Key PointsQuestionIs fluoropyrimidine-based systemic chemotherapy effective in treating patients with inoperable low-grade mucinous appendiceal adenocarcinoma?FindingsIn this randomized crossover trial that included 24 patients, there was no significant difference in tumor growth between treatment and observation time periods.MeaningThese findings suggest that patients with low-grade mucinous appendiceal adenocarcinoma did not derive clinically meaningful benefit from systemic fluoropyrimidine-based chemotherapy.