Radiation therapy physics
A comparison of postimplant dosimetry for 103Pd versus 131Cs seeds on a retrospective series of PBSI patients
Permanent breast seed implantation (PBSI) is an accelerated partial breast irradiation technique performed using stranded103Pd radioactive seeds (average energy of 21 keV, 16.97 day half-life). Since 2004, 131Cs brachytherapy sources have become clinically available. The 131Cs radionuclide has a higher energy (average energy of 30 keV) and a shorter half-life (9.7 days) than 103Pd. The purpose of this study was to determine whether or not there are dosimetric benefits to using 131Cs brachytherapy seeds for PBSI.
The prescribed dose for PBSI using103Pd is 90 Gy, which was adjusted for 131Cs implants to account for the shorter half-life. A retrospective cohort of 30 patients, who have already undergone a 103Pd implant, was used for this study. The treatments were planned using the Variseed treatment planning system. The air kerma strength of the 131Cs seeds was adjusted in all preimplantation treatment plans so that the V100 (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 131Cs seeds. The postimplant dosimetric parameters were compared using a two tailed t-test.
The prescribed dose for131Cs was calculated to be 77 Gy; this dose would have the same biological effect as a PBSI implant with 103Pd of 90 Gy. The activities of the 131Cs sources were adjusted to an average of 2.2 ± 0.8 U for 131Cs compared to 2.5 ± 1.1 U for 103Pd in order to get an equivalent V100 as the 103Pd preimplants. While the use of 131Cs significantly reduces the preimplant V200 (the volume within the target that receives 200% or more of the prescribed dose) compared to 103Pd 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 cm3 of tissue receiving 200% of the dose.
This analysis of 30 early stage breast cancer patients who underwent the PBSI procedure shows that there is a theoretical dosimetric advantage to using131Cs. However, in a realistic implant that will have seed misplacements and tissue changes, the use of 131Cs may not result in any clinically significant benefit.