Risk factors of level Ib lymph node metastasis and clinical outcome of its selectively prophylactic irradiation in nasopharyngeal carcinoma: A real-world study.

Purpose: To evaluate the risk factor of level Ib lymph node metastasis (LNM) and the clinical outcome of its selectively prophylactic irradiation (pRT) in nasopharyngeal carcinoma (NPC) patients treated with IMRT.

Methods: 518 NPC patients receiving radical IMRT were collected. The structures of primary tumor invasions and neck LNM levels were analyzed bilaterally to estimate the risk factors of level Ib LNM. Patients with level Ib LNM and submandibular gland (SMG) invasion received level Ib pRT. The level Ib recurrence-free survival (RFS_Ib), regional recurrence-free survival (RRFS), and the incidence of ≥ grade 2 xerostomia at 1-year post-IMRT were compared in negative level Ib LNM patients who omitted, received unilateral, or bilateral level Ib pRT.

Results: Thirteen (2.5 %) patients with 18 sides had level Ib LNM. Ipsilateral SMG invasion was an independent risk factor for level Ib LNM. With a median follow-up time of 98.0 months, the 5-year RFS_Ib, 5-year RRFS and the incidence of xerostomia ≥ grade 2 at 1-year post-IMRT in negative level Ib LNM patients who omitted pRT, received unilateral, bilateral pRT to the level Ib were 99.7 % vs.100 % vs. 97.5 % (P = 0.110), 98.0 % vs. 92.1 % vs. 95.1 % (P = 0.120) and 28.0 % vs. 38.3 % vs. 90.0 % (P < 0.001), respectively.

Conclusions: Our study revealed that ipsilateral SMG invasion was the independent risk factor for the level Ib LNM. Omitting pRT in patients without ipsilateral level Ib LNM and SMG invasion did not increase the RFS_IB and RRFS, and reduced the incidence of xerostomia. Further multi-center prospective randomized clinical trial is warranted.