Influence of change in anatomical volume on dose distribution during intensity-modulated radiotherapy for different stages of nasopharyngeal carcinoma

Objective To explore the change in anatomical volume during intensity-modulated radiotherapy (IMRT) for different stages of nasopharyngeal carcinoma (NPC) and its influence on dose distribution, and to assess the necessity to modify the IMRT plan. Methods Twenty-four patients with newly diagnosed NPC who received IMRT and chemotherapy were enrolled in the study, and were divided into early-intermediate group (12 cases) and locally advanced group (12 cases) according to the 2008 staging system for NPC. Each patient had a repeated CT scan at week 5 of radiotherapy, and target volume and organs at risk (OAR) were contoured. The dose distribution of the original plan shown on CT was calculated. Changes in target volume, OAR anatomical volume, and dose distribution were analyzed, and paired t-test and Spearman correlation analysis were performed. Results In the early-intermediate group, gross target volume of neck positive lymph nodes (GTVnd) was reduced during radiotherapy (P=0.059), and gross target volume of nasopharynx (GTVnx), high-risk clinical target volume (CTV1), and parotid volume were reduced significantly during radiotherapy (P=0.001, 0.012, 0.002, and 0.000, respectively). In locally advanced group, GTVnx, GTVnd, CTV1, and parotid volume were significantly reduced during IMRT (P=0.000, 0.000, 0.003, 0.003, and 0.000, respectively). Compared with the values before radiotherapy, the parotid dose increased significantly in the two groups during IMRT (P=0.044, 0.026, 0.033, and 0.026, respectively; P=0.024, 0.016, 0.030, and 0.015, respectively), and the increase in GTVnd dose was observed in the locally advanced group (P=0.029 and 0.049). Conclusions It is recommended to perform another CT scan for patients with locally advanced NPC at week 5 of radiotherapy and formulate a new IMRT plan to maintain target volume dose and guarantee a safe parotid dose. Key words: Nasopharyngeal neoplasms/intensity-modulated radiotherapy; Volumetric changes; Dosimetric changes