ESTRO 2020 Abstract Book

S516 ESTRO 2020

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 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.