TOMOTHERAPY - OPTIMIZED PLANNING AND DELIVERY OF RADIATION-THERAPY

In contemporary radiotherapy dose optimization, radiation beams and be am modifiers are iteratively selected until the dose distribution is a cceptable. Another approach, referred to as the ''inverse problem,'' i s: Given the dose prescription, compute the optimal set of photon beam s while preventing unphysical solutions such as negative beam weights, and iterate to achieve the prescription as closely as possible. This solution to this inverse problem, which uses image reconstruction math ematics, entails the delivery of large numbers of nonuniform beam inte nsities to produce uniform dose distributions. These dose distribution s can be arranged to conform very closely to even complex target volum es, yet spare surrounding sensitive tissue. Alternatively, the dose di stributions can be arranged to generously treat a regional field and ' 'conformally avoid'' overtreating sensitive volumes within the field. Multiple dose prescriptions can be delivered without additional effort . We propose that a practical way of delivering optimized dose distrib utions would be to intensity modulate a photon beam, using collimator leaves intersecting a slit field of radiation. Modulation is achieved by varying the time that the leaves are blocking the field. A practica l geometry to deliver such a beam is a computed tomography-like gantry configuration, which also lends itself to tomographic setup verificat ion and the potential for unprecedented accuracy in the verification o f dose delivered to the patient. We refer to such a delivery method as ''tomotherapy.'' Several types of tomotherapy simulations have been c onducted. A fully three-dimensional optimized treatment planning syste m using iterative filtered back-projection have been developed. We wil l present examples of conformal plans for breast and prostate radiothe rapy. We have constructed an experimental apparatus for simulating hel ical tomotherapy delivery by simultaneously rotating and longitudinall y translating a phantom past an intensity-modulated fan beam. A compar ison between a computation and an experimentally realized plan is pres ented. A Monte Carlo simulation of the angular distribution and energy fluence spectrum of 10-MV photons produced by a tungsten target have been used to estimate the optimized shape and mass of a primary shield ing required to meet regulatory standards. (C) 1995 John Wiley and Son s, Inc.