CONFORMAL RADIATION-THERAPY WITH FIXED SHAPED COPLANAR OR NONCOPLANARRADIATION BEAM BOUQUETS - A POSSIBLE ALTERNATIVE TO RADIOSURGERY

Purpose: Three-dimensional (3D) geometric conformation of the therapeu tic dose volume to the shape of a target tissue volume is the motivati on for both conformal radiotherapy and radiosurgery. Although noncopla nar arcs have a clear physical and geometric advantage over fixed fiel ds for small spherical targets, those advantages are reduced for large or irregularly shaped targets where static fields can be individually shaped. We have developed a system that allows efficient and flexible design and reliable delivery of customized ''bouquets'' of fixed nono pposed coplanar or noncoplanar shaped fields, resulting in highly unif orm dose distributions. This report describes our initial experience u sing beam bouquets to treat intracranial lesions. Methods and Material s: Patients with primary (11) or metastatic (4) intracranial lesions w ith a maximum diameter less than approximately 6 cm, most of whom cand idates for single-fraction radiosurgery, were treated with beam bouque ts of four to eight nonopposed coplanar or noncoplanar beams. Doses ra nged from 16-20 Gy in four fractions for recurrent lesions (8) to 45 t o 68 Gy in 25 to 34 fractions for primary lesions (7). The patients we re immobilized with custom foam head supports and face masks attached to a fixed base plate. Planning computed tomography scans were acquire d, from which the physician developed the custom beam bouquet using 3D treatment-planning tools. The bouquet was designed based primarily on geometric concerns. The bouquet was subsequently modified to add wedg e filters chosen by vector analysis of dose gradients to achieve unifo rm dose over the volume of beam crossfire. At the time of treatment, t he isocenter was placed using the instructions provided by the treatme nt-planning system and pretreatment orthogonal port films were compare d to digitally reconstructed radiographs (DRR) to assure proper isocen ter placement. For several situations, the 3D dose distributions resul ting from alternative coplanar and noncoplanar plans were compared. Re sults: Each patient was treated without incident, Daily pretreatment p ort films showed excellent reproducibility of isocenter placement in 8 7% of setups. With short follow-up (0-12 months), two patients with re current glioblastoma experienced clinical deterioration 2 to 4 weeks f ollowing treatment. One had increased edema on scans and responded to steroids. Six patients clinically improved following radiation therapy . Review of alternative treatment plans reveals that the relative util ity of coplanar vs. noncoplanar beams is likely dependent on the locat ion of the lesion. Noncoplanar beam bouquets are likely preferable to coplanar beams when the target is located in the central regions of th e head. Coplanar beams are likely adequate, and possibly preferable, f or peripherally located targets. Conclusion: The biological advantages of fraction and the physical advantages of radiosurgery are exploited with this approach. The use of multiple nonopposed coplanar or noncop lanar conformal wedged fields provides a uniform dose to the target an d acceptable dose gradient at the target edge. This technique may prov e to be an alternative to are-based radiosurgery in some settings and has the potential advantages that fractionation should improve the the rapeutic ratio, and each beam can be individually shaped to conform to irregularly shaped targets. Additional studies are underway to improv e this system and better define its utility.