SU-E-T-315: Dosimetric Effects of Couch Top Shift On VMAT Delivery in Absence of Indexing




To investigate dosimetric effects of couch top shift for volumetric-modulated arc therapy (VMAT) in absence of indexing of immobilization devices.


A total of twelve VMAT treatment plans were selected from three regions (lung, abdomen, and pelvis) to account for the variation of the patient position relative to the couch top. The treatment plans were generated using the Varian Eclipse system. A pinpoint ionization chamber (PTW TN31014) was placed at the center of 16-cm solid water phantom and the dose was delivered using the Varian TrueBeam STx with BrainLAB ExacTrac couch top. To simulate the day-to-day variation of the patient position relative to couch top, the couch top was laterally shifted up to 50 mm, with an increment of 5 mm from 0 to 20 mm; and of 10 mm afterwards, and the phantom was moved back to 0 cm shift for measurement. The dose was also delivered using a Varian tennis racket grid insert at 0 cm shift to simulate the absence of couch top. The treatment plans were delivered with 6, 10, and 15 MV photons using the same leaf sequencing to investigate the energy dependence of couch top shift. The dose difference was normalized to 0 cm shift for the regular couch top for comparison.


The percent difference of dose was found to increase with lateral shift for all energies; however, the average differences were close to 0% and the maximum difference was within 1% along the lateral shifts. The differences with the absence of couch top were 2.2±0.5% (6MV), 1.7±0.3% (10MV), and 1.6±0.2% (15MV), respectively.


The inclusion of couch top is recommended in treatment planning to minimize the dosimetric uncertainty between calculated and delivered dose even in absence of indexing of immobilization devices in VMAT delivery.