ESTRO 2020 Abstract Book

S496 ESTRO 2020

of both progression-free (PFS) and overall survival (OS) when compared to ET alone. In the metastatic setting, radiation therapy (RT) is often required with either palliative or ablative intent, most in case of oligometastatic patients. The aim of our study was to evaluate the safety and efficacy of concomitant RT and CDK4/6i in metastatic HR+/HER2- breast cancer patients. Material and Methods We analysed 23 patients (for a total of 25 radiation treatments) consecutively treated in Our Institution with I-II line CDK4/6i and RT between September 2017 and August 2019. Both hematologic and non-hematologic toxicity have been evaluated (according to CTCAE v.5.0), along with PFS and OS. Results Median age of the series was 59 years (range 37-80). At the time of metastatic disease diagnosis, 19 patients (83%) were postmenopausal. Sixteen patients were treated with palbociclib, 7 with ribociclib. Fifty-two percent of patients received letrozole (I line) and 48% received fulvestrant (II line) as ET. Bone was the most frequently treated RT site (76%), followed by central nervous system (12%) and liver (8%), with 3-dimensional conformal radiation therapy (3D- CRT) as the most commonly technique used (56%). In 9 cases (36%) RT was given concomitantly to CDK4/6i, while in 16 cases (64%) CDK4/6i were suspended before or during RT. At a median follow up of 10.7 months (range 1.2-23.1), OS was 95.7% and PFS 69.6%. RT localization, techniques, palliative or ablative intent, and combination between RT and CDK4/6i were not significantly associated with OS and PFS. The combination of both ablative and palliative RT and CDK4/6i was not significantly associated with CDK4/6i dose reduction and with any toxicity graded >G2. Hematologic adverse events were the most commonly reported. In detail, 44% of patients developed neutropenia, most commonly >G2 (35%). Thrombocytopenia was reported for the 17% of patients and anemia for 26% (G1). Non-hematologic toxicity was less frequent: two patients experienced G2 diarrhea, and the most common non-hematologic event was G1 asthenia (22%). Conclusion Our study showed the safety and the efficacy of RT with either palliative or ablative intent combined with CDK4/6i plus ET, with a limited number of adverse events >G2 and data on PFS/OS comparable to those currently available in literature. Further studies on larger series are strongly needed to confirm these encouraging findings. PO-0929 Mammary chain irradiation: can we reduce the risk of secondary cancer and ischaemic heart disease? S. Corradini 1 , V. Figlia 2 , C. Simonetto 3 , M. Eidemüller 3 , S. Naccarato 2 , G. Sicignano 2 , A. De Simone 2 , R. Ruggieri 2 , R. Mazzola 2 , C. Matuschek 4 , E. Bölke 4 , M. Pazos 1 , M. Niyazi 1 , C. Belka 1 , F. Alongi 1 1 LMU Munich, Department of Radiation Oncology, Munich, Germany ; 2 IRCCS Sacro Cuore Don Calabria Hospital, Department of Advanced Radiation Oncology, Negrar-Verona, Italy ; 3 Helmholtz Center Munich, Institute of Radiation Medicine, Munich, Germany ; 4 Heinrich Heine University- Medical faculty, Department of Radiation Oncology, Düsseldorf, Germany Purpose or Objective Aim of the present study was to comparatively estimate the risks of radiation-induced secondary lung and breast cancer and ischemic heart disease for different radiotherapy techniques (IMRT versus 3DCRT) in women with node-positive left-sided breast cancer, candidates for regional node irradiation (RNI). A special focus of the study was on the impact of the addition of internal mammary chain (IMC) irradiation on risk estimates. Material and Methods

For this risk modelling study, RNI contouring and re- planning was performed with a dose of 50Gy in 25 fractions. For each technique (3D-CRT and IMRT), two treatment plans were created, based on the inclusion of IMC (n=40). We calculated estimates of excess relative risk (ERR) and excess absolute risk (EAR) for radiation-induced lung and breast cancer and major coronary events using linear, linear-exponential and plateau models. Statistical analyses were conducted using Wilcoxon signed-rank tests to estimate statistical significance. Results The addition of IMC irradiation to RNI significantly increased the dose exposure of the heart, lung and contralateral breast in both, 3DCRT and IMRT plans (p=0.002, respectively). This correlated with an increase of the ERR for secondary lung cancer (58% vs 44%), secondary contralateral breast cancer (49% vs 31%) and ischemic heart disease (41% vs 27%) in IMRT plans, if the IMC was added as a target volume. However, the use of IMRT significantly reduced the mean cardiac dose as compared to 3DCRT and resulted in decreased ERR (64% 3DCRT vs 41% IMRT, p=0.002) and 10-year EAR for major coronary events in IMC irradiation (see figure 1). Nevertheless, even if using the IMRT technique, the estimated additional absolute 10-year cardiac risk from IMC irradiation was large (up to 4%), thus compromising the benefit of IMC irradiation for patients with high cardiovascular baseline risks. The application of IMRT significantly reduced mean lung dose exposure, resulting in a significant reduction of ERR for secondary lung cancer if the IMC was included (75% vs 58%, p=0.004). Regarding the ERR for secondary contralateral breast cancer, there was a significant increase in ERR through the addition of the IMC (p=0.002), without any influence of the RT technique.

Conclusion Although IMC irradiation has been shown to increase survival rates in node positive BC patients, it increased dose exposure of organs at risk in left-sided BC, resulting in significantly increased risks for secondary lung and contralateral BC and ischemic heart disease. In this setting, the adoption of IMRT seems advantageous when compared to 3DCRT. PO-0930 Wide tangents versus volumetric arc therapy to treat the internal mammary chain using breath hold D. Gujral 1 , S. Nazir 1 , B. Hunter 1 , P. McNaught 1 , L. Williams 1 , S. Porter 1 , S. Coughlan 1 , S. Cleator 1 1 Imperial College Healthcare NHS Trust, Department of Clinical Oncology, London, United Kingdom Purpose or Objective Inclusion of the internal mammary chain (IMC) radiotherapy (RT) for early breast cancer is increasingly more common to improve survival and, consequently,