To investigate key geometrical sampling properties and potential advantages of non-coplanar volumetric modulated arc therapy (VMAT) compared with coplanar VMAT. The study is motivated by cone-beam computed tomography (CBCT) reconstruction theory, particularly concepts related to sampling completeness. The analysis focuses on head and neck cancers (HNC) and geometric phantoms. Approach: VMAT was simulated using densely spaced, fluence-optimized static beams. Non-coplanar partial arcs with angular ranges of 32°, 64°, or 180° centred at predefined gantry angles were added orthogonally to the standard coplanar circular beam arrangement, which served as the reference configuration. Plan quality was evaluated using the composite objective value (COV), the minimum dose to the planning target volume (PTV), and the dose in cranio-caudal planes immediately adjacent to the PTV. The study included twenty patients with oro-, hypo-, or nasopharyngeal carcinomas with extended cranio-caudal target volumes (>15 cm) and complex anatomical organ at risk (OAR) proximity. In addition, idealised phantom cases (spherical or cylindrical PTVs with surrounding OAR structures) were analysed similarly to investigate controlled geometric configurations. Main results: Incorporation of non-coplanar arcs consistently improved plan quality. The most pronounced effect was observed for additional 180° arcs; however, even arcs with reduced angular range centered around the 0° gantry angle significantly increased PTV minimum dose while reducing dose to adjacent cranio-caudal tissues. These improvements are explained by the circle-and-arc beam configuration, which fulfils the Tuy condition known from CBCT theory by filling the Radon space shadow zone. Significance: Non-coplanar arcs with reduced angular ranges can enhance target coverage and organ sparing in elongated HNC targets. By transferring mathematical principles from CBCT reconstruction to radiotherapy planning, this work demonstrates how fulfilling the Tuy condition can improve VMAT dose distributions and may inform future developments in dynamic trajectory radiotherapy.
Razinskas et al. (Wed,) studied this question.