Purpose: To investigate the correct time point for re-planning by evaluating dosimetric changes in the parotid glands (PGs) during intensity-modulated radiotherapy (IMRT) in head and neck cancer patients.
Materials and methods: Patients with head and neck cancer treated with IMRT were enrolled. During treatment all patients underwent cone-beam computed tomography (CBCT) scans to verify the set-up. CBCT scans at treatment days 10, 15, 20 and 25 were used to transfer the original plan (CBCTplan I, II, III, IV, respectively) using rigid registration between the two. The PGs were retrospectively contoured and evaluated with the dose-volume histogram. The mean dose, the dose to 50 % of volume, and the percentage of volume receiving 30 and 50 Gy were evaluated for each PG. The Wilcoxon sign ranked test was used to evaluate the effects of dosimetric variations and values <0.05 were taken to be significant.
Results: From February to June 2011, ten patients were enrolled and five IMRT plans were evaluated for each patient. All the dosimetric parameters increased throughout the treatment course. However, this increase was statistically significant at treatment days 10 and 15 (CBCTplan I, II; p = 0.02, p = 0.03, respectively).
Conclusion: CBCT is a feasible method to assess the dosimetric changes in the PGs. Our data showed that checking the PG volume and dose could be indicated during the third week of treatment.