Direct measurement of relative stopping power maps of prosthesis devices and synthetic materials by proton computed tomography

Objective: Treatment planning in proton therapy requires an accurate estimation of Stopping Power Ratio relative to water (SPR) maps. Presently, about 4% of patients submitted to radiotherapy treatments have metallic implants, which are responsible for an incorrect determination of SPRs in prostheses and surrounding regions. This study presents the first application of the Proton Computed Tomography (pCT) technique, able to directly measure SPRs maps, on complex metallic implants. Approach: A homogeneous Ti6Al4V alloy sample, a set of metallic devices used for prostheses and an intra-vertebral titanium alloy implant have been inspected, by means of a prototype pCT system with a 5x20cm2 field-of-view developed by INFN Firenze (Italy), under a proton beam at Trento Proton Therapy Centre (APSS, Trento, Italy). For comparison, a Multi Layer Ionization Chamber has been used to independently determine the SPR mean value of the Ti6Al4V alloy sample. Main Results: Tomographic reconstructions of all devices and materials have been performed and SPR maps have been obtained. All pCT images and profiles, even of metallic components, are characterized by negligible artifacts. The fine spatial resolution of our pCT system, about 0.7 lp/mm, allowed us to resolve details within a millimeter scale. The internal grid of the meshed cage as well details of the screws’ head of the intra-vertebral titanium alloy implant are clearly visible. The SPR of the Ti6Al4V alloy sample measured with pCT, 3.14±0.02, compares well with what was measured by MLIC: 3.17±0.02. Significance: This study presents the first application of the pCT methodology to directly measure SPR maps of complex metal prostheses. The ability of pCT to correctly determine mean SPR values has been experimentally demonstrated. Furthermore, this technique was shown to reconstruct complex metal structures at the millimeter scale with negligible artifacts.