MRI Monitoring of Locally Advanced Rectal Cancer in Watch and Wait Patient.

Background: Watch and wait (W&W) is an alternative strategy for locally advanced rectal cancer (LARC) (T3-4/N+) patients. MRI-based evaluation of clinical complete response (cCR) and post-cCR disease monitoring remain uncertain. This study aims to investigate the association between MRI feature changes and survival outcomes in these patients.

Methods: LARC Patients achieving cCR after neoadjuvant chemoradiotherapy and opting for W&W were included. MRI features were recorded from pretreatment to the follow-up period, including tumor bed scarring on post-treatment T2-weighted imaging (T2WI), magnetic resonance tumor regression grade (mrTRG), and high-signal areas on diffusion-weighted imaging (DWI). The relationship between follow-up data and MRI feature changes over time was analyzed.

Results: 41 patients were included, and 5-year PFS and OS were 64.1% and 90.9%. For the 11 patients without cCR at the initial assessment, tumor signals decreased over time on both T2WI and DWI, with an increasing trend in scar formation on T2WI. Cumulative cCR rates at 7 weeks (± 1 week), 16 weeks (± 3 weeks), 24 weeks (± 4 weeks), and 32 weeks (± 3 weeks) were 72.5%, 80.5%, 95.1%, and 100.0%. Seven patients (17.1%) had a recurrence, with a disruption of the scar and the appearance of intermediate-intensity signals within the low-signal fibrotic area of the original tumor bed on T2WI. A newly developed high-signal area was observed on DWI.

Conclusions: The cCR rate increased over time. The scar sign on T2WI was more prevalent in patients with cCR, increased over time. MRI surveillance is essential for W&W patients but remains dynamic and challenging.