Carcinoma, Squamous Cell/drug therapy; Carcinoma, Squamous Cell/radiotherapy; Image Enhancement; Image Processing, Computer-Assisted; Lymphatic Metastasis/radiotherapy; Neoplasm Recurrence, Local/diagnostic imaging; Neoplasm, Residual/diagnostic imaging; Otorhinolaryngologic Neoplasms/drug therapy; Otorhinolaryngologic Neoplasms/radiotherapy; Positron-Emission Tomography; Tomography, X-Ray Computed; Adult; Aged; Aged, 80 and over; Carcinoma, Squamous Cell/diagnostic imaging; Carcinoma, Squamous Cell/pathology; Combined Modality Therapy; Disease Progression; Female; Fluorodeoxyglucose F18; Follow-Up Studies; Humans; Lymphatic Metastasis/diagnostic imaging; Lymphatic Metastasis/pathology; Male; Middle Aged; Neck Dissection; Neoplasm Staging; Otorhinolaryngologic Neoplasms/diagnostic imaging; Otorhinolaryngologic Neoplasms/pathology; Radiotherapy, Conformal; Sensitivity and Specificity