Evaluation of proton range differences in photon-counting and dual-energy computed tomography across imaging doses and anthropomorphic phantom sizes

<i>Objective.</i>In proton therapy, dual-energy computed tomography (DECT) has shown to improve proton range estimation and treatment planning, but it is not yet widely implemented clinically as technology differences may restrict application (e.g. field-of-view or temporal separation). Recently, photon-counting CT (PCCT) was introduced clinically. PCCT offers multi-energy CT imaging in a single scan and with low imaging dose options. Therefore, this study aimed to evaluate proton range differences between PCCT and DECT across varying anthropomorphic phantom sizes, tissue types, and imaging doses.<i>Approach.</i>Virtual monoenergetic images (70 and 180 keV) were generated from DECT (80/140 kVp) and PCCT (120 kVp) scans at three imaging doses (CTDI_vol,32cm: 5, 10, and 20 mGy) using a commercial head phantom and a 3D-printed abdomen phantom with exchangeable fat rings for varying dimensions. Proton therapy plans were created in RayStation (RaySearch laboratories) with varying proton beam configurations (e.g. crossing different tissues) to assess dependency of phantom size, tissue type, and proton energy. The proton range,<i>R</i>₈₀, was extracted for each plan, and range differences were evaluated both between imaging doses and between modalities (DECT vs PCCT) at 20 mGy (inter-modality).<i>Main results.</i>Under reduced imaging dose, DECT showed substantial proton range differences for the large phantom sizes. It resulted in median range differences up to 20.1 mm due to visible imaging artifacts. Similar trends were seen for long proton path lengths. In contrast, for PCCT, range deviations stayed below 0.5 mm across all phantom sizes and through all tissue types. Inter-modality range variations increased when protons traversed denser tissues or longer path lengths.<i>Significance.</i>This work showed consistent<i>R</i>₈₀estimations for PCCT across phantom sizes and traversing different tissue types, even under reduced imaging dose in PCCT. This offers possibilities to lower the imaging dose and thus cumulative patient dose, without compromising treatment accuracy.