Abstract
Purpose: The ultimate goal of IMRT QA should be to verify that the correct dose has been delivered to the patient. The purpose of this study is to evaluate for feasibility and reliability of volumetric transit dosimetry as a routine in‐vivo dosimetry method during the actual patient treatment for different sites. Methods: To evaluate volumetric transit dosimetry for IMRT delivery, prostate, H&N and lung IMRT cases were selected. During patient treatment, the EPID imager was extended to collect transit radiation through the patient. Collected transit dose images are imported and reconstructed as a volumetric dose distribution using the Dosimetry Check (Math Resolutions LLC, Maryland, USA) Exit Dose module. Dosimetry Check program to compare to the reconstructed volumetric dose including tissue heterogeneous correction from the transit EPID images. Also, to investigate the influence of daily setup or anatomy change, multiple transit dosimetry measurements were performed on different treatment days up to 5 fractions for prostate case. In addition, patientˈs verification plan was generated on a homogeneous CIRS IMRT phantom to verify the absolute dose and transit dosimetry algorithm check. Collected QA results were evaluated with 1) gamma passing rate (3 mm DTA and 3% dose) comparing calculated dose from exit dosimetry and treatment planning system (TPS) dose prediction, 2) difference of absolute point dose from exit dosimetry module compared to ion chamber measurements in the phantom. Results: The current implementation shows promise at predicting point doses from exit dose images acquired during patient treatment. However, 3D dose distributions calculated from exit dose images are not consistent in their agreement with the planned dose distribution. Conclusions: Further study is needed to explain the differences. Areas to consider include offsets in imager position, differences in dose calculation algorithm, couch attenuation effects and anatomy changes.
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