Individual beam sharpening improves composite dose fall-off near a target for non-isocentric cyberknife radiosurgery

Stereotactic radiosurgery (SRS) refers to a high dose of radiation delivered to a focal region of interest while maximizing the steep dose gradient to minimize dose to the surrounding normal tissues. Multiple factors can influence the dose fall-off, and the relative importance of such factors have not yet been characterized for non-isocentric Cyberknife SRS. Our aim was to investigate whether the composite dose fall-off near a target may be enhanced via sharpening the lateral beam profile (or penumbra) of each individual beam. Cyberknife beam profiles were fitted and parameterized to obtain a characteristic penumbra function for each collimator size. Simulated beam profiles with progressively sharper penumbras were then generated, and used to perform simulated treatment planning on seven pediatric intracranial arteriovenous malformations (AVMs) cases. Penumbra size was found to significantly influence the peripheral dose fall-off. Peripheral dose volumes were reduced by 5 to 10% with reductions in penumbra size ranging from 40 to 80%. Dose conformality and homogeneity were not significantly changed with decreasing penumbra size. Therefore, individual beam sharpening provides a straightforward way of improving the composite dose fall-off for non-isocentric Cyberknife SRS.