Toward adaptive proton therapy guided with a mobile helical CT scanner

Purpose: To evaluate the feasibility of image-guided adaptive proton therapy (IGAPT) with a mobile helical-CT without rails. Method: CT images were acquired with a 32-slice mobile CT (mCT) scanning through a 6 degree-of-freedom robotic couch rotated isocentrically 90 degrees from an initial setup position. The relationship between the treatment isocenter and the mCT imaging isocenter was established by a stereotactic reference frame attached to the treatment couch. Imaging quality, geometric integrity and localization accuracy were evaluated according to AAPM TG-66. Accuracy of relative stopping power ratio (RSPR) was evaluated by comparing water equivalent distance (WED) and dose calculations on anthropomorphic phantoms to that of planning CT (pCT). Feasibility of image-guided adaptive proton therapy was demonstrated on fractional images acquired with the mCT scanner. Results: mCT images showed slightly lower spatial resolution and a higher contrast-to-noise ratio compared to pCT images from the standard helical CT scanner. The geometric accuracy of the mCT was <1 mm. Localization accuracy was <0.4 mm and <0.3° with respect to 2D kV/kV matching. WED differences between mCT and pCT images were negligible, with discrepancies of 0.8 ± 0.6 mm and 1.3 ± 0.9 mm for brain and lung phantoms respectively. 3D gamma analysis (3% and 3 mm) passing rate was >95% on dose computed on mCT, with respect to dose calculation on pCT. Conclusion: Our study has demonstrated that the geometric integrity, image quality and RSPR accuracy of the mCT are sufficient for IGAPT.