Selectively Sparing the Submandibular Gland When Level IB Lymph Nodes Are Included in the Radiation Target Volume: A Safety Analysis in Cancers of the Oropharynx and Oral Cavity

Abstract

Submandibular gland metastases are extremely rare in head and neck cancer, even in the presence of level Ib lymph node (LN) involvement. In recent years, we have contoured the submandibular gland (SMG) and selectively attempted to limit its dose exposure even in patients in whom the level Ib LN station is targeted. This study reports our preliminary feasibility and safety experience with selective submandibular gland sparing and its dosimetric impact. We identified 174 patients with squamous cell cancer (SCC) of the oral cavity or oropharynx, with T1-2, N0-3, M0 disease in whom at least a single level Ib lymph node region was included in the target volume. All patients were treated from 2009 - 2014 with definitive or postoperative IMRT with or without chemotherapy. Patients with recurrent disease, or who were treated with re-irradiation or a split course technique were excluded. Patient, tumor and treatment related factors were abstracted from the medical record. The treatment plans for each patient were reviewed and verified for level Ib targeting (unilateral vs. bilateral) as well as if the submandibular gland was excluded from the target volume and sparing was attempted during planning. Mean doses were calculated for each submandibular gland and the oral cavity. A total of 174 patients met criteria for inclusion. Patients had a median age of 59 years and median KPS of 90 at diagnosis. 147 patients had SCC of the oropharynx and 27 patients had SCC of the oral cavity. 134 patients were treated definitively while 40 were treated post-operatively. Of the 174 included patients, 142 were treated with concurrent chemotherapy. Among the 190 level Ib LN stations that were deliberately targeted in the clinical treatment volume, 32 submandibular glands were contoured, excluded from the target volume and avoided during treatment planning. Mean doses to the spared SMG were able to be reduced by 12% (66.9 Gy vs. 58.9 Gy). In a subgroup analysis of 26 patients who had bilateral level Ib LN targeted, we compared the dosimetric outcomes of 4 patients in whom bilateral submandibular glands were avoided with those of 22 patients in whom bilateral submandibular glands were included in the target volumes. In addition to a 12% reduction in mean submandibular dose (66.3 Gy vs. 58.2 Gy), mean oral cavity dose was reduced by 14.5% (43.4 Gy vs. 37 Gy). None of these patients experienced any level 1b LN failures. Selective sparing of the submandibular gland when targeting the level 1b nodes in oral cavity and oropharynx cancer is feasible, reduces the mean doses to submandibular glands and oral cavity, and does not result in increased level 1b nodal failure rates. Future studies with larger numbers are needed to validate this preliminary finding and examine the impact of this technique on functional outcomes.