Purpose: Emerging evidence suggests that the heart is more radiosensitive than previously assumed; therefore, accounting for heart motion in radiation therapy planning is becoming more critical. In this study, we determined how much heart delineations based on 3-dimensional (3D) computed tomography (CT), 4-dimensional (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. Methods and Materials: The 3D and 4D CT scans of 10 lung cancer patients treated with stereotactic ablative radiation therapy were used. Median surfaces were derived from heart delineations of 3 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, and MIP volumes were calculated. Distances in the most extreme surface points (1.5 cm most superoinferior, 10% most right/left/anteroposterior) 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 3D, AVG, or MIP scans required extensions for representing the heart's full extent of motion, with 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 of the heart is required.
- Manchester Cancer Research Centre