Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

Purpose/Objective:

The standard approach for CT-number to stopping-power-ratio (SPR) conversion in particle therapy is the use of a heuristic stepwise translation, a so-called Hounsfield look-up table (HLUT). It is defined by each treatment facility individually and depends on both the calibration method and CT scan protocol. A recent survey has shown broad variability in these parameters [1], making a simple comparison on HLUT level unfeasible. Hence, we present a comprehensive experimental evaluation of inter-centre variation and absolute accuracy in SPR prediction within the European Particle Therapy Network.

Material/Methods:

A head and a body phantom with 17 tissue surrogate inserts were scanned consecutively at the participating centres using their individual clinical scan protocol. The inserts were tissue-equivalent concerning particles; their composition and SPR were blinded for the participants. The SPR calculation was performed using each centre’s CT scan and HLUT (Fig.1).The inter- centre variation and absolute accuracy in SPR prediction were quantified for each tissue surrogate individually and then summarised into the relevant tissue groups: lung, soft tissues and bones. Finally, to evaluate the integral effect on range prediction for typical clinical beams traversing different tissues, for three simplified beam paths the determined SPR deviations were accumulated according to their respective tissue distribution. So far, data from 9 out of 17 participating centres was available.

Results:

A 2σ inter-centre variation in SPR prediction of 5.7% and 5.5% relative to water was determined for the bone inserts in the head and body setup, respectively. Comparable results were achieved for the lung tissue surrogates (6.4% and 2.2%). In the soft tissue region an overall higher accuracy was achieved with a variation below 0.9% in both setups and a mean SPR prediction accuracy below 0.5%. In the head setup, both lung tissues and bones were overestimated in most centres, while in the body setup the bones were underestimated (Fig. 2A). For the three exemplary beam paths, inter-centre variations in relative range were 1.5% on average. In specific centres, range deviations from reference exceeded 1.5% (Fig 2B).

Conclusion:

Large inter-centre variations in SPR prediction were observed in low- and high density tissue surrogates. The differences in deviation for bone between the two setups indicate a strong influence of scanning parameters such as the level of beam hardening correction, potentially resulting in range shifts of clinical relevance. As the study allows for a direct attribution of the measured deviations to the calibration methods and scan protocols used by the individual centres, it stresses the need for inter-centre standardisation. While this work addresses the accuracy in SPR prediction under idealised study conditions, a direct conclusion on overall range accuracy in patients is not possible. The study is currently still ongoing.

[1] Taasti et al. 2018, phiRO 6 25-30