Radiation-induced volume changes in parotid and submandibular glands in patients with head and neck cancer receiving postoperative radiotherapy: a longitudinal study.

Objectives/hypothesis: To investigate changes in parotid and submandibular gland volumes of patients with head and neck cancer treated with postoperative intensity-modulated radiotherapy (IMRT) or conventional radiotherapy (CRT), and to relate the volume changes to different mean radiation therapy (RT) doses to the parotid and submandibular glands.

Methods: Between May 2007 and May 2008, 82 head and neck cancer patients (oral cavity cancer, 71; oropharynx cancer, 11) were treated with surgery and postoperative RT (IMRT, 40; CRT, 42) at our institution. Every patient underwent four computed tomography (CT) scans: one before RT, one after 3 weeks or at the 15th fraction of RT, one upon completing RT, and one at 2 months after RT; 39 of 82 patients had an additional CT scan at 6 months after RT. A dose-volume histogram was used to evaluate the mean volumes of patients' parotid and submandibular glands and mean RT doses to the glands on every CT scan. Altogether, 241 salivary glands (parotid, 162; submandibular, 79) were analyzed. The volume changes of the glands were evaluated against three levels of mean gland doses: <30 Gy, 30 to 50 Gy, and >50 Gy.

Results: The mean RT doses to spared parotid glands, spared submandibular glands, and irradiated submandibular glands in patients treated with IMRT were 22.21 Gy, 18.26 Gy, and 52.19 Gy, respectively. The mean doses to parotid and submandibular glands in patients treated with CRT were 50.22 Gy and 62.09 Gy, respectively. The average volume loss rates in parotid glands after 3 weeks of RT, at the end of RT, and 2 months after RT versus before RT were 20.01%, 26.93%, and 27.21%, respectively. The average volume loss rates in submandibular glands after 3 weeks of RT, at the end of RT, and 2 months after RT versus before RT was 11.49%, 16.76%, and 16.29%, respectively. Parotid and submandibular glands did not continue to shrink after completing RT. We observed more volume loss during RT in the parotid glands than in the submandibular glands. The average rates of volume loss during the first 3 weeks of RT (20.01% and 11.49%, respectively) were larger than in the last 3 weeks of RT (8.57% and 6.0%, respectively). No significant differences were observed in the mean volumes of both parotid and submandibular glands between the end of RT, 2 months post-RT, and 6 months post-RT (P > .05). Volume loss at higher doses (>30 Gy) to the glands was significantly larger than at low doses (<30 Gy; P < .001).

Conclusions: The parotid and submandibular glands shrunk during RT, but the parotid glands shrunk more than the submandibular glands. These gland volume reductions correlated significantly with the mean dose to the irradiated glands; the spared glands showed few changes.