Room: Exhibit Hall
Purpose: Stereotactic radiotherapyâ€™s goal is to precisely target cancerous cells while minimizing exposure to healthy tissue. Some instruments, like the GammaKnife, have a large geometrical field space coverage of beams to create a highly uniform dose within the focal spot with a sharp dose fall-off. It has been assumed that larger the solid angle from which the beams are focused to the focal spot, the better the resulting treatment plan. This study investigates how the uniformity and dose fall-off of the focal spot affect the final plan quality.
Methods: 1500 beams of 10mm in diameter were uniformly distributed in a spherical arrangement equidistant from a focal spot, focused on a central spherical target with a diameter of 3cm inside a digital ellipsoid water phantom. The beams converge to the focal spot from 4Ï€, 1Ï€, 0.5Ï€, 0.25Ï€ steradians and a single arc, creating single shot dose kernels of different quality in terms of dose uniformity. The weights and locations of different dose kernels were then optimized to provide conformal dose coverage. Dose coverage to the target and normal tissue doses, such as V50 and V20, were then compared.
Results: Optimizing the placement and weighting of the focal spots lessens the requirement of the focal spot quality. Little difference in normal tissue DVH is seen among plans generated with kernels. The largest drop of 53% is seen at V20 while there is less than a 5% difference at V50 between 4Ï€ and the single arc. Differences in dose falloffs is dependent on the direction of the beams.
Conclusion: It is suggested that while a highly uniform single shot improves the uniformity of the dose distribution outside of the target, due to the arrangement of the beams, it is not necessary to improve the dose conformity of the target.
Funding Support, Disclosures, and Conflict of Interest: Dr Cedric Yu is the founder of Xcision Medical Systems, LLC