Beam´s eye view geometric optimization of isotropic beam bouquets for stereotactic and radiotherapy treatment planning

A maxim of stereotactic radiosurgery and radiotherapy treatment planning is that the steepest dose gradient is obtained by using maximally spatially separated beam arrangements (bouquets), that is, beams which are separated from one another by the largest possible angles while avoiding opposed beams. Webb presented isotropically converging beam bouquets as an optimization starting point, rather than a complete solution, because these beam bouquets did not account for non-geometric considerations such as beam overlap with radiosensitive structures and beam deliverability concerns. We propose a treatment plan optimization method that optimizes beam directions from a starting point of a set of isotropic beams. The optimization process individually steers each beam to a best position, based on beam´s eye view (BEV) critical structure overlaps with the target plus BEV margin, and based upon the target´s cross sectional area presented at each beam position. The final, optimized beam arrangements maintain wide separation between adjacent beams while avoiding critical structures. For radiosurgery cases (single fraction, prescription isodose 60%-80%), satisfactory plans are usually generated by using equally weighted beams at the specified orientations. Radiotherapy (multiple fraction, prescription isodose 90%-98%) treatment plans generally require additional wedge optimization by the user. As shown by a radiosurgery plan example, this optimization method improves the critical structure sparing properties of an unoptimized isotropic beam bouquet, while maintaining the same degree of dose conformity and dose gradient. This optimization method has the potential to offer a simple means of designing conformally shaped beam radiation therapy plans for treatment of intra-cranial lesions