Background: Advanced head and neck squamous cell carcinomas frequently develop distant metastases to limited organs, including the lungs, bone, mediastinal lymph nodes, brain, and liver. Peritoneal carcinomatosis as an initial distant metastasis from hypopharyngeal squamous cell carcinoma is quite rare. Case presentation: A 75-year-old man diagnosed with hypopharyngeal squamous cell carcinoma and his clinical stage was determined as T2N2cM0. Notably, the right retropharyngeal lymph node surrounded more than half of the right internal carotid artery. Concomitant conformal radiation therapy was administered for the primary hypopharyngeal lesion, and the whole neck and tumor response was evaluated at this point according to our algorithm-based chemoradioselection protocol. As the tumor responses at both the primary and lymph nodes were poor, with the right retropharyngeal lymph node in particular demonstrating mild enlargement, we performed a radical surgery: pharyngolaryngectomy, bilateral neck dissection, and reconstruction of the cervical esophagus with a free jejunal flap. Then, postoperative CRT was performed. During these therapies, the patient developed a fever and mild abdominal pain, which was associated with an increased C-reactive protein level. Contrast-enhanced computed tomography from the neck to the pelvis demonstrated mild peritoneal hypertrophy and ascites with no evidence of recurrent and/or metastatic tumor formation. We initially diagnosed acute abdomen symptoms as postoperative ileus. However, cytological examination of the refractory ascites resulted in a diagnosis of peritoneal carcinomatosis. Owing to rapid disease progress, the patient died 1.5 months after abdominal symptom onset. Conclusions: The present case is the second reported case of head and neck squamous cell carcinoma with peritoneal carcinomatosis as an incipient distant metastasis. Therefore, peritoneal carcinomatosis should be considered a differential diagnosis when acute abdomen is noted during treatment for head and neck cancers.
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