The aim of this study is to assess the effects of Positron Emission Tomography (PET) associated with computed tomography (CT) on resource allocation (costs and savings) of the following treatment in radiotherapy for non small cell lung cancers (NSCLC) and Hodgkin's diseases. A national prospective study was conducted in nine hospitals. Two treatment decisions made on the basis of CT only or on PET associated with, were compared in a before-after design. The direct medical cost of using PET was assessed by micro-costing. The costs of new exams and the costs and savings associated with changes in the chosen treatment were calculated on the basis of reimbursement rates. The economic study was conducted over 2 years and included 209 patients (97 patients with Hodgkin's disease and 112 with NSCLC). The mean cost of using PET, corresponding to an extra cost, was approximately 800 euro (50% for the radionuclide FDG). Radiotherapy treatments were modified for only 10% of patients with Hodgkin's disease with a minor impact on treatment costs versus 40% of patients with lung cancer with a reduction in mean treatment cost of more than 500 euro.