It has also been shown that there is a correlation between 5FU plasma level, tumor response and toxicity [7, 8]. A Phase III study showed that there was a significant impact on the response rate (33.7% VS 18.3%) p = 0.004 if doses of 5-FU were adjusted to plasma levels of 5-FU versus body surface [9]. Median survival was 16 months in the control arm versus 22 months in the experimental arm (P = 0.08), with severe grade 3-4 toxicities statistically increased in the control arm (p = 0.03). An adaptation of doses of 5-FU to the body surface, as it is done today, is in conclusion insufficient to obtain reproducible plasma concentrations due to an interindividual variability of the metabolism, linked to the activity of the DPD .