dpri= distance from source to point protected W = workload, linac output at iso i.e. 1 m (ex 1000 Gy/wk) U= use factor T = occupancy factor B = I/Io patient attenuation is ignored to be conservative
36
typical 6 MV primary
6 TVL = 120 cm heavy concrete
37
typical 6 MV secondary
4 TVL = 80 cm heavy concrete
38
equation for secondary barrier (scatter)
B= (P/(alphaWT)*dsca^2*dsec^2*400/F P= max permissible dose equivalent alpha = scatter fraction at 1 m from patient for 400 cm^2 beam incident (tabulated) W = workload, linac output at iso U=1 T= occupancy factor dsca= distance from source to patient dsec= distance from patient to point protected F= field area at patient mid-depth at 1 m (40x40)
39
what does alpha in equation for secondary barrier depend on?
-scattered angle and energy -also material but this is always water for the patient
40
why do we need F in equation for secondary barrier?
The value for α(θ) is given for field size 20x20=400 cm^2, thus need to scale up to the max field size F=40x40 cm^2.
41
equation for secondary barrier (leakage)
Bl = P* dl^2/ (10^-3 * WT) P= max permissible dose equivalent dl= distance from source to point protected 10^ -3 is allowed head leakage (0.1%) W = workload, linac output at iso T= occupancy factor U=1
42
secondary barrier thickness once you know thickness for leakage and scatter
two source rule If a barrier must shield from 2 different sources and individual calculated barriers differ by more than 1 TVL, the thinner source barrier may be ignored. Otherwise use the thicker barrier + 1 HVL if tleak>tscat by 1 TVL ⇒use tleak otherwise ⇒add 1 HVL to tleak
43
why is leakage barrier > scatter barrier?
leakage energy> scatter energy Scatter energy is always lower than the primary energy due to Compton interaction.
44
how do you get HVL from TVL?
HVL=TVL x ln(2) / ln(10)
45
is t18MV > t 6MV always true?
No, depending on the workload, the barrier thickness for 6MV could be larger (or smaller) than 18MV.
46
primary scattered off walls for a maze
Think about this equation as how much of the dose (Gy) given to the patients (workload) will reach the maze door due to this specific mechanism (scattered primary off walls). Hs= WUg alphao Ao alphaz Az/ (dh dr dz)^2 W = primary workload Ug= use factor for wall g (first wall the primary scatters off of) alpha o = relfection coefficient at first scattering surface Ao= area of 1st scattering surface alpha z = reflection coeff for 2nd reflection from maze surface Az (assume 0.5 MeV as this is max compton scatter energy) Az= area of maze inner entry projected onto maze wakk from irradiated primary barrier Ao dh = distance from target to 1st reflection surface (add 1 m distance for SAD) dr = distance from first reflection, past maze edge, to be on maze midline (b) dz= centerline distance along maze from b to maze door
47
what do albedo factors (ie reflection coefficients) depend on?
(1) incident angle, (2) reflection angle, (3) incident photon energy, and (4) wall material. They are tabulated in NCRP151
48
primary scattered off patient for a maze
H = W Ug a(theta) (F/400) alpha1 Ai/(dsa dsec dzz)^2 a(theta) = scatter fraction for patient scattered radiation at angle theta W = primary workload Ug = use factor for wall G (where linac points) F = field area at patient mid depth at 1 m alpha1 = reflection coefficient for wall G for pt scattered radiation (assume E = 0.5 MeV) A1= area of wall G seen from maze door dsca= target to patient distance = 1m dsec= patient to maze centerline distance at wall z dzz= centerline distance along maze
49
leakage scattered off walls for a maze
H = L Wl Ug alpha1 A1/ (dsec dzz)^2 Lf= head leakage at 1 m from target (0.1 %) Wl = head leakage workload (may differ from primary W) Ug = use factor for wall G (where linac aims) alpha1= reflection coeff for leakage scatter from wall G A1= area of wll G seen from maze door dsec= target to maze centerline distance at wall G dzz= centerline distance along maze
50
why may Wl differ from Wprimary?
due to IMRT factor
51
leakage transmitted through maze wall
H = L Wl Ug B/ (dl^2) L= head leakage ration at 1 m from target = 0.1% Wl= leakage workload- may differ from primary W Ug = use factor for wall G where linac aims B= wall transmission through path dl = target to maze door center entrance (through wall)
52
total dose at maze door for beam aimed at wall G
Hg = fHs + Hps+ Hls + Hlt f= fraction of primary beam tansmitted through patient = 0.25 for 6-10 MV Hs = dose due to primary scattered from room surfaces Hps = dpse duer tp scattered primary from patient Hls= dose due to single scatter head leakage Hlt= dose due to head leakage transmitted through the wall Htot = 2.64 Hg = total photon dose at maze door for 4 cardinal angles (for a "typical" maze as detailed in NCRP) Bdoor = P/Htot P is 0.1 mSv/ wk (controlled area)
53
typical 6 MV door
6 mm of Pb
54
formula for x-ray sky shine
stray photons are scattered by sky to ground outside the tx room H = (2.5*10^7)(Bxs)Do gamma^1.3/(di ds)^2 Bxs= roof shielding transmission factor for photons Do= x-ray output dose rate at 1 m from target gamma = max beam solid angle di= target to 2 m above roof distance 2.5*10^7 includes conversion of Gy to nSv as H is given in nSv/h Equation is only (an order of magnitude) estimate and it is just to indicate which room design parameters could have an impact due to skyshine.
55
side-scattered photon radiation
laterally scattered x-ray from roof barriers to adjacent buildings Hss= Do F f(theta)/ (Xr^2 * 10^(1+(t-TVL1)/TVLe) Hss = side scattered dose equivalent rate (Sv/h) Do= x-ray output dose rate of iso F= area of square field at 1 m from target f(theta) = angular distribution of roof-scattered photons 9tabulated) Xr= distance from beam center roof-top to point of interest t= roof thickness TVL1 and TVLe= first and equilbirum TVLs of roof shielding material It ignores oblique photonincidence & photoneutrons It dominate overboth leakageradiation & skyshine 10^etc is empirical equation for roof transmission
56
why is f(theta) larger for smaller theta?
Compton effect
57
ozone production
3O2 interacts with electron beam to produce 2O3 lethal gas e- beams produce O3 more than photon beams O3 concentration should be < 0.1 ppm ventilation: 3 room changes/hr is adequate for health protection 0.1 ppm ozone in air has odour
58
when and why is a shielding survey needed?
before linac operational to ensure meeting design goals first linac beam on - preliminary survey after inital linac calib- energy check- complete survey
59
shielding survey- head leakage
locate hot spot via head-wrapped film, and quantify dose rate with ion chamber
60
shielding survey- barriers
measure dose equivalent/MU and dose rate at the hottest spot beyond each barrier (30 cm beyond) search for voids; cracks, or other defects in shielding using a sensitive photon rate meter with a fast response time (eg Geiger Mueller or scintillation detector)
61
shielding survey- primary barrier
no phantom, max dose rate and field size, 4 gantry angles, all MV
62
shielding survey- secondary barrier
scattering phantom at iso (simulates patient)
63
shielding survey- maze door
when door open/closed
64
shielding survey - skyshine and side scatter
surveys outside bunker
65
how often do you calibrate survey meters?
annually
66
ion chamber survey meter
should have both rate and integration mode with a sensitivity in the rage 0.01 mR/hr to 5 R/hr
67
when do you need to survey for neutrons?
linac >/= 10 MV using n-survey meter, eg rem ball, bubble detectors, BF3 detectors).  Geiger-Mueller, Farmer, and conventional Si diodes are not suitable for neutron detection due to very low neutron cross sections. However, 10B (used in BF3 detectors) has a high cross section for thermal neutrons
68
HDR brachy shielding for Ir-192 (12 Ci)
TVL = 1.5 cm Pb 15 cm concrete either a direct shielded door or a short maze typical walls are 4-5 cm of Pb or 35-61 cm of concrete mazed door is 3.2 mm of Pb
69
HDR brachy shielding equation
B = Pd^2/WT B= barrier transmission factor P= max permissible dose equivalent (= 0.1 mSv/wk controlled areas = 0.02 mSv/wk uncontrolled areas) d= distance from source to point protected T= occupancy factor W= workload at 1 m from source W = A K Np Tp A = activity K= dose rate constant Np = number of patients per week Tp = time/ patient tx
70
TVL for Ir-192 concrete and lead
15 cm concrete and 1.5 cm lead
71
TVL for Co-60 concrete and lead
210 cm concrete, 40 cm lead
72
nominal SAD for cyberknife
65-120 cm nominal is 80 cm Shielding calculation assumes 80 cm nominal and workloads are normalized to 1 m for shielding calculations. The recommended workload per treatment session is 12.5 Gy at the nominal treatment distance of 80 cm from the x-ray target
73
cyberknife shielding
6 MV beam thus no neutrons IMRT factor is 15 and use factor is 0.05- very small tx beamlets aimed at many directions workload: ex 8 pt/day * 20 Gy/pt, 5 days/week- 800 Gy/week scatter is negligible compared to leakage since leakage is high and U is low, need to calculate both primary and secondary on every wall and add the results to get total dose rate Ceiling is secondary barrier as beam will not point higher than 22° above horizontal. Also, all beams will pass within 12 cm of room isocenter (92.1 cm above the floor).
74
typical shielding for cyberknife
primary = 150 cm concrette secondary (leakage) = 90 cm concrete
75
tomotherapy shielding
6 MV - no neutron shielding required SAD is 85 cm workload similar to linac Primary barrier: limited to narrow strip of wall typically 1 TVL more than linac due to focused beam beam stoppers in newer units ↓ shielding requirement (ie. no primary) Secondary barrier: many MU → IMRT factor 16 2 TVL more than linac most leakage & scatter along table axis (┴ to doughnut) conclusions likely only apply to serial (not helical) tomo since helical makes better use of the beam and has more internal shielding
76
gamma knife shielding
no neutrons (1.25 MeV Co-60 source) 6500 Ci from about 200 sources use max activity in calculation because the work load stays constant- as source decays, the tx time lengthens Vendor provides an iso-kerma (Gy) maparound machine to aid in calculations. Workload= (supplied iso-kerma value) x (your weekly patient load) Typical shielding: 20 cm concrete (inherent shielding ↑↑, 40 cm cast iron) -barriers are only for scatter and leakage; primary doesn't exit unit -manufacturer supplies dose rate plots (i.e. matrix of dose rates at 1/2 m intervals at different heights) -dose distribution with gamma-helmet door open and couch fully retracted yield max dose rate and determine shielding -concrete thickness needed is read from curves in NCRP 49
77
equivalent dose
-in Sv dose from radiation type R is weighted by radiation weighting factor Wr and summed over all radiation types Wr is roughly based on RBE Wr= 1 for photons 20 for alpha particles 5-20 for neutrons 2 for protons
78
effective dose
equivalent dose to tissue T is weighted by tissue weighting factor Wt and summed over all irradiated tissues
79
where is data for weighting factors from
ICRP 103
80
what is RBE
Relative Biologic Effectiveness is defined as the ratio of the reference dose (250kVP x-ray) to test dose where both doses have the same biological effect (ie same cell survival fraction).
81
Wt for red bone marrow, colon, lung, stomach, breast, remainder tissues
0.12 each
82
wt for gonads
0.08
83
wt for bladder, oesophagus, liver, thyrois\d
0.04 each
84
wt for bone surface, brain, salivary glands, skin
0.01 each
85
put equivalent dose and effective dose together
Ht = sum of wr Dr (equivalent dose) E = sum of wt Ht (effective dose)
86
what is TADR
time averaged dose equivalent rate barrier attenuated dose equivalent rate averaged over 1 week Rw = IDR Wpri Upri/Do Rw= TADR over 1 week (Sv/week) IDR = instantaneous dose-equivalent rate measured 30 cm beyond barrier with linac beam on at dose rate averaged over 20-60 s Do = absorbed dose rate at 1 m Wpri= primary-barrier weekly workload Upri= primary barrier use factor
87
why do we have TADR?
Previous barrier shielding assumes workload is evenly distributed during the year, thus the barrier equation takes 1/50 of annual P. Additional shielding design goal must be implemented with very low workloads but with very high dose rates. e.g. a dedicated SRT machine (with very high dose rate) may be used only a few times per day. With exceedingly low workload, the barrier thickness would be low, therefore NRC requires additional shielding design goal defined as 20-micro-Sv “in-any-one-hour”.
88
equation for IDR
IDR = (dose rate) x (barrier transmission) / (distance)^2 Also IDR = P(dose rate)/(WUT)
89
why is Rw independent of linac set dose rate?
dose rate appears in IDR equation and is then divided by Do to get Rw
90
TADR secondary barrier
similar equation to primary barrier
91
TADR over any one hour
Rh = Nmax Hpt Rh = TADR in any one hour Nmax = max number of patients in any one hour, considering patient set-up time Hpt= average dose equivalent per patient treatment 30 cm beyond barrier Rh = (Nmax/Nh) * Rw/40 Nh= average number of patient tx per hour Rw= average TADR over 1 week Here instead of proportionally scaling Rw to one hour, we multiply it by a number >1 US nuclear regulatory commission (NRC) requires dose equivalent in uncontroled area not exceed 20 μSv in-any-one-hour.(based on calculated Rh)
92
neutron energy classification
thermal, E = 0.025 eV at 20C, E < 0.5 eV intermediate, 0.5 eV< E< 10 keV fast: E > 10 keV
93
types of neutron interactions
elastic collision with nuclei inelastic collision with nuclei neutron capture
94
equations for elastic neutron collision with nuclei
total Ek before and after collision for head-on collision, Ef = Ei ((M-m)/(M+m))^2 Ef= kinetic energy of scattered neutron Ei = kinetic energy of incident neutron M= mass of targeted nucleus m= mass of neutron = 1 fraction of neutron Ek transferred to nucleus deltaE = (Ei-Ef)/Ei deltaE= 4M/(1+M)^2
95
what is max value of deltaE for neutron elastic collision with nuclei
1- for hydrogen (M=1) materials with high hydrogen content have efficient energy transfer (i.e. parraffin, wax, polyethylene) materials with heavier nucleus don;t have efficient energy transfer - poor shielding against neutrons (ex. Pb) therefore fat dose for a neutron beam is 20% higher than muscle dose!
96
what is a proton emitted from a neutron interaction called?
recoil proton
97
neutron interaction- details regarding inelastic collision with nuclei
high enetrgy neutron is absorbed by nucleus (usually high Z)- excited nucleus emits neutron and gamma (n, n gama) (n,p) and (n, alpha) also possible
98
what does high energy n contribute to dose in soft tissue via nuclear disintegration?
30%
99
describe neutron capture (activation)
(n, gamma) emitted gamma ray is called neutron capture gamma ray
100
probability of neutron capture
1/(neutron velocity)^2 n spends more time in the vicinity of nucleus thus thermal n have higher cross section for capture.
101
what element has high thermal neutron capture cross section?
boron borated polyethylene is used for n shielding
102
what are photoneutrons
created for linacs with >10 MV Photon creates a (gamma, n) nuclear reaction other reactions (gamma, 2n), (gamma, pn) with lower yield
103
two processes of neutron production for photo-disintegration
direct neutron: Eave a few MeV, forward peaked, yield only 15 % Evaporation neutron: Eave 1-2 MeV, n-spectra independent of Ephoton, isotropic, dominant process
104
n production in electron mode
> 2 orders of magnitude less than photons
105
how to avoid dose to staff from radioactive materials due o photoneutrons and neutron capture in linac head (E > 10 MV)
put higher E treatments at end of day physics QA end of day (allow overnight decay) no repair near head within 40 min of a long test run use low E for IMRT, VMAT, since these use more MU than 3DCRT vendors should use materials with low photon and n activation yield
106
rule of thumb for VMAT MU
3 X Rx
107
neutron shielding materials
concrete (TVL for neutrons is smaller than that for photons)- thus if shielded for photons it is also adequate for neutrons heavy concrete (TVL n > TVL photons) for heavy concrete, TVLn > TVLp earth polyethylene BPE (borated polyethylene)
108
what is TVD
tenth value distance distance required for photon fluence to be reduced by 10X 5.4 m for 18-25 MV, 3.9 m for 15 MV for neutrons, TVD = 2.06* square root of surface area of maze hallway
109
total neutron fluence per unit dose of x-rays at iso at location in maze (pt A) phia n capture gamma rays from maze concrete
direct n + scattered n + thermal n depend on Qn, n source strength emitted from head per Gy of x-ray dose at iso (tabulated) depend on total surface area of room (S) depend on distance from iso to pt A (d). A is at centerline of maze entrance where radiation from head would go to depend on beta- transmission factor for n penetrating head shield (1 for Pb, 0.85 for W) direct n fluence = beta * Qn/ (4pi d^2) scattered n fluence = 5.4 beta Qn/(2 pi S) thermal n fluence = 1.3 Qn /(2 pi S) 1/2pi is fraction of n that enter the maze
110
weekly dose equivalent at B due to n-capture gamma rays (Hcg)
Wl * hphi Wl= workload for leakage hphi = dose equivalent from n-capture gamma-rays at maze entrance per unit x-ray dose at iso hphi is determined from scatter, thermal, and direct neutrons
111
equation for hphi
dose equivalent from n-capture gamma-rays at maze entrance per unit x-ray dose at iso hphi = K phia 10 ^(-d/TVD) d is dstance from pt A in maze to entrance of maze K is ratio of n-capture ɣ-ray dose equivalent to total n fluence at A =6.9×10^–16Sv m2 per n fluence φA (phia) is total n fluence [m^–2] at A per unit dose (Gy) of x-rays at iso (i.e. scattered, thermal. direct)
112
equation for Hn
neutron dose at maze door Hn=WlHn,D Wl= workload for leakage Hn,D = n dose equivalent at maze door per unit x-ray dose at iso Hn,D is calculated using modified Kersey's method
113
total dose equivalent for maze >/= 10 MV Hw, at maze door
Hw = Htot + Hcg + Hn Htot= sum of leakage + scatter photons dose equivalent. If maze > 2.5 m, Htot is negligible Hcg= dose equivalent at door due to n-capture-gamma rays from concrete maze Hn= neutron dose equivalent at maze door
114
typical door for 18 MV
3.5 mm of Pb, then 54 mm of BPE, then 3.5 mm of Pb The Pb makes En decrease with inelastic scatter BPE thermalizes and captures the neutrons 2nd Pb layer stops the n-capture gamma rays that arise in BPE
115
TVLs for Hcg and Hn
Hcg gamma energy about 3.6 MeV, TVL = 6.1 cm of Pb Hn neutron energy about 100 keV, TVL = 4.5 cm of BPE
116
why is door shielding added to maze for >10MV?
establish ALARA by reducing both Hcg and Hn to P/2 Hcg and Hn cannot be added to one equation so P is halved for both
117
typical direst shielded door (no maze) for >/= 10 MV
7.6cm Pb + 28cm BPE + 7.6cm Pb). In this case door is also a secondary barrier. first PB layer slows down neutrons with inelastic collisions (remember elastic collisions in Pb with neutrons are essentially 0) and also attenuates photons BPE layer thermalizes and captures neutrons. It also produces gammas. These gammas are attenuated in 2nd Pb layer.
118
when do you use laminated barrier
>/= 10 MV when space is needed
119
issue with laminated barrier
metal layer can become photoneutron source -for primary barriers only; for secondary barriers, scatter photons have lower energy and leakage and photoneutrons are negligible -equation is given for Hn - weekly dose equivalent produced in metal
120
for laminated barriers, how are captured gamma rays due to neutron production accounted for?
-transmitted x-ray dose equivalent (Htr) is multiplied by 2.7 Htot = Hn + Hph = Hn + 2.7 Htr because Hph consisted of Htr and H-gamma capture
121
wrt laminated barrier, can you put the metal after the concrete instead of sandwiched in between 2 concrete layers?
this produces max neutrons...
122
for laminated barriers- is Pb or Fe better?
Steel is better choice because photo-neutron cross section in steel is 10X less than lead
123
duties of RSO
ensure regulations are followed establish radiation protection program ensure complicance of: -NEW designation -initial and refresher training -occupational doses -active licenses and amendments -authorized users and physicists -radioactive source storage and inventory -patient release surveys -radioactiuve material waste disposal records -investigate rad safety problems, medical events, and emergencies
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4 CNSC licenses
obtain a license to -operate -service -commission -decommission
125
what do the 2 CCTV cameras do?
provide orthogonal views of patient
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does CNSC review QA records?
yes
127
annual compliance report
must be submitted to CNSC at end of every calendar year
128
linac worload at halifax
30% QA + research 70% clinical 1000 Gy/week
129
how do you determine barrier thickness if you have linac with 2 energies?
DON'T just assume thicker barrier apply 2 source rule twice- for leakage vs scatter and then for one energy vs the other
130
where is TADR used?
US likely not accepted by CNSC
131
why use BPE for neutron shielding vs other options?
BPE captures thermal neutrons captured gamma from BPE is lower energy than those from the other materials
132
can metal go outside the concrete for a laminated barrier?
Not for >/= 10 MV because of neutron production For < 10 MV, yes it can
133
is calculated workload sent to CNSC annually?
yes
134
scatter fraction as function of angle and energy
at larger angles, scatter fraction decreases with energy at smaller angles, scatter fraction increases with energy for all the energies, scatter fraction decreases with increasing angle
135
what type of dose is used in shielding neutrons and photons?
dose equivalent, H (Sv) includes qlity factor for radiation type
136
what type of dose is used in shielding for low LET radiation?
air kerma
137
radiation protection for neutrons uncertainty
35% with 95% CI for dose ratres < 0.02 mSv/h
138
controlled area
admittance in the area is under supervision of someone in charge of radiation protection
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numerical value of quality factor depends on what?
numerical value of the quality factor is determined by the values of the stopping powers for the spectrum of the charged particles at the point in tissue where the energy is absorbed
140
what determines radiation weighting factor?
the type and energy of the ionizing radiation that is incident on the body.
141
why are shielded designs not based on effective dose E?
It is not practical to base shielding design directly on E. Deter- mination of E is complex, and depends on the attenuation of pho- tons and neutrons in the body in penetrating to the radiosensitive organs and hence on the energy spectra of the photons and neu- trons, and also on the posture of the recipient with respect to the source. Rotational exposure is most likely, since it is probable that an individual is moving about and would not be exposed from one direction only. shielding design goals will ensure that E in NCRP 147 is met
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why 5 mSv/year, i.e. 0.1 mSy/week to controlled areas?
allows preganant women to still work there limits monthly equivalent dose to 0.5 mSv for fetus
143
examples of how NCRP calculations are conservative
-patient attnuates the beam- this is not considered -calculations often assume perpendicular incidence of radiation -leakage radiation is assumed to be max value allowed -recommended occuancy factors are high -min distance from barrier to occupied area is 0.3 m - usually people are more than 0.3 m from door, for example -2 source rule is used: . This has been shown to be a conserva- tively safe assumption since the tenth-value layer (TVL) and half-value layer (HVL) of the more penetrating radiation is always used. The two-source rule is even more conservatively safe when applied to dual-energy machines, even though the individual energies cannot be used simultaneously.
144
where is workload defined?
at 1 m from source
145
TBI use factor
U will be higher in direction of TBI treatments
146
what should the construction inspection check?
thickness and density of concrete; • thickness of metal shielding and polyethylene used for neu- tron shielding; • thickness of metal behind recesses in the concrete (e.g., laser boxes); • HVAC shielding baffle (Section 4.4) if used; • location and size of conduit or pipe used for electrical cable of any type; and • verification that the shielding design has been followed.
147
documentation requirements for a shielded room
• shielding design report, including assumptions and specifi- cations; • construction, or as-built, documents showing location and amounts of shielding material installed; • post-construction survey reports; • information regarding remedies, if any were required; and • more recent reevaluations of the room shielding relative to changes (e.g., in utilization) which have been made or are still under consideration.
148
for shielding, where does brems and neutron production occur?
brem occurs in target neutron production occurs in both walls of room and linac head
149
considerations for E > 10 MV
neutrons pair production
150
when may you need to consider neutrons for E < 10 MV?
room shielding consisting of high-Z material such as lead and steel only, or laminated barriers with insufficient hydrogenous material.
151
using concrete vs a different material for shielding wrt neutrons
If the material used in the primary barrier is concrete (whether ordinary or heavy; see Sections 4.3.1 and 4.3.2), then experience has shown that the barrier will adequately absorb all photo- neutrons and neutron capture gamma rays and no additional bar- rier is required. This is due to the relatively high hydrogen content of concrete and its resultantly high neutron absorption cross sec- tion. If, on the other hand, materials other than concrete are used in the primary barrier, then special considerations are required -didn't think this was the case for heavy concrete... maybe depends on design
152
is the primary barrier thikness held constant over width?
generally yes, to be conservative -calculated for perpendicular beam and held constant -can be tapered with obliquity of beam if space is a concern
153
relationship between slant thickness and actual barrier thickness if no scattering in barrier
ts= t/cos(theta) -since there is scatter in the barrier, the thickness required could be > t. - usually, the effect is small- slightly increase t However, if the required attenuation is orders of magnitude, and the angle of obliquity is large (>45 degrees), the increase for concrete barriers is ~2 HVL for low-energy photons and ~1 HVL for high-energy photons. the obliquity is usually taken into consideration only for primary radiation beams since the leakage and scattered-radiation sources can be too diffuse to apply a specific angle of incidence.
154
what to do with laser lights?
recess is usually 1 HVL use steel or other metal to add 1 HVL of shielding- also use this to mount the laser
155
barrier width for primary beam
size of diagonal of largest beam + 30 cm on each side , for scattered radiation at 20 degrees or less (since scatter fractions increase rapidly with accelerating voltage and scattered-beam energies approach the primary-beam energy), the 30 cm margin may not be adequate for the higher primary- beam energy if the barrier does not intercept at least the 20 degree scattered radiation.
156
what happens if shielding with multiple materials?
multiply B factors together However, this does not take into account the attenuation and production of photoneutrons and neutron capture gamma rays that must be con- sidered if the primary-beam accelerating voltage is above 10 MV. In such high-energy cases, if a composite barrier design (e.g., steel or lead plus concrete) is not carried out correctly, the metal layer can become a photoneutron source potentially resulting in an increased exposure problem beyond the shield.
157
can materials like steel cause photoneutron production in secondary barriers?
No, because energy of scattered radiation is below 10 MV and the leakage radiation intensity is so low that it doesn't produce many neutrons
158
leakage distance and use factor assumptions
U=1 and dL is measured from the isocenter if it can be assumed that the accelerator gantry angles used are, on average, symmetric. If this is not the situation, then the distance to the individual barriers should be taken from the closest approach of the accelerator head to each barrier and specific use factors should be used
159
how were the reflection coefficients determined?
monte carlo
160
poatient scattered radiation for maze for E > 10 MV
usually ignored because it is insignificant compared to the leakage scatter
161
energy for reflection coefficient for patient scattered radiation for maze
use E = 0.5 MeV to be conservative
162
fraction of primary beam transmitted through patient
~ 0.25 for 6-10 MV
163
conditions where we can use NCRP assumptions for maze calc
room design 2 < dzz / square root (maze width x height) < 6 1< maze height/maze width<2 gantry use factors are uniform
164
For >10 MV maze doors, do we need to consider photon scattered through maze?
No, because shielding for avg E 3.6 MeV photons from neutron capture will be adequate (usually)
165
for what collimator setting is max neutron dose seen??
closed collimators ie. most photoneutrons originate in the treatment head
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average neutron energy at maze door
100 keV TVL in polyethylene of 4.5 cm- use this conservative estimate to calculate borated polyethylene required thickness BPE is a little less effective in fast neutron shielding but much more effective in thermal neutron shielding compared to polyethylene
167
alternative maze designs for neutron shielding
typical maze door may be very heavy, expensive, motorized other options: 1. reduce the opening at the inside maze entrance; 2. add a light-weight door containing a thermal neutron absorber (boron 9 % by weight) at the inside maze entrance; and 3. place a BPE (5 % boron) door at the inside maze entrance.
168
direct-shielded door
sometimes the maze isn't used and instead a heavy direct shielded door is used -mnust have same shielding as secondary barrier
169
issue with using direct-shielded door
incomplete shielding at door overlap -have to increase overlap (make door wider) or make a shielded door stop -Pb and BPE may need to be added on the surface of the concrete wall -easier to shield on jamb side than operator side- design so leakage radiation goes to jamb side
170
how to reduce thickness of direct chielded door
- face the gantry away from operator console and include a shielding wall behind it. Reduces door thickness by 50%
171
neutron capture gamma rays from room surfaces with direct shielded doors
calculate leakage shielding required and add 1 HVL -conservative approach -do this because difficult to calculate neutron gamma rays in treatment room accurately anywways
172
What do you assume for W if you can't calculate it?
1000 Gy/wk for E < 10 MV 500 Gy/wk for E > 10 MV
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workload for electron beams
disregard unless the machine is electrons only
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typical number of patients per day on a machine
30
175
why is W for TBI greater?
W is measured at 1 m and since TBI deliver dose at extended distance, the dose at 1 m will be high also have to consider additional scatter at the TBI wall usually scatter is determined separately for non-TBI and for TBI
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why does IMRT factor not apply to scatter or primary work?
because dose absorbed by the patient is similar to that with conventional
177
parameters tupically used for scatter
90 degree angle and U= 1 conservatively safe result, since the increased intensity of small angle scatter relative to 90 degree scatter is generally offset by the much smaller use factor for the gantry angles producing the small angle scatter
178
what do you multiply neutron dose equivalents by in high energy rooms that use special procedures like TBI?
1.5
179
why is TADR used over IDR?
the use of a measured instantaneous dose-equivalent rate (IDR), with the accelerator operating at maximum output, does not properly repre- sent the true operating conditions and radiation environment of the facility. It is more useful if the workload and use factor are considered together with the IDR when evaluating the adequacy of a barrier. For this purpose, the concept of time averaged dose- equivalent rate (TADR) is used (american thing)
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what is TADR
barrier attenuated dose-equivalent rate aver- aged over a specified time or period of operation. TADR is propor- tional to IDR, and depends on values of W and U. There are two periods of operation of particular interest to radiation protection, the week and the hour.
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how to measure IDRscatter, leakage, and total
IDRscatter = IDRtotal - IDR leakage measure IDR leakage as dose 30 cm beyond a barrier with no phantom at iso and IDR total with a phantom at iso
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equation for Rw
IDR Wpri Upri/Do
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equation for Rh
Rh= Nmax * Hpt Nmax = maximum number of patient treatments in-any- one-hour with due consideration to procedure set-up time Hpt= average dose equivalent per patient treatment at 30 cm beyond the penetrated barrier Hpt is also equal to the time averaged dose equivalent per week (Rw) divided by the average number of patient treatments per week
184
pros and cons of heavy concrete
more high Z material added- attenuates more photons however doesn't attenuate more neutrons heavier and more expensive but can keep barrier thinner
185
pros and cons of lead
toxic, needs to be covered good against photons transparent to neutrons but slows down neutrons through inelastic scattering steel is more expensive but not toxic it is intermediate photon attenuator between concrete and lead
186
earth
consider like concrete, but with density of 1.5 g/cm3 hard to define!
187
addition of iron rebar to concrete
improves photon shielding and neutron shielding steel form ties also not concerning
188
usual material used for a baffle for photons
lead amount of radiation scattered from lead is less than that from lighter materials and the scattered radiation is more readily attenuated in lead thickness of lead where required should be at least equal to equivalent thickness of displaced concrete (ratio of TVLs)
189
typical shielding for door (photons) if there is maze
< 6 mm Pb
190
where to put duct in a room with maze?
through shielding above the door, where photon and neutron fluence is lowest ducts for low E linacs typically won't require shielding but those for high E linacs may depending on the length of the maze (< 2.2 m)
191
where to put ducts in room without maze?
along walls parrallel to gantry rotation
192
considerations for machine cables and water and electrical conduits?
usually none
193
chief drawback for Pb
toxic needs to be held in place by steel or concrete produces neutrons and doesn't have hgh cross section for neutron absorption
194
conduits, ducts through lead
more significant than through concrete because the % thickness of the Pb that the duct takes up is larger... dimension of the opening relative to the width of the barrier determines the absorp-tion of x rays that are diagonally incident on the barrier
195
issue when beam is aimed at junction between ground and floor
-groundshine little shield- ing is provided by the concrete floor slab when the beam is aimed at the junction between the wall and the floor. To rectify this prob- lem it will be necessary to add steel or lead to the floor in order to reduce the scattering path length under the wall. Alternatively, the lead and polyethylene wall can be extended into the floor. -neutrons not an issue because they are attenuated by concrete in floor
196
beamstopper
can now make primary barriers into secondary barriers -usually attenuates scatter with angle up to 30 degrees
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what is consideration in addition to sky shine radiation?
side-scattered photon radiation from ceiling barrier -neutrons produced in roof or oblique incidence aren't considered, but there are dominated by the scattered photon radiation
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shielding for dedicated intra-operative radiotherapy units
Linear accelerators that produce only electron beams are used within operating suites in which direct access to the tumor can be achieved. Shielding assessment of such a mobile electron accelera- tor was considered by Daves and Mills (2001) and they found that these IORT units could be used in standard operating rooms without added shielding if the machine on-time is restricted to ~30 min week–1. This results from: (1) the very low beam currents used for electrons only, (2) the low leakage radiation because no bending-magnets are employed, (3) the low bremsstrahlung pro- duction from the low-Z materials in the beam path, (4) the use of a compact beamstop beyond the tumor volume, and (5) low energy to eliminate neutron production
199
consideration for Co-60
have to ensure that source always returns to "safe" position since it is always on
200
shielding survey
ensure barrier thicknesses are adequate ensure IDR is acceptable search for linac head hot spots with film search for hot spot, cracks, at lasers, ducts etc primary barriers are surveyed without phantom in beam, secondary barriers are surveyed with phantom in beam, max field size check for skyshine, groundshine
201
why are neutron measurements difficult to do?
Neutron measurements inside the treatment room of a radio- therapy facility are fraught with difficulties because of photon interference from the primary and leakage photon beam and the fact that neutron detection is spread over many decades of energy ranging from thermal energies (0.025 eV) to several million elec- tron volts. No single detector can accurately measure neutron flu- ence or dose equivalent over the entire energy range. inside the room have to use passive detector because photon fluence overwhelms active detector outside the room, can use passive or active detector
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active neutron monitoring
Active neutron monitoring usually relies on slowing down fast neutrons or moderating them until they reach thermal energies. A thermal detector is then used to detect the thermal neutrons
203
rem-meters
Rem-Meters. Active detectors such as neutron rem- meters are useful in radiation fields for which the neutron spec- trum is not well characterized, since their response is designed to be proportional to the dose equivalent and therefore independent of neutron energy. Thus, in principle, no knowledge of the neutron spectrum is required. uses ICRP factors Typically, most rem-meters have a very large over-response in the intermedi- ate energy region, and give an adequate measure of dose equivalent between 100 keV and 6 MeV (Rogers, 1979). Therefore, it is impor- tant to know the spectrum, at least roughly, before any reliance can be placed on the instrument readings. usually consists of moderator (polyethylene) that slows down neutrons and a thermal neutron detector (ex. BF3, 3He) In the BF3 detector, the thermal neutrons are captured in the boron via the 10B(nth,α)7Li reaction. The alpha particle and recoil 7Li nucleus each produces a large pulse in the proportional counter. The large pulses are orders of magnitude higher than the pulses produced by photon interactions, and therefore can be discrimi- nated from the small pulses produced by photons in mixed fields
204
neutron spectrometers
thermal neutron detectors in spheres of varying size to deterine neutron spectrum or use scintiallation spectrometer with hydrogeneous scintillator- but photons will interfere or time of flight spectrometer A signal is produced at the point of creation of the neutron or when it first enters the detection system, and the time is measured until that neutron gets to a detector some distance away. The energy of the neutron can be determined by knowing the time taken to travel a given distance.
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passive monitoring for neutrons
-inside treatment rooms The various types of passive monitors are: moderated thermal-neutron activa- tion detectors, threshold activation detectors, TLDs, solid-state nuclear track detectors, and bubble detectors. Passive ther- mal-neutron monitors such as activation foils and TLDs can also be used inside a series of hydrogenous spheres of varying diameters to determine the neutron spectrum
206
difference between passive and active dosimeters
Dosimeters called passive are dosimeters that do not need an external source of energy to operate. They are integrating dosimeters : they give only an estimate of an overall cumulated dose. They do not measure instantaneous doses, unlike active dosimeters that are able to follow the variations of the exposure.
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what does moderator do
provides thermal neutron fluence that is proprtional to fast neutron fluence often used with activation foils (gold and indium) -neutron absorption in foil produces a radioactive nucleus
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solid state nuclear track detectors
detect neutrons mainly by sub-microscopic damage trails from the recoil nuclei of its constitu- ent atoms, namely hydrogen, carbon and oxygen. The damage trails or tracks can be revealed by a suitable etching proces track density is related to neutron dose
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bubble detectors for neutrons
A bubble detector consists of tiny super- heated droplets that are dispersed throughout a firm elastic poly- mer contained in a small sealed tube. The detector is sensitized by unscrewing the cap. Secondary charged particles are produced when the neutrons strike the droplets. The energy deposited by the charged particles causes the droplets to vaporize, producing bub- bles which remain fixed in the polymer
210
does 1000 Gy/week and 500 Gy/week for recommended workloads include QA and research?
yes very conservative since therapists do clinical and physicists do QA so nobody actually sees 1000 Gy/week
211
what do we aim for with shielding in canada?
divide CNSC regulations by 1/20
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is linar bremss. isotropic?
no predominantly forward but not exclusively this is where leakage comes from (through head shielding i.e. primary collimator)
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0.1% of output for head shielding is defined where?
at 1 m from source
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when do we check head leakage?
acceptance testing from vendor
215
how do you make heavy concrete?
add scraps of high Z material to concrete
216
what is leakage dose driven by?
MUs
217
what is B?
barrier transmission factor
218
why is U=1 for secondary barriers?
leakage and scatter are always there (isotropic)
219
does leakage or scatter dominate?
leakage, due to higher E
220
why 25 uSv/hr?
strange constraint remote possibility someone exceeds their occupancy somewhere and exceeds limit?
221
TOMO IMRT factor
16
222
IMRT factor at NSHA
2.5
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what TVLs do you use for door at maze for low E beam? for neutrons?
for 2 MeV beam for neutrons- for 100 keV neutrons and 3.6 MeV photons
224
what is background dose level in treatment room?
0.1-0.2 uSv/h i.e. high E machine returns to background after 48 h from beam-on (0.8 mGy/h 2 min after 30 min beam on)
225
why do we survey the shielding?
-ensure it is constructed per design -can also take density audit of material -also have to report shielding results to CNSC
226
what does additional bend in maze do?
reduce dose at door by 3 X
227
where does skyshine backscatter come from?
air
228
where does side scatter come from?
roof
229
why don't electrons cause photoneutron production?
-lower electron current (X3 order of magnitude) in electron mode vs current mode (since Bremstrahlung inefficient) -electrons have lower cross section for neutron production
230
how do you do shielding survey
-GM counter to look for hot spots -large volume ion chamber (i.e. survey meter) to measure dose rate at points of interest -do both energies -30 cm away from barrier -max FS, 45 degree colli -for primary, beam directed at all and no phantom -for secondary, phantom, and gantry in 4 angles- leakage highest where head is closest to wall
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key point about cyber knife
no isocenter no "primary"
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why are GM counters so sensitive
high Z high pressure high voltage
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brachy leakage requirement
<10 uSv/h at 10 cm, once source is shielded
234
why do heavy materials have low cross section for neutron elastic collisions?
like a ping pong ball hitting bowling balls the neutrons bounce off and don't transfer energy to the material
235
boron that has high cross section for neutron capture
enriched B10 -cross section is 3840 barns -in comparison B11 is 0.005 barns (i.e. 800,000X less) -concrete is around 5 barn? -Fe about 2 barns
236
n,alpha reactions of thermal neutrons
10B(n,alpha)7Li alpha is trapped in door gamma can escape
237
why are slow neutrons more likely to undergo neutron capture?
slower is easier to capture...
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gammas from concrete neutron capture vs boron
concrete = 3.6 MeV boron = 476 keV
239
what happens in inelastic collision
nuclear arrangement changes in some way- i.e. neutron, proton, electron released kinetic energy of system not preserved
240
what does neutron PDD look like?
similar to photons -that is why we can use TVL for neutrons
241
what does the moderating in neutron maze door?
polyethylene does the moderating Pb is only for photons thus can have Pb only on outside of the door if you want (but has to be on outside to get the ncapture gammas) -however putting some Pb in front does reduce high-energy neutrons in energy through inelastic interactions
242
activation products
-materials made radioactive by neutron activation -half life on order of minutes or hours -some products include 56 Mn (2.5 h), 24 Na (15 h), 28 Al (2 min), 62Cu (10 min), 64Cu (12 h), 187W(24 h), 57Ni (37 h)
243
photoneutron reaction template
aX(gamma,n)a-1X -produce evaporation neutrons and direct neutrons
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what materials in linac head have high photoneutron yields
Pb and W
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by how much does neutron dose at outer maze increase by colli is reduced from 40x40 to 0x0?
15%
246
barrier consists of concrete plus a Pb layer. Do neutrons produced in Pb fall off with IS?
No, they fall off as 1/d because Pb is a large volume source (not pt source)
247
what if you increase Pb thickness outside of concrete for neutron door? what if you replace Pb with steel?
neutron dose becomes even worse because Pb creates neutrons but doesn't have high cross section for capturing them -replacing Pb with steel will lower neutron production in metal by 10
248
layers of shielding for laminated barriers- steps taken
-assume there is concrete and Pb laminate -evaluate neutron dose outside shield and photon dose -if neutron/photon dose is higher than protection level, add BPE or concrete above Pb -x-ray ateneuation outside BPE or concrete is evaluated -multiply x-ray level by 2.7 to account for neutron-capture gammas -if photon dose is high, add another Pb layer -measure neutron dose. Add BPE or concrete if needed
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metal laminate inside the treatment room- how much does it increase neutron dose to patient?
2.3 for Pb 1.2 for steel
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items to evaluate during construction inspection
-thickness and density of concrete, metal, BPE -shielding behind recesses (lasers) -HVAC shielding -location of pipe for physics cables -inspection to determine if required shielding design as followed
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when is shielding survey done?
-time of first beam- preliminary survey to ensure no health concerns -once linac completely operational- complete shielding survey
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equipment needed for survey
-large volume ion chamber - rate and integration mode- 0-5 R/h, ie 0-0.05 Sv/h -geiger-muller (check for hot spots) -neutron survey meter -scattering phantom -film for photon head leakage measurement -moderated foil activation dosimeter and associated foil counting system if neutron head leakage is measured -ion chamber to spot check accelerator output
253
elastic scattering of neutrons by hydrogen contributes what % of dose from neutrons?
75% over fast neutron range
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does hydrogen undergo neutron capture
yes -does this in patient
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fast and slow neutron interactions in patient
fast: 1H(n,n)1H proton recoil (elastic) slow: 1H(n,gamma)2H neutron capture and 14N(n,p)14C nuclear reaction
256
rem meter calibration factor
relates instrument response to neutron dose equivalent -slows down neutrons and slow neutrons are detected -response RATE of detector per unit neutron flux as a function of neutron energy has a shape
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why can't the BF3 detectors be used in the treatment room>
photons are too intense- make an intense pulse when accelerator operated won't be able to measure neutrons
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what can be used for neutron surveys inside the room?
activation detectors -don't suffer from interference from pulse photons -gold or indium -induced radioactivity produced by slow neutron capture in foil is measured after irradiation -can also use moderator with activation detector (ex parraffin) -expose room to indium with moderator (get fast neutrons) and without (get thermal neutrons) -must correct for neutrons generated by photon interactions in cadmium shield
259
spots where you should survey for leakage?
-junction of ceiling and floor with primary walls -door and HVAC duct (especially for neutrons)
260
can bubble detectors be used in tx room?
yes but not in main beam, because photon interactions in dosimeter will generate neutrons that will be detected by the device
261
what is included in shielding evaluation report
-date of survey, person doing it etc -methods- survey techniques used, W, T, U etc -instruments used for survey-serial number and date of calibration -results-table of max dose eqivalent to be expected a various points -conclusions and recommendations -floor plan and correlating survey points
262
gamma knife head shield
40 cm of cast iron
263
typical workload for gamma knife unit
-4 targets per patient, 1 fraction -120 s to open door to move patient in and out of unit -1320 s to deliver four dose fractions -100x4 + 1320 = 1800 s door open time per patient
264
max gamma knife leakage rate when helmet door is closed
avg of 2.77 nSv/s
265
example calc for gamma knife unit barrier thickness
calculate dose per week at point when door is open (i.e. dose rate X time open) calculate leakage dose as leakage dose X 40 hour work week -add these 2 together
266
shielding for rooms that house brachy patients
B= Pd^2/(0.8f gamma A) 0.8 is transmission through patient for 137Cs radiation gamma is gamma factor for radium A is mg Ra equivalent in patient f is factor for 137Cs
267
dose rate in brachy maze fall off- does it follow IS?
-no, falls off slower (not pt source, maze not long enough?)
268
Workload for brachy source
gamma factor * f factor * activity * time
269
CT room shielding design
-manufacturer provides isodose distributions (dose per scan or dose per mA min at the pt being evaluated) -manufacturer supplies scatter plots -no primary beam can strike walls, only consider leakage and scatter -use W, kV and mA, T -W is ecxpressed as mA s per week -12000 mA minutes/wk is typical -Dose per week at a point is given by W * dose per scan * T -transmission value TR = P/D and is also Xs/Xo Xs is R per mA min at 1 m for shield thickness s Xo is R per mA min at 1 m for no shielding Xs= Xo *TR -shielding thickness required is read off a curve using Xs
270
shielding for CV simulators
basically treat as linac except W is in mA min -thickness based on transmission is determined from curves -seems like the same kV energy is used for primary, scattered, and leakage radiation (conservative)
271
range of reflection coefficients
0.05 to 0.0015
272
HVAC is above maze door. When does HVAC penetration need shielding baffle?
for short maze with d < 2 m -can use HVAC baffle, duct wrap, concrete baffle
273
air activation due to photoneutrons
14N(gamma, n)13N, half life = 600s 16O(gamma,n)15O, half life = 122 s each of these is positron emitter - exposes techs to positrons and annihilation photons (0.5 MeV) -presents minimum hazard because max risk is dose to skin from positrons, and this was calculated to be well below 0.5 Sv/yr- limit
274
issue with atrium
-dose equivalent on 3rd floor was higher than on 1st floor
275
water door
water filled with boric acid much cheaper than typical Pb and BPE door
276
how do you make a neutron detector only respond to fast neutrons?
surround moderator with cadmium because slow neutrons cannot penetrate cadmium
277
thin vs thick moderator
thin= higher detection efficiency for low energy neutrons thick= higher detection efficiency for high energy neutrons
278
neutron- moderated REM meters for dose equivalent rate measurements
possible to design a moderated detector whose counting efficiency (counts per neutron) varies with neutron energy in the same way that the dose equivalent per unit neutron fluence varies with neutron energy
279
TVD refers to what radiation?
photons and neutrons only in maze
280
do you use laminate on secondary barrier?
No, only on primary, not necessary for secondary laminate on inside part of linac room (so neutrons get absorbed in concrete)
281
US dose per year
6 mSv/year imaging (mostly CT) added 3 mSv/y to background
282
5-year period in CNSC
same for everyone Started Jan 1, 2001 and is 5 year increments from there
283
say you don't want to use personal TLDs but need to show CNSC your doses are within regs. What can you do?
-badge a subset of workers -area monitors
284
do pregnant workers have to inform RSO of being pregnant?
Not as of 2021
285
what can you do to accomodate pregnant worker
biweekly dosimetry put on lower energy linac (high energy linac has activation products near linac head, where therapist would work)
286
CNSC action level
-the licensee defines the action levels in the license. If exceeded, have to report to CNSC immediately and submit report in 21 days -At NSHA, action level is if a NEW gets 10% of annual limit
287
drawings required in license to construct
-circuit logic, intercom, CCTV system, beam on/off, enable/disable
288
Why is TVL for 3.6 MeV photons 6.1 cm but TVL for 6 MV photons 5.7 cm?
6 MV photons have average energy around 2 MeV- less energetic
289
density of concrete
2.35 g/cm3 regular 5 g/cm3 heavy
290
why is brachy all primary barriers?
radiation emitted in all directions DON'T say isotropic source
291
Can CNSC inspection be a surpise
yes, both types can
292
what can you do if workload higher than license
-ask CNSC to increase workload- apply for license amendment -not considered incident since you are informing them in compliance report
293
most likely neutron detector
bubble dector don't forget to talk about moderator!
294
what to do if RSO changes
license amendment because RSO is named in license
295
what detector to use for leak test?
well counter
296
radiation safety audiot
-internal or external -essentially do CNSC audit but its not from CNSC
297
code vs regulation
code is recommendation regulation is law
298
acceptable dose to the public in an emergency
20-100 mSv/year in planning
299
equation for surface contamination
C(surface) =( (cpm(gross)- cpm(background))/ efficiency )* area of wet wipe/detector area cpm is counts/min efficiency 33 % for scintillation counter -wipe is usually 100 cm2 -answer is given as cpm/100 cm2 usually -dpm is disintegration/min
300
what is smear test
used to estimate hazard from surface contamination by transmission of the contamination from the surface into the body via inhalation or digestion
301
skin reactions to dose
2 Gy: transient erythema (hours) 6 Gy: erythema (1-2 weeeks) 10 Gy: dry desquamation > 15 Gy: moist desquamation hair loss -deterministic for 3-5Gy -onset after 2-3 weeks -can be permanent for dose > 7 Gy
302
risk of mental retardation for fetus
0.4/Gy at 8-15 weeks -four times lower at 16-25 weeks -treshold of 0.3 Gy
303
when does microcephaly and general growth retardation occur?
< 16 weeks
304
when does growth retardation occur?
-congenital anomalies, neonatal death, temporary growth retardation for 1-6 weeks -permanent growth retardation > 6 weeks
305
summarize hematopoetic symptoms, GI symptoms, and cerebrovascular symptoms from radiation
hematopoetic= fever, chills, fatigue GI = nausea, diarrhea, vomiting, anorexia brain = disorientation, loos of coordination, seizure, coma