Consensus guide on CT-based prediction of stopping-power ratio using a Hounsfield look-up table

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
The individual challenges and best practices 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 exemplarily performed on three CT scanners from different vendors, proving its
feasibility for SPR prediction on both single-energy CT scans and virtual monoenergetic CT images derived
from dual-energy CT (Figure 2).
Conclusion
A comprehensive step-by-step guide on CT-based HLUT specification is described, representing a
consensus found within the ESTRO Physics Workshop and the EPTN WP5. The presented
recommendations and examples can contribute to increase the accuracy in proton range prediction for
treatment planning in individual proton centres and, following from this, reduced inter-centre variations
in SPR prediction and thus a better comparability of treatment data between different centres for multi-
centre clinical studies.