Practical considerations for the calibration of low energy/low activity seeds

The popularity of permanent brachytherapy implants for prostate cancer has resulted in the introduction of a large number of new low-energy photon-emitting radioactive seed designs and a large number of institutions working with these seeds. NIST has developed the Wide Angle Free Air Chamber (WAFAC) and a system to quickly process new seed designs through this calibration standard. The TG-40 Report recommends that these radioactive seeds have their calibration verified prior to the implant. Two issues unique to measuring these low energy/low activity seeds in reentrant well chambers used as transfer standards by ADCLs and clinics are (1) precise measurement at low currents and (2) the strong energy response of these chambers. Regarding the first problem, we have devised a system where we can time readings to set a reproducibility in current better than 0.1%. The second issue, the energy response of chambers of different models has not yet been adequately addressed. Currently the recommended procedure is to have transfer chambers calibrated for each model of seed to be used in the clinic. The data demonstrate that each model responds similarly for all four models of ¹⁰³Pd sources. For ¹²⁵I seeds the chamber response depends on the design of the seed. This information is helpful to clinical physicists to better understand their calibration and equipment needs for low energy brachytherapy sources.