The effect of delay in treatment on local control by radiotherapy.

Abstract

The objective of this study was to estimate the effect of delay in initiation of treatment on rates of local control by radiotherapy. A model of the effects of delay was developed based on the following assumptions: (a) that tumor growth rate is exponential, (b) that a predetermined radiotherapy regimen will kill the same fraction of clonogenic cells in a given tumor whether it is administered early or late, and (c) that the absolute number of cells surviving in a tumor is determined by Poisson statistics. Monte Carlo simulation was used to estimate the expected rate of decrease in local control associated with delay in a population of tumors, which was heterogeneous with respect to doubling time and initial volume. The model was applied to carcinoma of the tonsillar region. It was shown that at some point in the evolution of every case, the probability of local control decreases sharply over a relatively short period of time. The maximum rate of decrease in the probability of local control occurs at the 37% local control level when it reaches 25.5% per tumor doubling time. When heterogeneity with respect to doubling time and stage was taken into account, it was estimated that the local control rate would decrease by approximately 10% per month in a typical series of patients with carcinoma of the tonsillar region. It was concluded that delay in initiation of radiotherapy may be associated with a clinically important deterioration in local control rates. We recommend that waiting times for radiotherapy should be As Short As Reasonably Achievable (ASARA).