Second primary tumor of Head and Neck is most commonly seen in malignancy of -

"The site of the index cancer influences the most likely site of a second primary malignancy. As an example, in a series of 1257 patients with squamous cell carcinoma of the head and neck, patients with an index malignancy arising in the larynx were more likely to develop a second primary cancer in the lung, while those arising in the oral cavity were more likely to develop a second primary in the head and neck or esophagus. This relationship has been observed in other studies as well."