Purpose: In particle therapy treatment planning, dose calculation is performed using patient-specific ion stopping power ratio (SPR) maps to predict beam ranges. Improving patient-specific SPR prediction is therefore essential for accurate dose calculation and minimization of healthy tissue irradiation. In this study, we investigated the Philips Spectral CT 7500, a second-generation dual-layer spectral computed tomography (DLCT) system, as an alternative to conventional single-energy computed tomography (SECT) for patient-specific SPR prediction.
Methods: Using a double-layer detector, DLCT imaging simultaneously acquires two X-ray spectra of different energies for spectral imaging purposes, affording direct SPR prediction from quantitative measurements of relative electron density and effective atomic number. In contrast, indirect SECT-based SPR prediction relies on generic CT-number-to-SPR conversion tables. The performance of the Spectral CT 7500 in SPR prediction was characterized in tissue-equivalent materials and common non-tissue implant materials. Furthermore, end-to-end analyses of DLCT-based treatment planning were performed for proton, helium, and carbon ion therapies with anthropomorphic head and pelvic phantoms and compared against ionization chamber array measurements as the reference.
Results: For tissue-equivalent materials and non-tissue implant materials, DLCT was found to reduce uncertainty in SPR prediction over SECT. Mean deviations from measured SPR values were 0.7% (DLCT) and 1.6% (SECT) in tissue-equivalent materials. For anthropomorphic head and pelvic phantoms, DLCT-predicted dose distributions revealed higher 3D gamma passing rates than SECT-predicted dose distributions. Furthermore, in the DLCT-based treatment plans, measured distal-edge evaluation layers were within 1 mm of their predicted positions, demonstrating the accuracy of DLCT-based range prediction.
Conclusion: This study demonstrated that the Spectral CT 7500 may lead to better agreement between planned and delivered doses in particle therapy treatment planning compared to conventional SECT systems. Further comparative studies of DLCT- and SECT-based SPR prediction should be performed in biological tissue samples to assess its full clinical potential.
Stopping Power, Dual-energy Imaging, Treatment Planning
TH- External Beam- Particle/high LET therapy: Dual energy/spectral CT-based stopping power mapping