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Organ Doses From MV IGRT Using MV-MV and MV-CBCT

Y Li

Y Li12*, T Netherton12 , P Nitsch1 , P Balter1 , S Gao1 , A Klopp1 , L Court1 , (1) The University of Texas MD Anderson Cancer Center, Houston, TX, (2) The University of Texas MD Anderson Cancer Center UTHealth Graduate School of Biomedical Sciences, Houston, TX


TU-C1-GePD-J(A)-5 (Tuesday, August 1, 2017) 9:30 AM - 10:00 AM Room: Joint Imaging-Therapy ePoster Lounge - A

Purpose: To examine organ doses from MV IGRT using a prototype linac. IGRT using MV treatment beam has shown certain potential advantages over kV imaging, especially direct imaging of the treatment beam, and reduced equipment and QA requirements.

Methods: Four imaging techniques were evaluated using a prototype linac: MV-MV imaging and MV-CBCT, each with a low-dose and high-quality mode. All these techniques utilize the 6MV FFF treatment beam for imaging. An anthropomorphic thorax phantom (CIRS) was imaged and doses were measured using various imaging field sizes and isocenter locations. Imaging dose has been accounted for in treatment planning. Measurements and TPS calculations were compared to evaluate the accuracy of imaging dose modeling.

Results: For high-quality MV-MV mode, the mean extra-target doses of heart, left lung, right lung and spine were ranged 0.80-1.64cGy. Corresponding in-target doses were ranged 2.61-3.72cGy. Imaging dose from CBCT was less sensitive to the location of organ relative to the treatment field. For high-quality mode the mean organ imaging doses were ranged 6.51-8.45cGy. Ipsilateral breast dose was not sensitive to isocenter location, and its mean doses were measured as 9.65 and 3.83cGy for high-quality MV-CBCT and MV-MV, respectively. For contralateral breast, imaging dose from MV-MV relied greatly on the location of treatment region (left or right breast/chestwall). While using low-dose mode, both MV-MV and MV-CBCT imaging doses of normal tissue were reduced by an average factor of 2.02. The imaging dose calculated by TPS had good agreement with measurements, with an average difference of-0.07±0.19cGy. The maximum difference (-0.95cGy) was found in the lung for high-quality MV-CBCT.

Conclusion: The imaging dose to normal tissues due to MV imaging greatly depends on imaging techniques, imaging field size and isocenter location. TPS accurately models the MV imaging dose, which is incorporated into the calculated patient dose.

Funding Support, Disclosures, and Conflict of Interest: This study was funded by Varian Medical Systems.

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