We report a case of adenoid cystic carcinoma of the oral floor that was diagnosed nearly 3 years after the patient’s first visit to our institution. A 43-year-old woman presented with a 4-month history of pain in the tongue and left submandibular region. Clinical examination revealed palpable induration in the left sublingual and submandibular regions. Ultrasonography showed heterogeneous echoes in the left submandibular gland; however, computed tomography (CT) and serological examination revealed no abnormalities. Fine-needle aspiration cytology (FNAC) did not reveal any atypical cells. Ultrasonography performed after 6 months showed no changes, and the patient was discharged from the clinic. Subsequently, the patient experienced numbness of the tongue and difficulty in moving it and was examined again. Repeat FNAC of the sublingual region revealed a class III result, and CT/positron emission tomography-CT indicated oral cancer with lung metastasis (T4aN2bM1). Rapid pathological analysis of the cervical lymph nodes also led to the diagnosis of cervical lymph node metastasis from the adenoid cystic carcinoma. We performed resection of the left side of the oral floor, left hemiglossectomy, marginal resection of the mandible, bilateral neck dissection, and reconstruction using a pectoralis major myocutaneous flap. Histological examination of the surgical margin was positive for cancer cells microscopically, and the patient was initiated on postoperative chemoradiotherapy. Since then, there has been no evidence of local recurrence; however, the patient was initiated on pembrolizumab treatment due to an increase in the size of the lung metastasis. The probability of malignancy in sublingual tumors is high, and regular follow-up should be continued even when no definitive diagnosis has been made.
