Assessment of the impact of CT scan protocols on carbon ion radiotherapy treatment planning

The purpose of this study was to quantify the impact of computed tomography (CT) scan protocols and CT scanner on carbon ion radiotherapy treatment planning MATERIAL AND METHODS: A table was created for each of the five parameters to relate CT values to stopping power (SPR) (CT-SPR conversion table). The parameters changed in the four protocols, with the exception of the clinical protocol, are as follows: tube voltage, convolution kernel, CT scanner, and CT model. Treatment plans were calculated with each of the five CT-SPR conversion tables applied, and differences from the doses in the clinical protocol were evaluated by comparison of DVH parameters and gamma analysis.

The largest difference in CT values in the protocols with changing tube voltage compared to the clinical protocol was more than 200 HU for the high CT value material. The change in CT values due to the change in the reconstructed kernel was slight, and the difference in CT values for the CT scanner change or model change protocol was similar. Mean difference <1% for pelvic region plans for changes in DVH parameters. The difference in DVH parameters between CT scan protocols for treatment planning in the head and neck region compared to the pelvic region was large, with a maximum difference of 6.17%. Gamma analysis showed that a 3%/3mm tolerance resulted in a near 100% pass rate for both CT scan protocols, while 2%/2mm and 1%/1mm resulted in a 5%-10% or greater pass rate reduction.

The effect of changes in CT-SPR conversion table due to changes in CT scan protocols on carbon ion radiotherapy treatment planning was quantitatively evaluated, and the importance of calibration for each CT scan protocol in carbon ion radiotherapy was clarified.