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Short-course irradiation reduces the risk of local recurrence and showed overall survival improvement with a lower rate of early toxicity when compared to chemoradiation. Previous studies demonstrated that radiotherapy dose of neoadjuvant treatments is a significant predictor of achieving a local response. The purpose of this study is to measure the effects of stereotactic MR-guided adaptive radiotherapy (SMART) for rectal cancer patients in terms of early toxicity, radiological and pathological response.

Patients diagnosed with local advanced rectal cancer with positive lymph node staging, resectable (cT3 with > 5 mm extramural invasion and uninvolved MRF) or unresectable unfit for chemotherapy, underwent SBRT on rectal lesion and mesorectum with patological nodes using hybrid MR-Linac (MRIdian ViewRay). Treatment prescription at 80% isodose for the rectal lesion and mesorectum with pathological nodes was 40Gy (8Gy/fr) and 25 Gy (5Gy/fr),respectively,  delivered on 5 days (3fr/week). The gross target volume (GTV) was identified on a true fast imaging (TRUFI) MR scan acquired during simulation and prior to each fraction to adapt the treatment plan of the day. New plans were calculated and delivered every fractions because of rectal and bowel motion. An intrafraction motion management strategy was applied, consisting of a gating approach based on the real-time acquisition of a sagittal cine MRI during the whole delivery time (temporal resolution: 8 frames/s). Response assessment by MRI was performed 3 weeks after SMART, than patients fit for surgery underwent total mesorectal excision. Adverse effects of RT were scored according to the NCI Common Toxicity Criteria (CTC) scale, version 5.0.

Twenty patients underwent rectal SMART. No G3-5 toxicity was recorded. A moderate or good radiological response (mrTRG≤3) was observed in all of this patients. Twelve patients were elegible for total mesorectal excision. Mean interval  between the completion of SMART and surgery was  4 weeks.  Pathological downstaging occurred in all patients, 2 patients achieved complete response (pCR).  pCR occured with a prolonged time to surgery (>7 weeks). Patients baseline characteristics, pathological stage and response are listed in Table 1.

SMART for rectal cancer is well tolerated and effective in terms of tumor regression. This radiation treatment can help to achieve a complete pathological response in selected patients, especially when followed by delayed surgery. In addition, short-course schedule is less expensive and time saving, therefore SMART could be useful to treat a larger number of patients and to reduce patients’ waiting list. However, further studies are required to confirm these findings.