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Background
Multiple treatment options are available for the management of rectal adenocarcinoma, ranging from transanal local excision for early-stage disease to standard-of-care neoadjuvant therapy for locally advanced rectal adenocarcinoma, followed by surgical resection or organ-preserving surveillance (“watch and wait”) strategy for selected patients who achieve complete clinical response after neoadjuvant treatment. Rectal MRI plays a major role in evaluating treatment response and directing management following neoadjuvant therapy. Therefore, it is imperative for radiologists to be familiar with posttreatment MRI restaging of rectal adenocarcinoma.

Educational Goals / Teaching Points
This exhibit aims to provide an overview of surgical and nonsurgical management options for rectal adenocarcinoma, highlight the value of MRI in categorization of treatment response, underline commonly used terminology in restaging of rectal cancer, and illustrate common pitfalls and challenges faced in MRI restaging of rectal adenocarcinoma.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Acquiring an appropriately protocoled rectal MRI is key for accurate staging and restaging of rectal cancer. Following neoadjuvant therapy, tumor signal on T2-weighted imaging (T2WI) and DWI are used to categorize degree of treatment response into complete/near complete, incomplete or no response. Comparison with initial pretreatment staging MRI is necessary for accurate assessment. Common pitfalls seen following neoadjuvant chemoradiation include submucosal edema which can mimic residual tumor due to its intermediate/high T2 signal intensity. The presence of artifact on DWI related to gas in the rectum can confound the evaluation for viable tumor. ‘T2 dark-through’ due to scarring can also be mistaken for residual tumor on ADC. Therefore, T2WI should be interpreted in conjunction with DWI/ADC to differentiate residual tumor versus scar formation. Common challenges include evaluation of treatment response for mucinous adenocarcinoma and recognizing mucinous degeneration of rectal tumors, since MRI is unable to differentiate cellular from acellular mucin. In assessment of nodal response, it is important to understand the anatomic boundaries of pelvic lymph node compartments so as to accurately localize metastatic lymph nodes and apply size criteria. Familiarity with normal postoperative changes on MRI is key to recognizing tumour recurrence as well postsurgical complications such as bleeding or urinary tract injury.

Conclusion
MRI is a reliable tool in the evaluation of rectal adenocarcinoma following treatment. Awareness of current treatment trends, posttreatment imaging appearance, as well as recognizing potential pitfalls and technical limitations can help increase accuracy and mitigate interpretation errors.