ESTRO 2020 Abstract Book

S498 ESTRO 2020

Purpose or Objective Early stage breast cancer (BC) is currently managed with breast-conserving surgery (BCS) followed by postoperative irradiation. However, preoperative radiation in BC include potential advantages if compared with standard treatment sequence, and some experiences prove the feasibility of this approach. In our Institution, an exploratory study enrolling early BC patients treated with single fraction Cyberknife ® preoperative robotic radiotherapy is currently running (NCT03520894-ROCK trial). Nevertheless, there is no standardised approach for target delineation in this setting. For this reason, we assessed interobserver variability (IOV) in terms of target delineation within the trial and produced a consensus between 3 different operators involved in treatment planning. Material and Methods Overall, 5 patients have currently been included within the trial. Four patients already underwent single fraction 21 Gy preoperative radiotherapy followed by BCS. All patients had planning CT with and without contrast enhancement.Gross Target Volume (GTV) was delineated on contrast enhanced planning CT scan co-registered with Magnetic Resonance Imaging (MRI) obtained in treatment position. Clinical Target Volume is obtained expanding GTV by 1.5cm, excluding 5 mm from skin, thoracic wall, and pectoral muscle. Three different operators involved in the trial (VDC, GF, and SC) independently delineated GTV for all included patients . Simultaneous Truth and Performance Level Estimation (STAPLE) algorithm was used for analysis of contour agreement by obtaining a probabilistic estimate of the“true” contour (GTV-multi) (Figure 1). Dice similarity score (DSC) was calculated between each single operator and GTV-multi and between the 3 different operators. After reviewing the results, a consensus meeting was held in order to establish critical issues to GTV delineation in this setting.

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 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

Results Mean DSC between single operators GTV and GTV-multi was 0.9, 0.86 and 0.82 for VDC, GF, and SC, respectively (Table 1). Mean DSC between the single operators coupled was 0.77, 0.73, and 0.71 for VDC and GF, VDC and SC, GF and SC,respectively. After the consensus meeting, all operators agreed that a Window Width and Level of 230 and 40 HU were the best parameters to locate the target 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.