Wk09 Brachytherapy Practce And Planning Flashcards
What is the most common source for HDR brachytherapy?
Ir192
What are the common dose rate and activity for HDR brachytherapy?
Dose rate: >12Gy/hr
Activity: 4 to 12 Ci
What is the half life of iridium 192
74days
Need replace the source every 4-6 months
What is the decay chain of iridium?
Beta decay to excited state of Pt-192
Then electron capture to Os-192
What is the half value layer of iridium?
HVL 2.5mm Pb and 44.5mm concrete
Name 4 common Ir192 afterloaders
MicroSelectron
Flexitron
BEBIG
Varisource
What are the treatment parameters for HDR brachytherapy
- Dwell positions
- Dwell times
What is the shielded safe made of?
Tungsten or depleted uranium
How much air kerma rate is reduced using the shielded safe?
The shielded safe reduce the air kerma rate to 1-4uGy/h
What is the check cable drive consist of?
- Check cable stepper motor
- Shaft encoder
- Cable guide tube
What is the source cable drive consist of?
- Source stepper motor
- Shaft encode
- Wire-in switch
- Emergency stoop motor
- Cable guide tube
What is the indexer consist of?
- Indexer stepper motor
- Indexes disc
- Indexer channel encoder
- Treatment tube
- Locking ring
- Channel selection tube
What are the safety features in a brachytherapy suite?
- Emergency switches
- Emergency crank
- Door interlock
- Audio / visual system
- Radiation monitor / treatment on indicator
- Emergency service instruments
- Back-up battery
What are the 3 major categories of brachytherapy
- Interstitial
- Intracavitary
- Superficial
What is the common dose scheme for combined MRI/CT guided BT for CA cervix?
4 frs of BT with 7 Gy, in 2 applications n consecutive weeks.
Which guideline guides brachytherapy dose calculation geometry ?
AAPM TG-43
What are the limitations of TG-43 guideline?
- homogeneous water medium is assumed
- full scatter condition assumed
- no electrons is taken into account
- full 3D source geometry is not taken into account
What is the use of shielded applicators? (Made by tungsten)
Significantly reduce dose to OARs (rectum / bladder)
How can we evaluate a brachytherapy treatment plan?
- dose points
- dose distribution
- dose volume histogram parameters
What is IPSA and HIPO?
IPSA = inverse planning by simulated annealing (dwell positions and dwell time varies dramatically)
HIPO = hybrid inverse planning optimisation
(more user friendly and realistic / applicable)
What components do TG-43 2D formalism consist?
Source strength
Dose rate constant
Geometry function
Radial dose function
Anisotropic function
Where is the reference point of TG-43 2D formalism ?
1cm from the source at 90degrees
A typical 10 Ci Iridium192 = _______________ U
40000
What is the unit of source strength ?
1 ugym2/h
What is the dose rate constant influenced by?
Influenced by the radionuclide and the design of source model
What do the geometry function takes into account?
It accounts the inverse square law, neglecting scatter and attenuation
What do the radial dose function takes into account?
It accounts for dose fall off in the transverse plane due to photon scattering and attenuation, does not include inverse square fall off which is handled by the geometry function
Anisotropy function is always _________ for a perfect point source?
Unity
