The aim of this prospective study was to evaluate the dosimetric benefits of the 3-phase adaptive radiation therapy (ART) technique for head and neck cancer (HNC). In addition, it was aimed to examine the changes in skin dose due to the anatomical changes that occurred in the patients during radiation therapy with Thermoluminescent dosimeters (TLDs). Fifteen patients with head and neck cancer were included in this prospective study. TLDs were placed on the patient's neck at 5 different points determined during computed tomography (CT). A new CT was recommended for patients with a skin dose increase of more than 10% and a weight loss of more than 5% during treatment. Control CTs CT14 and CT24 were obtained in the patient due to weight loss and increase in skin dose. Offline adaptive replans were generated after fractions 14 and 24 and used for further processing (Aplan 1 and Aplan 2, respectively). All patients received 3-stage ART. Accordingly, iPlan Aplan1 and Aplan2 were applied between 0–14, 15–24 and 25–33 fractions, respectively. Dose-volume histograms (DVHs) were compared for adaptive (ART: iPlan + Aplan1+Aplan2) and non-adaptive (non-ART) plans. Lateral neck diameter (LND) change was statistically significant, with a decrease of 7.35% (p < 0.001) and 5.44% (p < 0.001) for CT14 and CT24, respectively. The mean weight loss for CT14 compared to CT0 was 7.80%. Weight loss in CT24 compared to CT14 was 5.65%. As a result of the measurements made with TLDs on different days between 0–15 fractions during the application of iPlan, a 15.25% increase in skin dose was measured. During Aplan 1 and Aplan 2 applications, the increase in skin dose was measured as 10.56% and 2.83%, respectively. For ART, the ipsilateral parotid and contralateral parotid Dmean doses were decreased by 1.62 Gy (p < 0.001) and 2.25 Gy (p < 0.001), respectively. During treatment, an increase in skin dose of more than 10% and a weight loss of more than 5% should be considered as a trigger point for initiation of ART. Repeat CT scanning and replanning after fractions 15 and 25 is recommended during intensity modulated radiotherapy (IMRT) in patients with HNC. In conclusion, the 3-phase ART approach provides dosimetric benefits to patients with HNC.