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

Jae Hyuk Choi; Behzad Asadi; John Simpson; Jason A Dowling; Stephan Chalup; James Welsh; Peter Greer

doi:10.1016/j.ejmp.2022.11.011

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

Phys Med. 2023 Jan:105:102507. doi: 10.1016/j.ejmp.2022.11.011. Epub 2022 Dec 17.

Authors

Jae Hyuk Choi¹, Behzad Asadi², John Simpson², Jason A Dowling³, Stephan Chalup⁴, James Welsh⁵, Peter Greer⁶

Affiliations

¹ School of Information and Physical Sciences, University of Newcastle, Newcastle, New South Wales, Australia. Electronic address: jaehyuk.choi@uon.edu.au.
² Department of Radiation Oncology, Calvary Mater Hospital, Newcastle, New South Wales, Australia.
³ School of Information and Physical Sciences, University of Newcastle, Newcastle, New South Wales, Australia; Commonwealth Scientific and Industrial Research Organisation, Australian E-Health Research Centre, Herston, Queensland, Australia.
⁴ School of Information and Physical Sciences, University of Newcastle, Newcastle, New South Wales, Australia.
⁵ School of Engineering, University of Newcastle, Newcastle, New South Wales, Australia.
⁶ 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.

PMID: 36535236
DOI: 10.1016/j.ejmp.2022.11.011

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 (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_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 ΔD_iso with R² = 0.6, and the 95 % prediction interval for ΔD_iso between -1.2 and 1 %. The ΔWED¯ results showed an improved linear trend (R² = 0.8) with a narrower 95 % prediction interval from -0.8 % to 0.8 %.

Conclusion: ΔWED¯ highly correlates with ΔD_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: Dosimetric QA; MRI-only radiotherapy; Synthetic CT; Water equivalent depth.

MeSH terms

Algorithms
Humans
Magnetic Resonance Imaging* / methods
Male
Radiotherapy Dosage
Radiotherapy Planning, Computer-Assisted* / methods
Tomography, X-Ray Computed / methods