Abstract： Objective:To investigate the value of low-energy virtual monoenergetic image (VMI) at 45 keV in visualizing the primary tumor and T staging of hypopharyngeal squamous cell carcinoma.Methods:The clinical and imaging data of 58 patients with hypopharyngeal squamous cell carcinoma from April 2018 to January 2020 at Eye & ENT Hospital, Fudan University were analyzed retrospectively. All the patients underwent a venous phase contrast-enhanced dual-source dual-energy CT scan before treatment. The VMI at 45 keV and standard linearly blended image (30% 80 kV+70% 140 kV) were acquired from dual-energy post-processing software. One senior radiologist and one junior radiologist independently assessed the visibility of the tumor on the 45 keV VMI and standard linearly blended image using a 5-point Likert rating scale. Furthermore, the senior radiologist assessed the visibility of the tumor at each subsite (piriform fossa, posterior pharyngeal wall, postcricoid region) and determined the invasion depth of the tumor (extension to esophagus, invasion to strip muscles and prevertebral muscles) and performed the T staging of the primary tumor using the two sets of images blindly. The accuracy of T staging was calculated, using pathological T staging (surgical cases) or clinical T staging (non-surgical cases) as the gold standard. The image scores of the two sets of images were compared using Wilcoxon rank sum test. McNemar-Bowker test was used to compare the accuracy of T staging using the two sets of images.Results:The overall image scores of the 45 keV VMI and standard linearly blended image from the senior radiologist were 3.5 (3, 4) and 3 (2, 3) respectively ( Z=-7.03, P<0.001), and the scores from the junior radiologist were 3 (3, 4) and 2 (2, 3) ( Z=-6.93, P<0.001). The scores of the 45 keV VMI were significantly higher than those of the standard linearly blended image in visualizing tumors in the piriform fossa, posterior pharyngeal wall, and postcricoid region, as well as in detecting invasion to the strip muscles ( P<0.05). There was no significant difference in the scores of the two sets of images in determining whether the tumor extended to esophagus or invaded prevertebral muscles ( P>0.05). Referring to pathological and clinical T stage, the accuracy of T staging determined by the 45 keV VMI and standard linearly blended image was 87.9% (51/58) and 81.0% (47/58) respectively, and the difference was not significant (χ 2=3.33, P=0.189). Conclusions:The 45 keV VMI is superior to the standard linearly blended image in visualizing tumors and detecting invasion to the strip muscles of hypopharynx squamous cell carcinoma. However, the accuracy of determining T staging using 45 keV VMI is slightly improved than that of standard linearly blended image, and the difference is not statistically significant. In determining whether the tumor extends to esophagus or invades prevertebral muscles, 45 keV VMI shows no significant advantage over standard linearly blended image.