ESTRO 2021 Abstract Book

S907

ESTRO 2021

PO-1090 Tumor bed boost volume delineation and planning post-oncoplasty following multidisciplinary approach T. Wadasadawala 1 , J. Manjali 1 , G. Chitkara 2 , R. Pathak 1 , P. Thakkar 2 , S. Joshi 3 , R. Krishnamurthy 1 , N. Nair 2 , V. Parmar 2 , R. Sarin 1 1 Tata Memorial Center, Radiation Oncology, Mumbai, India; 2 Tata Memorial Center, Surgical Oncology, Mumbai, India; 3 Tata Memorial Center, Surgical Oncologist, Mumbai, India Purpose or Objective Oncoplastic surgery (OS) has been increasingly utilized as additional strategy to improve aesthetic outcome after post breast conservation surgery (BCS). However, traditional methods of tumour bed boost (TBB) delineation for radiotherapy (RT) in post-OS may lead to inaccuracy. In an attempt towards better definition of TBB, we report a multidisciplinary approach with close co-operation between a surgeon and radiation oncologist to delineate an ideal TBB volume. Materials and Methods We retrospectively reviewed patients with invasive breast cancer who underwent BCS+OS and subsequent RT planning from April 2019 and September 2020 in our hospital. As a pattern of care, patients were reviewed by a surgical oncologist (SO) and radiation oncologist (RO) on the day of Computed Tomography (CT) based RT planning. Tumor Bed (TB) was delineated by radiation oncologist in consultation with surgeon, taking into consideration the surgical procedure, surgical clips (placed in tumor bed and cavity walls) and the radiological appearance of the flap. The primary endpoint was to delineate an ideal TB (CTVtb and PTVtb) in the various OS techniques, which are Type I and Type II - displacement (D) and replacement (R). The secondary endpoints included volumetric estimation and dosimetric comparisons. Results We identified 49 patients in the above study period. There were 17 and 32 cases of type I and type II procedures. In type I oncoplasty, the line of re-approximation was delineated as CTVtb along with the surgical clips. In type II D, the advancement of flap/breast tissue led to realignment of tumor bed and cavity walls. In coordination with SO, CTVtb was contoured by taking into account the location of the index tumor, steps of the surgical procedure and the clips placed along the walls and base. In type II R (19 patients), the most commonly used flap was Latissimus Dorsi (LD, 63.1%). Technically the flap would not be a part of the target volume, we delineated the tumor bed including the cavity walls along the line of contact with the whole flap and the surgical clips. An anisotropic margin was given to the CTVtb considering a safety margin of 2 cm minus the width of excision. The mean volumes of CTVtb, PTVtb and CTVbr (breast) along with ratios (CTVtb/ PTVtb and PTVtb/ CTVbr) are described in Table 1. The mean CTVtb and PTVtb was comparatively higher in Type II R compared to type I and Type II D, though not statistically significant. Of the 49 patients, 32 were treated with a whole breast dose of 40Gy/15# followed by sequential TBB, while 17 were treated using 26Gy/5# (FAST FORWARD) with simultaneous boost. The mean target coverage was 92.3% and 87.8% for CTVtb and PTVtb respectively. Target coverage (CTVtb and PTVtb) was superior in photon-based boost plans as compared with that of electron (p = 0.001). https://www.estro.org/ESTRO/media/Abstracts/173/ea28abf9-bf49-419f-9174- c40756cdad3b.jpeghttps://www.estro.org/ESTRO/media/Abstracts/173/ea28abf9-bf49-419f-9174- c40756cdad3b.jpeg

Conclusion Multi-disciplinary approach for TBB planning is necessary for better understanding of OS and the accurate TBB delineation in such patients.

PO-1091 Positive axillary non-sentinel nodes predictors in breast cancer patients after neoadjuvant therapy E. La Rocca 1 , C. Arrobbio 2 , F. Colombo 2 , S. Gandini 3 , M. Zaffaroni 2 , V. Galimberti 4 , S. Kahler-Ribeiro-Fontana 4 , S. Dicuonzo 2 , D.P. Rojas 2 , M.C. Leonardi 2 , R. Orecchia 5 , B.A. Jereczek-Fossa 2,6 1 IEO European Institute of Oncology IRCCS, Radiation Oncology, Milan, Italy; 2 IEO, European Institute of Oncology, IRCCS, Radiation Oncology, Milan, Italy; 3 IEO, European Institute of Oncology, IRCCS, Division of Epidemiology and Biostatistics, Milan, Italy; 4 IEO, European Institute of Oncology, IRCCS, Division of Breast Cancer Surgery, Milan, Italy; 5 IEO, European Institute of Oncology, IRCCS, Scientific Directorate, Milan, Italy; 6 University of Milan, Department of Oncology and Hemato-oncology, Milan, Italy

Purpose or Objective To assess the rate of positive non-sentinel nodes (non-SLNs) after neoadjuvant chemotherapy (NAST) in breast