Purpose: Commissioning online adaptive radiotherapy platforms presents a unique but critical challenge as guidance is lacking and deformable phantoms are exceedingly rare. We present a novel adaptive commissioning process as implemented for the Varian Ethos using standard clinical resources and our resulting recommendations.
Methods: Ethos deforms the planning CT to the daily CBCT to create a synthetic CT (synCT). The adaptive plan-of-the-day and original plan are calculated on this synCT, and users select which plan to use at each treatment. To validate synCT generation and dose calculation, we conducted end-to-end tests using nine-field IMRT plans, a CIRS IMRT Thorax phantom, pinpoint ionization chamber, and Gafchromic film. Four common scenarios were evaluated: weight-loss, weight-gain, internal target displacement, and changes in gas. (1) Weight-Loss: Phantom simulated with 6cm of bolus. For treatment, 0.5-4cm bolus was incrementally removed adaptive session. (2) Weight-Gain: Phantom simulated without bolus. For treatment, 0.5-2cm bolus was added. (3) Target Displacement: Phantom simulated without bolus. For treatment, CTV was displaced 1.0cm laterally. (4) Changes in Gas: Phantom simulated with/without rods removed to simulate gas. To mimic changes in gas at treatment, the rods within the phantom were removed/reinserted at different locations.
Results: All point measurements agreed within +/-2.5% of calculated dose. The average deviation was -0.93%. Relative film measurements passed gamma analysis (3%/3mm, pass rate>90%) for all scenarios except the adapted plan for 2cm weight gain (89.56% pass rate). Body habitus changes on synCTs versus bolus changes were within 1cm of expected values and demonstrated reasonable deformations for all scenarios.
Conclusion: Our process for commissioning the adaptive workflow successfully validated the dose calculation for online treatment plans. Based on our results, we recommend that weight gain of 2cm+ requires re-simulation. This commissioning process used commonly available equipment and, therefore, can be applied in other clinics commissioning online adaptive platforms.
Funding Support, Disclosures, and Conflict of Interest: Kelly Kisling acknowledges honoraria and speaker fees from Varian Medical Systems. Kevin Moore acknowledges consulting fees and honoraria from Varian Medical Systems. Xenia Ray acknowledges honoraria and speaker fees from Varian Medical Systems and a research agreement with Varian Medical Systems.