Investigation of a water equivalent depth method for dosimetric accuracy evaluation of synthetic CT.

Phys Med

School of Information and Physical Sciences, University of Newcastle, Newcastle, New South Wales, Australia; Department of Radiation Oncology, Calvary Mater Hospital, Newcastle, New South Wales, Australia.

Published: January 2023

Purpose: To provide a metric that reflects the dosimetric utility of the synthetic CT (sCT) and can be rapidly determined.

Methods: Retrospective CT and atlas-based sCT of 62 (53 IMRT and 9 VMAT) prostate cancer patients were used. For image similarity measurements, the sCT and reference CT (rCT) were aligned using clinical registration parameters. Conventional image similarity metrics including the mean absolute error (MAE) and mean error (ME) were calculated. The water equivalent depth (WED) was automatically determined for each patient on the rCT and sCT as the distance from the skin surface to the treatment plan isocentre at 36 equidistant gantry angles, and the mean WED difference (ΔWED¯) between the two scans was calculated. Doses were calculated on each scan pair for the clinical plan in the treatment planning system. The image similarity measurements and ΔWED¯ were then compared to the isocentre dose difference (ΔD) between the two scans.

Results: While no particular relationship to dose was observed for the other image similarity metrics, the ME results showed a linear trend against ΔD with R = 0.6, and the 95 % prediction interval for ΔD between -1.2 and 1 %. The ΔWED¯ results showed an improved linear trend (R = 0.8) with a narrower 95 % prediction interval from -0.8 % to 0.8 %.

Conclusion: ΔWED¯ highly correlates with ΔD for the reference and synthetic CT scans. This is easy to calculate automatically and does not require time-consuming dose calculations. Therefore, it can facilitate the process of developing and evaluating new sCT generation algorithms.

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http://dx.doi.org/10.1016/j.ejmp.2022.11.011DOI Listing

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