This study proposes a CT number (CTN) -to-stopping power ratio (SPR) calibration method that can be directly integrated into the photon therapy workflows and evaluates its robustness against tissue variations, as well as dosimetric deviations compared with current calibration methods for scanned proton therapy. To develop the CTN-to-SPR calibration curve, relative electron densities (REDs) derived from a phantom with tissue-substitute inserts were applied to the parametric regression model (PRM) constructed from 53 standard human tissues. PRM stability was tested by varying the mass density and elemental composition of the reference tissues and recalculating SPRs from RED and CTN. For dosimetric evaluation, treatment plans for head-and-neck, chest, and pelvis were recalculated using a PRM-derived HLUT (CTN-to-SPRPRM) without re optimization on the Siemens SOMATOM X. cite and GE Revolution CT ES, and compared with CTN-to-SPRclin. Dose-volume histograms (DVHs) assessed the dosimetric impact. RED-based SPR was expressed as SPR=-0. 0173ρₑ³+0. 0057ρₑ²+1. 0163ρₑ, (RMSE ~0. 82% (0. 68% without air). CTN‑to‑SPRPRM and CTN‑to‑SPRclin showed similar shapes with minor head-body differences; averaged CTNs were used. CTN-to-SPRPRM gave higher SPRs in lung and bone, but lower in soft tissue, with differences within 3% between -200 and +200 HU, >25% at -800 HU, and 1. 23% and 0. 11% at 1000 HU for Siemens and GE, respectively. Hydrogen and calcium variations caused the largest RMSE increases (1. 58% and 6. 00%), and trends were consistent across sexes. DVH deviations were <3% overall, except -5% in left parotid and -4. 2% in lung Dmean. The proposed CTN-to-SPRPRM method demonstrates clinical feasibility and ease of integration into the photon therapy workflows.
Yagi et al. (Mon,) studied this question.