In this study it will be determined whether the rate of severe toxicity associated with fluoropyrimidine treatment (capecitabine or 5-fluorouracil) can be significantly diminished by individualized dosing of fluoropyrimidines based on upfront phenotypic assessment of dihydropyrimidine dehydrogenase (DPD) deficiency.

In this study a phenotypic approach will be studied to determine the additional value of pretreatment uracil level-guided dose individualization in wildtype patients. Patients with a pretreatment serum uracil concentration above 16 ng/ml will be treated with a 50% reduced fluoropyrimidine starting dose. The pretreatment serum uracil levels in DPYD variant carriers will be assessed retrospectively and non-interventional. Additionally, the effect of a higher dose reduction in c.1236G>A and c.2846A>T DPYD variants carriers (50% instead of 25%) will be studied.

Inclusion Criteria:

1. Pathologically confirmed malignancy for which treatment with a fluoropyrimidine is considered to be in the patient's best interest
2. Patient need to be of Western descent
3. Age ≥ 18
4. Able and willing to give written informed consent
5. WHO performance status of 0, 1 or 2
6. Able and willing to undergo extra blood sampling for study related analysis
7. Adequate baseline patient characteristics, in the opinion of the treating physician (complete blood count, hepatic function which involves serum bilirubin, AST, ALT, and renal function)

Exclusion Criteria:

1. Prior treatment with fluoropyrimidines
2. Patients with known substance abuse, psychotic disorders, and/or other diseases expected to interfere with study or the patient's safety in the opinion of the treating physician
3. Patients treated with the combination of a fluoropyrimidine and irinotecan