Abstract Book

ESTRO 37

S380

Material and Methods In the ABLATIVE study we recruited women with unifocal, estrogen receptor positive and Her2neu negative tumor of maximum 2 cm (50-70 years) or 3 cm (≥70 years), N0(sn), WHO ≤2 and no indication for (neo)adjuvant chemotherapy. Diagnostic breast MRI was performed to confirm unifocality and tumor size. Contrast-enhanced radiotherapy planning CT and MRI scans were performed in supine radiotherapy position and co-registered for target delineation. Investigational treatment consisted of single dose partial breast irradiation. The prescribed dose was 20 Gy to gross tumor volume (GTV) and 15 Gy to clinical target volume (CTV , GTV plus 20 mm margin). A 3 mm planning target volume (PTV) margin was applied to both GTV and CTV. Patients underwent breast- conserving surgery (BCS) 6 months after RT, response was monitored through MRI assessed by dedicated breast radiologists using tumor morphology and contrast enhancement kinetics. Pathologic response was assessed by three dedicated breast pathologists according to EUSOMA criteria. Radiation induced toxicity was reported according to CTCAE 4.03 criteria. Results Between May 2015 and January 2017 we included 15 patients who fulfilled inclusion criteria with a median clinical tumor size of 13 mm. Two patients were treated with pre-operative endocrine treatment following radiotherapy. After BCS, pathologic complete response (pCR) was observed in 5 patients and near pCR (<10% residual tumor cells) in 5 patients. Partial response (PR, 10-50% residual tumor) and stable disease (SD, >50% residual tumor cells) was observed in 3 and 2 patients, respectively. In all 5 patients with pCR and in 3 patients with near pCR, radiologic complete response (rCR) was reported at 6 months follow-up. No radiologic complete response was observed in patients with PR or SD (figure1, table 1). Furthermore, no clinical or radiologic disease progression was observed in any patient. Radiation- induced toxicity was mostly grade 1, i.e. fibrosis (93%) and breast pain (20%). Grade 2 toxicity was less common, i.e. pain breast (7%), lymphedema (7%) and pain chest wall (7%), and was transient in all patients.

near pCR (n=5)

PR or SD (n=5)

pCR (n=5)

Radiologic response

complete

5

3

-

No radiologic complete response -

2

5

Table 1: Radiologic and pathologic response at 6 months following MRI-guided single dose pre-operative ablative radiotherapy. Conclusion MR-guided single dose pre-operative ablative RT in low- risk breast cancer patients resulted in pCR in 5 out of 15 patients after an interval of 6 months. Response monitoring using MRI showed individual variation in response and relation to pCR. Further optimization and individualization of response monitoring and interval between RT and surgery is necessary. PO-0741 Implementation of contouring guidelines can be achieved by online learning through a dummy run L. Levy 1 , C. Petit 1 , C. Rossier 1 , A. Ruffier 1 , E. Bronsard 1 , M. Palmu 2 , N. Bonnet 3 , S. Michiels 4 , S. Rivera 1 1 Institut Gustave Roussy, radiotherapy, Villejuif, France 2 ESTRO, Office, Brussels, Belgium 3 UNICANCER, UNITRAD, Paris, France 4 Institut Gustave Roussy, Biostatistics and epidemiology, Villejuif, France Purpose or Objective Variability in contouring is a weak point in modern radiotherapy, whose heterogeneity can even compromise the results of randomized clinical trials. The use of contouring guidelines may improve quality in clinical trials. Compliance with guidelines can be assessed through a dummy run (DR). Delineation in the multicentric randomized phase III HypoG-01 trial, which compares moderately hypofractionnated with normofractionated loco-regional radiotherapy in early breast cancer, was based on ESTRO breast contouring guidelines. The aim of the present study was to assess the implementation of contouring guidelines through a DR based on a blended learning approach. Material and Methods Investigators (INVs) of the 27 participating centers were invited to an online interactive workshop (WS). Online teaching consisted in a lecture presenting the ESTRO breast and various organs at risk (OAR) contouring guidelines. Corresponding publications of the guidelines were sent to all INVs. Following the WS, each INV was asked to delineate a DR in the FALCON-EduCase online system, before entering patients in the trial. Two senior radiation oncologists reviewed all contours comparing them to a reference contour according to guidelines for target volumes and OARs. The DICE index (DI), upper and lower limits of the contours and predefined qualitative measures were used as endpoints. Observed deviations have been classified as major or minor. Centers that did not pass were asked to modify their contours and resubmit there DR until validation. Results As of today, 17 out of 27 HypoG-01 trial investigating centers have submitted at least once the DR. The median number of submission was 2 (range 1-4). At first submission, the average number of major and minor deviations was 2.1 (+/- 1.9) and 8.2 (+/- 5) respectively. In validated submissions, the number of deviations dropped to 0.2 (+/- 0.6), and to 2.1 (+/- 2.4) for major and minor deviations respectively. Among all submissions, the most frequent major deviations concerned brachial plexus (29.9%), level 4: CTVn-L4 (20.5%) and internal mammary nodes: CTVn-IMN (9.1%). The most frequent

Figure 1: A) diagnostic MRI; B) radiologic complete response; C) near pCR. D) diagnostic MRI; E) radiologic complete response; F) pCR.