Dosimetric experience with accelerated partial breast irradiation using image-guided interstitial brachytherapy

Background and purpose: To present our dosimetric findings with image-guided high-dose-rate interstitial brachytherapy of breast cancer. Materials and methods: Treatment plans of 28 patients with early-stage breast cancer treated with interstitial partial-breast irradiation were selected for the study. The total dose was 30.1 Gy in 4.3-Gy fractions given twice daily. Before implantation all patients underwent CT imaging with the purpose of target volume definition and catheter placement planning. Postimplant CT scanning was done for dose planning. Geometrical and graphical optimizations were performed. Quantitative dosimetry on the target volume and critical structures was performed. Retrospectively, conformal dose plans using dose point optimization were made and compared with the treatment plans. Results: Three-, four-, and five-plane implants were used in 10, 13 and 5 patients, respectively. The median number of catheters was 14 (range 8-22). The mean volume of the PTV was 63.1 cm3 (range 17.2-124 cm3) and on average, 91% (range 90-96%) of the PTV received the reference dose. The mean minimum dose received by at least 90% of the PTV was 102% (range 99-107%) and the conformality index 0.68 (range 0.51-0.82). The dose non-uniformity ratio and dose homogeneity index for target were 0.33 (range 0.25-0.41) and 0.64 (range 0.50-0.76), respectively. The mean maximum dose to skin, lung and heart was 53%, 42% and 21%, respectively. Volumes of the lung and heart receiving 5 Gy were 42.6 cm3 and 8 cm3, respectively. Conclusions: Dosimetric results using our implant technique based on two sets of CT scanning seem to be acceptable with respect to target coverage, dose homogeneity and conformality