Interstitial brachytherapy is used in several areas of radiation oncology. The most commonly encountered treatment of this type in Australia is prostate brachytherapy, either low or high dose rate.
Permanent Implant, Low Dose Rate Brachytherapy
This is commonly referred to as 'seed brachytherapy' and involves the insertion of numerous sealed sources into the prostate gland. The most commonly used sources are Iodine-125 and Palladium-103. Both of these sources are characterised by emission of low kilovoltage energy photons, giving the highest concentration of dose within close vicinity of the seeds. The major difference between the two sources is the half life; Iodine-125 has a half life of 60 days whereas palladium-103 has a half life of just 17 days. This has some implications for:
- Radiation Protection - 125I is active for significantly longer than 103Pd, meaning that patients must not be cremated for at least a year if they receive 125I implants. Staff will also receive a higher dose when performing 103Pd implants.
- Radiobiological Effectiveness - The slower release of dose by the 125I seeds has potential negative consequences radiobiologically if it is used for more rapidly growing tumours. Some centres advocate the use of 103Pd seeds for Gleason 7 disease to counter this as the dose is released more rapidly. There has been no detectable effect when comparing 125I with 103Pd.
A newer model of permanent LDR implants contains 131Cs. This isotope of Cesium, not to be confused with 137Cs which has a much longer half life, has a short half life of 10 days. It has not been compared extensively with the 125I and 103Pd seeds. Importantly, most of the dose is delivered early which means that quality assurance dosimetry should be performed in the perioperative period (0-1 days) before oedema has settled.
Temporary Implant, Low Dose Rate Brachytherapy
Temporary Implant, High Dose Rate Brachytherapy
Useful references for Prostate Brachytherapy
- A review article by Crook J, published in 2011. Available on PubMed here