Mike's notes Flashcards
radiation safety accessories
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
CNSC annual dose limits
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
shielding max permissible dose (P)
NEW: 1 mSv/yr
public: 0.05 mSv/year
NCRP suggests 5 mSv/yr in controlled area, 1 mSv/yr in uncontrolled area
what is CNSC
canadian nuclear safety commission
what is nrc
nuclear regulatory commission (US)
pregnant NEW dose
4 mSv from declaration to end of pregnancy
PER CNSC FINAL ANSWER
diagnostic dose
3 mSv/yr
50% from CT and 25 % from nuc med
background radiation
1mSv year from cosmic (0.3 mSv), terrrestrial (0.3 mSv), and internal (0.4 mSv)
2 mSv from Radon
ALARA limits vs ICRP60 limits
ALARA limits are 1/20 those of ICRP60 or CNSC
what do you do if radiation level is > 25 uSv/h?
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
properties of leakage radiation
depends on design
limitied to 0.1% of primary beam
originates from target
assumed to be isotropic
properties of scatter radiation
assumed to come mostly from patient
use largest field size fr measurement (40x40)
Assumed to be isotropic
when is neutron shielded needed?
E >/= 10 MV
barrier types
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
typical barrier thickness
Primary 2.1 - 2.4 m
Secondary 0.9 - 1.2 m
Thickness depends on energy, workload, occupancy, and distance.
why do we use 35x35 instead of 40x40 for max field size?
max field size not perfectly square (clipped corners)
-35x35 with collimator rotated 45 degrees
DONT USE THIS QUESTION
why is hydrogen content of shielding material important?
for neutron shielding
different shielding materials
concrete
heavy concrete
steel
lead
earth, dry packed
TVL
TVL = ln(10)/u
u is broad beam linear attenuation coefficient
more shielding needed with broad beam vs narrow beam to stop additional scatter
relate TVL to HVL
TVL =HVL * ln(10)/ln(2)
for broad beam, why are subsequent TVLe < TVL1?
TVLe is subsequent (equilibrium) TVL
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.
concrete primary TVL1, TVLe
TVL1 = 37 cm, TVLe = 33 cm for 6 MV
TVL1 = 45 cm, TVLe = 43 cm for 18 MV
concrete leakage and scatter TVL for 6 MV
leakage TVL1 = 34 cm, TVLe = 29 cm
scatter TVL = 17 cm
concrete leakage and scatter TVL for 18 MV
leakage TVL1 = 36 cm, TVLe = 34 cm
scatter TVL = 19 cm
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
barrier thickness t expressed in number of TVLs (n)
n = log10(1/B)
B = I/Io
t= TVL1 + (n-1)TVLe
barrier thickness as function of TVL1 and TVLe
t = TVL1 + (n-1)TVLe
TVL1 and TVLe for steel
10 cm at 6 MV, 11 cm for 18 MV
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
use factor for all secondary barriers
1
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
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
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
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.
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
typical 6 MV primary
6 TVL = 120 cm heavy concrete
typical 6 MV secondary
4 TVL = 80 cm heavy concrete
equation for secondary barrier (scatter)
B= (P/(alphaWT)dsca^2dsec^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)
what does alpha in equation for secondary barrier depend on?
-scattered angle and energy
-also material but this is always water for the patient
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.
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
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
why is leakage barrier > scatter barrier?
leakage energy> scatter energy
Scatter energy is always lower than the primary energy due to Compton interaction.
how do you get HVL from TVL?
HVL=TVL x ln(2) / ln(10)
is t18MV > t 6MV always true?
No, depending on the workload, the barrier thickness for 6MV could be larger (or smaller) than 18MV.
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
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
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
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
why may Wl differ from Wprimary?
due to IMRT factor
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)
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)
typical 6 MV door
6 mm of Pb
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.
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
why is f(theta) larger for smaller theta?
Compton effect
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
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
shielding survey- head leakage
locate hot spot via head-wrapped film, and quantify dose rate with ion chamber
shielding survey- barriers
measure dose equivalent/MU and dose rate at the hottest spot beyond each barrier (30 cm beyond)
search forvoids; cracks, or other defects in shielding using a sensitivephoton rate meterwith a fastresponse time (egGeiger Muellerorscintillation detector)
shielding survey- primary barrier
no phantom, max dose rate and field size, 4 gantry angles, all MV
shielding survey- secondary barrier
scattering phantom at iso (simulates patient)
shielding survey- maze door
when door open/closed
shielding survey - skyshine and side scatter
surveys outside bunker
how often do you calibrate survey meters?
annually
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
when do you need to survey for neutrons?
linac >/= 10 MV
usingn-survey meter, egrem 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
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
HDR brachy shielding equation
B = Pd^2/WT
B= barrier transmission factor
P= max permissible dose equivalent (= 0.1 mSv/wkcontrolled areas
= 0.02 mSv/wkuncontrolled 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
TVL for Ir-192 concrete and lead
15 cm concrete and 1.5 cm lead
TVL for Co-60 concrete and lead
210 cm concrete, 40 cm lead
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
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).
typical shielding for cyberknife
primary = 150 cm concrette
secondary (leakage) = 90 cm concrete
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
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
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
effective dose
equivalent dose to tissue T is weighted by tissue weighting factor Wt and summed over all irradiated tissues
where is data for weighting factors from
ICRP 103
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).
Wt for red bone marrow, colon, lung, stomach, breast, remainder tissues
0.12 each
wt for gonads
0.08
wt for bladder, oesophagus, liver, thyrois\d
0.04 each
wt for bone surface, brain, salivary glands, skin
0.01 each
put equivalent dose and effective dose together
Ht = sum of wr Dr (equivalent dose)
E = sum of wt Ht (effective dose)
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
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”.
equation for IDR
IDR = (dose rate) x (barrier transmission) / (distance)^2
Also IDR = P(dose rate)/(WUT)
why is Rw independent of linac set dose rate?
dose rate appears in IDR equation and is then divided by Do to get Rw
TADR secondary barrier
similar equation to primary barrier
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)
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
types of neutron interactions
elastic collision with nuclei
inelastic collision with nuclei
neutron capture
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
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!
what is a proton emitted from a neutron interaction called?
recoil proton
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
what does high energy n contribute to dose in soft tissue via nuclear disintegration?
30%
describe neutron capture (activation)
(n, gamma)
emitted gamma ray is called neutron capture gamma ray
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.
what element has high thermal neutron capture cross section?
boron
borated polyethylene is used for n shielding
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
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
n production in electron mode
> 2 orders of magnitude less than photons
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
rule of thumb for VMAT MU
3 X Rx
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)
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
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
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
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)
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
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
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
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
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
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.
when do you use laminated barrier
> /= 10 MV
when space is needed
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
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
wrt laminated barrier, can you put the metal after the concrete instead of sandwiched in between 2 concrete layers?
this produces max neutrons…
for laminated barriers- is Pb or Fe better?
Steel is better choice because photo-neutron cross section in steel is 10X less than lead
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
4 CNSC licenses
obtain a license to
-operate
-service
-commission
-decommission
what do the 2 CCTV cameras do?
provide orthogonal views of patient
does CNSC review QA records?
yes
annual compliance report
must be submitted to CNSC at end of every calendar year
linac worload at halifax
30% QA + research
70% clinical
1000 Gy/week
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
where is TADR used?
US
likely not accepted by CNSC
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
can metal go outside the concrete for a laminated barrier?
Not for >/= 10 MV because of neutron production
For < 10 MV, yes it can
is calculated workload sent to CNSC annually?
yes
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
what type of dose is used in shielding neutrons and photons?
dose equivalent, H (Sv)
includes qlity factor for radiation type
what type of dose is used in shielding for low LET radiation?
air kerma
radiation protection for neutrons uncertainty
35% with 95% CI for dose ratres < 0.02 mSv/h
controlled area
admittance in the area is under supervision of someone in charge of radiation protection
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
what determines radiation weighting factor?
the type and energy of the
ionizing radiation that is incident on the body.
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
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
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.
where is workload defined?
at 1 m from source
TBI use factor
U will be higher in direction of TBI treatments
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.
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.
for shielding, where does brems and neutron production occur?
brem occurs in target
neutron production occurs in both walls of room and linac head
considerations for E > 10 MV
neutrons
pair production
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.
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
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
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.
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
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.
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.
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
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
how were the reflection coefficients determined?
monte carlo
poatient scattered radiation for maze for E > 10 MV
usually ignored because it is insignificant compared to the leakage scatter
energy for reflection coefficient for patient scattered radiation for maze
use E = 0.5 MeV to be conservative
fraction of primary beam transmitted through patient
~ 0.25 for 6-10 MV
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
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)
for what collimator setting is max neutron dose seen??
closed collimators
ie. most photoneutrons originate in the treatment head
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
alternative maze designs for neutron shielding
typical maze door may be very heavy, expensive, motorized
other options:
- reduce the opening at the inside maze entrance;
- add a light-weight door containing a thermal neutron
absorber (boron 9 % by weight) at the inside maze entrance;
and - place a BPE (5 % boron) door at the inside maze entrance.
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
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
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%
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
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
workload for electron beams
disregard unless the machine is electrons only
typical number of patients per day on a machine
30
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
why does IMRT factor not apply to scatter or primary work?
because dose absorbed by the patient is similar to that with conventional
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
what do you multiply neutron dose equivalents by in high energy rooms that use special procedures like TBI?
1.5
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)
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.
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
equation for Rw
IDR Wpri Upri/Do
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
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
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
earth
consider like concrete, but with density of 1.5 g/cm3
hard to define!
addition of iron rebar to concrete
improves photon shielding and neutron shielding
steel form ties also not concerning
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)
typical shielding for door (photons) if there is maze
< 6 mm Pb
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)
where to put ducts in room without maze?
along walls parrallel to gantry rotation
considerations for machine cables and water and electrical conduits?
usually none
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
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
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
beamstopper
can now make primary barriers into secondary barriers
-usually attenuates scatter with angle up to 30 degrees
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
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
consideration for Co-60
have to ensure that source always returns to “safe” position since it is always on
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
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
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
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
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.
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
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.
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
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
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
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
what do we aim for with shielding in canada?
divide CNSC regulations by 1/20
is linar bremss. isotropic?
no
predominantly forward but not exclusively
this is where leakage comes from (through head shielding i.e. primary collimator)
0.1% of output for head shielding is defined where?
at 1 m from source
when do we check head leakage?
acceptance testing from vendor
how do you make heavy concrete?
add scraps of high Z material to concrete
what is leakage dose driven by?
MUs
what is B?
barrier transmission factor
why is U=1 for secondary barriers?
leakage and scatter are always there (isotropic)
does leakage or scatter dominate?
leakage, due to higher E
why 25 uSv/hr?
strange constraint
remote possibility someone exceeds their occupancy somewhere and exceeds limit?
TOMO IMRT factor
16
IMRT factor at NSHA
2.5
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
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)
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
what does additional bend in maze do?
reduce dose at door by 3 X
where does skyshine backscatter come from?
air
where does side scatter come from?
roof
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
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
key point about cyber knife
no isocenter
no “primary”
why are GM counters so sensitive
high Z
high pressure
high voltage
brachy leakage requirement
<10 uSv/h at 10 cm, once source is shielded
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
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
n,alpha reactions of thermal neutrons
10B(n,alpha)7Li
alpha is trapped in door
gamma can escape
why are slow neutrons more likely to undergo neutron capture?
slower is easier to capture…
gammas from concrete neutron capture vs boron
concrete = 3.6 MeV
boron = 476 keV
what happens in inelastic collision
nuclear arrangement changes in some way- i.e. neutron, proton, electron released
kinetic energy of system not preserved
what does neutron PDD look like?
similar to photons
-that is why we can use TVL for neutrons
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
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)
photoneutron reaction template
aX(gamma,n)a-1X
-produce evaporation neutrons and direct neutrons
what materials in linac head have high photoneutron yields
Pb and W
by how much does neutron dose at outer maze increase by colli is reduced from 40x40 to 0x0?
15%
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)
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
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
metal laminate inside the treatment room- how much does it increase neutron dose to patient?
2.3 for Pb
1.2 for steel
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
when is shielding survey done?
-time of first beam- preliminary survey to ensure no health concerns
-once linac completely operational- complete shielding survey
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
elastic scattering of neutrons by hydrogen contributes what % of dose from neutrons?
75% over fast neutron range
does hydrogen undergo neutron capture
yes
-does this in patient
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
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
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
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
spots where you should survey for leakage?
-junction of ceiling and floor with primary walls
-door and HVAC duct (especially for neutrons)
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
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
gamma knife head shield
40 cm of cast iron
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
max gamma knife leakage rate when helmet door is closed
avg of 2.77 nSv/s
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
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
dose rate in brachy maze fall off- does it follow IS?
-no, falls off slower (not pt source, maze not long enough?)
Workload for brachy source
gamma factor * f factor * activity * time
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
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)
range of reflection coefficients
0.05 to 0.0015
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
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
issue with atrium
-dose equivalent on 3rd floor was higher than on 1st floor
water door
water filled with boric acid
much cheaper than typical Pb and BPE door
how do you make a neutron detector only respond to fast neutrons?
surround moderator with cadmium because slow neutrons cannot penetrate cadmium
thin vs thick moderator
thin= higher detection efficiency for low energy neutrons
thick= higher detection efficiency for high energy neutrons
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
TVD refers to what radiation?
photons and neutrons
only in maze
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)
US dose per year
6 mSv/year
imaging (mostly CT) added 3 mSv/y to background
5-year period in CNSC
same for everyone
Started Jan 1, 2001 and is 5 year increments from there
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
do pregnant workers have to inform RSO of being pregnant?
Not as of 2021
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)
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
drawings required in license to construct
-circuit logic, intercom, CCTV system, beam on/off, enable/disable
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
density of concrete
2.35 g/cm3 regular
5 g/cm3 heavy
why is brachy all primary barriers?
radiation emitted in all directions
DON’T say isotropic source
Can CNSC inspection be a surpise
yes, both types can
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
most likely neutron detector
bubble dector
don’t forget to talk about moderator!
what to do if RSO changes
license amendment because RSO is named in license
what detector to use for leak test?
well counter
radiation safety audiot
-internal or external
-essentially do CNSC audit but its not from CNSC
code vs regulation
code is recommendation
regulation is law
acceptable dose to the public in an emergency
20-100 mSv/year in planning
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
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
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
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
when does microcephaly and general growth retardation occur?
< 16 weeks
when does growth retardation occur?
-congenital anomalies, neonatal death, temporary growth retardation for 1-6 weeks
-permanent growth retardation > 6 weeks
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
