Abstract
Radiation dose-response curves play a fundamental role in the attempts to optimize radiotherapy, and it is a major task in clinical and experimental radiation research to characterize and quantify the factors that determine the position and shape of dose-response curves. A convenient measure of the steepness of radiation dose-response curves is the normalized dose-response gradient, gamma, which represents the increase in response, in percentage points, for a 1% increase in dose. Theoretically, the normalized dose-response gradient should increase with increasing clonogenic cell number or, assuming a constant clonogen density, with increasing tumor volume. The aim of this study was to test this hypothesis over a range of tumor volumes and to study how this relationship is affected by heterogeneity in tumor oxygenation.