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Abstract Background Anatomical variations in head and neck cancer during IMRT leads to volume shrinkage, results in dosimetric variations in tumour and normal tissue including parotid glands, with a risk of radiation toxicities. Methods 30 patients with a stage II–IV head and neck squamous cell carcinoma (HNSCC) were treated with definitive IMRT‐SIB and concomitant chemotherapy. Volumetric and dosimetric variations were evaluated during the period of IMRT by recalculating and obtaining dose‐volume histograms of re‐contoured target volumes and parotid glands on repeat CT scans taken multiple times during treatment (CT1, CT2, CT3 and CT4). Results Result showed significant (p < 0.001) mean decrease in both primary and nodal tumors volume with time whereas increase (p < 0.01 or p < 0.001) in respective V100 (%) and D2% (Gy). The mean parotid gland dose increased (p < 0.01 or p < 0.001) with time, whereas parotid gland volume and distance between plan isocenter and centre of mass of parotid glands decreased (p < 0.05 or p < 0.001) with time. Patient's mean weight and neck circumference both decrease (p < 0.001) with time whereas ECOG score increase (p < 0.001) with time. The mucosal toxicity increased significantly (p < 0.001) with time. The change in both weight and neck circumference showed significant (p < 0.001) and direct (positive correlation) association with change in parotid gland volume. Conclusion If the PTV and normal anatomy are changing with time, adaptive IMRT would be beneficial radiation dose delivery where possible.