Feasibility study of pharyngeal constrictor muscle sparing in proton radiotherapy for nasopharyngeal carcinoma based on MRI/CT fusion image

Abstract

Abstract Background/Purpose: Radiotherapy (RT) for head and neck tumors did not limit the dose of pharyngeal constrictor muscle (PCM) as organs at risk (OARs). This study is to segment the PCM using magnetic resonance (MR) and CT fusion images and investigate the feasibility and dosimetric characteristics of PCM sparing in nasopharyngeal carcinoma (NPC) proton radiotherapy. Methods: We collected CT and MR simulation images of 20 patients with NPC retrospectively. The PCM was contoured on the MR and CT fusion images. And it was divided into superior pharyngeal constrictor muscle (PCM-s), middle pharyngeal constrictor muscle (PCM-m) and inferior pharyngeal constrictor muscle (PCM-i) according to anatomical structure. Four RT plans were created for each patient, which included intensity-modulated photon therapy (IMRT), PCM sparing IMRT (IMRT-S), intensity-modulated proton therapy (IMPT), and PCM sparing IMPT (IMPT-S). Various dosimetric parameters among the four plans were compared. Result: The four plans can meet the target coverage rate. CI, HI of IMPT and IMPT-S were significantly lower than IMRT and IMRT-S (p<0.05). There was no statistical difference in dosimertic parameters of the PCM-s, PCM-m and PCM-i between IMRT and IMPT (p>0.05). However, compared with IMRT, for PCM-s, IMRT-S and IMPT-S reduced the Dmean, Dmax, V50, V60, V65 by 3.9 Gy, 2.1Gy, 0.3%, 0.8%, 17.5%, 24.3% and 10.7 Gy, 4.6 Gy, 9.4%, 33.8%, 34.3%, 37.4% respectively; For PCM-m, they respectively reduced 4.4 Gy, 4 Gy, 22.5%, 3.4%, 7.1%, 0.4% and 10.1 Gy, 10.3Gy, 66.5%, 22.7%, 7.8%, 4.0%; For PCM-i, they respectively reduced 5 Gy, 6.1 Gy, 67.3%, 4.4%, 0.3%, 0% and 17.2 Gy, 17.5 Gy, 95.6%, 6.6%, 0.3%, 0% (p<0.05). IMPT-S had the best sparing for PCM. Conclusions: IMRT for NPC has demonstrated reduction in PCM dose while using PCM as an OAR in the optimization. However, IMPT for NPC can achieve optimal PCM sparing. We recommend that PCM should be contoured as an OAR for NPC to reduce the probability of complications associated with dysphagia and improve overall survival (OS) rate of patients while there is no compromise on target dose coverage and other OARs dose.