Real patient data based cross verification of kilovoltage and megavoltage CT calibration for proton therapy

Purpose: We propose a methodology to evaluate the stoichiometric calibration method on MVCT against the corresponding kVCT calibration using patient data.

Methods: Stoichiometric calibrations were conducted for a MVCT and a kVCT scanner, respectively. We retrospectively analyzed kVCT and MVCT images of 21 patients by picking small tissue volumes in kVCT images and performing image registration to locate the tissue volumes in corresponding MVCT images. We computed the difference between the mean proton stopping power derived through kVCT and MVCT calibration, taking into account the uncertainties in calibration, imaging, and image registration.

Results: kVCT and MVCT calibration curves were in good agreement for soft tissues such as muscle and brain, but showed statistically significant difference (p < 0.05) in stopping power of adipose (2.4 ± 1.7%) and bony structures such as spongiosa, and cranium (-3.2 ± 1.4 and -3.1 ± 2.1%, respectively).

Conclusion: The MVCT calibration might not agree with the corresponding kVCT calibration for some tissues.