ESTRO 2020 Abstract Book

S500 ESTRO 2020

distant metastasis occurred, and all patients are alive. The average volume of the surgical cavity, clinical target volume (CTV) and planning target volume (PTV_EVAL) was 7.8 cm 3 (range: 1.75-27.3 cm 3 ), 53.8 cm 3 (range: 23.9- 103.5 cm 3 ) and 75 cm 3 (range: 36.9-135.9 cm 3 ) respectively. The mean value of the PTV/whole breast volume ratio was 0.09 (range: 0.03-0.19). No grade 2 or worst acute side-effect was detected. Grade 1 (G1) erythema occurred in 6 (17.1 %) patients, while G1 oedema was observed in 5 (14.3%) cases. G1 pain was reported by 2 (5.7 %) patient. Cosmetic outcome was excellent in 20 (57.1%) and good in 15 (43.9%) patients. Conclusion SAPBI with CK is a reproducible and feasible technique for the delivery of external beam APBI following BCS for the treatment of low-risk, early-stage invasive breast carcinoma. Our early findings are promising, CK-SAPBI delivered with four daily fractions is well tolerated by the patients. PO-0936 Comparison of local recurrence rates after two different APBI techniques, a prospective study D. Jacobs 1 , M. Mast 2 , G. Speijer 3 , N. Horeweg 1 , A. Petoukhova 2 , U. Fisscher 2 , M. Straver 4 , E. Coerkamp 5 , H.M. Hazelbag 6 , J. Merkus 7 , E. Roeloffzen 8 , L. Zwanenburg 8 , M. Fiocco 9 , C. Marijnen 1 , P. Koper 2 1 Leiden University Medical Center LUMC, Radiotherapy, Leiden, The Netherlands ; 2 Haaglanden Medical Center, Radiotherapy, The Hague, The Netherlands ; 3 Haga Hospital, Radiotherapy, The Hague, The Netherlands ; 4 Haaglanden Medical Center, Surgery, The Hague, The Netherlands ; 5 Haaglanden Medical Center, Radiology, The Hague, The Netherlands ; 6 Haaglanden Medical Center, Pathology, The Hague, The Netherlands ; 7 Haga Hospital, Surgery, The Hague, The Netherlands ; 8 Isala, Radiotherapy, Zwolle, The Netherlands ; 9 Leiden University Medical Center LUMC, Statistics, Leiden, The Netherlands Purpose or Objective Accelerated partial breast irradiation (APBI) aims to reduce treatment burden whilst maintaining adequate local control and is an alternative to whole breast irradiation for low-risk early stage breast cancer patients. The optimal APBI technique remains to be defined. We aimed to evaluate the ipsilateral breast tumour recurrence (IBTR) after two different APBI techniques; intraoperative electron radiotherapy (IORT) and external photon beam- APBI (EB-APBI). Material and Methods Between 2011 and 2016, women ≥60 years with breast carcinoma or DCIS of ≤30 mm undergoing breast conserving therapy were included in a prospective multi-centre cohort study. Clinical nodal involvement, >pN1a or positive surgical margins were not allowed. IORT (1x23.3 Gy prescribed at 100%) was applied in one hospital and EB- APBI (10x3.85 Gy daily within 6 weeks post-operatively) in 2 other hospitals. Primary endpoint was IBTR at 5 years after lumpectomy, IBTR comprising all recurrences in the ipsilateral breast irrespective of localization (in or out of field recurrence). A competing risk model was used to estimate the cumulative incidence of IBTR for treatment techniques. To assess the difference between the cumulative incidences Fine and Gray’s test was applied. Univariate Cox-regression models were estimated to identify risk factors for IBTR. Results Among patients recruited, 268/316 (85%) patients were eligible for IORT and 207/300 (69%) were eligible for EB- APBI. Patient and tumour characteristics and median follow-up (computed with reverse Kaplan Meier) are shown in table 1. Differences can be largely explained by increased ineligibility in EB-APBI patients due to the availability of definitive pathology results in this group. At 5 years the cumulative incidence of IBTR was 11.8(95%

confidence interval 7.5-16)% after IORT and 3.9(0.7-7.1)% after EB-APBI. The difference in cumulative incidence of IBTR between treatment groups was statistically significant (p=0.006, figure 1). Surgical margins <2mm (compared to ≥2mm) were identified as risk factor for IBTR (HR 2.3 95%CI 1.2-4.7). Histology, grade, pN, estrogen receptor, Her2Neu expression, tumour size, and planned systemic therapy were not associated with IBTR.

Conclusion The cumulative incidence of combined in and out of field IBTR was significantly higher after IORT than after EB-APBI in this cohort of early stage breast cancer patients. With the exception of surgical margins, no associations between clinical risk factors and IBTR was found, hampering the identification of a subgroup wherein IORT would yield satisfactory recurrence rates. PO-0937 Partial breast re-irradiation with IMRT for local recurrence after whole breast radiotherapy S. Arculeo 1,2 , S. Frassoni 3 , I. Cavallo 1,2 , S. Dicuonzo 2 , M.A. Gerardi 2 , A. Morra 2 , V. Dell'Acqua 2 , F. Cattani 4 , S. Comi 4 , P. Veronesi 5 , F. Pansini 4 , V. Galimberti 5 , C. Fodor 2 , V. Bagnardi 3 , R. Orecchia 6 , M.C. Leonardi 2 , B.A. Jereczek- Fossa 1,2 1 University of Milan- Italy, Department of Oncology and Hemato-oncology, Milan, Italy ; 2 European Institute of Oncology, Department of Radiation Oncology- European Institute of Oncology- Via Ripamonti 435- 20141- Milan- Italy, Milan, Italy ; 3 University of Milan-Bicocca, Department of Statistics and Quantitative Methods- University of Milan-Bicocca- Milan- Italy, Milan, Italy ;