Inter-center variability in CT-to-SPR conversion in particle therapy: Survey-based evaluation

Purpose/Objective
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 conversion is applied to treatment planning CTs to finally derive the proton range in patients. Currently, CT scan protocols for treatment planning are not standardized in image acquisition and reconstruction parameters. Hence, the CT-to-SPR conversion (Hounsfield look-up table, HLUT), depending on the former parameters, has to be defined by each center individually. Aiming to access the current status of inter-center differences, this investigation is a first step towards better standardization of CT-based SPR derivation.
Material/methods
A questionnaire was sent out to particle therapy centers involved in the EPTN and a few 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 (BH) on the HLUT 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 (10 in Europe, 2 in the US). Scan parameters, especially reconstruction kernel and beam hardening correction, as well as the HLUT generation varied widely between centers. Eight of the twelve centers applied a stoichiometric calibration method, while three defined the HLUT entirely based on tissue substitutes, and one center used a combination of both. All facilities performed a piecewise linear fit to convert CT numbers into SPRs, but the number of line segments used varied from 2 to 11 (Table 1). Nine centers had investigated the influence of BH, and seven of them had evaluated the size dependence of their conversion. All except one center had validated their HLUT experimentally, but the validation schemes varied widely. A few things were though found to be common for most centers: 1) CT scans were most commonly acquired at 120 kVp, 2) all centers individually customized their CT-to-SPR conversion, and 3) dual energy CT was seen as a promising technique to reduce CT-related uncertainties (Figure 1).
Conclusion
In general, a large inter-center variability in implementation of CT scans, image reconstruction and especially in CT-to-SPR conversion was found. The benefit of a future standardization is obvious: It would reduce the time-intensive site-specific efforts as well as 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.