Brachytherapy Flashcards
What is the most common HDR source?
Ir192
What is the half-life of Ir192?
74 days
What is the gamma factor of Ir192?
4.69 R cm^2/(mCi h)
What is the decay energy of Ir192?
380 keV
What is the TVL of Ir192?
1.2 cm pb or 15 cm of concrete
What is the HVL of Ir192?
0.25 cm Pb
What is the general TG43 equation?
Dose rate at distance r and angle theta = SK * Lambda * GL(r,theta)/GL(r0,theta0) * gL(r) * F(r,theta)
what is “r” in TG43
distance from the center of the source to the point of interest
what is r0 in TG43?
reference distance = 1 cm
What is theta in TG43?
The angle between the line connecting the center of the source to the point of interest and the longitudinal axis of the source.
What is theta0 in TG43?
The reference angle = 90 degrees or pi/2 radians
What is SK in TG43?
Air Kerma Strength
How is SK found in TG43?
It’s the air Kerma rate at a distance of d in vacuo (so ignoring attenuation and scatter) due to photons above a cut-off energy (so excluding low-E contamination) TIMES d^2
What is the delta cut-off typically for determining the air Kerma rate?
5 keV
What are the units of SK in TG43?
U = cGy*cm^2/h
What is Lambda in TG43?
Dose rate constant = dose rate at the reference location (r0,theta0)/SK
What are the units of Lambda in TG43?
cGy/(h*U)
Where do you get SK for TG43?
It’s what you should get with your source? From the company?
Where do you get Lambda for TG43?
It’s published and you look it up based on the radionuclide and source model
What is GL(r,theta) in TG43?
Geometry function, it’s your inverse square correction
What is gL(r) in TG43?
Radial dose function, accounts for dose falloff due to scattering and attenuation and includes the effects of encapsulation, self-attenuation, etc. BUT is only along the transverse plane
Where do you get GL in TG43?
Calculate it based on whether you’re using the point-source or line-source approximation.
Where do you get gL(r) in TG43?
Either look it up on the table or use a 5th order polynomial fit
What is F(r,theta) in TG43?
Anisotropy function - takes into account internal shielding of source that causes the emission to not be totally isotropic
What is F(r,90) in TG43?
1
What is the most common prostate seed source?
I-125
What is the half-life of I-125?
60 days
What is the decay energy of I-125?
28 keV
What is the typical activity of an HDR source?
10 Ci
~ how often is an Ir192 source changed?
quarterly (every 3 months)
Why is Ir192 used for HDR?
because of its high specific activity, you can have a high dose rate from a small source (450 Ci/g)
What is the clinical dose rate from Ir192?
700 cGy/min @ 1 cm
How would you calculate a typical workload for an HDR room?
W = gamma factor (R/mCi/h @ 1m) * 0.97 cGy/R * A (mCi) * treatment time
How many seeds do you need to assay for a brachytherapy seed treatment?
10% of the total
Write up your radionuclide table thing
Why does TG43 only go up to a 2D equation?
Because brachytherapy seeds are cylindrically symmetric, and so is the dose distribution around them. So you rotate that 2D dose around the longitudinal axis and get a 3D distribution.
What is the active-length limit of TG43?
sources up to 1 cm long
What does TG43 calculate?
Dose rate to water, in water, at a point (r, theta) from the center of the source.
at what distance d is air kerma strength determined for TG43?
along the transverse plane at a distance far enough such that your source can be approximated by a point source
typically done in air at a distance of 1 m
What does Sk * lambda get you in TG43?
Takes you from kerma in air to dose to water in water at your reference location (1 cm, theta = 90)
What is a standard prescription for a prostate seed treatment?
145 Gy
What are required safety precautions for a brachytherapy implant?
- Put up a radioactive materials sign
- survey the room before
- survey the room/occupants/items removed after
- give patient release instructions/rad safety precautions
Packaging and transportation of radioactive material
10 CFR 71
How is air kerma strength measured at NIST?
using a wide-angle free air chamber with an aluminum filter to filter out the low-energy photons originating in the source encapsulation
How is air kerma strength measured at an ADCL?
using a calibrated well chamber, that’s calibrated against a set of NIST-calibrated sources
How is air kerma strength measured clinically?
using a well-type chamber that has been calibrated for a particular source design at an ADCL
What is a common dose-rate constant?
~1 cGy/hr/U (1.1, etc.)
What is a typical workload for brachytherapy?
20-30 cGy/wk
What is your expected exposure @1 m from an LDR brachytherapy source?
20-50 mR/hr
What mode is typically used for transrectal ultrasound for prostate seed implants?
B-mode (brightness mode) - producing a 2D image of the anatomy surrounding the transducer
Where is point A in the manchester system?
2 cm up from the flange (cervical os) and 2 cm lateral from the tandem midline
What does point A in the manchester system roughly correspond to?
Primary prescription point, approximately where the uterine vessels cross the ureter, the most dose-limiting structures in the area.
Where is point B in the manchester system?
2 cm up from the flange (cervical os) and 5 cm lateral from the patient’s midline
What does point B in the manchester system represent?
meant to represent dose to the pelvic lymph nodes
What are the TG128-recommended tests?
ReD GraDes Are VoluNTary
- Resolution
- Distance
- Grayscale
- Depth of penetration
- Area measurement
- Volume measurement
- Needle/Template alignment
- Treatment planning computer
What are the definitions of HDR and LDR?
HDR > 12 Gy/hr at Rx point
LDR > 2 Gy/hr at Rx point
What are the recommended QA tests for an afterloader from TG56
SKA RAn Punk’s DOME At Viking Speed
- Survey meter
- Key interlock
- Activity
- Radiation Alarm
- Position accuracy
- Dwell timer & Door interlock!!!
- Obstruction/Misconnect
- Emergency stop
- Audio/Visual
- Source alert
What exposure rate do I expect at the surface of a patient who just got a brachytherapy prostate seed implant?
1.5 mR/h
What should you bring with you to a brachy implant?
- radiation materials sign
- Survey meter (geiger counter + ion chamber)
- lead pig
- tweezers
- the source for the patient
What line do you have to draw on the floor for an LDR treatment?
2 mR/h
