VOLUME AND HETEROGENEITY DEPENDENCE OF THE DOSE-RESPONSE RELATIONSHIPFOR HEAD AND NECK TUMORS

Based on the Poisson statistics of cell kill a model for the response of heterogeneous tumours to non-uniform dose delivery have been develo ped. The five parameters required to characterize the response are the 50% response dose, D-50, the normalized dose-response gradient, gamma , the tumour heterogeneity factor, h, the relative volume, upsilon and the extra daily dose required to counteract the tumour cell prolifera tion, delta. The model has been fitted to data from a number of clinic al investigations to allow the derivation of clinically relevant radia tion response parameters for head and neck tumours. The D-50 value for T2 larynx cancers is 59.9 Gy in 41 days with a relative standard devi ation of 2.1 Gy and the gamma value is 2.9 with a relative standard de viation of 0.3. The value of delta, which is most consistent with the clinical data for laryngeal tumours, is 0.35 Gy/day and this value sho uld be used if the treatment time is changed from the 41 days normaliz ation. The heterogeneity factor, h, is close to zero for laryngeal tum ours which indicates that their response is basically governed by Pois son statistics. Nasopharyngeal tumours, on the other hand, exhibit h v alues around 0.2 which indicates that these tumours are more heterogen eous in their internal organization and so are their responses to radi ation.