A comparison of postimplant dosimetry for (103)Pd versus (131)Cs seeds on a retrospective series of PBSI patients.

Purpose: Permanent breast seed implantation (PBSI) is an accelerated partial breast irradiation technique performed using stranded (103)Pd radioactive seeds (average energy of 21 keV, 16.97 day half-life). Since 2004, (131)Cs brachytherapy sources have become clinically available. The (131)Cs radionuclide has a higher energy (average energy of 30 keV) and a shorter half-life (9.7 days) than (103)Pd. The purpose of this study was to determine whether or not there are dosimetric benefits to using (131)Cs brachytherapy seeds for PBSI.

Methods: The prescribed dose for PBSI using (103)Pd is 90 Gy, which was adjusted for (131)Cs implants to account for the shorter half-life. A retrospective cohort of 30 patients, who have already undergone a (103)Pd implant, was used for this study. The treatments were planned using the Variseed treatment planning system. The air kerma strength of the (131)Cs seeds was adjusted in all preimplantation treatment plans so that the V(100) (the volume within the target that receives 100% or more of the prescribed dose) were equivalent at time of implantation. Two month follow-up CT scans were available for all 30 patients and each patient was reevaluated using (131)Cs seeds. The postimplant dosimetric parameters were compared using a two tailed t-test.

Results: The prescribed dose for (131)Cs was calculated to be 77 Gy; this dose would have the same biological effect as a PBSI implant with (103)Pd of 90 Gy. The activities of the (131)Cs sources were adjusted to an average of 2.2 ± 0.8 U for (131)Cs compared to 2.5 ± 1.1 U for (103)Pd in order to get an equivalent V(100) as the (103)Pd preimplants. While the use of (131)Cs significantly reduces the preimplant V(200) (the volume within the target that receives 200% or more of the prescribed dose) compared to (103)Pd by 13.5 ± 9.0%, the reduction observed on the 2 month postimplant plan was 12.4 ± 5.1% which accounted for seed motion, implantation inaccuracies and tissue changes. This translates into an absolute reduction of 4.1 cm(3) of tissue receiving 200% of the dose.

Conclusions: This analysis of 30 early stage breast cancer patients who underwent the PBSI procedure shows that there is a theoretical dosimetric advantage to using (131)Cs. However, in a realistic implant that will have seed misplacements and tissue changes, the use of (131)Cs may not result in any clinically significant benefit.