Palladium-103 - UW-L Brachy CoursePalladium 103 theraseedsPalladium-103 - UW-L Brachy CourseElectron capture decay of Pd 103


Relevant historical data: Palladium 103 is a radioactive isotope of the element palladium. Discovered in 1803 by William Wollaston. The name arises rom the Greek goddess of wisdom (Pallas) and after an asteroid. Palladium-103 is considered a nuclear by product and is not under the NRC jurisdictions. It is created by bombarding gold in a nuclear accelerator. Its emitting energy drops off dramatically after its third half life and is considered without energy after ten half-lives.
Chemical/Radioactive Composition: Chemical Symbol: Pd
Atomic # = 46
Mass Number = 103
Energy Characteristics: Decays by electron capture to rhodium 103, emitting xrays with 21KeVs of energy.
Exposure Rate Constant: The exposure rate is 1.48 (R cm^2mCi-1hr-1). The air kerma strength is 1.30 cGy-cm^2/mCi-hr.
Half-life Properties: Has a 17 day half life. It decays about 4% in one day. Pd sources deliver 90% of the dose in about 2 months.
Forms available for use: Most designs consist of a Pd source encapsulated in a titanium tube. Available as either loose seeds or stranded seeds. Dosimetrically the two are the same. It is recommended that stranded seeds be avoided in the region of the apex for prostate patients. Incidence of seed migration is decreased with stranded seeds as compared with loose seeds. An Ophthalmic plaque is a small, dish-shaped device that contains a radioactive source,they are available in numerous sizes ranging from 12mm to 20mm. Pictures of both can be seen above.
HVL in lead: HVL in lead is .013mm. The tenth value layer is 0.05mm. Exposure from Palladium-103 can typically be reduced by 97% or more with a .06 mm lead sheet.
Measurement/Calibrations/QA: Pd-103 are available in activities up to 2 mCi and produce initial dose rates up to 20cGy/hr.
Used in formula/calculation: A doctor decides that the implant will not be inserted on the day it was calculated for. They decide to move the case to the next day. To calculate how much Pd-103 is decaying per day the following would be used:
Therefore, Pd-103 decays about 4% per day
Therefore, in one week Pd-103 would be 75% of its original value.
Uses in Radiation Oncology: Used for the treatment of prostate cancer and uveal melanoma. It is used in permanent LDR brachytherapy for prostate treatment. It has been suggested that Palladium is more effective against dedifferentiated tumors than I 125. Patients treated with Palladium have shown more intense radiation proctitis in the first month after implantation, but recover from the radiation symptoms faster. Prostate patients who recieve Pd-103 as a boost receiv e 40-50 Gy of external beam with an additional 30-40 Gy from the seed implant. For those who brachytherapy is the primary treatment the dose is usually 115-120 Gy. In 1990, palladium-103 (Pd-103) became available for the first time for treatment of intraocular tumors. Multiple dose comparisons (versus iodine-125) showed that palladium-103 (Theragenics Corp. Buford, Georgia) offered a more favorable intraocular dose distribution in most cases. It delivered slightly more irradiation to the tumor while allowing less to be absorbed by the patients surrounding normal structures. Less radiation also reaches the operating surgeon and health-care personnel.
Treatment Planning: The average activity used to treat prostate cancer is 1.32 mCi (range .50 to 1.90 mCi) to improve dose homogeneity within the prostate as well as decrease dose to the rectum and urethra. Low-energy radioactive eye-plaques contain rice-sized palladium-103 seeds that emit low energy photon (23-28 KeV) radiation. This radiation is effectively blocked by the gold-backing of the plaque creating a directional source. Typically, the radioactive eye-plaque is then sewn onto the eye as to cover the intraocular tumor shadow, plus a 2-3 mm "free-margin." This extra margin is used to make sure that the tumor is always under the plaque during radiation. With the plaque in place, radiation is continuously delivered over 5 to 7 days, and then removed.
One other interesting fact: Due to the source design and the low energy x-rays emitted the anisotropy for Pd-103 is high and the dosimetry is very complex.


Brachytherapy by Phillip Devlin, ,

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