ESTRO 2023 - Abstract Book

S1018

Digital Posters

ESTRO 2023

guidelines. For each patient, the following anatomic features were assessed: CLB volume (cc), breast height measured at the sagittal plane, central breast to breast distance (CBB) calculated as the minimum distance between the contoured breasts at the central axial slice. Dose-volume histograms were generated and the following dosimetric parameters were calculated: Dmean, Dmax and V4 Gy to CLB. Statistical associations between patient’s anatomic features and dosimetric parameters to CLB were analysed using Pearson correlation and linear regression. Results Mean age was 46 (28-71) years. Eighty-six percent of patients underwent regional lymph nodes irradiation. The mean Dmax/Dmean to CLB was 10.12 Gy/0.78 Gy. The mean V4 Gy to CBL was 0.99% ranging between 0.02% and 8.77%. Substantial interpatient variability in CLB exposure was noticed with Dmax/Dmean ranging between 0.51 Gy/0.18 Gy and 40.12 Gy/2.79 Gy. The CBB distance largely varied in this cohort from 2 cm to 13.35 cm with a mean value of 4.77 cm. The CBB distance was strongly correlated to Dmean and V4 Gy to CLB, with Pearson coefficients of 0.53 and 0.5 (p<0.001), respectively. For every 1 cm increase in CBB distance, Dmean to CLB decreased by 41%. No statistical association was found between CBB distance and Dmax to CLB. Neither CLB volume nor CLB height were statistically correlated to CLB doses Conclusion This study showed that CBB distance is a reliable predictor of CLB exposure in 3D conformal breast cancer radiotherapy. Personalized estimation of CLB doses from planning CT data allows early patient’s selection for CLB sparing radiotherapy techniques, especially in patients with high risk of contralateral breast cancer. 1 Technical University Munich, Medical School, Klinikum rechts der Isar, Department of Radiation Oncology, Munich, Germany; 2 Deutsches Konsortium für translationale Krebsforschung (DKTK) , Partner Site Munich, Munich, Germany; 3 Institute of Radiation Medicine, Helmholtzzentrum München, Munich, Germany Purpose or Objective There has been a progressive de-escalation of axillary surgical therapy within the framework of breast cancer treatment due to the questionable oncological benefit and the associated toxicity of axillary lymph node dissection. Together with the improving resolution of diagnostic imaging, morphologically abnormal lymph nodes can be detected more frequently. In lymph-node positive patients, locoregional treatment of the lymph node drainage system has the potential to improve the oncological outcome. If resection of the remaining suspect lymph nodes is not feasible, a simultaneous boost to the lymph node metastases (LN-SIB) can be applied. We aim to provide preliminary data on the outcome and safety of this technique as there is no data on this yet. Materials and Methods 48 breast cancer patients with remaining suspect lymph nodes after surgery who received adjuvant radiotherapy of the chest/chest wall (50.4 Gy in 28 fractions) and an integrated boost within the lymph node system (median dose: 58.8 Gy / 2.1 Gy (range 56-63 Gy / 2-2.25 Gy)) were included in the current study (2010-2020 at TU München). The treatment decisions were discussed interdisciplinary with consideration of another individual surgical intervention before consensus of radiotherapy. The brachial plexus was contoured in every case and the dose within the plexus PRV (+0.3-0.5mm) was limited to an EQD2 of 59 Gy. All patients received structured radio-oncological and gynecological follow-up (FU) by clinically experienced physicians. Intervals for radio-oncological FU were at 0 weeks, 6 weeks, 3 months, 6 months and subsequent annually after irradiation. There were no severe late side effects ( ≥ III°) observed during the follow-up period. The most frequent chronic side effect was fatigue. One patient (2.1 %) developed pain and mild paresthesia in the ipsilateral arm indicating brachial plexopathy After a FU of 557 days (41 to 3373 days), in 8 patients a recurrence was observed (16.7%). In 4 patients the recurrence involved the regional lymph node system. Hence, local control after a median FU of 557 days was seen in 44 patients (91.6 %). Conclusion Our analyses indicate a predominantly low to moderate acute or late toxicity after LN-SIB and reasonable locoregional control. LN-SIB-irradiation may be considered as a potential treatment option if re-resection of residual lymph nodes after initial surgery is not feasible. Patients receiving a LN-SIB must be informed about a higher risk of brachial plexopathy, especially if a large volume is included in the LN-SIB. PO-1274 Simultaneous integrated boost within the lymphatic drainage system in breast cancer A. Peiler 1 , S.T. Klusen 1 , R. Asadpour 1 , S.E. Combs 1,2,3 , K.J. Borm 1 Results The median FU time was 557 days (82.4 weeks) and ranged from 41 to 3373 days. Overall, 28 patients developed I°, 18 patients II° and 2 patients III° acute toxicity.