Differences in lower cranial nerve complications predicted by the NTCP model between RTOG and reduced-volume IMRT planning in radiotherapy for nasopharyngeal carcinoma

Background: The aim of this study was to assess the differences in lower cranial nerve (LCN) complications predicted by the normal tissue complication probability (NTCP) model between Radiation Therapy Oncology Group (RTOG) and reduced-volume intensity-modulated radiotherapy (IMRT) planning for nasopharyngeal carcinoma (NPC) radiotherapy. Methods: A total of fifty patients with NPC were divided into two groups according to T-stages of T1-2 and T3-4, and the LCNs of each patient were contoured on CT simulation images. The targets were contoured based on the RTOG 0225 clinical trial and a working committee for clinical stage NPC in China in 2010. The NTCP differences in LCNs between the two plans were calculated. Results: The LCN volume of the 50 patients was 10.07 cc. The Dmax and Dmean of LCNs in RTOG plans were significantly larger than those in reduced-volume plans (7,453 vs. 7,401 cGy, 6,740 vs. 6,436 cGy, P=0.004, 0.000), and these values were lower in the T1-2 group than in the T3-4 group (7,390 vs. 7,464 cGy, 6,442 vs. 6,733 cGy, P=0.019, 0.000). NTCP in RTOG plans was significantly higher than that in reduced-volume plans (59.98% vs. 51.62%, P=0.000), among which NTCP was significantly lower in the T1-2 group than in the T3-4 group (51.72% vs. 59.88%, P=0.002). There were strong correlations of NTCP with Dmean and irradiation volume for more than 6,600 cGy (R=0.847, P=0.000; R=0.841, P=0.000). Conclusions: the clinical T-stage, a high Dmean and a large irradiation volume are important factors in predicting LCN complications. Of the two most common IMRT guidance plans in China, the LCN NTCP based on the reduced-volume plan is significantly lower than that based on the RTOG plan.