Brachytherapy Flashcards
(22 cards)
What is brachytherapy?
Use of radioactive sources at ‘short distance’ to treat tumours, positioned at very short distances to the tumour.
What are three types of brachytherapy?
- Surface moulds
- Interstitial
- Intracavitary
Why is brachytherapy used?
To give a localised treatment (dose distribution) with a high dose near to the radioactive sources (near to the tumour), with a rapid fall off further away (1/r^2 dominates in brachytherapy).
What are the features of an ‘ideal’ brachytherapy source? (9)
- Energy range of 0.2-0.4 MeV
- Half-life (long for temporary, short for permanent)
- No charged particle emission
- No gaseous decay
- High specific activity
- Chemically non-toxic and insoluble
- Mechanically stable
- Easily moulded and formed into different shapes
- Sterilisable
What is afterloading?
The ability to deliver a radioactive source into a catheter or applicator after it has been correctly positioned for treatment.
What is manual afterloading?
Applicators are correctly positioned and sources are manually loaded into the applicator.
What is low dose rate remote afterloading?
Applicators and afterloading machine form a closed system when they are attached. A pressure system forces the pellets from the safe into the applicators in trains - often under positive air pressure.
What is high dose rate remote afterloading?
A single stepping source system moves the source to pre-determined positions using a stepper motor/cable system.
What are the benefits of HDR brachytherapy?
High activity sources reduce treatment times from hours to minutes.
How is the dose distribution of brachytherapy obtained?
Dwelling the source at different positions for different times.
What are the safety features of a HDR brachytherapy system?
- Back-up secondary timer
- Automatic check of the transfer tube/catheter system before exposure
- Distance check
- Operating system to check the source has returned properly
- Back-up power supply
- Manual source return in the event of complete power failure
- Automatic retention of treatment data and history in the event of power failure
- Alarm and status code system to alert user to faults
What are the safety features of a HDR brachytherapy treatment room?
- Thick concrete walls with a maze entrance
- Maze needs careful design due to the radiation not being collimated
- Control desk outside treatment room
- Controlled area under IRR17
- Radiation warning lights independent of machine
- Audible alarm when the source is exposed
How is a HDR brachytherapy source calibrated?
Two independent calibrations, with different equipment sets, each compared to the manufacturers AKR. Then a definitive calibration is issued.
How often is a HDR brachytherapy source calibrated?
For every new source (source of systematic error), then at routine quarterly service.
What is intracavitary brachytherapy?
“Via cavity”
Involves the insertion of radioactive source into natural body cavities with an applicator to protect the source. Tumour receives a high dose whereas distant tissues receive less.
When is intracavitary brachytherapy used (which site)?
Gynae (cervix).
When is LDR brachytherapy used for prostate cancer?
For low-risk, localised disease.
How is LDR brachytherapy implemented for prostate cancer?
Tiny radioactive I-125 seeds are permanently implanted within the prostate.
How is cervical bracytherapy implemented?
- An applicator is inserted into the vagina
- The patient is imaged (MRI) with applicator in situ
- Clinician outlines tumour and OARs
- Physicist plans dose distribution
How long after insertion are I-125 seed effectively inert?
The seeds are effectively inert after 1 year.
How is HDR brachytherapy implemented for prostate cancer?
Ir-192 treatment using needles under GA.
When is HDR brachytherapy used for prostate cancer?
Can be used alone for intermediate patients or as a boost for high-risk patients.
