# Topogram and 3DCT geometry calibration for image-guided proton therapy with in-room CT-on-rails

## TL;DR

This paper introduces a method to improve the accuracy of proton therapy by calibrating in-room CT scans and topograms, enabling precise patient positioning.

## Contribution

A novel calibration workflow combining CT-on-rails and robotic tables to achieve sub-millimeter patient positioning accuracy.

## Key findings

- Geometric distortions in 3DCT and topogram images were corrected using affine models validated with laser tracker data.
- Phantom experiments showed sub-millimeter accuracy in 3D/3D and 2D/3D registration.
- A novel algorithm for digitally reconstructed topograms enabled high-precision treatments without isocenter verification.

## Abstract

•Corrected 3DCT and topogram distortions using affine transform models.•Introduced fan-beam model for digitally reconstructed topogram generation.•Achieved sub-millimeter accuracy in 3D/3D and 2D/3D patient positioning.•Enabled high-precision treatments without verification imaging at the isocenter.

Corrected 3DCT and topogram distortions using affine transform models.

Introduced fan-beam model for digitally reconstructed topogram generation.

Achieved sub-millimeter accuracy in 3D/3D and 2D/3D patient positioning.

Enabled high-precision treatments without verification imaging at the isocenter.

In-room computer tomography (CT) on-rails scanners are available in proton therapy centers but often limited to control imaging, due to geometric distortion and lack of integration. We present a calibration method combining CT-on-rails with a robotic table to achieve sub-millimeter patient positioning accuracy. Geometric distortions in 3DCT and topogram images were corrected using affine models and validated with laser tracker data. Phantom experiments simulating clinical conditions showed errors below 0.15 mm for 3D/3D and 0.3–0.55 mm for 2D/3D registration, the latter using a novel algorithm for digitally reconstructed topograms. The workflow enables accurate positioning without isocenter verification, supporting high-precision radiotherapy.