Brachytherapy Flashcards
What equation would be most appropriate for calculating the BED for treatment involving a permanent radioactive implant
BED = (initial dose rate/decay constant){1+[initial dose rat/((repair rate constant + decay constant)(a/B))]}
Which of the following radiobiological processes contributes to the inverse dose-rate effect?
A. Repair of sublethal damage
B. Accumulation of cells in S phase
C. Proliferation
D. Repair of potentially lethal damage
E. Redistribution
E
Decreasing the dose rate over the range from 1 Gy/min to 0.01 Gy min generally results in an increase in the surviving fraction following irradiation with a fixed dose of radiation due to repair of sublethal damage (SLDR). The inverse dose-rate effect is the observation that, as the dose rate declines further over a critical range, cellular survival decreases as the same fixed dose is delivered. This effect relates to reassortment/redistribution of cells into the radiosensitive phase of the cell cycle by progressing through the DNA damage-induced G2 block. It would not be anticipated that SLDR, the accumulation of cells in S phase, proliferation, or repair of potentially lethal damage (PLDR) would cause an inverse dose-rate effect, since each of these processes would increase cell survival.
All of the following are used for brachytherapy implants EXCEPT:
A. Cesium-137
B. Iridium-192
C. Iodine-125
D. Iodine-131
E. Gold-198
D
Cesium-137, Iridium-192, Iodine-125, and Gold-198 are all available for brachytherapy implants. Iodine-131 is an unsealed isotope that is administered systemically for diagnostic or therapeutic purposes.
Which of the following has a half-life of 30 years?
A. Cesium-137
B. Iridium-192
C. Iodine-125
D. Iodine-131
E. Gold-198
A
Cesium-137 has a half-life of 30 years. Iridium-192 has a half-life of 74.2 days. Iodine-125 has a half-life of 60.2 days. Iodine-131 has a half-life of 8.0 days. Gold-198 has a half-life of 2.7 days.
Iridium-192 is characterized by the following EXCEPT:
A. It is the most widely used radionuclide for brachytherapy procedures in the US.
B. It has a small source size.
C. It is used for permanent implants.
D. The lower photon energy makes radiation protection easier than radium or cesium.
E. It is available for use with computer-controlled remote after-loaders.
C
Iridium-192 is the most widely used radionuclide in part because of its convenience, its small size, its low photon energy simplifying radiation protection, and its ability to be used in remote afterloaders. It is used for temporary implants and is not used for permanent implants.
The principal reason for choosing brachytherapy rather than external beam radiation is:
A. Brachytherapy is associated with lower risk of exposure to hospital staff.
B. Implant of a brachytherapy source within the tumor provides a distinct geometrical advantage for sparing the surrounding normal tissues.
C. Brachytherapy from a single dwell position provides a uniform dose within the implanted tissue.
D. The dose rates that can be achieved with brachytherapy are more cytotoxic compared to that with external beam in all tumor types.
E. A precise brachytherapy implant delivers similar dose rate to both normal tissues and the tumor.
B
The principal reason for choosing brachytherapy rather than external beam radiation is that a brachytherapy implant within the tumor provides a distinct geometrical advantage for sparing the surrounding normal tissues.
Brachytherapy is associated with higher risk of exposure to hospital staff. There is a steep dose rate gradient around an implanted radioactive source that results in a variation in the dose and associated cell killing (Answer Choice C).
Dose rates associated with brachytherapy are equal to or less than external beam radiation and would not be more cytotoxic (Answer Choice D). The cell killing is highest immediately adjacent to the source, within the tumor. This has the advantage in a well-placed implant of lower dose rate to the normal tissues that are at a greater physical distance from the sources (Answer Choice E).
Brachytherapy:
A. Allows for a low dose rate treatment minimizing toxicities to surrounding normal tissues.
B. Is used predominantly for retinal tumors.
C. Requires the use of antibodies for targeting the radionuclide to the tumor.
D. Can be used to treat larger tumors than can be treated with external beam irradiation.
E. Can be activated with microwave radiation.
A
One major advantage of brachytherapy is that it is able to treat the tumor and reduce toxicities to normal tissues because of the dose-rate effect. It is used predominantly for prostate and cervical carcinomas. While antibodies CAN be used for targeting, more often the brachytherapy is delivered as a seed therapy or interstitial implant. Brachytherapy is not activated in general, and microwave would not do it in any case.
