Intraductal oncocytic papillary neoplasm （IOPN） exhibits characteristic histological features.　Despite its tumor grade, the prognosis of IOPN is relatively favorable.　Unlike intraductal papillary mucinous neoplasm （IPMN）, IOPN rarely harbor genetic alterations of KRAS or GNAS, which are common driver alterations in IPMN.　Instead, rearrangements involving PRKACA or PRKACB have been identified as the driver genomic alterations of IOPN.　Therefore, IOPN is classified as a distinct entity separate from IPMN.　Based on these characteristic genomic alterations, the histological diversity of IOPN has been revealed.　Distinguishing between IOPN and IPMN is important, and it is essential to recognize the morphological variations of IOPN.

Epulis, a gingival mass, is a clinical umbrella term for benign reactive proliferative lesions.　Although frequently encountered in dental and oral surgical practice, its recognition in general medical settings remains limited, and its detailed pathological features and disease concept are not widely understood.　This article aims to clarify the histopathology of this lesion, which has traditionally been regarded as unique to the gingiva, including its historical background.　Furthermore, it demonstrates that each type of epulis is not an independent disease confined to the gingiva, but corresponds to well-recognized histopathological entities.

Severe Fever with Thrombocytopenia Syndrome （SFTS） is a tick-borne viral hemorrhagic fever caused by the SFTS virus （SFTSV）.　In recent years, fatal SFTS cases in cats have been reported, exhibiting pathological features strikingly similar to those in humans.　Moreover, instances of animal-to-human transmission have also been documented, highlighting the necessity of a One Health approach to address this zoonotic disease.　This review integrates and compares histopathological findings from human and feline SFTS cases to provide a comprehensive understanding of the pathogenic mechanisms underlying this emerging viral infection.　Furthermore, the pathological study on SFTS emphasizes the vital necessity of performing pathological analyses from both human and veterinary viewpoints in the investigation of the pathogenesis of zoonoses such as SFTS.　Pathological research on emerging infectious diseases is crucial for establishing the fundamental components of infectious disease control:diagnostics, treatment, and prevention.　Considering that numerous emerging infectious diseases with pandemic potential are zoonotic, it is essential to establish a research framework for “One Health infectious disease pathology” that promotes collaboration between human and veterinary pathology to prepare for future emerging infectious disease threats.

Oral mucosal lesions show various gross findings.　In most cases, clinicians perform biopsies to confirm the diagnoses of epithelial neoplastic lesions, including oral squamous cell carcinoma and oral epithelial dysplasia, or to exclude them.　Therefore, pathologists should consider various possibilities as well as inflammatory lesions, odontogenic tumors, salivary gland tumors, and others for the pathological diagnosis.　In this review, we are showing current trends of oral epithelial tumors and remaining challenges, followed by differential diagnosis or pitfalls in routine pathological diagnosis.

Mucinous adenocarcinoma is a salivary gland carcinoma characterized by mucin production without the features of other salivary gland tumor types.　The patient was an elderly man in his 80s with masses on the right upper lip and right submandibular area.　Histologically, both tumors were adenocarcinomas showing papillary and tubular growth patterns composed of tall columnar epithelial cells.

Immunohistochemically, the tumor cells were positive for NKX3.1, MUC5AC and MUC5B, and showed diffuse, strong positivity for p53.　Sanger sequencing revealed an AKT1 p.Glu17Lys gene variant.　Although Alcian blue-PAS-positive-mucinous cells were limited to 10% of the tumor cells, this case was diagnosed as mucinous adenocarcinoma originating from the right upper lip after excluding other tumor types.

Mucinous adenocarcinoma demonstrates a broad histological spectrum, with varying mucinous cell ratio.　The update of diagnostic criteria and tumor classification are warranted.

Branchioma is a rare benign soft tissue tumor thought to originate from the branchial arches of the neck.　The patient was a previously healthy woman in her 50s who presented with a mass in the left clavicular region.　A biopsy was performed but failed to yield a definitive diagnosis, leading to marginal excision of the tumor.　Histologically, the lesion was composed of spindle cells, epithelial nests, and adipocytes, and was diagnosed as branchioma based on these features.　Formerly referred to as ectopic hamartomatous thymoma （EHT）, the tumor was reclassified and named “branchioma” in the 2024 WHO classification.　While diagnosis is relatively straightforward when the entire tumor is excised, it can be challenging when only a limited sample is obtained by biopsy.　Although rare, its predilection for the supraclavicular region highlights the importance of including branchioma in the differential diagnosis of subcutaneous tumors in this area.

Pseudoinvasion/epithelial misplacement （PEM） generally refers to a tubular adenoma that has deviated into the submucosa.　PEM is generally seen in the sigmoid colon and descending colon, rarely in the other colon resion.　We report a case of PEM arising in the transverse colon.　The patient was an 83-year-old man, visited his local doctor with a chief complaint of anemia.　A close examination revealed tumors on the anterior wall of the middle body of the stomach and on the hepatic flexure of the transverse colon, respectively, and the resection was performed for these tumors as double cancer.　The gastric tumor was an adenocarcinoma, whereas the tumor of the transverse colon showed a tubular adenoma that deviated into the submucosa, so-called PEM.

A woman in her 80s received immune checkpoint inhibitor （Nivolumab） for gastric cancer.　She was found in cardiopulmonary arrest at home 83 days after receiving Nivolumab.　Upon arrival at the hospital, an electrocardiogram showed ventricular fibrillation, and a coronary angiography ruled out myocardial infarction.　Autopsy revealed lymphocytic myocarditis with prominent infiltration of CD8-positive T cells, which was consistent with the histology of immune check point inhibitor-associated myocarditis previously reported.

The patient was a female in her 50s.　A mass lesion with bone destruction was found in the right maxilla.　Macroscopically, the mass was grayish-white in color with indistinct borders with the surrounding area.　Histologically, atypical large cells with well-defined nucleoli and eosinophilic cytoplasm were proliferating in a cord-like or solitary arrangement, similar to epithelioid rhabdomyosarcoma.　However, atypical cells showed positive staining of ALK, and FISH revealed TFCP2 split signals and FUS-TFCP2 fusion signals, which confirmed the diagnosis of rhabdomyosarcoma with TFCP2 rearrangement.

The patient was an 8-year-old girl.　She underwent right lobectomy and lymph node dissection for right thyroid follicular carcinoma, and lymph node metastasis was found from the time of the initial surgery.　Genetic testing detected a somatic DICER1 mutation.　Germline DICER1 mutations were not tested.　After surgery, the patient developed progressive lymph node metastasis and recurrence, requiring additional treatment.　This case is noteworthy in that it is the first reported case of pediatric follicular thyroid cancer with a DICER1 mutation and lymph node metastasis.　It is also the third youngest reported case of follicular thyroid cancer with a DICER1 mutation.