Carcinoma/drug therapy; Colorectal Neoplasms/drug therapy; Deoxycytidine/analogs & derivatives; Dihydrouracil Dehydrogenase (NADP)/genetics; Drug-Related Side Effects and Adverse Reactions/genetics; Fluorouracil/analogs & derivatives; Polymorphism, Single Nucleotide; Adult; Aged; Aged, 80 and over; Antimetabolites, Antineoplastic/administration & dosage; Antimetabolites, Antineoplastic/adverse effects; Antimetabolites, Antineoplastic/therapeutic use; Biomarkers, Pharmacological/metabolism; Biomarkers, Tumor/genetics; Capecitabine; Carcinoma/genetics; Case-Control Studies; Clinical Trials, Phase III as Topic; Cohort Studies; Colorectal Neoplasms/genetics; Deoxycytidine/administration & dosage; Deoxycytidine/adverse effects; Deoxycytidine/therapeutic use; Dihydrouracil Dehydrogenase (NADP)/metabolism; Disease Progression; Dose-Response Relationship, Drug; Female; Fluorouracil/administration & dosage; Fluorouracil/adverse effects; Fluorouracil/therapeutic use; Haplotypes; Humans; Male; Middle Aged; Polymorphism, Single Nucleotide/physiology; Randomized Controlled Trials as Topic; Retrospective Studies; Treatment Outcome