The 5th edition of the WHO Classification of Head and Neck Tumours was published online in 2022. The edition introduces new lists of oral potentially malignant diseases （OPMDs） and HPV-associated oral epithelial dysplasia, which are now classified as separate diseases from conventional oral epithelial dysplasia （OED）. In the 5th edition, the three-tiered classification system for OED is maintained, but the binary system is also mentioned. In addition, the 5th edition adds histological features that consider the proliferative potential and differentiation of cells concerning the structural and cytological characteristics seen in OED. Squamous cell carcinoma was listed at the beginning of the chapter on “Tumours of the oral cavity and mobile tongue” in the 4th edition of the WHO classification, but in the 5th edition, it is moved to the end of the chapter. Verrucous carcinoma and carcinoma cuniculatum, which have been treated as subtypes of squamous cell carcinoma, are now listed as separate items based on their characteristic clinical and pathological features, as well as pathogenesis. This report outlines the changes made to the classification of oral epithelial tumors in the 5th edition of the WHO Classification of Head and Neck Tumours.

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It is well known that the World Health Organization （WHO） Classification of Head and Neck Tumours is an essential tool and glossary for standardizing diagnostic practices worldwide. This report provides an overview of the revisions to the WHO Classification of Head and Neck Tumours （5th edition） regarding oral potentially malignant disorders and oral epithelial dysplasia related to superficial lesions among epithelial tumors in the oral cavity. In 2017, the fourth edition of the WHO Classification of Head and Neck Tumours described oral potentially malignant disorders for the first time. A major topic at that time was the integration of the concepts of previous precancerous conditions and precancerous lesions. In this revision, three disorders were deleted, one was incorporated into a familial cancer syndrome, and three new disorders were added: proliferative verrucous leukoplakia, oral lichenoid lesions, and oral graft-versus-host disease. Oral epithelial dysplasia is defined as a lesion with oral potentially malignant disorders and carries a risk of malignant transformation to squamous cell carcinoma. The grading of oral epithelial dysplasia has become a histopathological evaluation tool for potentially malignant disorders of the oral cavity. On the other hand, the revisions to oral epithelial dysplasia may be interpreted as contradictory. We outline the issues and the Oral Superficial Cancer Working Group’s thinking on oral epithelial dysplasia.

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Papillary thyroid cancer （PTC） and oral cancer share a common lymph node affiliation. In patients with both types of cancers, it may be difficult to preoperatively identify which cancer has metastasized to the cervical lymph nodes. The treatment of oral cancer with detected PTC metastases in the cervical lymph nodes is controversial. Here, we report three cases of incidentally detected PTC in neck dissection specimens of patients with oral cancer.
Case 1：A 61-year-old woman with squamous cell carcinoma （SCC） of the right side of the tongue （cT4aN2cM0） underwent subtotal glossectomy, bilateral conservative neck dissection, and reconstruction using a rectus abdominis musculocutaneous flap. No cervical lymph node metastases of SCC were found, but five cervical lymph node metastases of PTC were found.
Case 2：A 79-year-old woman with SCC of the right mandibular gingiva （cT4aN0M0） underwent segmental mandibulectomy, right-sided conservative neck dissection, and reconstruction with a rectus abdominis musculocutaneous flap and titanium plate. Cervical lymph node metastases of SCC and PTC were observed.
Case 3：A 61-year-old woman underwent conservative right-sided neck dissection after surgery for mucoepidermoid carcinoma of the right lower lip. Cervical lymph node metastasis of mucoepidermoid carcinoma and PTC were observed.
The present study suggests the significance of performing histological diagnosis, including fine-needle aspiration of the thyroid gland and cervical lymph nodes, when PTC is suspected during systemic examination of patients with oral cancer.

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Angiomatoid fibrous histiocytoma （AFH） is a soft tissue tumor of intermediate malignancy that occurs most frequently in the extremities, but rarely occurs in the oral cavity. We report a case of the myxoid variant of AFH that occurred in the anterior mandibular region and was difficult to diagnose. The patient was a 17-year-old male. He was referred to our department with a swelling of gingiva of the right mandibular anterior teeth. An elastic soft mass of 20 mm diameter with a clear border and normal surface mucosa was found in the lingual gingiva of the right mandibular anterior teeth. No root resorption was observed on panoramic X-ray images, but resorption of the lingual cortical bone and cancellous bone between the right lower teeth was observed on CT images. MRI showed low signal intensity on T1-weighted images and high signal intensity on T2-weighted images. A biopsy was performed but no definitive diagnosis was made, and it was reported to be a myxoid tumor. Tumor resection was then performed under general anesthesia, and a diagnosis of AFH was made. Two years have passed since the operation, and no recurrence or metastasis has been observed. This tumor is difficult to distinguish from other tumors based on clinical and imaging findings, and because it exhibits a variety of histopathological findings, a definitive diagnosis cannot always be obtained from a biopsy. FISH is required for a definitive diagnosis, and was used to make a definitive diagnosis in the present case.

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