Evaluation of CT calibration curve impact on proton range accuracy in treatment planning

Abstract

Introduction: In proton therapy, the accuracy of dose calculation and proton range determination is critically dependent on the conversion of CT number to proton stopping power ratio (SPR). Any inaccuracy in the CT calibration curve can lead to proton range uncertainties and potential deviations in dose distribution within the target and surrounding organs at risk. Therefore, verification of the accuracy of the CT calibration curve used in treatment planning is essential to ensure precise dose delivery. Objective: To evaluate the accuracy and reliability of the CT calibration curve currently implemented in the proton therapy treatment planning system at the Department of Radiology, King Chulalongkorn Memorial Hospital, The Thai Red Cross Society. Methods: A calibration curve was generated by correlating CT numbers with the proton stopping powers of Gammex tissue-equivalent materials. The resulting curve was applied in the treatment planning system to calculate proton ranges, which were then compared with measurements obtained using a Giraffe dosimeter on a Varian ProBeam system. Both phantom and real tissue samples were evaluated for comparison. Results: No statistically significant difference was found between calculated and measured proton ranges (P = 0.21). The mean differences of R80 and R90 in the phantom and real tissue were 1.53 ± 3.85 mm and 0.51 ± 3.08 mm, and 1.54 ± 3.82 mm and 0.83 ± 2.43 mm, respectively. Six of the Gammex materials met the AAPM TG-185 criteria, while the remaining materials showed consistent results with previous studies using the same CT and proton systems. Conclusion: The current CT calibration curve used in the proton therapy planning system at King Chulalongkorn Memorial Hospital demonstrates acceptable accuracy and reliability for clinical dose calculation and treatment planning applications.