Purpose: The use of PET/CT for monitoring treatment response in cancer patients after chemo- or radiotherapy is a very promising approach to optimize cancer treatment. However, the timing of the PET/CT-based evaluation of reduction in viable tumor tissue is a crucial question. We investigated how to plan and analyze studies to optimize this timing.; Methods: General considerations about studying the optimal timing are given and four fundamental steps are illustrated using data from a published study.; Results: The optimal timing should be examined by optimizing the schedule with respect to predicting the overall individual time course we can observe in the case of dense measurements. The optimal timing needs not to and should not be studied by optimizing the association with the prognosis of the patient.; Conclusions: The optimal timing should be examined in specific 'schedule optimizing studies'. These should be clearly distinguished from studies evaluating the prognostic value of a reduction in viable tumor tissue.;