Presentation Optimum size of a calibration phantom for x-ray CT to convert Hounsfield units to stopping-power-ratios in charged-particle therapy treatment planning

Inaniwa, Taku  ,  Tashima, Hideaki  ,  Kanematsu, Nobuyuki

In charged-particle therapy treatment planning, the volumetric distribution of stopping power ratios (SPRs) of body tissues relative to water is used for patient dose calculation. The distribution is conventionally obtained from computed tomography (CT) images of a patient.using predetermined conversion functions from the CT numbers to the SPRs. One of the biggest uncertainty sources of patient SPR estimation is insufficient correction of beam hardening arising from the mismatch between the size of the patient cross section and the calibration phantom for producing the conversion functions. The uncertainty would be minimized by selecting a suitable size of the cylindrical water calibration phantom referred to as an "effective size" of the patient cross section, L.effective. In this study, we investigated the L.effective for pelvis, abdomen, thorax, and head and neck regions by simulating an ideal CT system using volumetric models of the reference male and female phantoms. The L.effective values were 23.3, 20.3, 22.7 and 18.8 cm for the pelvis, abdomen, thorax, and head and neck regions, respectively.
57th Annual Conference of the Particle Therapy Co-operative Group (PTCOG57)

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