Most commonly in radiation therapy, treatments are delivered in a co-planar geometry. Numerous advantages have been reported of adding non-coplanar beams to the treatment plan. The aim of this study was to compare current state-of-the-art VMAT and CyberKnife treatment plans to that of a novel linac design developed at Stanford which utilizes a static conical beam arrangement that allows the inclusion of a full ring diagnostic CT with shared isocenter with the treatment beams. Four clinical cases, prostate, lung, head/neck, and pediatric brain, were selected and treatment plans were generated with 45 or 60 (to the longitudinal axis of the patient) conical beam IMRT and compared with co-planar 90 VMAT plans. Double cone, with beams entering from both superior and inferior directions, and single cone geometries were evaluated. Plans were optimized in RayStation using an in-house developed script to minimize operator bias between the different techniques. Non-coplanar CyberKnife IMRT plans for the pediatric and prostate case were optimized separately in MultiPlan and compared to conical geometry plans. In the prostate case, increased mean dose to the rectum (2.3-3.7 Gy) and bladder (9.5-14.5 Gy) but decreased dose to the femoral heads (femurs) (7.1-10 Gy) were found with the conical arrangement compared to 90 VMAT. Only minor dosimetric differences were found in the lung case, while selective sparing of organs at risk was found with 45 degrees or 60 degrees conical arrangement in the pediatric brain and head/neck cases. For the prostate case, a reduction in mean doses to the bladder and rectum of 6% (2 Gy) and 18% (5.2 Gy), respectively, was found when comparing the CyberKnife to the 60 degrees conical plan, in favor of the CyberKnife plan, but with an increase in integral dose and reduced conformity. An increase in integral dose and reduced conformity was also found for the pediatric brain case when comparing CyberKnife and 60 degrees conical plan. Minor benefits were found with double cone compared to single cone geometry. Comparable treatment plan quality could be achieved between conical beam arrangement and 90 degrees (coplanar) VMAT and CyberKnife (non-coplanar) IMRT, demonstrating the promise of this novel beam geometry. The use of this beam geometry allows volumetric image-guidance with full ring imaging and a common isocenter for simultaneous treatment and imaging.
