Correlation between the reduction of parotid gland and gross tumor volume and their exposure dose in patients with different metastasis status of lymph node receiving intensity-modulated radiotherapy for nasopharyngeal carcinoma

Abstract

Objective The aim of this study was to explore the correlation between the reduction of parotid gland and gross tumor volume and their exposure dose in patients with different metastasis status of lymph node receiving intensity-modulated radiotherapy (IMRT) for nasopharyngeal carcinoma, and to determine the best time for replanning. Methods Thirty patients with NPC treated with IMRT were divided into three groups according to lymph node diameter: lymph-node negative (group A), lymph-node positive and diameter ≤3 cm (group B), and diameter >3 cm (group C). The initial computed tomography (CT) scan was CT0. Each patient was rescanned weekly in the same position at the 5th, 10th, 15th, 20th, 25th and 32nd fraction (CT1, CT2, CT3, CT4, CT5 and CT6, respectively). Results CT0 showed no difference in initial parotid volumes among the three groups (P>0.05). Between-group analysis showed a significant difference in parotid volume changes after CT4 between group A and B (t=2.12-2.46, P<0.05) and between group A and C after CT3 (t=2.19-3.25, P<0.05). According to within-group analysis, significant parotid volume changes occurred in groups A and B after CT3 (t=2.16-4.11, P<0.05) and in group C after CT2 (t=2.37-5.20, P<0.05), compared with CT0. There were significant changes when CT4 compared with CT2 (t=2.16, 3.34, P<0.05) in groups C. Conclusions Patients with larger lymph nodes need earlier replanning of IMRT, at the 10th and possibly again at the 20th fraction, while patients with negative or smaller lymph nodes do not require replanning until the 15th fraction. Key words: Nasopharyngeal carcinoma; Volume change; IMRT; Lymph node diameter