Change in volumes and radiation doses of parotid and submandibular glands during intensity modulated radiation therapy (IMRT) for nasopharyngeal carcinoma

Abstract

Purpose/Objective(s): To investigate the changes in volumes and radiation doses to parotid and submandibular glands during IMRT for nasopharyngeal carcinoma in an attempt to justify re-planning in the mid-course of IMRT to minimize radiationinduced xerostomia. Materials/Methods: 33 consecutive patients with stage III to stage IVB nasopharyngeal carcinoma (AJCC Staging Manual 6th Edition) who received concurrent chemoradiation were included in this study. Computed tomography (CT) scans were performed for IMRT planning purposes (PLCT) and at mid-course of IMRT (MCCT) with head and neck immobilized by the same thermoplastic cast. Volumes of parotid and submandibular glands were outlined in these two set of CT images. Treatment plans were generated by planning systems for PLCT. PLCT of all patients were then co-registered with their corresponding MLCT. The treatment plans for PLCT were then copied to MCCT preserving the same beam shapes, angles and energies of PLCT. Dose distribution on MCCT was then re-calculated using this re-normalized plan so that differences in radiation dose will be due to anatomical and positioning changes rather than differences in dose calculation algorithms. Volumes of parotid and submandibular glands on PLCT and MCCT together with dosimetric parameters including D95, D50, D05, D01 and minimum, mean, maximum doses were compared by paired sample t-tests. Results: All 33 patients had their volumes of parotid and submandibular glands significantly reduced on MCCT (Table 1). The minimum and mean doses, D50, D05 and D01 of both parotids were significantly increased during mid-course while there was no difference of the dosimetric parameters of both submandibular glands before and during mid-course. Conclusions: The major salivary glands shrank and the parotids received a higher radiation dose during IMRT for nasopharyngeal carcinoma. This signifies that adaptive radiotherapy with re-planning should be seriously considered during course of IMRT as normal organs can receive an unexpected higher dose leading to more profound complications.