ESTRO 2020 Abstract book

S546 ESTRO 2020

PO-0937 Partial breast re-irradiation with IMRT for local recurrence after whole breast radiotherapy S. Arculeo 1,2 , S. Frassoni 3 , I. Cavallo 1,2 , S. Dicuonzo 2 , M.A. Gerardi 2 , A. Morra 2 , V. Dell'Acqua 2 , F. Cattani 4 , S. Comi 4 , P. Veronesi 5 , F. Pansini 4 , V. Galimberti 5 , C. Fodor 2 , V. Bagnardi 3 , R. Orecchia 6 , M.C. Leonardi 2 , B.A. Jereczek- Fossa 1,2 1 University of Milan- Italy, Department of Oncology and Hemato-oncology, Milan, Italy ; 2 European Institute of Oncology, Department of Radiation Oncology- European Institute of Oncology- Via Ripamonti 435- 20141- Milan- Italy, Milan, Italy ; 3 University of Milan-Bicocca, Department of Statistics and Quantitative Methods- University of Milan-Bicocca- Milan- Italy, Milan, Italy ; 4 European Institute of Oncology, Department of Medical Physics, Milan, Italy ; 5 European Institute of Oncology, Division of Breast Surgery- European Institute of Oncology- Milan- Italy, Milan, Italy ; 6 European Institute of Oncology, Scientific Directorate- European Institute of Oncology- Milan- Italy, Milan, Italy Purpose or Objective Although the standard treatment of in-breast recurrence (IBR) after breast conservative surgery (BCS) and whole breast radiotherapy (WBRT) is still represented by mastectomy, over the last decade there has been an increasing attitude towards performing a second BCS followed by further radiotherapy (RT) with different techniques (intraoperative RT, brachytherapy, external beam RT). The aim of the study is to evaluate acute toxicity and local control of partial breast re-irradiation (re-PBI) with intensity modulated RT (IMRT), using a hypofractionated scheme. Material and Methods Eligibility criteria included patients previously treated with WBRT who experienced IBR and were operated on with second BCS. Re-irradiation was limited to the tumor bed and was performed using either TomoTherapy® IMRT with helical modality or BrainLab-VERO® IMRT step-and-shoot. Planning target volume (PTV) was generated by clinical target volume (CTV) with a margin of 5 mm. Daily image guided RT was applied by megavoltage fan beam computerized tomography (CT) for TomoTherapy® and kilovoltage cone beam CT for VERO®. For target volume, the PTV planning objectives were V100%≥95%, V95%≥98%, V90%≥100%, Dmax≤110%. Toxicity was evaluated using RTOG/EORTC criteria. Results Between 6/2012 and 3/2018, 59 patients were treated with re-PBI. Prescription dose was 37.05 Gy in 13 fractions. Fifteen (25%) patients were treated with TomoTherapy® and 44 (75%) with VERO®. Overall, median follow-up was 38.7 (1-80) months. Data of chronic toxicity > 1 year are available for 44 (74%) patients; 5%,10%, 5% of patients report fibrosis G3, atrophy G2 and teleangectsia G2, respectively. Ten (17 %) patients showed a subsequent oncologic event 24±12 months after the retreatment: 1 patient had a second IBR distant from the re-PBI field (1.6% cumulative incidence of IBR), 1 patient had regional recurrence, 7 patients developed distant metastasis (lymph nodes, lung, bones and brain) and 1 had other primitive tumors. Conclusion Re-PBI after second BCS represents a feasible alternative to mastectomy with regard to local control, showing good toxicity profile. Longer follow-up is needed to evaluate late toxicity and to establish the role of this treatment modality in local control. Longer follow-up is needed to evaluate late toxicity and to establish the role of this treatment modality in local control. This abstract is the basis for a future article.

Results Among patients recruited, 268/316 (85%) patients were eligible for IORT and 207/300 (69%) were eligible for EB- APBI. Patient and tumour characteristics and median follow-up (computed with reverse Kaplan Meier) are shown in table 1. Differences can be largely explained by increased ineligibility in EB-APBI patients due to the availability of definitive pathology results in this group. At 5 years the cumulative incidence of IBTR was 11.8(95% confidence interval 7.5-16)% after IORT and 3.9(0.7-7.1)% after EB-APBI. The difference in cumulative incidence of IBTR between treatment groups was statistically significant (p=0.006, figure 1). Surgical margins <2mm (compared to ≥2mm) were identified as risk factor for IBTR (HR 2.3 95%CI 1.2-4.7). Histology, grade, pN, estrogen receptor, Her2Neu expression, tumour size, and planned systemic therapy were not associated with IBTR.

Conclusion The cumulative incidence of combined in and out of field IBTR was significantly higher after IORT than after EB-APBI in this cohort of early stage breast cancer patients. With the exception of surgical margins, no associations between clinical risk factors and IBTR was found, hampering the identification of a subgroup wherein IORT would yield satisfactory recurrence rates.