S40 CARO 2016 _________________________________________________________________________________________________________ patients with vaginal cancer and three with vulva cancer who received SBRT, either with radical or palliative intention. Conclusions: SBRT experience in gynecological tumours lacks homogeneity. Close to 400 patients treated with SBRT for locoregional disease were found in the literature and at least six different clinical scenarios were described. A high rate (> 20%) of late

more »

... /4 GI toxicity was seen in patients with recurrent gynecological pelvic tumours when salvage was attempted with SBRT possibly due to multifactorial reasons. Purpose: The ideal timing for post-implant dosimetry in permanent breast seed implant (PBSI) is yet unknown and is performed inconsistently across the country, limiting the ability to compare dosimetric indices among centres. The purpose of this study is to determine the most appropriate time to perform this post-implant analysis. Methods and Materials: Patients underwent four post-implant CT scans: 0, 15, 30, and 60 days after their seed implant. Each post-implant scan was deformably registered to the planning scan to obtain the seroma contour, which was reviewed and adjusted as necessary by a radiation oncologist. An evaluation PTV was defined to be a 5 mm isotropic expansion of the adjusted CTV contour, trimmed to the chest wall muscle and skin. Standard post-plans using the TG-43 calculation formalism were completed on each scan, considering dosimetric parameters for the CTV (V100) and evaluation PTV (V90, V100, and V200). As a reference, accumulated dose was determined by deformably summing the dose from all four time points to the day 0 postimplant scan, taking into account the decay of the seeds to weight the dosimetric contribution from each time point. Each time point was compared to the reference accumulated dose by sum-of-squared residuals and absolute differences for each dosimetric index. Results: Five patients have completed all four post-implant CT scans. The PTV V200 showed the most significant disagreement between the accumulated dose and each individual postplan (median absolute disagreement: 7.3%, range: 0.7 -16.8%), while the CTV V100 showed fairly consistent agreement for all time points (median absolute disagreement: 0.5%, range: 0.0 -5.3%). The day 15 scan showed the smallest sum-of-squared residuals for both the CTV V100 and the PTV V200; 51% and 52% lower than the next best time point, respectively, when considering the entire cohort. Other time points, however, still showed similar CTV V100 values, while other dosimetric indices had more variation in both time and between individual patients. Conclusions: For the five patients who have completed all four requisite scans, the PTV V200 showed the largest variation when compared to the reference accumulated dose, while the CTV V100 had fairly good agreement for all time points. The time point which best agrees with the reference accumulated dose is not unanimous for all patients; further patient accrual is ongoing and required to recommend the most appropriate time point for the population.