Dose gradient index (DGI) is a tool used to evaluate radiation dose gradient outside the target. This study aimed to analyze the consistency of this tool through the long course of radiotherapy due to patient anatomical changes, such as body weight loss and tumor shrinkage.

A total of 30 patients diagnosed with different head and neck cancer were treated with the simultaneous integrated boost intensity-modulated radiation therapy technique; the patients underwent new computed tomography (CT) simulations after 10 and 20 treatment sessions. The gradient index was compared for the initial, reconstructed, and adaptive plans.

All patients showed a significant decrease (p < 0.001) in weight at reconstructed CT1 (RCT1) and reconstructed CT2 compared with original CT. Also, primary gross tumor volume was significantly decreased (p < 0.001) at reconstructed CT1 and reconstructed CT2. In the dosimetric part, all organs showed a significant increase in dose delivery at reconstructed plans (Rplans) compared with the original plan (Oplan). Meanwhile, at adaptive plans (Aplans), all organs showed a significant decrease in dose delivery compared with Oplan. The DGI was significantly increased at Rplan1 and Rplan2 compared with Oplan, with a median value of 29 (15.5–41.3) and 30 (15.1–38) at Rplan1 and Rplan2, respectively, and 25.8 (14.9–37.6) at Oplan. Whereas DGI value decreased significantly at Aplan1 compared with Oplan, and then insignificantly increased at Aplan2 compared with Aplan1.

DGI must be evaluated during the intensity-modulated radiation therapy course in the treatment of head and neck cancers, which clearly varies significantly as a result of a patient's anatomical changes during the radiotherapy course, and it can be improved or maintained to its original value by using adaptive planning strategy.