A NUMERICAL-SIMULATION OF ORGAN MOTION AND DAILY SETUP UNCERTAINTIES - IMPLICATIONS FOR RADIATION-THERAPY

Purpose: In radiotherapy planning, the clinical target volume (CTV) is typically enlarged to create a planning target volume (PTV) that acco unts for uncertainties due to internal organ and patient motion as wel l as setup error, Margin size clearly determines the volume of normal tissue irradiated, yet in practice it is often given a set value in ac cordance with a clinical precedent from which variations are rare, The (CTV/PTV) formalism does not account for critical structure dose, We present a numerical simulation to assess (CTV) coverage and critical o rgan dose as a function of treatment margins in the presence of organ motion and physical setup errors, An application of the model to the t reatment of prostate cancer is presented, but the method is applicable to any site where normal tissue tolerance is a dose-limiting factor. Methods and Materials: A Monte Carlo approach was used to simulate the cumulative effect of variation in overall tumor position, for individ ual treatment fractions, relative to a fixed distribution of dose, Dis tributions of potential dose-volume histograms (DVHs), for both tumor and normal tissues, are determined that fully quantify the stochastic nature of radiotherapy delivery, We introduce the concept of Probabili ty of Prescription Dose (PoPD) isosurfaces as a tool for treatment pla n optimization, Outcomes resulting from current treatment planning met hods are compared with proposed techniques for treatment optimization, The standard planning technique of relatively large uniform margins a pplied to the CTV, in the beam's eye view (BEV), was compared with thr ee other treatment strategies: (a) reduced uniform margins, (b) nonuni form margins adjusted to maximize normal tissue sparing, and (c) a red uced margin plan in which nonuniform fluence profiles were introduced to compensate for potential areas of reduced dose. Results: Results ba sed on 100 simulated full course treatments indicate that a 10 mm CTV to PTV margin, combined with an additional 5 mm dosimetric margin, pro vides adequate CTV coverage in the presence of known treatment uncerta inties, Nonuniform margins can be employed to reduce dose delivered to normal tissues while preserving CTV coverage, Nonuniform fluence prof iles can also be used to further reduce dose delivered to normal tissu es, though this strategy does result in higher dose levels delivered t o a small volume of the CTV and normal tissues, Conclusions: Monte Car lo-based treatment simulation is an effective means of assessing the i mpact of organ motion and daily setup error on dose delivery via exter nal beam radiation therapy, Probability of Prescription Dose (PoPD) is osurfaces are a useful tool for the determination of nonuniform beam m argins that reduce dose delivered to critical organs while preserving CTV dose coverage. Nonuniform fluence profiles can further alter criti cal organ dose with potential therapeutic benefits, Clinical consequen ces of this latter approach can only be assessed via clinical trials. Copyright (C) 1997 Elsevier Science Inc.