Computerized design of target margins for treatment uncertainties in conformal radiotherapy

Purpose: We describe a computerized method of determining target margins fo r beam aperture design in conformal radiotherapy plans. Materials and Methods: The method uses previously measured data from a popu lation of patients to simulate setup error and organ motion in the patient currently being planned. Starting with a clinical target volume (CTV) and n ontarget organs from the patient's planning CT scan, the simulation is repe ated many times to produce a spatial probability distribution for each orga n in the treatment machine coordinate system. This is used to determine a p rescribed dose volume (PDV), defined as the volume to receive the prescribe d dose, which encompasses the CTV while restricting the volume of nontarget organs within it, according to planner-specified values. The PDV is used t o design beam apertures using a conventional margin for beam penumbra. Results: The method is applied to 6-field prostate conformal treatment plan s, in which the PDV encloses the prostate and seminal vesicles while limiti ng the enclosed rectal wall volume, The effect of organ motion is assessed by applying the plans on subsequent CT scans of the same patients, calculat ing probabilities for tumor control (TCP) and normal tissue complication (N TCP), and comparing with plans designed from a physician-drawn planning tar get volume (PTV). Although prostate TCP and rectal wall NTCP are found to b e similar in the two sets of plans, TCP for the seminal vesicles is signifi cantly higher in the PDV-based plans. Conclusions: The method can improve the dose conformality of treatment plan s by incorporating population-based measurements of treatment uncertainties and consideration of nontarget tissues in the design of nonuniform target margins. (C) 1999 Elsevier Science Inc.