SU‐E‐T‐501: Immobilization System Design and Testing for Proton Therapy

Purpose: To evaluate the quality of new immobilization devices for our clinical proton treatment program. Methods: At the Loma Linda University proton treatment center, we are developing and integrating new immobilization devices (Qfix Systems, LLC) into our clinical proton treatment program. For proton therapy applications it is desirable to use devices that exhibit a very thin water equivalent thickness (WET) to minimize beam penumbra, while the immobilization device itself also needs to be uniform in nature, with the WET accurately represented in the planning CT. Device inhomogeneities or incorrect representation of the device within the treatment planning system will result in systematic range errors. An important task in commissioning this system is validating the WET uniformity of these immobilization devices, which are typically too large to be submerged into a water tank. The WET of these devices was evaluated with a new method that measures the shift in the distal edge of a proton beam using a variable thickness range shifter apparatus that attaches to the proton snout. Results: Testing has been completed on various immobilization devices with excellent WET uniformity results. For example, the head and neck device produced a WET of 2.705mm +/− 0.015mm. The measurement technique was also very reproducible with a standard deviation of 5.7×10‐4cm for multiple measurements of the same location Conclusions: The immobilization devices that have been developed and are being integrated into our proton program meet the stringent requirements of proton therapy. The method of WET evaluation developed in this work can be adapted by other centers for evaluation of their own immobilization devices prior to deployment in proton therapy applications.