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Colorectal cancer remains a major cause of cancer-related morbidity and mortality. In rectal cancer, accurate pretreatment risk stratification is essential for guiding neoadjuvant treatment and surgical planning. Magnetic resonance imaging (MRI) plays a central role in this process, but the interpretation and prognostic value of mesorectal tumor deposits and other malignant nodular structures remain incompletely understood. In parallel, many patients undergoing rectal cancer surgery require a permanent colostomy, and parastomal hernia is a common long-term complication. The roles of prophylactic mesh, stoma placement and abdominal wall changes, including rectus abdominis muscle atrophy, are incompletely understood.

The overall aim of this thesis was to expand knowledge in two clinically relevant areas of colorectal cancer: MRI-defined and histopathological mesorectal nodular disease as markers of metastatic risk in rectal cancer, and factors associated with parastomal hernia formation after permanent colostomy.

Paper I investigated the correlation between pretreatment MRI and histopathology for detecting tumor deposits and differentiating them from lymph node metastases in patients with rectal cancer who had not received neoadjuvant therapy. MRI had limited ability to identify histopathologically defined tumor deposits, particularly when deposits were small, and differentiation from metastatic lymph nodes was challenging.

Paper II evaluated whether MRI-defined mesorectal nodular structures were associated with outcome in rectal cancer. Increasing numbers of strictly defined suspicious lymph nodes and MRI-defined tumor deposits with venous continuity were associated with worse cancer-free and overall survival, suggesting that the burden of confidently malignant mesorectal nodular disease on MRI may provide prognostic information even when precise pathological classification is uncertain.

Papers III and IV investigated parastomal hernia after permanent colostomy. Paper III found rectus abdominis muscle atrophy to be more frequent after prophylactic mesh placement and associated with lower odds of parastomal hernia. Paper IV, based on data from a prospective randomized multicenter trial, did not confirm the previously observed association between mesh, rectus abdominis muscle atrophy and reduced hernia risk.

In conclusion, this thesis shows that pretreatment MRI cannot reliably reproduce the histopathological distinction between tumor deposits and metastatic lymph nodes, but that MRI-defined malignant mesorectal nodular burden may still have prognostic value. The findings also indicate that associations between prophylactic mesh, abdominal wall changes after stoma construction and parastomal hernia risk may be context-dependent, highlighting the need for standardized definitions, careful endpoint selection and further validation before changes in clinical practice are implemented.