FP2.1 - The accuracy of radiological staging in rectal cancer patients post-long course chemoradiotherapy treatment

Abstract Aim Accurate post Long-course chemoradiotherapy treatment (LCCRT) allows safe selection of patients to rectal preservation without delaying resection in those with residual cancer. The aim of this study was to assess the accuracy of radiological staging in these patients. Methods Demographic and treatment data from consecutive patients receiving LCCRT for rectal cancer between May 2020 to December 2021 was collected. Local radiological staging post-LCCRT treatment was compared to pathological staging. Where patients had complete radiological response (CRR) and opted for watch and wait (WW) those with demonstrable local recurrence, within 12 months, were assumed to have been under-staged. Results Of 61 patients, 37 underwent surgical resection following LCCRT and 24 watch and wait for CRR. Post-LCCRT imaging was taken at a median of 10.86 weeks (IQR, 8.86- 13.00) post completion of treatment and resections 16.57 weeks (IQR 9.71 – 23.00) post-imaging. Four WW patients were diagnosed with local recurrence at a median of 56.54 weeks of follow-up (IQR, 39.9 – 67.90). The ratio of patients radiologically staged as T0:T1:T2:T3:T4 was 15%: 0%: 28%: 46%: 10%; whilst the pathology specimens demonstrated 20%: 5%: 12%: 49%: 15% respectively. Nodal staging (N0:N1:N2) was 78%: 20%: 2% with histology showing 71%: 24%: 5% respectively. Conclusion In our low-numbered study T-staging of patients with T0 and T1 disease had highest rates of over-staging such that 2 (5%) patients for potential watch and wait were not recognised as CCR by MRI alone. Nodal staging appeared more accurate although a tendency to under-stage N0 disease was seen.