(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: In treatment planning systems (TPS), a single CT calibration curve is used to convert Hounsfield Units (HU) to mass density and stopping power ratios for proton therapy dose calculations. The dosimetric differences from protocol-specific calibration curves are investigated in this study.
Methods: The conversion between HU and mass density was measured for two clinical CT protocols, head and body, at the time of commissioning. A third conversion, the average of the two protocols, is used within the TPS. Dose calculations were performed using the TOPAS Monte Carlo transport code for each of the three conversions on two different patients. The two patients were scanned in the CT simulator using the head protocol and the body protocol. Dosimetric comparisons were made using ICRU report 50 defined D95, V95, and D1 of the CTV. The dosimetric comparisons were made with calculations performed on the scanned protocol compared to the averaged conversion. As a test of the most extreme case of mismatched conversion, the results using head and body protocol conversions were compared to those with scanned protocol.
Results: When calculated using the head protocol conversion, the cranial patient exhibited differences of -0.04%, 0.8% and 0.002%, in D95, V95, and D1 with respect to calculations performed on the protocol averaged conversion. The patient with a body site treatment demonstrated differences of -0.01%, 0.3%, and -0.03% in D95, V95, and D1 dose with respect to calculations performed on the protocol averaged conversion. Even in the extreme case of mismatched conversion and scanned protocols, differences were 0.01%, 0.8%, 0.1% for the cranial site and -0.01%, 0.2%, and -0.04% for the body site.
Conclusion: Due to the negligible dosimetric differences, use of an averaged HU to stopping power conversion among clinical protocols from a common CT scanner is appropriate.
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