Influence of Magnetic Resonance Simulation on Target Volume Delineation in Local Advanced Nasopharyngeal Carcinoma

Abstract

Currently computerized tomography (CT) is widely used in the design of treatment plan but has limitations in showing anatomical structures for nasopharyngeal carcinoma, and magnetic resonance imaging (MRI) can show accurate relationship of the tumors with their surrounding tissue structures owing to its high soft tissue resolution. This study analyzed the influence of magnetic resonance simulation (MRI-sim) on target volume delineation on local advanced nasopharyngeal carcinoma (LA-NPC). The clinical data of 24 patients with LA-NPC was collected from one institution. Each patient was made scanned by computerized tomography simulation (CT-sim) and MRI-sim in the same condition on the same day. Target volumes ware delineated based on CT and MRI respectively. The data was analyzed by paired t test. Gross tumor volumes (GTVs) in the target areas on CT and MRI were significantly different (P=.001). The average volumes (cm3) of GTV-CT and GTV-MRI were 162.74 ± 55.42 and 155.35 ± 54.46 respectively. In the 24 cases, 19 cases were diagnosed of skull base bone invasion (SKB) in MRI and 15 cases in CT. In the SKB cases, GTVskb-CT and GTVskb-MRI had significantly differences (P=.001), and in the non-SKB cases, GTVskb-CT and GTVskb-MRI had no significantly differences (P=.501). There were remarkable differences of target areas (cm3) of cervical lymph nodes based on CT and MRI (92.57 ± 48.76 and 84.50 ± 46.60, P=.000). All of the patients were diagnosed in MRI, and 6 were not in CT. The volumes (cm3) of retropharyngeal lymph node (RPN) of GTVrpn-CT and GTVrpn-MRI were 2.03 and 2.67 respectively (P=.028). Compared with CT, MRI shows a broader base of skull base bone invasion range and a higher detection rate. The general target volume based on MRI is smaller than that based on CT, which indicates that MRI-based radiation may protect the surrounding normal tissues and reduce radiation side effects.