Background: Previous studies showing higher mortality after elective surgery performed on a Friday were based on administrative data, known for insufficient case-mix adjustment. The goal of this study was to investigate the risk of adverse events for patients with colon and rectal cancer by day of elective surgery using clinical data from the Dutch ColoRectal Audit. Patients and Methods: Prospectively collected data from the 2012-2015 Dutch ColoRectal Audit (n=36,616) were used to examine differences in mortality, severe complications, and failure to rescue by day of elective surgery (Monday through Friday). Monday was used as a reference, analyses were stratified for colon and rectal cancer, and case-mix adjustments were made for previously identified variables. Results: For both colon and rectal cancer, crude mortality, severe complications, and failure-to-rescue rates varied by day of elective surgery. After case-mix adjustment, lower severe complication risk was found for rectal cancer surgery performed on a Friday (odds ratio, 0.84; 95% CI, 0.72-0.97) versus Monday. No significant differences were found for colon cancer surgery performed on different weekdays. Conclusions: No weekday effect was found for elective colon and rectal cancer surgery in the Netherlands. Lower severe complication risk for elective rectal cancer surgery performed on a Friday may be caused by patient selection.

Background: It is unclear whether emergency weekend colon and rectal cancer surgery are associated with worse outcomes (ie, weekend effect) because previous studies mostly used administrative data, which may insufficiently adjust for case-mix. Materials and Methods: Prospectively collected data from the 2012-2015 Dutch ColoRectal Audit (n=5,224) was used to examine differences in 30-day mortality and severe complication and failure-To-rescue rates for emergency weekend (Saturday and Sunday) versus Monday surgery, stratified for colon and rectal cancer. Analyses were adjusted for age, sex, body mass index, Charlson comorbidity index, American Society of Anesthesiologists classification score, tumor stage, presence of metastasis, preoperative complication, additional resection for metastasis or locally advanced tumor, location primary colon tumor, type of rectal surgery (lower anterior resection or abdominal perineal resection), and type of neoadjuvant therapy (short-course radiotherapy or chemoradiotherapy). Results: A total of 5,052 patients undergoing colon cancer surgery and 172 undergoing rectal cancer surgery were included. Patients undergoing colon or rectal cancer surgery during weekends had significantly more preoperative tumor complications compared with those undergoing surgery on a weekday. Additionally, differences in year of surgery and location of primary tumor were found for colon cancer surgery. Emergency colon cancer surgery during the weekend was associated with increased 30-day mortality (odds ratio [OR], 1.66; 95% CI, 1.10-2.50) and severe complications (OR, 1.29; 95% CI, 1.03-1.63) compared with surgery on Monday. Estimates for emergency weekend rectal cancer surgery were similar but not statistically significant, likely explained by small numbers. Conclusions: Weekend emergency colon cancer surgery was associated with higher mortality and severe complication rates. More research is needed to understand which factors explain and contribute to these differences.