A treatment planning method to correct dose distributions distorted by setup verification fields.

Abstract

Portal images of conformal treatment fields are often not suitable for setup verification purposes because they contain insufficient bony structures. Therefore, additional rectangular fields are frequently applied for setup verification purposes. It is the aim of this study to reduce the dose distortions induced by these extra fields by appropriately adjusting the beam weights and wedge angles of the treatment fields. A second treatment plan for the setup verification session is generated, with an identical beam setup as the original plan, but which also includes two orthogonal setup verification fields. An algorithm has been developed, based on vector analysis methods, that adjusts the beam weights and wedge angles of the treatment fields in such a way that both the dose at the isocenter and the dose homogeneity over the planning target volume (PTV) are conserved. The algorithm has been applied to three clinical cases. The number of MUs for the setup verification fields, using a liquid-filled electronic portal imaging device, varied between 16 MU in the head and neck region up to 34 MU for lateral images in the pelvic region. In all cases, the method yielded a treatment plan including two orthogonal setup verification fields with a similar dose distribution over the PTV as the original treatment plan without the setup verification fields. The dose distortions resulting from the acquisition of orthogonal verification imaging can be neutralized by modifying the original beam weights and wedge angles of the treatment fields.