Commissioning a single photon counting CT simulator for radiotherapy planning: Methods, challenges, and initial...

Photon counting computed tomography (PCCT) offers distinct advantages over standard energy-integrated detector (EID) CT by offering energy-resolved measurements, enhanced spatial resolution, and improved soft-tissue contrast. While PCCT shows great utility in diagnostic imaging, its application as a radiotherapy simulator has not been explored. Commissioning procedures for EID-based CT simulators are well-documented, but commissioning a PCCT scanner in a radiotherapy setting has not been investigated.

We investigated whether PCCT can be used for radiotherapy simulation by evaluating the geometric accuracy, image quality, dosimetric performance, and relative electron density calibration of a Siemens Naeotom Alpha PCCT scanner with a commercially available carbon-fiber couch top.

Commissioning followed AAPM Task Group 66 guidelines, involving electromechanical evaluation, image quality assessment, and radiation safety verification. The CATPHAN 604 phantom was used to evaluate CT number accuracy, uniformity, spatial resolution, low contrast detectability, and geometric accuracy across multiple clinical protocols. Dosimetric measurements were performed using CTDI phantoms with a 100-mm pencil ion chamber. Relative electron density (RED) calibration was performed using an Advanced Electron Density phantom at selected tube voltages (120 and 140 kVp), slice thicknesses (0.4-10 mm), and reconstruction filters. Statistical analyses included repeated-measures ANOVA with post-hoc pairwise comparisons to assess protocol and position effects on Hounsfield unit values.

Geometric accuracy measurements revealed in-plane spatial linearity of 50.00 ± 0.06 mm (nominal 50 mm). CT number accuracy aligned with manufacturer specifications, except for bone, using specific protocols. The mean discrepancy between measured and nominal CTDI_vol was 3.8 ± 4.6%, with excellent agreement on the dose-length product (0.9 ± 2.3%). Relative electron density measurements suggested statistically significant, but clinically insignificant, variations across protocols (mean HU shifts: -23.0 to 7.9 HU). Position-dependent effects suggested mean HU reductions up to 23.2 HU at peripheral locations. Iterative metal artifact reduction (iMAR) differentiated Titanium and Stainless Steel metallic implants without significantly affecting surrounding tissue CT numbers (P > 0.08).

The Siemens Naeotom Alpha PCCT scanner satisfies the requirements noted in task group reports for a radiotherapy simulator. The system's enhanced spatial resolution, spectral imaging capabilities, and flexible retrospective reconstruction have the potential to offer advantages for radiation oncology applications, including improved target delineation and treatment planning accuracy.