Evaluation of Radiation Doses in Ribs during HDR MammoSite®Treatment

Purpose: To estimate the risk of short-term effects (2-year followup) in women treated with HDR MammoSite® brachytherapy devices with special attention to ribs dose. Materials and Methods: 37 patients’ treatment and 164 CT studies done before treatment were reviewed for analysis. The median age of analyzed group is 61.5 years, range 50 to 83 years. 24 patients (65%) with left breast and 13 patients with right breast were treated in this group. For each patient a small balloon with 4-5 cm diameter was inserted. MammoSite® catheter was placed at the time of surgery or postoperatively. The treatment was given twice a day with 6-hour breaks for 5-7 days. Dose prescribed was 3.4 Gy per fraction (total treatment 10 fractions) at 1 cm from the surface of implanted balloon (planning target volume). For all cases pre-treatment CT (AcQsim, Royal Philips Electronics, Amsterdam, The Netherlands) scans were performed as a confirmation the suitability of treatment one or two days after implantation. CT studies were done with 2 or 3 mm thickness. CT-based plan was created using Brachyvision System Planning SoftwareVersion 7.3.10. A daily CT evaluations which consists of balloon volume, PTV volume, air and seroma volume, implant symmetry and skin distance were done. A decision about treatment was taken on the basis of this evaluation and plan by oncologist. Ribs were contoured on all slices which have PTV. The dose volume histogram (DVH) was generated for each CT study. Analysis of ribs’ volumes includes minimum and maximum dose and doses delivered to 0.1 cm3, 1 cm3 and 2 cm3 of the ribs were done. 2 Gy equivalent dose (EQD2) was calculated for these volumes using linear–quadratic formula. All those data were compared with patient followup. Results: Mean tumor size value was 1.16 cm. Average treatment PTV volume in study was (92.3 ± 11.1) cm3 with range 80.5 to 115.4 cm3. Mean values of Dose Homogeneity Index (DHI) and Coverage Index (CI) were 0.67 and 89.6%, respectively. Average ribs contoured volume was (8.6 ± 2.7) cm3, ranges from 4.3 to 19.5 cm3. All doses presented below are estimated from DVH received from all plans. Average minimum dose in the ribs was (0.75 ± 0.24) Gy and maximum average dose was (4.3 ± 1.60) Gy. The doses delivered to 3 different volumes of the ribs: 0.1 cm3, 1 cm3 and 2 cm3 were (3.88 ± 1.36) Gy, (3.15 ± 1.02) Gy and (2.75 ± 0.87) Gy respectively. The EQD2 calculations were done with α/β= 3Gy for: 0.1 cm3 - 53 Gy EQD2, for 1 cm3 - 38,7 Gy EQD2 and for 2 cm3 - 31.6 Gy EQD2. Conclusions: In our study we try to estimate the risk of ribs injury after MammoSite® treatment in relation with dose delivered to the ribs during treatment. During the 2-year followup of the analyzed group, only one patient complained of discomfort and a weak pain around the treatment area. Longer followup can be useful for estimation of the risk of ribs and cardiological late effects after APBI treatment with MammoSite® brachytherapy devices.