ESTRO 2020 Abstract Book

S499 ESTRO 2020

years) with clinical stage distribution of I (N=19),II (N=10), or III (N=1). Breast cancer subtype is (ER+ or PR+)/HER2- in 25, HER2+ in 4, and triple-negative in 1. OPR techniques included 25 reduction mammoplasties and 5 local tissue rearrangements, of which 6 were performed by the breast surgeons and the remainder by plastic surgeons. IOERT was delivered using 6 MeV electrons (N=3), 9 MeV electrons (N=8) or 12 MeV electrons (N=19) to a median depth of 3.1 cm (range, 1.8-3.6 cm) with a median applicator size of 5.5 cm (range, 4-7 cm). The complication rate was 10%(N=3): 2 hematomas requiring evacuation and 1 breast cellulitis requiring IV antibiotics. To date, 28 of the 30 patients have received adjuvant WBI (N=24 WBI to 40 Gy/15 and N=4 WBI+RNI to 50 Gy/25) at a median of 7.7 weeks (range, 5.3-12.3 weeks) after IOERT. The remaining 2 patients are completing chemotherapy. Conclusion Given the low rate of postoperative complications, this study has been deemed safe and will continue to the accrual goal of 176. This is the first prospective evaluation of the use of an IOERT boost in patients undergoing L+OPR, and the final results may result in a new treatment paradigm for women undergoing OPR. PO-0934 Evaluation of nodal outlining based on ESTRO guidelines in breast radiotherapy planning C. Welgemoed 1 , E. Spezi 2 , A. Chowdhury 1 , P. Riddle 1 , C. Amy 1 , R. McLauchlan 1 , G. Dorothy 1 , E. Aboagye 3 1 Imperial College Healthcare NHS Trust, Radiotherapy, London, United Kingdom ; 2 Cardiff University, School of Engineering, Cardiff, United Kingdom ; 3 Imperial College London, Surgery and Cancer, London, United Kingdom Purpose or Objective The nodal field-based planning technique in breast radiotherapy was superseded in 2016 by a volume-based technique. This technique requires outlining of nodal volumes, allowing for improved dose conformance to treatment volumes and minimising radiation to surrounding normal tissues. However, volume outlining can betedious and time-consuming, requires expert skills and confer large inter- and intra-user variability 1, 2, 3. . Yang described how the use of guidelines improved outlining accuracy, nonetheless inter- and intra-variability still exist due to differences in training, image quality and clinical experience levels 3 . The purpose of this work was to evaluate local inter- and intra-user variability in manual outlining for nodal target volume delineation. Accuracy outcomes were expected to demonstrate the quality of local outlining practice; potentially inform future training needs, and contribute to quality improvement of radiotherapy planning and treatment delivery. Material and Methods 4 Clinicians were exposed to equal image quality conditions and retrospectively outlined nodal volumes, as well as the brachial plexus on 12 anonymised CT data sets. The clinicians comprised of 2 consultant breast oncologists (DD and EE) and 2 specialist registrars (AA and BB). The European Society for Radiotherapy and Oncology guidelines 4 were followed when outlining the level 4 to 1, inter-pectoral and internal mammary nodes. Their outlines were compared to 12 “gold standard outlines” (clinician approved outlines), and the Jaccard coefficient (JCI) calculated to measure conformance to the “gold standard” outlines. A Jaccard coefficient value of 1.0 resembles 100% conformance to the ‘Gold Standard’ outline and 0.8 would be seen as reasonably acceptable 5,6 . Results Manual outlining differences for level 4 and 1 are demonstrated in figure 1. Results show a larger intra- observer range for level 4 than level 1 nodes. Furthermore, the variation of outlined volumes between clinicians is not statistically significant (JCI range 0-56–0.61 and 0.65-0.75

for level 4 and 1 nodes respectively). However, this importantly also demonstrates a universal failure to meet the ‘gold standard’, which is more pronounced with level 4 nodal volumes. Analysis from the x, y and z coordinates indicates risk of under-dosing the nodal volumes and exposing critical organs. The JCI for nodal 01 outlined by clinician DD was 0.23 (Fig.1). 3D analysis for the same volume demonstrated under-outlining of 1.8cm inferiorly and 1.4cm laterally and at depth. Consequently, the volume is 8.6 cubic centimetre smaller than the “gold standard” which may be of clinical significance (Fig.2).

Conclusion ‘From this study it can be reasoned that in order for this volume-based technique to deliver its predicted clinical improvement, further adjustments are required through amendment of current guidelines and employment of automated and computerised systems’ (or artificial intelligence systems). PO-0935 SAPBI for early stage breast cancer-– Feasibility and Early Experiences of a Phase II Clinical Study N. Mészáros 1 , V. Smanykó 1 , T. Major 1 , G. Stelczer 1 , L. Jánváry 1 , E. Kovács 2 , M. Bahéry 2 , Z. Zaka 1 , D. Pukancsik 3 , Z. Takácsi-Nagy 1 , C. Polgár 1 1 National Institute of Oncology, Radiotherapy, Budapest, Hungary ; 2 National Institute of Oncology, Radiology, Budapest, Hungary ; 3 National Institute of Oncology, Breast and Sarcoma Surgery, Budapest, Hungary Purpose or Objective To report the implementation, dosimetric results and early experiences of stereotactic accelerated partial breast irradiation (SAPBI) following breast conserving surgery (BCS) for low-risk early stage invasive breast cancer. Material and Methods Between November 2018 and October 2019, 35 patients with low risk early invasive (St I-II) breast cancer underwent BCS were enrolled in our phase II prospective study. SAPBI was performed with Cyber-Knife (CK) M6 machine, to a total dose of 25 Gy in 4 daily fractions of 6.25 Gy. Respiratory movements were followed with implanted gold markers and Synchrony system. Corrections for patient displacement and respiratory movement during treatment were performed with the robotic arm. Early side effects, cosmetic results and dosimetric parameters were assessed. Results All treatment plans met the predefined study specific dose constraints for PTV coverage and dose homogeneity, and all patients received the planned four fractions of CK- SAPBI. No protocol violation occurred. At least three fiducials were tracked in all cases. At a median follow-up of 6 months (range: 1-12) no loco-regional recurrence or