Clinical Outcomes of Conventional Low Anterior Neck Field and Intensity Modulated Radiation Therapy in Oropharyngeal Cancer Patients

Abstract

Radiotherapy (RT) to the uninvolved low neck can be delivered either by whole-neck intensity-modulated radiotherapy (WN-IMRT), or by matching a photon low anterior neck (LAN) field to the IMRT field. Potential benefits of the latter approach are the ability to deliver lower radiation dose to midline structures and improved dose distribution in the superior neck. However, the uncertainty of dose delivery at the match line and inadequate cover to the posterior low neck with this technique, has raised concern regarding the possibility of neck recurrences. Dosimetric studies comparing WN-IMRT with IMRT+LAN field have been published but clinical data on safety of the LAN field technique is lacking. The purpose of this study is to report outcomes following elective radiation to the lower neck using IMRT+LAN field in patients with squamous cell carcinoma (SCC) of the oropharynx (OP) and to evaluate recurrence risk at or below the match line with this technique. This is a single-centre retrospective study of OP SCC patients treated with bilateral neck IMRT between April 2005 and November 2014. p16 immunohistochemistry was performed prospectively after 2010. Our policy suggests using a LAN field in all cases with uninvolved low neck nodes (low level 3 and levels 4a/b, 5a-c) with the matching line placed at least 2cm below the inferior end of GTV. The LAN field delivers 45.5Gy in 21 fractions over 4.5 weeks). The half-beam IMRT fields encompass the high risk CTV (macroscopic disease with an isotropic margin of 10mm and involved nodal levels) treated to 65Gy in 30 fractions over 6 weeks, and the low-risk CTV (upper neck uninvolved nodes) treated to 54Gy in 30 fractions over 6 weeks. Out of 221 OP cancer patients treated with curative RT, 148 (67%) were treated with IMRT+LAN field. Median follow up was 41 months. Median age was 58 years (range, 26-86). 36 (24%) had T1, 59 (40%) T2, 21 (14%) T3 and 32 (22%) T4 disease. 20 (14%) had N0, 19 (13%) N1, 16 (11%) N2a, 73 (49%) N2b and 20 (14%) N2c disease. Of the 81 patients for whom p16 immunohistochemistry was available, 64 (79%) were p16 positive and 17 (21%) p16 negative. 109 (74%) of patients received induction chemotherapy with cisplatin and 5-fluorouracil, 128 (87%) received concomitant chemotherapy (cisplatin 29, carboplatin 98, cetuximab 1). After a median follow up of 41 months, the 3-year recurrence-free survival (RFS), regional recurrence-free survival (RRFS) and overall survival (OS) were 81.4%, 92.6% and 88.9%. The 5-year RFS, RRFS and OS were 77.6%, 86.2% and 91.2% respectively. No recurrences were seen in the LAN field and no recurrences were seen in the match line between the IMRT fields and the matched LAN field. RT to uninvolved lower neck nodal levels in selected OP SCC patients using a LAN field leads to excellent outcomes with no recurrences seen at the field match line or the low neck.