A feasibility study of using virtual noncontrast images synthesized from dual-energy computed tomography for radiotherapy treatment planning.

Background: In the traditional computed tomography (CT) simulation process, patients need to undergo CT scans before and after injection of iodine-based contrast agent, resulting in a cumbersome workflow and additional imaging dose. Contrast-enhanced spectral CT can synthesize true contrast-enhanced (TCE) images and virtual noncontrast (VNC) images in a single scan without geometric misalignment. To improve work efficiency and reduce patients' imaging dose, we studied the feasibility of using VNC images for radiotherapy treatment planning, with true noncontrast (TNC) images as references and explored its dosimetric advantages compared to using TCE images. Specifically, this study examined tumors near bones, including cases of bone metastasis and myeloma. Methods: A total of 54 patients (20 patients with cervical cancer, 15 patients with esophageal cancer, and 15 patients with laryngeal cancer, and 4 patients with bone metastasis or hip replacement) who underwent non-contrast-enhanced and contrast-enhanced spectral CT simulation were retrospectively enrolled between July 2023 and March 2024. The study was approved by the institutional review board. The CT images were acquired using a second-generation fast kilovoltage peak-switching CT. Treatment plans for photon radiotherapy were optimized and calculated using TNC images and recalculated based on TCE and VNC images. To evaluate image and dosimetric equivalent, several metrics, including Hounsfield unit (HU) value differences, gamma pass rates and dose-volume histogram (DVH) parameters of planning target volume (PTV), and organs at risk (OARs), were compared. Results: In terms of HU value difference, for the majority of patients, the HU value differences of the PTV between TCE and TNC images (36.7±23.9 and 27.8±2.1 in esophageal and laryngeal cancer, respectively) were greater than those between VNC and TNC images (10.59±25.8 and 3.55±1.9 in esophageal and laryngeal cancer, respectively). Regarding dosimetry, the gamma pass rates between VNC and TNC were 1 in 2%/2 mm and 3%/3 mm. Most DVH parameter differences were less than 1% between the VNC and TNC plans and between TCE and TNC plans. Meanwhile, in some blood-rich OARs such as heart and small intestine, VNC shows dosimetric potential compared to TCE based on the statistically significant DVH parameters differences. Conclusions: By analyzing radiotherapy treatment plans with target areas located in different locations, including tumors near bones such as bone metastasis, we preliminarily verified the feasibility of using VNC images for photon dose calculation. This approach can effectively improve clinical workflow and reduce the image dose to patients.