Commissioning of a relative stopping power to Hounsfield unit calibration curve for a Mevion proton radiation treatment unit

A calibration curve relating relative stopping power (RSP) to Hounsfield units (HU) for proton treatment planning was commissioned. Synthetic tissue equivalent materials were imaged with varied scan parameters on two separate CT units to account for potential variations in Hounsfield units. All the materials were measured for their relative stopping power (RSP) with different energy proton beams from a Mevion passive double scattering proton unit, and the results were compared to Bethe-Block calculated values. The calculated relative stopping power (RSP) was found to match measured data within 2.5% in all cases except for low density lung. Hounsfield units (HU) were found to vary with scan parameters by less than 2.14% for near-tissue materials, and up to 7.44% for lung and bone. HU values between CT scanners were found to be comparable.