3099 - Investigation of Using Virtual Non-Contrast Images from Photon-Counting Computed Tomography for Radiotherapy Treatment Planning

Purpose/Objective(s):

In common clinical practice, contrast-enhanced CT scans using iodine-containing agents are used to enhance the target-to-tissue contrast for target delineation while non-contrast images are required for accurate radiotherapy treatment planning and dose calculation. The repeated scan introduces extra exposure to patients. Patient motion in subsequent scan also cause misalignment on the position of target volumes and organs-at-risk (OARs). There is an imperative need to perform accurate target delineation and precise dose calculation within a single scan. This study aims to investigate the feasibility of utilizing Photon-counting CT-derived virtual non-contrast (VNC) images to substitute the conventional non-contrast images for radiotherapy treatment planning.

Materials/Methods:

This study employed CT simulation scans by PCCT from ten patients undergoing photon radiotherapy treatment with VMAT, including five patients with abdominal tumor, three with breast tumor and two with thoracic tumor. Treatment plans were originally planned by non-contrast images. Subsequently, the identical beam parameters from original plans were applied to recalculate the dose distribution on VNC images without reoptimization. To appraise the feasibility of utilizing VNC images for photon dose calculation, various metrics were applied to analyze the dosimetric equivalence between the two plans, including the Hounsfield unit (HU) of target volumes and OARs, the gamma comparison, and the variations in dose-volume-histogram (DVH) parameters.

Results:

Mean HU values of planning-target-volume (PTV) and most of OARs in VNC images were not significantly different from those in non-contrast images in all cases. Average absolute differences of PTV were 5.92HU, 14.82HU and 19.93HU in abdominal, thoracic and breast cases respectively. Lungs showed the largest difference (up to 45HU). Regarding the dosimetric comparison, the gamma passing rate between original plans and recalculation plans using VNC images with criteria 2%/2mm and 10% dose threshold approached 99% in all abdominal and breast cases. For the thoracic cases, it only achieved approximately 85%. It was mainly attributed to the anatomical misalignment between the two image series due to the breathing motion. Deformable registration was then performed and achieved a gamma passing rate of 99%. Meanwhile, DVH parameter differences including D95% and Dmean for PTV, Dmean and Dmax for OARs were within 1% in most cases.

Conclusion:

Our findings demonstrated the agreement in dosimetric parameters between conventional non-contrast plan and VNC plan. This result preliminarily supports the feasibility of using PCCT-derived VNC images to substitute the non-contrast images for radiotherapy treatment planning. Non-contrast scans can be eliminated, leading to improved clinical workflow and reduced patient dose.