Patient-specific quality assurance for proton depth dose distribution using a multi-layer ionization chamber in a single-ring wobbling method

The use of a multi-layer ionization chamber, Zebra, in patient-specific quality assurance (QA) for proton depth dose distributions in a single-ring wobbling method is investigated. The depth dose distributions measured using Zebra are compared with those calculated using the treatment planning system (TPS), XiO-M, and measured using an ionization chamber with a motorized water phantom system. Because the TPS only provides point doses, the average doses are calculated using in-house software. The detector size-corrected depth dose distributions are obtained by determining the average of the dose distributions from the TPS over a cylindrical region similar to the size of the Zebra detectors. The calculated depth dose distributions from the cases with a simple compensator shape are in good agreement with those obtained from the TPS without performing volume averaging; however, a 15% difference was shown when compared with those from the cases with a complex compensator shape. Then, the measurements are compared with the detector size-corrected depth dose distributions, showing an improved agreement within 3% for the highly steep dose gradient regions. Although there are some field size limitations, the Zebra system is a useful device for the fast measurement of patient-specific QA for depth dose distributions in wobbled proton beams. However, careful consideration is required for complex dose distribution fields, because the measurements obtained using Zebra cannot be directly compared to the depth dose distributions from the TPS owing to the finite detector size of the Zebra chamber.