Protecting the heart: a practical approach to account for the full extent of heart motion in radiotherapy planning

Purpose: Emerging evidence suggests that the heart is more radio-sensitive than previously assumed; therefore accounting for heart motion in radiotherapy planning is becoming more critical. In this study, we determined how much heart delineations based on 3D CT, 4D average projection (AVG) and maximum intensity projection (MIP) images should be extended to represent the full extent of heart motion during 4D imaging acquisition.Materials and Methods: 3D and 4D CT scans of ten lung cancer patients treated with stereotactic ablative radiotherapy (SABR) were used. Median surfaces were derived from heart delineations of three observers on the 3D CT, AVG, MIP and 25% exhale scans. Per patient, the 25% exhale contour was propagated on every phase of the 4D scan. The union of all 4D phase delineations (U4D) represented the full extent of heart motion during imaging acquisition. Surface distances from U4D to 3D, AVG, MIP volumes were calculated. Distances in the most extreme surface points (1.5cm most superior/inferior, 10% most right/left/anterior/posterior) were used to derive margins accounting only for systematic (delineation) errors.Results: Heart delineations on the MIP were the `closest' to the full extent of motion, requiring only 2:5 mm margins. Delineations on the AVG and 3D scans required margins up to 3.4 and 7.1 mm, respectively. The largest margins were for the inferior, right and anterior aspects for the delineations on the 3D, AVG and MIP scans, respectively.Conclusion: Delineations on either 3D, AVG or MIP scans required extensions to represented the heart's full extent of motion; the MIP requiring the smallest margins. Research including daily imaging, to determine the random components for the margins, and dosimetric measurements, to determine the relevance of creating a planning organ at risk volume (PRV) of the heart, is required