ESTRO 2020 Abstract book

S545 ESTRO 2020

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 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.

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