Computed Tomography (CT) imaging is an important step in treatment planning for proton therapy. The conversion from Hounsfield Unit (HU) to stopping power ratio (SPR) accounts for more than half of the 3.5% error that is considered in treatment planning. The aim of the study is to investigate the efficacy of photon-counting CT (PCCT) for predicting the SPR in proton therapy. Its performance will be compared to single-energy CT (SECT) and dual-energy CT (DECT).

Fresh tissue samples—steak, ground beef, and bone marrow—were secured in 3D-printed holders to minimize air inclusion and maintain tissue stability during measurements. These
samples were secured in a head and a body configuration of a phantom and CT scans were conducted using standard clinical protocols. SPR values were derived from the CT data
using automated DirectSPR software for the DECT and PCCT scans, and converted from HUs to SPR using a Hounsfield look up table (HLUT) for the SECT scans. Results were validated
against actual SPR measurements obtained via proton beam measurements.

Two measurement series have been performed. For the validation of the first measurement series, proton beam energies of 150 and 175 MeV both resulted in an SPR value of 1.00 for the
ground beef sample. For the steak sample, SPR values of 1.02 and 1.03 were found. The CT-based SPR predictions resulted in errors up to 1.5% for ground beef and 3.5% for steak. For
the validation of the second measurement series, SPR values for ground beef (0.89-0.92), steak (0.96-0.98), and bone marrow (0.87-0.91) were obtained. The percentage error between the CT-based SPR predictions and the validation measurements showed no substantial differences between the CT-modalities, except for SECT providing lower SPRs for the bone marrow samples than DECT and PCCT. The validation measurements of the second series resulted in lower SPR values than found in literature and the first
series, indicating possible methodological inconsistencies. Factors such as sample heterogeneities and measurement errors might have contributed to these inconsistencies, but do not account for these inconsistencies completely. These results highlight the need
for further research with standardized phantoms and animal tissue samples to evaluate PCCT’s true potential in combination with DirectSPR prediction.

From this study it cannot be concluded that PCCT demon-
strates an advantage over SECT and DECT as CT-modality for
the HU-SPR conversion in proton therapy treatment planning