ESTRO 2022 - Abstract Book

S1012

Abstract book

ESTRO 2022

Comparative outcome data after intraoperative radiotherapy(IORT) and whole breast irradiation (WBI) for breast cancer at >10ys median follow-up are rare. We present a large, mature single-institution matched-pair comparison reporting survival and relapse rates in patients treated with either modality. Materials and Methods Complete datasets for 258 IORT patients treated between 2000 and 2010 were matched with 258 patients postoperatively treated with WBI by age/histology/tumor size, grading/lymph-node-status/hormone receptors/type of adjuvant therapy/surgical margins and treatment date. WBI was performed with 2 tangential fields to the whole breast (50Gy/25 fractions) and with a 9-12MeV direct-electron-field boost to the tumor bed (10-16Gy/5-8 fractions). IORT was performed on a non-dedicated Linac (green-line-setup) with direct 8-12MeV electron fields (21Gy prescribed to 90%-isodose). Relapse at surgical intervention site was classified as true local recurrence (LR). All recurrences in the treated breast (any quadrant) were classified as Ipsilateral Recurrence (IR). Results Median follow-up was 157 months (12-251) for the IORT group, 154 months (31-246) for the WBI group. IR were 26 after IORT and 12 after WBI. LR for IORT and WBI groups were 16 and 6, respectively. Cumulative incidence of IR at 5, 10 and 15 years was 2.4%, 7.9% and 12.7% for IORT and 1.2%, 4.1% and 5.0% for WBI (p=0.02, HR 2.2 CI 95% 1.1-4.4). Cumulative incidence of LR at 5, 10 and 15 years was 1.6%, 5.1% and 8.3% for IORT and 0.4%, 2.1% and 2.5% for EBRT (p=0.02, HR 2.8 CI 95% 1.1-7.0). Overall survival (OS) at 5, 10 and 15 years was 96.9%, 91.7% and 78.9% for IORT and 96.1%, 90.2% and 76.6% for WBI (n.s.). Disease-free survival (DFS) at 5, 10 and 15 years was 95.3%, 89.5% and 74.6% for IORT and 94.1%, 86.6% and 75.0% for WBI (n.s.). No differences in non-breast-cancer-related deaths, second-cancer-incidence and cardiac events (arrhythmia or ischemic disease) were recorded in both treatment groups. When analyzed according to ASTRO-criteria for accelerated partial breast irradiation (APBI), outcome was better in the APBI-suitable group than in the entire cohort and the APBI-unsuitable group (IR/LR cumulative incidence at 5, 10 15 years of 0%/0%, 6.2%/3.3% and 7.3/3.3% for IORT and 0%/0%, 2%/2% and 3.1%/2% for WBI in “suitable” patients). Conclusion According with published randomized trial data, IR-rate was higher after IORT than after WBI if no stringent patient selection was performed. Second-cancer-incidence and cardiac events did not differ between IORT and WBI. In patients suitable for APBI according to ASTRO-criteria, similar IR-, LR- and OS-data indicate that IORT is a viable alternative to WBI. S. Curto 1 , J. Androulakis 1 , M. N.D. Machielse 1 , K. Sumser 1 , M. M. Paulides 2 , M. Franckena 1 , L. Koppert 3 , A. Jager 4 , G. C. Van Rhoon 1 1 Erasmus MC, Radiotherapy, Rotterdam, The Netherlands; 2 Eindhoven University of Technology, Electrical Engineering, Eindhoven, The Netherlands; 3 Erasmus MC, Surgery, Rotterdam, The Netherlands; 4 Erasmus MC, Medical Oncology, Rotterdam, The Netherlands Purpose or Objective Randomized clinical studies have shown the benefit of adding thermotherapy, increasing tumor temperature up to 40-44°C, as a sensitizing agent for radiotherapy and chemotherapy. Thermotherapy treatment planning (TTP) has proved to be a valuable tool in the treatment decision making and treatment implementation [Kok, 2021, Int. J. Hyperthermia 1;38(1)]. Accurate patient representative computational models are essential for the TTP process. However, patient representative models for the treatment of tumours in the intact breast have not yet been extensively investigated. The objective of this work is to investigate the feasibility of performing TTP as an adjuvant strategy to preoperative radiotherapy or neo-adjuvant chemotherapy considering wide variation of breast and tumor characteristics. Materials and Methods MR images of 25 invasive breast cancer patients treated in Erasmus MC were evaluated. Tissues were segmented and 3D breast patient models were generated. Variability in terms of breast volume, fat content, and tumor volume was evaluated using the generated models. A generic thermotherapy applicator consisting on 12 half-wavelength dipole antennas distributed over two rings was generated. Specific absorption rate (SAR) based TTP was performed on four models representative of small breast volume, large breast volume, high fat content and low fat content patients. Target-Hotspot- Quotient (THQ) and target coverage (TC) by the 25% (TC25), 50% (TC50) and 75% (TC75) normalized SAR contour were evaluated. Results Different tissues were not clearly identifiable in the MR images of three patients. This resulted in 22 patients eligible for model generation. Figure 1 shows an example of a generated breast model, SAR and thermal distribution. The generated repository contains models with a large range of breast volume (154.1-1336.5 ml), fat content (30.48-62.64 %) and tumor volume (1.44 – 39.26 ml). The THQ and TC obtained in the evaluated four representative patient models (Table I) show the feasibility of performing TTP and indicate the feasibility of good quality thermotherapy in all cases (THQ ≥ 0.74,TC25=100%). PO-1192 Treatment planning for thermotherapy as adjuvant strategy to breast cancer preoperative radiotherapy