(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: To evaluate a tolerance level of Hounsfield Units (HU) of the CT scanner for radiotherapy treatment planning
Methods: An electron density phantom (GAMMEX RMI 465) was scanned using a CT scanner (GE LightSpeed). In a treatment planning system (Varian Eclipse ver 5.3), a 5 cm thick slab representing a different material was inserted in a virtual water phantom, 30 x 30 x 15 cm³. Dose calculation was done with 6- and 15-MV photon energies: 10 x 10 cm² field size of at 10 cm depth with 100 cm source-to-axis distance. For each material, dose variation within ± 1% was calculated and its corresponding HU was estimated inversely to generate the lower and upper levels of HU values for both energies. The tighter level was selected to represent tolerance level of HU values of the relative electron density table. To validate the dosimetry change from an inhomogeneous correction, two CT-density curves were generated, commissioning ± tolerance. Three clinical plans, lung, prostate, and head & neck, were tested to confirm the dosimetry change.
Results: The dose variation from the lower and upper levels of relative electron densities in a virtual water phantom were -0.70 ± 0.18% and 0.77 ± 0.22% for 6- and 15-MV photon energies, respectively. Using the newly-created CT scanners in terms of electron density curve, commissioning ± tolerance, the changes of mean PTV dose were 0.98% and 0.47%, 0.01% and 0.01%, and -0.31% and 0.45% for lung, prostate, and head & neck, respectively.
Conclusion: The tolerance level for HUs of relative electron density data was evaluated from both 6- and 15-MV photon energies. To maintain dose variation within 1% for both photon beams, it is recommended a tighter tolerance level of HU of electron density curve as a part CT QA.
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