(Sunday, 7/29/2018) 3:00 PM - 6:00 PM
Room: Exhibit Hall
Purpose: To evaluate the use of the Catphan® CT phantom as a convenient means of independent megavoltage CT (MVCT) QA for a TomoTherapy unit. Although the vendor-supplied Tomotherapy ("Cheese") phantom provides a collection of plugs for assessing spatial resolution and a variety of inhomogeneities, the CatPhan has the advantage of being ubiquitous in the industry and independent of a particular therapy device vendor.
Methods: A series of measurements of the CatPhan 504 phantom were acquired and compared with results obtained from the vendor-supplied phantom. MVCT studies on the TomoHDAâ„¢ system were acquired with the three provided resolutions: fine, medium, and course.These studies were exported from the Tomotherapy unit to the SNC Machine QA server for analysis. We assessed geometric distortion, spatial resolution, uniformity, contrast, noise, slice thickness, low contrast, and the HU value for air, Teflon, Delrin, acrylic, polystyrene, Low density polystyrene, and polymethylpentene (PMP). These were compared with HU results for a range of inhomogeneity values using the TomoTherapy phantom with appropriate inserts.
Results: The geometric distortion, spatial resolution, uniformity, contrast, and noise were reproducible relative to baseline measurement.The Hounsfeld units obtained for MVCT generally were closer to zero (water) relative to vendor specification and kVCT comparison, but were stable.Data collection routinely took more than twenty minutes, with another ten minutes for processing and analysis.
Conclusion: A limitation of CatPhan substitution for the Cheese phantom is that the inhomogeneity regions use a different set of densities, so a direct comparison is not possible. This use of an independent phantom mitigates any questions related to using the vendor's own phantom. The CatPhan is suitable for use when complemented by additional tools to meet the expectations for Tomotherapy quality assurance as found in TG-148.
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