ESTRO 2023 - Abstract Book

S170

Saturday 13 May

ESTRO 2023

Universitaria Udine, Pathology, Udine, Italy; 5 Azienda Sanitaria Universitaria Udine, Medical Physics, Udine, Italy; 6 Azienda Sanitaria Universitaria Udine, Radiology, Udine, Italy Purpose or Objective We are conducting a phase II clinical trial to investigate the safety of single fraction Partial Breast Irradiation (PBI) post breast conserving surgery using the GammaPod system, a novel technology designed to deliver a highly focused dose to the breast under stereotactic localization. The preliminary results of the first 40 patients, corresponding to the completion of the first stage of the trial, are reported. Materials and Methods The study was designed using the 2-stage Simon method. In the first stage, 40 patients are enrolled. If there are more than 6 Grade 2 or higher toxicity events during the first stage of the study, the study will be closed due to excess toxicity; otherwise, accrual is to continue until 148 patients are enrolled. Eligibilities include ≥ 50 years of age with early stage (pT1- T2 pN0) invasive ductal carcinoma, and ≥ 70 years of age with lobular carcinoma or pN1a invasive ductal carcinoma. From February to September 2022, 40 patients were enrolled. CTV was based on NSABP B-39/RTOG 0413 guidelines; PTV was obtained by a 3 mm isotropic expansion of the CTV to sufficiently account for the stereotactic geometric uncertainty of the device. A single dose of 17.5 Gy was delivered to the 95% of the PTV using the GammaPod system, with imaging, planning and dose delivery mostly completed within a 1 hour session. Toxicity was graded according to the Common Toxicity Criteria (v4.0), including pain, erythema, hyperpigmentation and fibrosis. Toxicity was evaluated by a radiation oncologist at 1, 3, and 6 months. Results With a median follow-up of 5 months (range, 1-8 months), no case of Grade 2 or higher acute toxicity was found, only three patients (7.5%) showed Grade 1 skin toxicity. The median age was 70 years (range, 50-88 years). The tumor phenotypes were as follows: 37 (93%) ER/PgR positive-Her2 negative, and 3 (7%) ER/PgR positive-Her2 positive. Seven (18%) patients had a Ki-67>20%. Five (13%) were T1a, 16 (40%) were T1b, and 19 (47%) T1c tumors. Thirty-seven (93%) were N0. Thirty- four (85%) patients received hormonal therapy, 1 (2%) received chemotherapy, trastuzumab and hormonal therapy, and 5 (13%) received no adjuvant systemic therapy. The mean PTV was 50 cc (range, 13-107 cc). The median mean radiation dose delivered to the PTV was 18.5 Gy. The median D2 (maximum dose) to the skin was 12 Gy. Conclusion The highly focused dose distribution with sharp dose fall-off combined with a small PTV margin due to stereotactic localization allowed a single dose of 17.5 Gy to be delivered safely with a very low acute toxicity profile. To confirm these preliminary results, we will continue accrual up to 148 patients and following-up to assess long-term toxicity and local- regional control. PD-0235 Variation in incidental dose to the axilla in patients treated with whole breast radiotherapy alone F. Wilson 1,2 , T. Jaikuna 3 , C. Anandadas 2 , D. Azria 4 , S. Gutierrez-Enriquez 5 , D. De Ruysscher 6 , M. Lambrecht 7 , T. Rancati 8 , T. Rattay 9 , C. Talbot 9 , B. Rosenstein 10 , E. Sperk 11 , A. Vega 12 , L. Veldeman 13 , C. West 14 , A. Webb 15 , J. Chang-Claude 16 , P. Seibold 17 , E. Vasquez-Osorio 18 , M. Aznar 18 1 University of Manchester, Radiotherapy Related Research Group, Division Cancer Sciences , Manchester, United Kingdom; 2 Christie NHS Foundation Trust, Clinical Oncology, Manchester, United Kingdom; 3 University of Manchester, Radiotherapy Related Research Group, Division Cancer Sciences, Manchester, United Kingdom; 4 Montpellier University, Radiation Oncology, Montpellier Cancer Institute, Montpellier, France; 5 Vall d'Hebron Barcelona Hospital Campus, Hereditary Cancer Genetics Group , Barcelona, Spain; 6 Maastricht University Medical Center, Radiation Oncology (Maastro Clinic), Maastricht, The Netherlands; 7 Leuvens Kanker Instituut, Radiotherapy-oncology, Leuven, Belgium; 8 Fondazione IRCCS Istituto Nazionale dei Tumori, Prostate Cancer Program, Milan, Italy; 9 University of Leicester, Leicester Cancer Research Centre, Leicester, United Kingdom; 10 Icahn School of Medicine at Mount Sinai, Radiation Oncology, Genetics and Genomic Sciences, New York, USA; 11 University of Heidelberg, Radiation Oncology, Mannheim, Germany; 12 Hospital Clínico Universitario de Santiago, Fundacion Publica Galega Medicina Xenomica, Santiago de Compostela, Spain; 13 Ghent University Hospital, Radiation Oncology, Ghent, Belgium; 14 University of Manchester, Translational Radiobiology Group, Division of Cancer Sciences, Manchester, United Kingdom; 15 University of Leicester, Genetics and Genome Biology, Leicester, United Kingdom; 16 German Cancer Research Center (DKFZ), Division of Cancer Epidemiology, Heidelberg, Germany; 17 German Cancer Research Center (DKFZ), Division of Cancer Epidemiology, Heidelberg, Germany; 18 University of Manchester, Radiotherapy Related Research Group, Division of Cancer Sciences, Manchester, United Kingdom Purpose or Objective Incidental dose to the axilla in breast only radiotherapy (RT) can vary significantly with patient positioning, body habitus and field placement. Although extent of axillary surgery is the major risk factor for lymphoedema and shoulder morbidity, the dose to the axillary lateral thoracic junction (ALTJ), in the superior-lateral axilla, has previously been identified as a potential organ at risk for lymphoedema (1) (Fig. 1A). We report on incidental doses to shoulder and axillary structures in a prospective multi-centre cohort of patients treated with breast-conserving surgery. We also present a novel method to evaluate dose to regions of interest which could facilitate validation of new subregions of interest for RT toxicity. Materials and Methods Patients were selected from the REQUITE study (www.requite.eu) with the following criteria: full dose distribution available (primary + boost if delivered), supine position. Exclusion criteria were: prescribed axillary RT. To assess doses in this cohort, all individual CTs and dose distributions were non-rigidly deformed to a single reference patient (NiftyReg), focusing on the breast region and 5cm superior and inferior to the breast. On this reference patient, lymph node regions (according to ESTRO guidelines), ALTJ (according to (1)) and humeral head were contoured and agreed by 2 clinical oncologists. The global accuracy of image-registration was assessed using normalised correlation (NCC). Dosimetric parameters for each structure were calculated for each including D2%, D50%, D95% and D98%. Results 171 patients were available for analysis, 114 treated to the whole breast (WB) and 57 treated to WB plus tumour bed simultaneous integrated boost (SIB) (Table 1). The global accuracy of image registration was excellent (NCC=0.9, ideal value 1). Doses to shoulder and axillary structures showed a large variation both within and between centres (Fig. 1B-D).