MODULATED BEAM CONFORMAL THERAPY FOR HEAD AND NECK TUMORS

Purpose: The goal of modulated-beam conformal therapy is to reduce the dose to healthy tissue and sensitive structures around a uniformly ir radiated target volume, Multiple intensity-modulated fields offer impr oved tissue-sparing dose distributions, New computer-based systems for planning and delivering such treatments may soon be available from di fferent commercial sources that will make the formulation of an intens ity-modulated treatment plan and its execution widely available at any treatment facility that has the resources to acquire the necessary eq uipment, This work reports on a study of the integration of two such s ystems. Methods and Materials: Treatment planning was done using a com mercially available inverse planning algorithm based on simulated anne aling. The plans arbitrarily assumed nine coplanar x-ray beams at nono pposed gantry angles, Intensity modulation was computed for each beam, The modulated field at each gantry angle was broken down into a serie s of uniform (nonmodulated) subfields, which could be delivered as a s equence to produce the desired dose distribution, Because a large numb er of subfields was delivered, a multileaf collimator (MLC) was used f or held shaping, This allowed rapid and accurate held shaping for trea tments made up of several hundred subfields, Computer control of the M LC and linear accelerator allowed delivery of doses less than .01 Gy p er subfield. Treatment was delivered on a prototype, computer-controll ed accelerator and MLC system, Resulting dose distributions were analy zed using film and an anatomically specific, homogeneous phantom, Resu lts: The treatment plans were evaluated using dose-volume histogram an alysis, The plans provided acceptably uniform irradiation of the targe t volume without exceeding dose tolerances for nearby critical structu res, The plans were successfully delivered by a prototype dynamic MLC, The time needed to deliver a sequence of subfields at one gantry angl e ranged from 0.7 to 2.0 min, Isodoses from film agreed reasonably wel l with planned isodose distributions, Conclusions: It is feasible to p lan and deliver fixed gantry, modulated-beam conformal therapy for hea d and neck tumors with systems being developed commercially, The plann ed dose distributions exhibit significant potential for sparing closel y spaced normal tissue structures in the head and neck. (C) 1997 Elsev ier Science Inc.