L 14: Brachytherapy Flashcards
(65 cards)
1
Q
Exposure rate is defined as
A
It is the rate per hour at a point of 1cm from 1mCi point source.
2
Q
Sealed Sources
* Ra 226 : High energy, Gamma Emmiter
* I 192 : High energy, Gamma Emmiter
* Cs 137 : High energy, Gamma Emmiter
* I 125 : Low energy, Electron capture
* Pd 103: Low energy, Electron capture
* Co 60 : High energy, Gamma Emmiter
A
Unsealed Sources
Beta radiation
Short half life, limiting risk for environment and systemically
* I 131
* Y90
* P32
3
Q
Ir-192
Used in HDr
Imp to memorize
A
- T1/2 = 73.8 days
- Exposure rate constant = 4.69
- Photon Energy = 0.38 MeV ( uses gamma-rays)
- 73.8 and 0.38 (half life and energy)
- Produced by Neutron bombardment of Ir-191
- B-ve decay
- Less energy so less shielding required
4
Q
Ra-226
A
- T1/2 = 1600 years
- Exposure rate constant = 8.25 mg/hour
- Photon Energy = 0.83 MeV
- Difficult to shield so not commonly used, replaced by Ce-137 - LDR and Ir-192 -HDR
- Alpha emitter
- Decays to Radon: 4 days 1/2 life
5
Q
Co-60
Used in GK SRS
A
- T1/2 = 5.26 years
- Exposure rate constant = 13.07
- Photon Energy = 1.55 MeV
- Produced by Neutron bombardment
6
Q
I-125
Used for prostate seeds
A
- T1/2 = 59.4 days (~60 days)
- Exposure rate constant = 1.46
- Photon Energy = 28 KeV (Low energy x-rays)
- Produced by Neutron bombardment
- Electron capture, characteristic x-rays
- Anisotropic dose distribution
7
Q
Pd-103
Used in Prostate seeds
A
- T1/2 = 17 days
- Exposure rate constant = 1.48
- Photon Energy = 21 MeV (Low energy x-rays)
- Produced by Neutron bombardment
- Electron capture, characteristic x-rays
Dose is delivered at a much faster rate.
Very similar to I 125: Dose, exposure rate and Photon Energy
Faster dose delivery due to shorter half life
8
Q
Cesium-137
A
- T1/2 = 30 years
- Photon Energy = 662KeV
- Exposure rate constant = 3.26
2 % decay per year
Gamma Emitter
2.53 mCi of Cesium ~ 1mg of Ra
9
Q
What all are used for calibration of HDR sources
A
- Activity
- Exposure rate at a specified distance
- Equivalent mass of radium.
- Apparent activity.
- Air-Kerma Strength
10
Q
equivalent mass of radium
A
1 micro Gym^2/hr = 0.138mg Ra.
11
Q
Strontium-90
A
T 1/2 = 28.8 years
12
Q
Ytterbium-169
A
- T 1/2 = 32 days
- Photon Energy = 100 KeV
13
Q
Americium-241
A
- T 1/2 = 432 years
- Photon Energy = 60 KeV
14
Q
Gold
A
- T 1/2= 2.7 days
- Photon Energy = 412 KeV
15
Q
Breast Brachy, where is the dose prescribed to?
A
1 cm from the balloon surface
16
Q
Peterson-Parker (Manchester) system
A
- Designed to deliver a uniform dose to a plane or volume.
- Originally for Ra needles now replaced by Ir-192 ribbons.
- Needle/Ribbon spacing <=1cm.
- Uncrossed ends reduce the area/volume of +/-10% ( this means source distribution is non uniform (Peripherally loaded) to create uniform dose distribution)
- Between 25-100 cm^2 activity = 50-50 between periphery and center area
- Cannot be used for isotopes that has a low energy. Energy needs to be reasonably close to Ra 226
- Has crossed ends, needles/catheters run perpendicular to each other.
17
Q
The Quimby system
similar to peterson parker, has crossed ends
A
- The Quimby system is characterized by a uniform distribution of sources of equal linear activity.
- It results in a higher dose in the middle of the implant.
- Dose specification for a Quimby implant is different from that for the Paterson-Parker system
- Uses crossed end needles/catheters
- Uniform loading so can have central hot spot
18
Q
The Paris System
A
- Has multiple paraller needles/catheters
- Uniform loading, identical dose for all needles
- Uniform spacing of all needles
- Central hot spot noted
- PArallel ends: no crossing of needles
- It is designed for removable implants of long line sources such as Ir-192.
19
Q
The computer system
A
The computer system is similar to the Paris system except for dose specification.
Most mordern of all
Used mainly for prostate
20
Q
Exposure rate constant
A
- Depends on radioisotope used
- Dose at 1cm distance from 1 mCi source
- Rinken cm^2/mCi/hour
21
Q
Types of Brachy dose distribution
A
- Geometry Factor
- Anisotropy Factors
- Radial Factor
22
Q
Geometry Factor (G)
TG-43
A
- Represents the spatial distribution and inverse square dose fall off with distance G = 1/r^2 for point source
23
Q
Anisotropy Factor (F)
TG-43
A
- Dose varies with angle to source.
- It is observed as decreased dose at the ends of a line source/ brachy device.
- Angular dependence of photon scatter and absorption
- Attenuation is parallel to the seed axis
- F = This correction factor compensates for variation in attenuation with angle to the source
- More energy = more isotropy; Less energy = more Anisotropy
24
Q
Radial Dose Factor (gr)
TG-43
A
- Describes the radial dose fall off in water than air
- Describes the absorption and scatter along the transverse plane due to photon scatter and attenuation from the medium.
- It is a constant in an inherent value depending on the source and encapsulation used.
- gr = 1 for high energy gamma emitters
- gr «_space;1 for low energy photon emitters
25
LDR
* 0.4-**2Gy/Hour**
* 0.6-3.33 cGy/Min
26
HDR
* >20cGy/Min
* **>12Gy/Hour**
27
At what dose is a sealed source considered leaking
>185Bq
>0.005uCi
* wipe test is done to test for this every 6 months
28
Equivalent Ra mass
29
Air Kerma Strength
* Recently used to calculate the activity
* Air kerma rate x distance square
* Sk = (k l)-(l ^2)
* Unit = Micro gray m^2/hour
30
1 mg Radium equivalent
7.227
31
Point source
1/r^2
32
Linear Square
1/r
33
Brachytherapy source0 Task group
AAPM **TG-43** - 1995
AAPM **TG-43 U1** 2004, updated
34
Dose rate in a medium (water) depends on what factors
* f
* G
* gr
35
Dose calculation
Formula important from Dr. Ahmad's notes
36
Eye Plaque
Some plaques may need wipe testing
37
Formula
A1 x T1 = A2 x T2
38
Crossing needles
Not used in
* Paris system
* Computer system
39
Daily QA HDR source positioning accuracy distance
1mm
40
FLASH ultra HDR dose rates
40Gy/s
41
Generated in nuclear reactors
Co-60
Sr-90
Ir-192
Cs-137
F-18
42
AAPM specification of brachytherapy source strength
air kerma strength
43
HDR
* **Well type ionization chamber** is used for source strenght measurements
* source strangth measurements should agree +/- 5%
* Position accuracy +/- 1mm
44
HDR applicator diameter
* Decreases the dose at the applicator surface
45
AAPM TG 43 Brachytherapy dose calculations include
* Spatial distribution of activity within the source
* Photon absorption in the medium
* Photon scattering in the medium
* Filtration of photons traveling through the source encapsulation.
46
Brachytherapy source calibration routinely done by
**well type ionization chamber**
47
Naturally occuring
* Ra 226
* Ra 223
* Rn 222
48
Fission by-product
* Cs 137
* I 131
* Sr 90
CIS
49
Neutron Bombardment
* Au 198
* Ir 192
* Sm 153
* I 125
* Pd 103
* Sr 89
* Co 60
* P 32
50
Proton Bombardment
* I 123
* Ga 68
* F 18
* O15
* C11
* H3
F-O-C-H-I ( like Mochi)
51
Cyclotron
F18
52
Nuclear reactor
Sr 90
I 131
Co 60
53
To find equivalent mass of radium
X / 8.25
* 8.25 = exposure constant for radium
54
high specific rate activity =
small source size
55
56
57
OPtion A,
* This indicates that radiation is most attenuated along the long axis of the seed and strongest perpendicular to it.
* Option A: Seed is lying horizontally — max emission would be perpendicular to that (up/down), and minimal along the ends. This matches the F(θ) plot.
58
Low Energy isotopes
* I-125
* Pd-103
* Cs-131
Decay by electron capture
59
Unsealed sources
* Sr-90
* Lu-177
* Y-90
* P-32
Decay by B minus emission
60
Ra-223 (Xofigo)
Only unsealed source that decay s by **alpha emission**
61
Tc-99m
**Only** isotope that decays via **internal conversion**
62
Produced by Neutron activation
COPD-Irl
* C0-60
* Pd-103
* Ir-192
* I-125
63
Produced by Nuclear fission byproducts
Scissor
* Sr-90
* Cs-137
* I-131
64
Naturally occuring
Ra-226
65
Exposure rate constant depends on
Specific radioisotope and type of encapsulation used
