ESTRO 36 Abstract Book

S508 ESTRO 36 2017 _______________________________________________________________________________________________

Purpose or Objective Locally advanced tumors with conservative surgery have a higher relapse rate than early tumors. We analyze the clinical outcome of HDR brachytherapy boost in patients at high risk for tumor size, in terms of local control, adverse effects and cosmetic results. Material and Methods Between February 1999 and October 2011, forty two patients with 43 tumours, consecutively diagnosed with cT3 infiltrative breast carcinoma were treated with neoadjuvant systemic treatment, conservative surgery and Whole Breast Irradiation (WBI) (50 Gy) followed by High Dose Rate (HDR) interstitial brachytherapy boost (3 x 4.4 Gy at 85% isodose) in two days, with rigid needles. Survival rates were calculated using the Kaplan-Meier method, and the Cox proportional hazards model to demonstrate the infuence of tumor response to neoadjuvant chemotherapy. Results Median age was 48 years (30-77). Median follow-up was 95 months (8-201). The average lesion size was 56.7 mm (50- 100) before receiving any treatment. Local Control (LC) at 5 and 10 years was 87.1%. Overall Survival (OS) at 5 and 10 years was 85.7% and 72.4% respectively. Cancer- Specific Survival (CSS) to 5 and 10 years was 85.7% and 75.8%. Disease-Free Survival (DFS) was 74,4% and 62,7% at 5 and10 respectively. Twenty-five tumor lesions (58 %) had a complete response after neoadjuvance. There were no significant differences in terms of local control depending on the tumor response to neoadjuvant chemotherapy (p = 0.66). Nor concerning overall survival (p = 0.52) or cancer- specific survival (p = 0.74). Grade 1 early toxicity was 38.5% and Grade 2 was 12.8%. There were no early Grade 3-4 toxicity. For late toxicity, 7/43 (16.3%) of patients had fibrosis. Some of the patients reported induration from surgery. There were no trophic skin changes. Good or excellent cosmesis was recorded in 95.3% of patients. Conclusion Adding HDR brachytherapy boost to conserving therapy allows preservation of breast in 87% of locally advanced breast tumors (cT3) at 10 years, with good cosmetic outcome. This technique is effective and well tolerated. PO-0925 Timing of post-implant analysis in permanent breast seed implant: results from a serial CT study E. Watt 1 , M. Peacock 2 , L. Conroy 1 , S. Husain 3 , A. Frederick 1 , M. Roumeliotis 3 , T. Meyer 3 1 University of Calgary, Department of Physics & Astronomy, Calgary- Alberta, Canada 2 University of British Columbia, Division of Radiation Oncology, Vancouver- British Columbia, Canada 3 University of Calgary, Department of Oncology, Calgary- Alberta, Canada Purpose or Objective Permanent breast seed implant (PBSI) is a novel, one-day procedure for the treatment of early-stage breast cancer. In this technique, stranded 103 Pd seeds are permanently implanted in a volume surrounding the post-lumpectomy seroma. Post-implant dosimetry is used to assess implant quality, but the timing for this analysis is performed inconsistently across cancer centres. The use of different time points for analysis limits the ability to combine results for long-term outcome studies. The purpose of this study is to determine the most appropriate timing for Ten patients underwent CT scans at 0 (immediately after), 15, 30, and 60 days post-implant. Each post-implant CT scan was deformably registered to the planning scan to obtain the seroma contour (clinical target volume, CTV) using MIM Maestro TM (MIM Software, Inc., Cleveland OH). This contour was reviewed and adjusted as necessary by a radiation oncologist. Using the TG-43 dose calculation formalism, a postplan was generated for each scan. For post-implant dosimetry. Material and Methods

comparison, a model of the total accumulated dose to the target was calculated by summing the dose contributions from each time point. This was accomplished by deformably registering each post-implant CT scan and associated dose to the day 0 CT scan, scaling the dose contribution according to the seed activity at the time of the scan. A dose evaluation volume (DEV) was defined on all scans as a 5 mm isotropic expansion of the CTV trimmed to skin and chest wall muscle. Dosimetric indices for the CTV (V100) and DEV (V90, V100, and V200) were compared between each individual postplan and the accumulated dose using either a paired t-test or a Wilcoxon signed rank test, whichever carried more power given the distribution of the data. Residuals were also calculated, defined as the difference in dosimetric indices for a given time point and the accumulated dose model. As either a positive or a negative residual represents a deviation from the model, the median of the errors (where each error is the absolute value of the residual) was also calculated for each time point. Results The residuals for the DEV V100 and V200 for all 10 patients at each time point are shown in Figures 1 and 2, respectively. A statistically significant difference was observed between the day 60 scan and the accumulated dose for the DEV V90, V100, and V200 (paired t-test); no other significant differences were found. The smallest median (range) error occurred for the day 15 CT scan (as demonstrated in Figures 1 and 2); 2.4% (0.2-7.3%) and 4.5% (0.6-15.9%) for the DEV V100 and V200, respectively. Conclusion The results of this study indicate that the day 15 scan is the most representative of the accumulated dose delivered to target volumes in PBSI. For a 10-patient cohort, the median error was found to be at a minimum for the DEV V100 and V200 for the day 15 time point when compared to the day 0, 30, and 60 scans.