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

Purpose: Portal images of conformal treatment fields are often not suitable for setup verification purposes because they contain insufficient bony str uctures. Therefore, additional rectangular fields are frequently applied fo r setup verification purposes. It is the aim of this study to reduce the do se distortions induced by these extra fields by appropriately adjusting the beam weights and wedge angles of the treatment fields. Methods and Materials: A second treatment plan for the setup verification s ession 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 bea m 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. Results: 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 field s with a similar dose distribution over the PTV as the original treatment p lan without the setup verification fields. Conclusion: The dose distortions resulting from the acquisition of orthogon al verification imaging can be neutralized by modifying the original beam w eights and wedge angles of the treatment fields. (C) 2000 Elsevier Science Inc.