Characterizing the Reliability of a Novel Upright CT for Proton Therapy

TL;DR

This study evaluates the reliability of an upright CT scanner for proton therapy dose calculations and introduces a simplified phantom setup for faster quality assurance, demonstrating comparable accuracy to traditional methods.

Contribution

It provides evidence that upright CT is reliable for proton therapy planning and proposes a simplified phantom configuration for routine QA.

Findings

Upright CT shows excellent stability over multiple sessions.

Simplified phantom configuration reduces scanning time significantly.

Dose calculation accuracy is maintained with the new setup.

Abstract

Purpose: To evaluate reliability of upright CT for proton dose calculation and feasibility of a simplified phantom configuration for accelerated routine QA. Methods: A calibration phantom was scanned on an upright CT following consensus guidelines for 14 sessions/7 months. CT number repeatability was assessed by standard deviation (SD). Stopping power ratio (SPR) look-up table was derived. Phantom size dependency was assessed. The simplified phantom configuration was scanned for 15 sessions/8 months. Repeatability was assessed. CT numbers and SPR were compared with consensus configuration. Both configurations were scanned on a recumbent CT to validate the findings. An anthropomorphic phantom was scanned on upright and recumbent CT. Targets were drawn mimicking spine and prostate tumor. Proton plans were developed using pencil beam scanning techniques and robust optimization. Equivalence of dose calculation were assessed via controlled comparisons. Results: Simplified configuration measured all CT numbers in 1 scan vs 5 for consensus guidelines. Upright CT demonstrated excellent longitudinal stability (inter- and intrasession SD <4.9 HU and 1.6 HU, respectively). Size dependency was identified with significant (p<.05) differences in CT numbers, propagated to $\Delta$SPR <5.3%. Significant (p<.05) differences were found comparing upright CT numbers measured by 2 configurations ($\Delta$SPR<2.6%). Recumbent CT showed smaller $\Delta$SPR (<0.7%). Both dosimetric comparison showed local differences (<8% of prescription dose) while clinical equivalence was found with target coverage differences <0.2% and gamma pass rates=100% at 3 mm/3% for all controlled comparison of different CT machines and phantom configurations. Conclusions: The upright CT demonstrated reliability to support adaptive proton therapy. The simplified configuration shows feasibility for rapid QA.