Iodine and Palladium Seed Measurement

For Prostate Cancer Treatment

• Calibration of Individual Seeds
   
• Relative Calibration of Individual Palladium Seeds
   
• Batch assay of up to 500 Iodine or Palladium Seeds
   

Individual Seed Measurements

Single seed measurements are performed with the Source Holder for Single LDR Seeds, SeeDOS Code 70016. This insert has a 1.2mm inner diameter Teflon® tube which positions the seed at the most sensitive area of the HDR 1000 Plus Ion Well Chamber. The smooth Teflon tube allows easy removal of the seed. Ion chamber calibrations from the University of Wisconsin ADCL are available for many iodine and palladium seeds.

Batch Assay Measurements

LDR brachytherapy treatment for prostate cancer uses high numbers of seeds. A treatment centre must sometimes receive up to 500 seeds if several cases are imminent. Individual seed calibration is too cumbersome. For these large numbers of seeds, a random sampling of at least 10% of the seeds should be calibrated².

Measuring a quantity of seeds at the same time is performed with the Source Holder for LDR Seed Batch Assay, PN 70022(not shown). This source holder holds up to 500 of the iodine or palladium seeds typically used for prostate cancer treatment.

A batch of 6711 iodine seeds of activity 0.35 mR gives the following nominal signals.

Self absorption resulting from a large number of seeds being placed in the insert is significant. Because of this self absorption, correction factors must be experimentally obtained for each type of LDR seed used. This method is described very well in "Verification of manufacturer-supplied ¹²⁵I and ¹⁰³Pd air-kerma strengths." Mellenberg and Kline; Medical Physics. 22(9):1495-1497, 1995. Note, a correction factor for each isotope as shown in Fig. 2 should be determined for each individual well chamber upon receipt of the chamber.

Before each assay procedure, a constancy check should be performed to assure the integrity of the calibration system¹. This should be done with a long half-life isotope such as caesium.

Verification of Calibration

For low dose rate brachytherapy, the standard of practice has shifted so that the manufacturer's calibration of the seed alone is no longer adequate, "However, it is the responsibility of the institution to verify that this calibration is correct. the institution should compare the manufacturer's stated value with the institution's standard."²

References
1. "Verification of manufacturer-supplied ¹²⁵I and ¹⁰³Pd air-kerma strengths". Mellenberg & Kline; Medical Physics. 22(9):1495-1497, 1995.
2. "Comprehensive QA for radiation oncology: Report of AAPM Radiation Therapy Committee Task Group 40". Medical Physics. 21(4):581-618, 1994.

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