Pitfalls in estimating the influence of overall treatment time on local tumor control

Variations in the overall treatment time of human tumors occur for a number of reasons: differences in standard treatment schedules among centers, occurrence of public holidays during treatment, machine maintenance or breakdown, and decisions made by physicians to change treatment for individual patients based on patient or tumor response. It is now widely accepted that tumor proliferation can occur during treatment, and many efforts have been made to quantify the effects of varying the treatment time. Some of the many problems associated with obtaining quantitative estimates of the time factor are discussed. The aim of this paper is to illustrate that estimates of the time factor for tumor repopulation should be regarded with skepticism and used with extreme caution.