Purpose: The objective of this study was to estimate the effect of delay in initiation of treatment on rates of local control by radiotherapy.
Methods and materials: 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.
Results: 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.
Conclusions: 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).