Background: The use of Contrast Agents (CAs) in radiotherapy enhances target volume contouring accuracy, but may influence dosimetric outcomes for the Planning Target Volume (PTV) and Organs-at-Risk (OARs).
Objective: This study aimed to investigate the dosimetric effects of Oral Contrast Agent (OCA) in Helical Tomotherapy (HT) treatment plans for patients with Esophageal Cancer (EC).
Material and Methods: In this experimental study, a total of 15 patients (8 females, 7 males; mean age 59.35±8.42 years) underwent two Computed Tomography (CT) scans: non-Contrast-Enhanced CT (non-CECT) and CECT. The original HT plans were performed on CECT images using the Precision Treatment Planning System (TPS), and then original plans were transferred into non-CECT images using the patient QA plan module. Dosimetric parameters, including Homogeneity Index (HI), Conformity Index (CI), D_x, V_x, and gamma index were compared between two plans.
Results: Dosimetric evaluation showed an increase in PTV dose metrics in CECT images, with significant differences in D₉₈, D₉₅, and D_min. Also, a significant difference was observed in HI (0.03±0.01 to 0.05±0.02) and CI (0.85±0.06 to 0.80±0.08) indices. Notably, the use of OCA led to reduced mean radiation doses to the OARs (heart, lungs, and spinal cord) demonstrating a statistically significant decrease in mean lung dose (-0.14±0.25 Gy).  
Conclusion: Our findings demonstrate that the use of OCAs in Helical Tomotherapy Plans significantly enhances dosimetric outcomes by improving target volume coverage and reducing radiation exposure to critical organs.