ESTRO 2020 Abstract book

S562 ESTRO 2020

spontaneous rib fractures during their follow-up. The incidence steadily increased for 4 years after treatment. In multivariate analysis, chemotherapy (p<0.001) and RT (p<0.001) were significant risk factors for spontaneous rib fractures. In the subgroup analysis of patients who were treated with RT, 159 (18%) rib fractures occurred: 124 (78%) patients in ipsilateral breast and 35 (22%) in contralateral breast. Ipsilateral rib fractures occurred in 83 patients receiving tumor bed boost, of which 79 (95%) occurred in the boost field. Multivariate analysis of RT subgroup showed that hypofractionated RT (p<0.001) was a significant risk factor for spontaneous ipsilateral rib fractures. Conclusion Most of the rib fractures that occur after treatment were spontaneous. The significant risk factors for spontaneous rib fractures in breast cancer patients were chemotherapy and RT. Hypofractionated RT showed increased risk of ipsilateral rib fractures. PO-0969 Stereotactic body radiotherapy in extracranial oligometastatic or oligoprogressive breast cancer F. Weykamp 1 , L. König 1 , K. Seidensaal 1 , T. Forster 1 , P. Hoegen 1 , S. Akbaba 1 , M. Susko 2 , A. Schneeweiß 3 , J. Debus 1 , J. Hörner-Rieber 1 1 Heidelberg University Hospital, Department of Radiation Oncology, Heidelberg, Germany ; 2 University of California, Department of Radiation Oncology, San Francisco, USA ; 3 National Center for Tumor diseases, Center for Gynecological Oncology, Heidelberg, Germany Purpose or Objective Oligometastatic disease (OMD) and oligoprogressive disease (OPD), describe tumor states with a limited number of metastases. In contrast to other disease states, treatment of OMD or OPD has not yet become common for breast cancer. We sought to understand the outcomes and toxicities of this treatment paradigm. Material and Methods We retrospectively analyzed female breast cancer patients with extracranial OMD or OPD at a single institution who received stereotactic body radiotherapy (SBRT) for their respective metastatic lesions between 01/2002 and 07/2019. Survival analysis was performed using the Kaplan Meier method with logrank test being used for evaluation of significance. Toxicity was evaluated using the Common Terminology Criteria for Adverse Events (CTCAE v. 5.0). Results During the period of review, 46 patients with 58 lesions met criteria for inclusion, with 34 treatments (58.6%) taking place after the year 2016. OMD was present in 32 patients (70%), and OPD in 14 patients (30%), with no significant difference in patient or tumor characteristics between these groups. Overall, patients had one (80.4%), two (17.4%) or three (2.2%) lesions treated with SBRT, with all OPD patients being treated to a single lesion. Treatment sites were bones, liver, lung (n = 19 (33%) for each site), and the adrenal gland (n = 1 (2%)). Median biological effective dose (BED at a/b = 10) was 81.6Gy (range: 45.0-112.5Gy) and median planning target volume was 36.6cc (range: 3.8 – 311.0cc). At 1- and 2-years, the local control rate (LC) was 92.2% and 88.5 %, the general tumor control rate excluding the SBRT lesion 68.6% and 43.9%, and the overall survival (OS) was 85.4% and 62.1% respectively. LC was significantly inferior in patients with OPD (figure 1) p=0.009, superior in ER+ disease with p=0.013, and showed a strong positive trend in patients with bony lesions as their SBRT site (figure 2) with p=0.078. No significant difference in OMD or OPD considering the general tumor control rate excluding the SBRT lesion and the OS was found. Analysis of post-treatment toxicity showed 8 patients (17.4%) developed grade I toxicities, and 1 patient (2.2%) grade II toxicities, without any grade III+ complications.

Conclusion SBRT in breast cancer patients with extracranial OMD or OPD is well tolerated with excellent LC. The increase in utlization since 2017 points toward a growing acceptance of SBRT for OMD and OPD in breast cancer. The inferior, yet still acceptable local control rate of OPD patients, possibly reflects a different tumor biology. PO-0970 Tectum11 peptide in prevention and treatment of radiodermatitis in breast cancer E. Fernandez Lizarbe 1 , C. De la Pinta 1 , E. G. Arias- Salgado 2 , M. Martín 1 , T. Muñoz 1 , R. Hernanz 1 , V. Duque 1 , P. Barrionuevo 1 , S. Sancho 1 , R. Perona 2 1 University Hospital Ramon y Cajal, Department of Radiation Oncology, Madrid, Spain ; 2 Instituto de Investigaciones Biomedicas CSIC/UAM., CIBER de enfermedades raras. IDIPaz, Madrid, Spain Purpose or Objective To evaluate efficacy of Tectum11 peptide in prevention and treatment of radiodermatitis in breast cancer patients. Tectum11 activates telomerase activity, stabilizes and may rise the ability of the skin's stem cell to repair DNA damage decreasing apoptosis and minimizing fibrosis activation. Material and Methods This prospective, double-blind, randomized trial, compares basic moisturizing radiation cream with or without Tectum 11. Early stage breast cancer patients, receiving radiotherapy for the breast but not for the axilla, were eligible (15x267 cGy). Tumour bed boost if needed is performed either with brachytherapy (2x5Gy) or concomitant external beam radiotherapy (15x320 cGy). Randomly assignment 1:1 to moisturizing cream (Arm-A) vs Tectum 11 cream (Arm-B), from 1 week before to 1 week after radiation. Acute skin toxicity was evaluated using RTOG toxicity criteria in week 1,2,3 and 4, chronic skin