ESTRO 2021 Abstract Book

S1031

ESTRO 2021

from completion of SCRT was 24 months. 40% of patients were female with median age at diagnosis of 59 years. 90% of patients were ECOG 1 or less. 60% of tumors were low lying rectal (0-5cm from the anal verge), 33% middle (5-10cm), and 7% high (10+ cm). Stage breakdown included 7% stage II, 83% stage III, and 10% stage IVA and 14 patients had involvement of the circumferential resection margin on initial staging. 80% of patients received at least 8 cycles of consolidation chemotherapy with the vast majority (93%) of patients receiving mFOLFOX and the remaining patients (7%) receiving CAPOX. 12 patients (40%) had a cPR and underwent surgery with 50% receiving APR and 50% receiving LAR. Mean OS was 35 months (95% CI 29.0-40.3 months) with 1 and 2-year OS at 82.7% and 71%, respectively. PFS at 1 and 2 years were 76% and 58%, respectively. DMFS at 1 and 2 years was 100% and 80%, respectively. Two patients (7%) developed failures at the site of boosted LPLNs. Six patients (20%) developed a local regrowth at rectal primary site, 5 which were receiving non- operative management. The LFFS at 1 and 2 years of 86% and 77%, respectively. Conclusion These early data indicate that LPLN management with a simultaneous integrated boost in the setting of SCRT- CH with non-operative intent may result in effective loco-regional control in patients with locally advanced rectal cancer. PO-1250 The utility of short-course radiotherapy (SCRT) for watch-and-wait strategy (w&w) in rectal cancer K. Paciorek 1 1 on behalf of the Polish Colorectal Study Group, I Department of Radiotherapy, Maria Sklodowska-Curie National Research Institute of Oncology, Warsaw, Poland Two prospective observational studies were pooled to post-hoc assess the utility of SCRT for w&w. Both studies evaluated the w&w in the consecutive patients: the first single-institutional study in elderly with a small tumour and the second multi-institutional study in all the patients after standard of care preoperative radiotherapy. In total, 490 patients were analysed. SCRT alone or with subsequent 6 weeks of 5-Fu and leucovorin chemotherapy (SCRT+CT) or chemoradiation (50 Gy, 2 Gy per fraction with bolus 5-Fu and leucovorin) was given to 40.6%, 40.2%, and 19.2% of the patients, respectively. Generally, SCRT was most often given for intermediate-risk cancers or the patients unfit for chemotherapy: SCRT+CT or chemoradiation for high-risk cancers and chemoradiation for low-risk cancers. The median intervals between the start of radiation and the tumour response assessment for SCRT, SCRT+CT, and chemoradiation were 10.0, 10.4, and 13.2 weeks, respectively. 73 patients had a clinical complete response (cCR), of whom 71 underwent w&w with the median follow-up of 24 months. Results cCR was observed in 14% of the patients after SCRT, 13% after SCRT+CT, and 22% after chemoradiation. After accounting for tumours’ and patients’ characteristics, the multivariate logistic analysis showed that the association between the schedule of radiotherapy and cCR was insignificant (p=0.176). The table below shows results in the subgroups categorized according to the tumor size. The regrowth rates at 2 years were 30.2% after SCRT, 13.0% after SCRT+CT and 36.0% after chemoradiation (p=0.561). Purpose or Objective To assess the utility of SCRT (5×5 Gy) for w&w strategy. Materials and Methods

% of patients with cCR (number of patients with cCR / number of patients in a subgroup)

Small tumours 5 × 5 Gy

26.1 (18/69) 37.8 (17/45) 44.7 (17/38) 10.3 (7/68) 9.8 (5/51) 9.4 (3/32) 3.3 (2/60) 2.0 (2/100) 4.2 (1/24)

5 × 5 Gy + chemotherapy

Chemoradiation

Intermediate tumours 5 × 5 Gy

5 × 5 Gy + chemotherapy

Chemoradiation

Large tumours 5 × 5 Gy

5 × 5 Gy + chemotherapy

Chemoradiation

small tumours - involvement ≤50% of bowel wall circumference and ≤4 cm of the length large tumours - circular cancers or with ≥7 cm tumour length intermediate tumours - all other cancers Three patient were excluded because of the lack of data.

Conclusion Our results suggest that SCRT/SCRT+CT can be used for w&w.

PO-1251 Upfront surgery vs. neoadjuvant chemoradiation in upper rectal cancers R. Engineer 1 , J. Poddar 2 , D. Pandey 3 , A.L. Desouza 4 , S. kumar 5 , V. Ostwal 6 , A. Ramaswamy 7 , A. Saklani 8 1 Tata Memorial Centre, Radiation Oncology, Mumbai, India; 2 Tata memorial Hospital, radiation oncology, Mumbai, India; 3 Tata Memorial hospital, surgical oncology, Mumbai, India; 4 Tata Memorial hospital, Surgical oncology, Mumbai, India; 5 Tata Memorial Hospital, Radiodiagnosis, Mumbai, India; 6 Tata memorial Hospital,