Inter-centre variability of CT-based range prediction in particle therapy: Survey-based evaluation

Purpose: To assess the inter-center variability of the conversion between CT number and particle stopping power ratio (SPR), a survey-based evaluation was carried out in the framework of the European Particle Therapy Network (EPTN). The CT-to-SPR conversion (Hounsfield look-up table, HLUT) is applied to treatment planning CT scans to finally derive the particle range in patients. Currently, CT scan protocols for treatment planning are not standardized regarding image acquisition and reconstruction parameters. Hence, the HLUT depends on the selected scan settings and must be defined by each center individually. Aiming to access the current inter-center differences, this investigation is a first step towards better standardization of CT-based SPR derivation.
Methods: A questionnaire was sent to particle therapy centers involved in the EPTN and two centers in the United States. The questionnaire asked for details on CT scanners, acquisition and reconstruction parameters, the calibration and definition of the HLUT, as well as body-region specific HLUT selection. It was also assessed whether the influence of beam hardening was investigated and if an experimental validation of the HLUT was performed. Furthermore, different future techniques were rated regarding their potential to improve range prediction accuracy.
Results: Twelve centers completed the survey (ten in Europe, two in the US). Scan parameters, in particular reconstruction kernel and beam hardening correction, as well as the HLUT generation showed a large variation between centers. Eight of twelve centers applied a stoichiometric calibration method, three defined the HLUT entirely based on tissue substitutes while one center used a combination of both. All facilities performed a piecewise linear fit to convert CT numbers into SPRs, yet the number of line segments used varied from two to eleven. Nine centers had investigated the influence of beam hardening, and seven of them had evaluated the object size dependence of their HLUT. All except two centers had validated their HLUT experimentally, but the validation schemes varied widely. Most centers acquired CT scans at 120 kVp, all centers individually customized their HLUT, and dual-energy CT was seen as a promising technique to improve SPR calculation.
Conclusions: In general, a large inter-center variability was found in implementation of CT scans, image reconstruction and especially in specification of the CT-to-SPR conversion. A future standardization would reduce time-intensive institution-specific efforts and variations in treatment quality. Due to the interdependency of multiple parameters, no conclusion can be drawn on the derived SPR accuracy and its inter-center variability. As a next step within the EPTN, an inter-center comparison of CT-based SPR prediction accuracy will be performed with a ground-truth phantom.