ESTRO 2020 Abstract book

S544 ESTRO 2020

within 30 days of IOERT). We deemed a >15% incidence rate of these complications unacceptable. Based on the exact binomial distribution with a hypothesized probability of success of 0.15, the study would be halted if ≥9 of 30 patients experienced a complication. Results The first 30 patients were enrolled from 10/6/2017- 8/22/2019. The median age is 55 years (range, 40-74 years) with clinical stage distribution of I (N=19),II (N=10), or III (N=1). Breast cancer subtype is (ER+ or PR+)/HER2- in 25, HER2+ in 4, and triple-negative in 1. OPR techniques included 25 reduction mammoplasties and 5 local tissue rearrangements, of which 6 were performed by the breast surgeons and the remainder by plastic surgeons. IOERT was delivered using 6 MeV electrons (N=3), 9 MeV electrons (N=8) or 12 MeV electrons (N=19) to a median depth of 3.1 cm (range, 1.8-3.6 cm) with a median applicator size of 5.5 cm (range, 4-7 cm). The complication rate was 10%(N=3): 2 hematomas requiring evacuation and 1 breast cellulitis requiring IV antibiotics. To date, 28 of the 30 patients have received adjuvant WBI (N=24 WBI to 40 Gy/15 and N=4 WBI+RNI to 50 Gy/25) at a median of 7.7 weeks (range, 5.3-12.3 weeks) after IOERT. The remaining 2 patients are completing chemotherapy. Conclusion Given the low rate of postoperative complications, this study has been deemed safe and will continue to the accrual goal of 176. This is the first prospective evaluation of the use of an IOERT boost in patients undergoing L+OPR, and the final results may result in a new treatment paradigm for women undergoing OPR. PO-0934 Evaluation of nodal outlining based on ESTRO guidelines in breast radiotherapy planning C. Welgemoed 1 , E. Spezi 2 , A. Chowdhury 1 , P. Riddle 1 , C. Amy 1 , R. McLauchlan 1 , G. Dorothy 1 , E. Aboagye 3 1 Imperial College Healthcare NHS Trust, Radiotherapy, London, United Kingdom ; 2 Cardiff University, School of Engineering, Cardiff, United Kingdom ; 3 Imperial College London, Surgery and Cancer, London, United Kingdom Purpose or Objective The nodal field-based planning technique in breast radiotherapy was superseded in 2016 by a volume-based technique. This technique requires outlining of nodal volumes, allowing for improved dose conformance to treatment volumes and minimising radiation to surrounding normal tissues. However, volume outlining can betedious and time-consuming, requires expert skills and confer large inter- and intra-user variability 1, 2, 3. . Yang described how the use of guidelines improved outlining accuracy, nonetheless inter- and intra-variability still exist due to differences in training, image quality and clinical experience levels 3 . The purpose of this work was to evaluate local inter- and intra-user variability in manual outlining for nodal target volume delineation. Accuracy outcomes were expected to demonstrate the quality of local outlining practice; potentially inform future training needs, and contribute to quality improvement of radiotherapy planning and treatment delivery. Material and Methods 4 Clinicians were exposed to equal image quality conditions and retrospectively outlined nodal volumes, as well as the brachial plexus on 12 anonymised CT data sets. The clinicians comprised of 2 consultant breast oncologists (DD and EE) and 2 specialist registrars (AA and BB). The European Society for Radiotherapy and Oncology guidelines 4 were followed when outlining the level 4 to 1, inter-pectoral and internal mammary nodes. Their outlines were compared to 12 “gold standard outlines” (clinician approved outlines), and the Jaccard coefficient (JCI) calculated to measure conformance to the “gold standard” outlines. A Jaccard coefficient value

lesion. Moreover, cranio-caudal and lateral margins are related to a higher degree of uncertainty and may significantly influence Clinical Target Volume (CTV) after expansion (i.e., dorsal and ventral CTV margins may be unaffected by GTV IOV after skin and pectoral muscle exclusion). Delineation of retro-areolar targets may be more questionable, and MRI co-registration is mandatory for those cases.

Conclusion Low IOV was detected within operators. However, careful analysis of diagnostic imaging (i.e. MRI) and clinical examination is needed to increase reliability in target delineation of pre-operative BC radiotherapy. PO-0933 Prospective Evaulation Of Iort Boost In Women Undergoing Lumpectomy With Oncoplastic Reconstruction J. Bazan 1 , J. Stephens 1 , D. Agnese 2 , R. Skoracki 3 , J. Reiland 4 , K. Arneson 5 , G. Gupta 6 , K. Gallagher 7 , S. McElroy 1 , K.U. Park 2 , V. Grignol 2 , C. Lee 2 , G. Sisk 3 , S. Schulz 3 , M. Chetta 3 , S. Jhawar 1 , J. Grecula 1 , D. Martin 2 , W. Carson 2 , W. Farrar 2 , M. Carlson 8 , N. Gupta 8 , J. White 1 1 The James Comprehensive Cancer Center- The Ohio State University, Radiation Oncology, Columbus, USA ; 2 The James Comprehensive Cancer Center- The Ohio State University, Surgical Oncology, Columbus, USA ; 3 The James Comprehensive Cancer Center- The Ohio State University, Plastic Surgery, Columbus, USA ; 4 Avera Cancer Institute, Breast Surgery, Sioux Falls, USA ; 5 Avera Cancer Institute, Radiation Oncology, Sioux Falls, USA ; 6 University of North Carolina - Chapel Hill, Radiation Oncology, Chapel Hill, USA ; 7 University of North Carolina - Chapel Hill, Surgical Oncology, Chapel Hill, USA ; 8 The James Comprehensive Cancer Center- The Ohio State University, Radiation Oncology- Division of Physics, Columbus, USA Purpose or Objective In women amenable to breast conserving surgery, lumpectomy (L) followed by adjuvant whole breast irradiation (WBI) is the standard of care. Randomized trials have demonstrated that a lumpectomy cavity (LC) boost reduces recurrence but increases breast fibrosis (BF). Oncoplastic reconstruction (OPR) techniques are increasingly being used to help improve or preserve cosmetic outcomes. Women that undergo OPR represent especially difficult cases for LC delineation on CT scans used for adjuvant WBI planning. We designed a multi- center prospective protocol incorporating an intraoperative electron radiation therapy (IOERT) LC boost in women undergoing L+OPR with a hypothesis that IOERT boost followed by WBI will result in acceptably low rates of grade 3 BF. Here, we present the results of our protocol specified interim safety analysis. Material and Methods This is a single arm phase II study (NCT0297912) of IOERT boost (8 Gy x 1 to the 90% isodose line) followed by adjuvant WBI (40 Gy/15 fractions or 50 Gy/25 fractions) or adjuvant WBI+regional nodal irradiation (RNI) [50 Gy/25 fractions]. Inclusion criteria include stage I-III breast cancer with intent to undergo L+OPR with LC boost. The primary endpoint is the 1-year rate of grade 3 BF. The first 30 patients will be used to determine the safety of the study (rate of complications involving the breast treated with IOERT resulting in readmission/return to surgery