Cancer of unknown primary, Chemoradiation, Head neck cancer, Neck dissection, Unknown primary, Adult, Aged, Aged, 80 and over, Carcinoma, Squamous Cell, Chemoradiotherapy, Disease-Free Survival, Head and Neck Neoplasms, Humans, Kaplan-Meier Estimate, Lymphatic Metastasis, Male, Middle Aged, Neck Dissection, Neoplasms, Unknown Primary, Retrospective Studies, Survival Rate, Young Adult