Dose Coverage Variation Caused by Setup Uncertainties in VMAT-TBI Treatment

Abstract

Volumetric Modulated Arc Therapy Total Body Irradiation (VMAT TBI) offers several advantages over conventional TBI techniques, including reduced cost of the treatment room (a normal-sized vault versus a large shielded vault), increased patient comfort during treatment, lung sparing without the need for physical blocks, and a more homogeneous dose distribution. In VMAT-TBI treatment, plans at multiple isocenters are utilized to cover the entire body, and patients are positioned with the aid of image guidance. However, aligning the patient's entire body during setup can be challenging due to its pliability, and the setup time is heavily dependent on the tolerance allowed by the image guidance. Therefore, studying the variation in dose coverage caused by setup uncertainties in VMAT-TBI treatments can help optimize the clinical workflow and determine the optimal tolerance for patient positioning. New plans were generated to simulate the uncertainties that occur during treatment setup for each patient. These plans were created by shifting the original VMAT TBI plans at the head, chest, abdomen (or pelvis) isocenters by 5mm and 1cm in the left-right (LR), inferior-superior (IS), and anterior-posterior (AP) directions, respectively. Dose DicomRT files were exported, and the dose change due to the shifts was analyzed. The statistical quantification of the percentage of the body that experienced a dose change of over 2%, 5%, and 10% of the prescription due to the shifts from the original plans was calculated for all patients. Histograms were generated, showing the percentage of body getting dose change of 1-2%, 2-3%, 3-5%, 5-7%, 7-10%, 10-15%. The table below displays the percent volume receiving a dose change of 2%, 5%, and 10% of the prescription for a 5mm shift. Among the shift directions, the dose change is most sensitive in the IS direction, with similar impact observed the in LR and AP directions. Among different sites, the chest experiences the largest dose change, followed by the pelvis. For a 5mm shift in the IS direction, the average percent volume receiving a dose change of 2%, 5%, and 10% in the chest is 9.25%, 2.64%, and 0.27%, respectively. For a 1cm shift, the numbers are 12.23%, 6.75%, and 1.29%. In the pelvis (abdomen), these values are 9.03%, 1.67%, and 0.17% for a 5mm shift and 13.28%, 6.1%, and 0.85% for a 1cm shift. For head plans, the values are 2.72%, 0.9%, and 0.14% for a 5mm shift and 3.77%, 1.66%, and 0.53% for a 1cm shift. Accurate alignment in the chest region is crucial in VMAT TBI treatment. Efforts should be made to minimize shifts over 1cm in the IS direction.