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

Published:December 17, 2022DOI:https://doi.org/10.1016/j.ejmp.2022.11.011

      Highlights

      • Conventional image similarity metrics are insufficient to assess the quality of sCT.
      • Water equivalent depth (WED) calculation can evaluate dosimetric quality of sCT.
      • The mean WED difference ( Δ W E D ¯ ) between the sCT and reference CT can be calculated.
      • The Δ W E D ¯ can facilitate the development process of the new sCT generation method.

      Abstract

      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 ( Δ W E D ¯ ) 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 Δ W E D ¯ were then compared to the isocentre dose difference ( Δ D iso ) 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 Δ D iso with R2 = 0.6, and the 95 % prediction interval for Δ D iso between −1.2 and 1 %. The Δ W E D ¯ results showed an improved linear trend (R2 = 0.8) with a narrower 95 % prediction interval from −0.8 % to 0.8 %.

      Conclusion

      Δ W E D ¯ highly correlates with Δ D iso 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.

      Keywords

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