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Motivation Large variations in stopping-power ratio (SPR) prediction from computed tomography (CT) across European proton centres were observed in recent studies. To standardise CT-based SPR prediction using a Hounsfield look-up table (HLUT), a step-by-step consensus guide, created within the ESTRO Physics Workshop 2021 in a joint effort with EPTN-WP5, is presented. Methods The HLUT specification process includes six steps: Phantom setup, CT acquisition, CT number extraction, SPR determination, HLUT specification, and HLUT validation. Appropriate phantom inserts are tissue-equivalent for both X-ray and proton interactions and are scanned in head- and body-sized phantoms to mimic different beam hardening conditions. Soft tissue inserts can be scanned together, while scanning bone inserts individually reduces imaging artefacts. For optimal HLUT specification, the SPR of phantom inserts is measured and the SPR of tabulated human tissues is computed stoichiometrically. The HLUT stability is increased by including both phantom inserts and tabulated human tissues. Piecewise linear regressions of CT numbers and SPRs are performed for four tissue groups (lung, adipose, soft tissue, and bone) and then connected. Finally, a thorough validation is performed. Results The best practices and individual challenges are explained comprehensively for each step. A well-defined strategy for specifying the connection points between the individual line segments of the HLUT is presented. The guide was tested exemplarily on three CT scanners from different vendors, proving its feasibility on both single-energy CT and virtual monoenergetic images from dual-energy CT. Conclusion The presented step-by-step guide for CT-based HLUT specification with recommendations and examples can increase the clinical range prediction accuracy and reduce its inter-centre variation.

Background and purpose: Studies have shown large variations in stopping-power ratio (SPR) prediction from computed tomography (CT) across European proton centres. To standardise this process, a step-by-step guide on specifying a Hounsfield look-up table (HLUT) is presented here. Materials and methods: The HLUT specification process is divided into six steps: Phantom setup, CT acquisition, CT number extraction, SPR determination, HLUT specification, and HLUT validation. Appropriate CT phantoms have a head- and body-sized part, with tissue-equivalent inserts in regard to X-ray and proton interactions. CT numbers are extracted from a region-of-interest covering the inner 70% of each insert in-plane and several axial CT slices in scan direction. For optimal HLUT specification, the SPR of phantom inserts is measured in a proton beam and the SPR of tabulated human tissues is computed stoichiometrically at 100 MeV. Including both phantom inserts and tabulated human tissues increases HLUT stability. Piecewise linear regressions are performed between CT numbers and SPRs for four tissue groups (lung, adipose, soft tissue, and bone) and then connected with straight lines. Finally, a thorough but simple validation is performed. Results: The best practices and individual challenges are explained comprehensively for each step. A well-defined strategy for specifying the connection points between the individual line segments of the HLUT is presented. The guide was tested exemplarily on three CT scanners from different vendors, proving its feasibility. Conclusion: The presented step-by-step guide for CT-based HLUT specification with recommendations and examples can contribute to reduce inter-centre variations in SPR prediction.

Purpose/Objective Studies within the European Particle Therapy Network (EPTN) have shown a large variation in the estimation of proton stopping-power ratio (SPR) from computed tomography (CT) scans across European proton centres. To standardise the SPR prediction process, we present a step-by-step guide on the Hounsfield look-up table (HLUT) specification process. This consensus guide was created within the ESTRO Physics Workshop 2021 on CT in radiotherapy in a joint effort with the EPTN Work Package 5 (WP5). Material/Methods The HLUT specification procedure is divided into six steps (Figure 1): 1) phantom setup, 2) CT scanning, 3) CT number extraction, 4) SPR determination, 5) HLUT specification, 6) HLUT evaluation. For each step, considerations and recommendations are given based on literature and additional experimental evaluations. Appropriate phantom inserts are tissue-equivalent for both X-ray and proton interactions and are scanned in head- and body-sized phantoms to mimic different beam hardening conditions. Soft tissue inserts can be scanned together, while bone inserts are scanned individually to avoid imaging artefacts. CT numbers are extracted in material-specific regions-of-interest covering the inner 70% of each phantom insert in-plane and several axial CT slices in scan direction. For an appropriate HLUT specification, the SPR of phantom inserts is experimentally determined in proton range measurements at an energy >200 MeV, and the SPR of tabulated human tissues is computed stoichiometrically at 100 MeV. By including both phantom inserts and tabulated human tissues in the HLUT specification, the influence of the respective dataset-specific uncertainties are mitigated and thus the HLUT accuracy is increased. Piecewise linear regressions are performed between CT numbers and SPRs for four individual tissue segments (lung, adipose, soft tissue and bone) and then connected with straight lines. A thorough but simple validation is finally performed. Results