ESTRO 2020 Abstract book

S561 ESTRO 2020

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 Concomitant administration of Palbociclib with radiation therapy was reasonably well tolerated in our series of 30 patients. Caution should be exercised when a large volume is irradiated, and when the dose per fraction is high. It remains unsure whether the interruption-causing side effects of Palbociclib was based on synergistic toxicity with radiation therapy. We suggest that our findings are to be confirmed in prospective registration studies collecting larger number of patients. PO-0968 Spontaneous rib fractures after breast cancer treatment D. Kim 1 , J.S. Kim 1 , K.H. Kim 1 1 Seoul National University Hospital, Radiation oncology, Seoul, Korea Republic of Purpose or Objective Spontaneous rib fractures are defined as fractures without apparent blunt-force trauma. This study aimed to evaluate the incidence and risk factors of spontaneous rib fractures after treatment in breast cancer patients. Material and Methods We retrospectively reviewed 1265 patients with breast cancer who underwent surgery in 2015 at our institution and were followed up with at least three times of bone scan. The endpoint was spontaneous rib fractures detected by bone scan. Among these patients, 15 patients were identified with rib metastasis and 13 had a trauma history. In this study, 867 (69%) patients were treated with radiotherapy (RT): hypofractionated RT (n=548; 63%), conventional fractionated RT (n=302; 35%), and accelerated partial breast irradiation (APBI, n=23; 3%). Regional nodal irradiation were conducted in 303 (35%) patients and 647 (75%) patients underwent tumor bed boost. Results Median follow-up time was 37.5 months (range, 0.2-56 months). Two-hundred-nine (16.5%) patients experienced

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), 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