Room: Karl Dean Ballroom B1
Purpose: Multileaf collimator (MLC) tracking adapts radiotherapy deliveries to intrafractional anatomical changes. Thereby, it commonly assumes that the tumor rigidly shifts. For the first time, this study investigates the dosimetric benefit of accounting for changes in tumor shape during MLC tracking.
Methods: We have expanded our in-house MLC tracking software to facilitate tracking based on a 3D deformable vector field (DVF) as input signal. Initially, the DVF is combined with a mask of the PTV plus a 1cm margin. Next, the beam shape, described as a polygon, is deformed by shifting its vertices according to the average deformation along their beam's-eye-view projections. Finally, the MLC's leaves are re-fitted to the deformed polygon.Using the XCAT digital phantom, we generated 4DCT images of three representative cases exhibiting lung tumor deformations: a tumor stretching by 5mm in superior-inferior direction, a tumor rotating by 45Â°, and differential motion of 5mm between a tumor and an involved mediastinal lymph node. For each case, we created a 7-beam IMRT treatment plan for a VersaHD linac (Elekta AB, Stockholm, Sweden) delivering 60Gy in 30 fractions. Dose delivery to all ten breathing phases was simulated either with deformation tracking, translation-only tracking, or without tracking. Finally, the dose was accumulated on the peak-exhale phase using Elekta ADMIRE, research version 2.0, and we assessed the tumor dose coverage.
Results: Averaged over all investigated cases, tumor coverage was similar to the planned dose for deformation tracking and translation-only tracking (Î”D98-GTV: +0.1Gy and -0.1Gy), while it was lower without tracking (Î”D98-GTV: -1.9Gy) The largest dosimetric advantage of deformation tracking was seen for the stretching tumor (Î”D98-GTV: 0.0Gy, -0.4Gy and -1.9Gy).
Conclusion: Overall, the incremental benefit of using MLC tracking to correct for changes in target shape appears small, although it may increase tumor dose coverage for large target deformations.
Funding Support, Disclosures, and Conflict of Interest: We acknowledge financial and technical support from Elekta AB under a research agreement. Research at the ICR is supported by Cancer Research UK under program C33589/A19727 and C33589/A19908. We acknowledge NHS funding to the NIHR Biomedical Research Centre at the RMH and ICR.