This pilot study will take place at Health Sciences North (HSN) in Sudbury. Patients will be identified and screened for inclusion into the study once referred to a general or colorectal surgeon for resection of their rectal cancer. This may occur before or after preoperative chemoradiotherapy (CRT) but randomization will take place after CRT.

The main question of this study is whether or not delaying surgery after preoperative CRT increases disease-free survival. The standard 6-7 week waiting period will be compared to 10-11 weeks, which was the time-frame identified in the Dutch Surgical Colorectal Audit as increasing the likelihood of pCR the most. This trial will also be able to determine if an increased waiting interval increases the rate of sphincter-preservation, local recurrence, and pCR. While similar to the GRECCAR-6 trial, our primary outcome is disease-free survival (a more clinically relevant outcome) and is the first North American clinical trial of this nature.

To maximize the utility of this trial, immunohistochemical staining will be performed on pre-CRT tumor samples to prospectively validate the scoring system proposed by Hur et al. [28] The question being whether or not expression of these biomarkers can predict which patients will experience pCR and disease-free survival.

The pilot will be constructed as a prospective, open-label clinical trial, with patients being randomized to undergo surgery at 6-7 weeks or 10-11 weeks after completion of preoperative CRT. The intention is that once preliminary data is available, other centres in Ontario will be recruited. This will not only help answer our research question, but also create a large, prospective database of rectal cancer patients. The Primary outcome (disease-free survival) will be assessed at 3 years.

Inclusion Criteria:

• Adult patients (>18 years)
• T3/4N0M0 or TxN+M0 rectal cancer below or at the peritoneal reflection, confirmed on clinical exam, histology, pelvic MRI, and CT chest/abdo
• Patients who have completed a CRT protocol for the above tumor at HSN
• Informed consent given
• Surgical resection with TME planned

Exclusion Criteria:

• Metastatic disease
• Failure to complete preoperative CRT
• Unable to give free and informed consent
• Unable to comply with the requirements of the study
• Previous malignancy other than nonmelanoma skin cancer, papillary or follicular thyroid cancer
• Inflammatory bowel disease
• Hereditary colorectal cancer

This study will include patients at Health Sciences North who have been referred to a general or colorectal surgeon for resection of their rectal cancer.