Exhibit Hall | Forum 4
Purpose: To investigate the dosimetric effects of patient positioning in Volumetric Modulated Arc Therapy (VMAT) for Total Body Irradiation (TBI) through a treatment plan uncertainty evaluation.
Methods: Eighteen VMAT TBI treatment plans were evaluated as part of a prospective single-institution stage II clinical trial. A range of patient positioning offsets were simulated in the lateral (x), vertical (y), and longitudinal (z) directions, and dosimetric changes to planning target volume (PTV) coverage and organs at risk (OARs) were investigated. New plans were created with the simulated shifts for all eighteen patients without re-optimization; including 5mm or 10mm shifts in each direction.
Results: Lung mean dose increased an average of 7.7cGy, 4.1cGy, and 2.9cGy when simulating a 5mm shift in the x,y,z directions (respectively) across eighteen patients; 31.4cGy, 17.3cGy, 16.2cGy for 10mm shifts in x,y,z. Target coverage V100% decreased an average of 0.3%, 0.1%, 0.2% for 5mm shifts, and 1.2%, 1.1%, 0.5% for 10mm shifts in x,y,z. PTV-D98% decreased 0.9%, 0.5%, 0.4% when shifted 5mm; 3.8%, 2.5%, 1.2% when shifted 10mm in x,y,z. Mean dose to the left kidney increased 6.3cGy, 12.2cGy, 0.1cGy for 5mm, and 27.3 cGy, 37.7cGy, 9.7cGy for 10mm shifts in x,y,z. Right kidney mean dose increased 10.4cGy, 9.6cGy, 0.3cGy for 5mm, and 34.5, 31.6, 11.6cGy for 10mm.
Conclusion: Plan uncertainty evaluation of eighteen patient plans showed that lateral offsets in patient positioning resulted in the most significant dosimetric effects to Lung Dose and Target Coverage. Based on these results, special attention should be paid to patients’ lateral alignment. Given that the lungs are the main OAR of concern for VMAT-TBI patients, lateral imaging residuals (especially in the chest area) should be minimized and prioritized with a suggested action limit of 5mm. Additionally, for single-direction 5 or 10mm offsets, our dosimetric goals were still met for all eighteen patients.
Funding Support, Disclosures, and Conflict of Interest: Dr. Gerber would like to disclose consulting work for Invus LLC and Group Dynamics, not directly related to this project.