ESTRO 2020 Abstract Book

S515 ESTRO 2020

of both free breathing (FB) and ABC mDIBH were done. Tangent field treatment plans with a dose prescription of 40 Gy/15 Fr were generated for each patient, in both scans. Target coverage, dose to the heart, LAD, and the left lung were documented with dose-volume histograms. Results Statistical Package for the Social Sciences, version 20 software, was used for analysis and the level of significance was set at P < 0.05. Mean heart dose was 308.5cGy with FB and 159cGy with ABC ( P < 0.0001). Mean dose to the LAD was reduced by 53.81% (1320.64 cGy vs. 606.56 cGy, P < 0.001). Target coverage was equal in both the plans. Conclusion We report that the use of ABC mDIBH technique resulted in a significant reduction in cardiac dose and hence can be considered as a promising tool for cardiac sparing. PO-0966 Partial breast irradiation at the 1.5T MR- Linac: treatment time and electron stream effect C. De Colle 1 , M. Nachbar 2 , D. Mönnich 2,3 , S. Boeke 1 , C. 1 Department of Radiation Oncology. University Hospital and Medical Faculty. Eberhard Karls University Tübingen. Tübingen. Germany, Radiooncology, Tübingen, Germany ; 2 Section for Biomedical Physics. Department of Radiation Oncology. University Hospital and Medical Faculty. Eberhard Karls University Tübingen- Tübingen. Germany, Radiooncology, Tübingen, Germany ; 3 German Cancer Consortium DKTK- partner site Tübingen and German Cancer Research Center DKFZ- Heidelberg. Germany, Radiooncology, Tübingen, Germany Purpose or Objective Low-risk breast cancer patients can be effectively treated with adjuvant external beam partial breast irradiation (PBI). The newly clinically introduced magnetic resonance guided linear accelerators (MRL) offer in this setting a better visualization of the tumour bed and the possibility of daily treatment plan adaption. Beside these advantages, challenging aspects i.e. the in-room treatment time and the electron stream effect (ESE, which might be responsible for increased out-of-field dose, especially on the chin) need to be investigated. We report the analysis of the first 11 patients treated with PBI at the 1.5T MRL in terms of treatment time and chin dose. Furthermore, factors which might affect the ESE were investigated on phantom. Material and Methods Eleven patients were treated with PBI at the 1.5T MRL (Elekta AB, Stockholm, Sweden), with dose/fractionation as for the IMPORT low protocol. To investigate and minimize the ESE, a 1cm bolus was placed on the chin. In vivo dosimetry was performed during one fraction using Grafchromic® EBT3 films placed on the top of the bolus and underneath. Results were compared with the simulated dose distribution (treatment planning system Monaco 5.4 (Elekta AB)). In vivo dosimetry was also performed on a patient treated with PBI at the conventional linac (Synergy, Elekta AB) with verification through cone beam CT. To test if the breast volume and position and the target position influence the ESE, we used the Rando Alderson Phantom and artificial breast prosthesis with different size (275ml and 130ml). Different planning CTs/MRs were performed with small prosthesis central/caudal positioned, larger prosthesis central/caudal positioned and larger prosthesis cranial positioned. For each size and position a central and a lateral located target were contoured. Plans were generated and the chin dose calculated. Results PBI was feasible in all the patients. Treatment plans were adapted based on the patient’s position. The mean in- room treatment time was 25.5 minutes (range, 22.9-30.5), Gani 1 , N. Weidner 1 , V. Heinrich 1 , J. Winter 2 , S. Tsitsekidis 2 , O. Dohm 2 , D. Thorwarth 2 , D. Zips 1,3

with a mean of 3.4 min for the pre-treatment imaging and 4.5 min for the plan adaption. Mean doses (sum for 15 fractions and 40.05 Gy PTV dose) measured on top and underneath the bolus were 2.48 and 0.53 Gy, respectively. The mean values calculated were 3.25 and 0.55 Gy, respectively. The phantom simulation showed a higher chin dose when the target was located medially. At the conventional Linac the dose measured on top and underneath the bolus was 0.93 and 0.81 Gy, respectively. Conclusion PBI with the 1.5T MR-Linac is feasible. The ESE is responsible for an out-of-field dose to the chin, which can be modelled by the TPS Monaco and effectively reduced by a 1cm Bolus. Using the bolus, this dose is not higher than the dose at the conventional linac, where no ESE is present, but daily CBCT is used for IGRT. PO-0967 Toxicity of concurrent Palbociclib and Radiation therapy in metastatic breast cancer patients A. Beddok 1 , A. Arsene-Henry 1 , B. Porte 2 , K. Cao 1 , L. Bazire 1 , N. Scher 1 , J. Otz 3 , M. Minsat 3 , F. Bidard 2 , A. Fourquet 1 , P. Cottu 2 , P. Poortmans 1 , Y. Kirova 1 1 Curie Institute, Radiation Oncology, Paris, France ; 2 Curie Institute, Medical Oncology, Paris, France ; 3 Curie Institute, Radiation Oncology, Saint Cloud, France Purpose or Objective Palbociclib, a small-molecule inhibitor of cyclin- dependent kinases (CDK4 and CDK6), combined with letrozole increases progression-free survival among patients with previously untreated ER-positive, HER2 negative advanced breast cancer. However, the data on the safety of the concomitant association between Palbociclib and radiation therapy are scarce. The purpose of our study was to retrospectively evaluate the tolerance of the first patients treated with this concomitant combination at Curie Institute. Material and Methods Between April 2017 and August 2019, 30 women with ER- positive, HER2 negative metastatic breast cancer received locoregional and/or symptomatic irradiation at a metastatic site concurrently with Palbociclib at a daily dose of 125 mg, from d1 to d21 every 28 days. Palbociclib was always associated with an endocrine treatment: Letrozole (with or without an LHRH analogue) or Fulvestrant. Median patients’ age at the time of diagnosis of metastases was 66 years (range, 35-86 years). Thirty- five sites were irradiated: nine patients received post- operative locoregional radiation therapy, including the chest wall or breast and lymph node areas, and 26 sites of metastases were irradiated: 17 at the spine, 7 peripheral skeletal lesions, 1 brain lesion and 1 choroidal lesion. The dose prescribed for locoregional mammary radiotherapy was 50 Gy in 25 fractions and varied for the treatment of metastatic sites: 20 Gy in 5 fractions (n=13), 30 Gy in 10 fractions (n=10) and 8 Gy in 1 fraction (n=2). The brain metastasis was stereotactically treated (1 fraction of 18 Gy). The primary endpoint was toxicity scored according to the common terminology criteria for NCI adverse events, version v4.0. Results The median follow-up since the end of radiation therapy was 11 months (1 – 28 months, SD 7). Mean number of days of Palbociclib during radiation therapy was 8.8 days (range, 1 to 24 days). The most common acute toxicities were radiodermatitis (12/35, including 2 grade 2) and neutropenia (12/35, including 9 grade 2). Palbociclib had to be stopped during the radiation therapy of two patients (2/30): one patient treated locoregionally (bilateral breast and lymph nodes irradiation) developed a grade 3 radiodermatitis and febrile neutropenia, another treated locoregionally developed grade 2 dysphagia (loss of 4 kg in 1 month). Of the 19/30 patients with a follow-up of more than 6 months none developed late toxicities. Conclusion