Magnetic Resonance Imaging in treated rectal cancer: response patterns and impact on treatment strategy

Magnetic resonance imaging (MRI) is the modality of choice for staging and evaluating response to neoadjuvant treatment in rectal cancer. Its ability to differentiate between post-treatment fibrosis, residual tumor, and complete tumor response is essential for surgical planning and for selecting candidates for organ-preserving strategies. Proper interpretation of findings requires familiarity with specific patterns such as desmoplastic fibrosis, the “split scar sign,” the T2 dark-through effect, and the assessment of mucin in both mucinous and non-mucinous tumors. The MRI-based tumor regression grade allows response categorization with clinical and prognostic relevance. However, several diagnostic challenges remain, including overestimation of residual tumor due to heterogeneous fibrosis or edema, difficulty in distinguishing cellular from acellular mucin, and motion or susceptibility artifacts. Understanding these imaging patterns and limitations is crucial for accurate interpretation, to avoid overtreatment and optimize patients selection for non-operative management. This work describes the main MRI findings after chemoradiotherapy, illustrated with clinical cases, aiming to support radiologic diagnosis and individualized clinical decision-making.