A longitudinal evaluation of early anatomical changes of parotid gland in intensity modulated radiotherapy of nasopharyngeal carcinoma patients with parapharyngeal space involvement

INTRODUCTION: Radiotherapy of nasopharyngeal carcinoma patients with parapharyngeal space (PPS) involvement may deliver high dose to the parotid gland. This study evaluated parotid gland changes during and up to 3 months after radiotherapy. METHODS: Kilovoltage computed tomography (CT) scans of head and neck region of 39 nasopharyngeal carcinoma patients with PPS involvement were performed at pre‐radiotherapy, 10th, 20th and 30th fractions and 3 months after treatment. The parotid glands were contoured in pre‐radiotherapy planning CT scan and in subsequent scans. Dice similarity coefficient (DSC), percentage volume change and centroid movement between the planning CT and the subsequent CTs were obtained from the contouring software. In addition, the distance between medial and lateral borders of parotid glands from the mid‐line at various time intervals were also measured. RESULTS: The ipsilateral parotid gland received a mean dose of about 5 Gy higher than the contralateral side. The mean DSC and parotid volume decreased by more than 30% at 20th fraction and reached the minimum at 30th fraction. Partial recovery was observed at 3 months after treatment. The centroid displacement followed a similar pattern, which moved medially and superiorly by an average of 0.30 cm and 0.18 cm, respectively, at 30th fraction. The changes in ipsilateral gland were slightly greater than the contralateral side. CONCLUSIONS: Substantial volume change and medial movement of parotid gland were observed with slightly greater magnitude in the ipsilateral side. Adaptive radiotherapy was suggested at around 15th to 20th fraction so as to optimise the original dose distribution of the plan.