lymphoma. His condition improved after the administration of CODOX-M/IVAC chemotherapy, and subsequent PET-CT imaging showed there had been a complete response. However, the lesion recurred, and he died one year after his first visit. Narrowing the differential diagnosis in the present case would have been difficult, especially given that the invisible malignant lymphoma occurred in the head and neck area. However, if a patient has a subjective symptom such as mental paresthesia, when cancer cannot be ruled out clinically, the tumor may be a hematopoietic malignancy, a malignant lymphoma, or possibly a distant metastasis of another organ malignancy. In such cases, performing an early general examination such as a PET-CT may lead to a more rapid diagnosis.