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On call > brachy stuff > Flashcards

brachy stuff Flashcards

1
Q

constraints and objectives for permanent implant prostate PDR

A

CTV: V100%> 95% so D90% will be > 100%
CTV: V150% < 50%
rectum: D2cc < Rx, D0.1cc < 150%
urethra: D10< 150%, D30<130%

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2
Q

who is eligible for SABR prostate with fiducials?

A

low-intermediate risk
-36.25 Gy/5 to PTV, 40Gy/5 to CTV, on alternate days
-if patient has hip prosthesis, there would be too much artifact in the CT

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3
Q

seed activity

A

0.2-0.4 mCi for I-125
1-2 mCi for Pd-103

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4
Q

survey readings in brachy

A

Brachy: >1 mSv/hour; indicative of source completely outside of the afterloader). Normal background is <0.01 mSv/hour in the treatment room when the source is completely in the safe.

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5
Q

gyne 1500/3 bladder and rectum constraints

A

bladder 620 cGy
rectum 420 cGy

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6
Q

APBI brachy dose

A

o HDR balloon brachytherapy dose: 34 Gy b.i.d. x 5 days
o OARs (skin, lung, heart) must be far enough away; with small breast this may be difficult to achieve.

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7
Q

HDR monotherapy prostate experiments dose levels: 13.5 Gy x2 fx

A
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8
Q

Ir-192 vs 60Co decay

A

Ir-192 decays 1%/day, Co-60 decays 1%/month

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9
Q

initial dose rate of a permanent implant

A

= prescription dose/ average life of the source
average life is 1.44X the half-life

  • Pd-103- 21.3 cGy/h
  • I-125: 7 cGy/h
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10
Q

standard brachy skin prescriptions for surface applicators

A

Leipzig, Valencia, and eBT
40Gy/8
42Gy/6

achieves BED of 60-71.4 Gy

treatment usually delivered every other day

prescription are prescription points are all over map, some at surface, some 3-5 mm from skin surface

-most common is 3-4 mm prescription depth for Leipzig, valencia, and eBT applicators

~10%/mm gradient (PDD)- so skin surface if prescrbing to 3 mm depth is 130-150%

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11
Q

standard brachy skin interstitial treatment dose

A

30 Gy/10fx

used for lesions more than 5 mm deep (surface brachy would give unacceptable high skin dose)

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12
Q

valencia vs leipzig

A
  • Valencia has FF between source and skin surface = increased treatment time than Leipzig but better dose profile
  • typically made of high Z material (thus can be artifacts if CT is used for imaging)

Have to use Monte Carlo because TG43 doesn’t model the metal

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13
Q

does skin applicator have to be flush to skin?

A

yes, otherwise there would be an air gap
-dose fall off for air gap of 1 mm can be 10%

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14
Q

what should dose to the skin surface be limited to for skin brachy?

A

125% for flaps and 140% for custom molds

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15
Q

why use brachy for skin vs kV or electrons?

A

skin brachy is useful if the skin has a complicated surface

kV and electrons- need flat surface. With skin, can use freiburg flap, moulds

skin brachy can be interstitial- less dose to skin surface compared to kV. Also less dose to organs past skin with brachy compared to EBRT

Valencia/Leipzig can be tough for skin surface as PTV has to fit within 3 cm diameter device and need flat surface- however, faster dose fall-off than ortho and will “stick” to target

freiburg flap-can get more heterogeneous dose distribution compared to using kV or electrons

no brachy near eyes (skin too fragile)- skin in general is risky-can disfigure face

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16
Q

standard of care for gyne brachy planning

A

mri

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17
Q

can you use PET-CT bunker for HDR suite?

A

No, because PET-CT is shielded for 0.5 MV photons whereas Ir-192 spectrum includes photons of higher energy than 0.5 MV

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18
Q

is 60 cc rule in prostate more relevant for HDR or LDR?

A

LDR - with HDR it is easier to spare OARs with optimization

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19
Q

you cannot see anterior part of prostate- what can you do?

A

decrease US frequency to get increased penetration at expense of resolution

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20
Q

building new brachy program- major consideration?

A

what source to use

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21
Q

Ir-192 vs I-125 g(r) fall-off

A
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22
Q

In machester system, why is Point A relative to tandem whereas point B is relative to patient body?

A

point A (which represents the crossing of the uterine artery and the ureter) is best approximated relative to the uterus while point B (which represents the pelvic lymph nodes) is best approximated relative to the patient’s body. This difference can be significant when the uterus is tilted relative to the pelvis.

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23
Q

vault dose in this clinic

A

monotherapy: 21/3
11/2 brachy with 45/25 EBRT
ring and tandem: 28/4 HDR with 45/25 EBRT (for gyne not vagi)

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24
Q

why can’t you use vault to treat beyond 5 mm?

A

plan would be too hot

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25
Q

how does the disk size for the vault affect the plan?

A

large disk- difficult to get homogeneous dose at upper part because side has 2 cm diameter whereas top is only 12 mm (for 4 cm cylinder); top tends to be hot. Dr. Bowes likes to not include the first dwell position to improve homogeneity.
small disk- hard to get homogeneous dose
-larger disk = impact of IS differences along vault less significant since IS is less significant at larger distances

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26
Q

how is ICRU system different than manchester system?

A

ICRU: relate the dose distribution to the target volume rather than to a specific point. (more like ext beam)

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27
Q

when is vaginal cuff (vault) treatment indidcated?

A

post hystorectormy
endometrial cancer
1) patients with grade 1 or 2 cancers with > 50 % myometrial invasion
2) 2) patients with grade 3 cancers with < 50 % myometrial invasion

usually if invasion > 50 %, get EBRT plus brachy boost (in addition to operation)
if invasion < 50%, just get brachy boost (in addition to operation)

-studies showed that radiotherapy post hysterectomy prevented relapses, mostly in vaginal cuff
-brachy = less toxicity compared to EBRT

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28
Q

textbook EQD2 constraints for rectum, sigmoid, bladder in cervix treatment

A

75 Gy EQD2 for rectum and sigmoid
90 Gy EQD2 for bladder

29
Q

2 most common indications of gyne brachy

A

post-operative endometrial
cervix

30
Q

is cervical brachy therapy ever used as monotherapy?

A

rarely, if early-stage cancer

31
Q

LDR dose for gyne

A

40 Gy in 2 insertions (2 fractions of 20 Gy)
-0.2-4 Gy/h dose rate

32
Q

guidlines for dose to OARs for gyne brachy

A

American brachy association calculates EQD2 for patient-specific plans
-typically, for 7 Gy fraction, 4Gy to hottest 2 cc of OARs is reasonable

33
Q

usual number of sources used in LDR tandem and ovoids

A

3 in tandem and one in each ovoid
-hard to shape dose compared to with HDR

34
Q

does ABS recommend still using a tandem if doing interstitial?

A

yes, to prevent a cold spot

35
Q

when does american brachy association recommened LDR or PDR boost for gyne?

A

2 applicatiors to allow for reduction in tumor volume and improved tumor coverage wth 2nd application
-first should be within 4-6 weeks of start of EBRT
-2nd should be 1-2 weeks later

36
Q

how long after surgery do you do the vaginal vault?

A

at least 4 weeks

37
Q

what if the vaginal vault cylinder is too small?

A

-can be air gaps or folds leading to underdosage of target

38
Q

who is candidate for vagi interstitial brachy?

A

patients with stage I-IVA vaginal cancers or recurrent cervical, endometrial, or vulvar carcinoma in vagina with vaginal lesions > 0.5 cm thick

39
Q

why use tandem and cylinder?

A

narrow vagina
treat varying lengths of vagina if there is spread of disease

40
Q

what is included in the low risk CTV in cervical cancer? Intermediate risk? High risk?

A

CTV-TLR comprises the whole
parametria, the whole uterus, the upper part of the
vagina, and the anterior/posterior spaces toward the bladder and rectum

CTV-THR = CTV-Tadapt that includes the GTV-Tres, the
whole cervix, and adjacent residual pathologic tissue,
if present

CTV-TIR = The CTV-TIR represents the GTV-Tinit as superimposed on the topography at the time of brachytherapy, together with a margin surrounding the anatomical cervix border (CTV-THR) in areas without an initial GTV-Tinit

41
Q

dose fall-off from vault plan

A

1 cm to go from 200% to 100 %, 1 cm to go from 100% to 50 %

42
Q

If someone is 25 cm away from the source during an emergency, how much dose would they receive in 5 minutes while resolving the issue?

A

3-6 cGy

43
Q

required range of G-M and ionization survey meters

A

G-M: 0.1-100 mR/h
ionization: 1-1000 mR/h

44
Q

what correlatives with urethral toxicity?

A

V150
V200
prostate size

45
Q

typical edema values

A

30% post implantation
50% within a day of implantation
10% 30 days after implantation

46
Q

Miami-type applicator

A

vaginal cylinder plus tandem. The vaginal cylinder has six channels in this case, allowing for asymmetric dose distributions within the vagina.

47
Q

how to QA the position of the dwell in the applicator

A

can use fluoro- see end of tube and source location
-can use film or EPID- mark end of tube and see where dose distribution is centered

48
Q

how to calculate transport index

A

determine the max radiation level in mSv/h at 1 m distance from external surfaces of package. This value multiplied by 100 gives the TI. Values shall be rounded up to the first decimal place, except that values of 0.05 may be considered zero.

49
Q

categorize I-white, II-yellow, III-yellow

A

o TI = 0 and max radiation level at any point on external surface <= 0.005 mSv/h  I-white
o TI > 0 but <=1 and max radiation level at any point on external surface > 0.005 mSv/h but <= 0.5 mSv/h  II-Yellow
o TI >1 but <=10 and max radiation level at any point on external surface > 0.5 mSv/h but <=2 mSv/h  III-Yellow
o TI > 10 and max radiation level at any point on external surface > 2 mSv/h but <= 10 mSv/h  III-Yellow, Must be transported under exclusive use.

  • Where the TI satisfies the condition for one category but the surface radiation level satisfies the condition for a different category, assign it to the higher category.
50
Q

necessary markings for transport of radioactive source

A
  • names and adresses of consigner and consignee
  • UN number (7)
  • radionuclides, their chemical form, and the max activity of the contents
  • category I, II, or III
  • transport index (for categories II and III only)
  • certificate of approval from various relevent authorities
  • specifify exclusive use if relevant
51
Q

assumptions of TG-43

A

No source-to-source shielding effects
all tissues in and around implant are water equivalent
scattering volume within patient is equivalent to that used in the concensus data sets (at least 5 cm of water-equivalent material surrounds the point of calculation)

52
Q

what does F(r,theta) approach with increasing radial distance?

A

unity
for all angles except 0

53
Q

where are concensus data for the sources taken from?

A

averaged experimental data are averaged with MC data

54
Q

what was TG-43 specifically written for?

A

interstitial brachy

55
Q

fundamental problem with pre TG43 protocols

A

based upon photon fluence around the source
in free space, whereas clinical applications require dose distributions
in a scattering medium such as a patient. Determination
of two-dimensional dose distributions in a scattering
medium from a knowledge of the two-dimensional distribution
of photon fluence in free space is easily accomplished
only for a point isotropic source. An actual brachytherapy
source exhibits considerable anisotropy

-could not handle non-point sources

TG43 solves this issue by measuring dose distribution in water equivalent phantoms

56
Q

what is 1-D anisotropy factor?

A

-average 2D variant for each radius

57
Q

overall uncertainty in dose rate at a point around a source TG43

A

10%

58
Q

for what r does TG43 formalism using a polar coordinate system break down?

A

r < L/2
points are inside the source capsule

59
Q

issue with lack of heterogneity corrections in TG43

A

high energy source: nearly same behaviour as water
low energy source: importance of PE effect increases as energy decreases

I125- difference of 9-20 % in lung vs water

60
Q

predominant effect on dose

A

distance

61
Q

FINAL extrapolation/interpolation guidelines per TG43 S1

A

F- linear-linear interpolation using 2 nearest adjacent points
for r< rmin, use F(rmin). For r> rmax, use F(rmax)

Phi(r) - log linear using adjacent data points. This differs from TG43U. Gives equation for r< rmin. For r> rmax, use Phi(rmax)

g(r)- log linear usding adjacent neighbours

  • use nearest g(rmin) for r< rmin
  • for r> rmax, there is equation with exponential function
  • for gp, interpolate gl instead (as gp changes drastically with r), then multiply by ratio of gp/gl
62
Q

where does TG 43 not work?

A
  • inhomogeneous
  • not enough scatter material
  • at small r where line source approximation breaks down
  • inter-source shielding effects and shielding from applicators
  • orientation of seeds unknown (argue can use pt dose approximation or 1D anisotropic function)
  • transit time not accounted for
63
Q

Unit of air kerma strength

A

uGym2/h

64
Q

air kerma strengths of Ir-192 sources

A

29000-41000U

65
Q

I-125 air kerma strength

A

6.3 U (6711)
51 U (6702)

66
Q

Pd-103 air kerma strength

A

2.6 U

67
Q

Co-60 air kerma strength

A

15000-18000U

68
Q

Cs137 Sk

A

56 U

69
Q

planning guidelines for HDR prostate brachy

A

V100%> 95%
V200%<11%
Urethra D10%< 118%
Urethra Dmean < 125%
Rectum V80%<0.5cc

On call (23 decks)

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