A case of a large irritation fibroma of the hard palate with immunohistochemical analysis

We report a rare case of a large irritation fibroma of the hard palate in a 63-year-old Japanese man. The patient had been aware of a small, asymptomatic palatal mass for over 10 years, which gradually enlarged and eventually caused a pronunciation disorder. Clinical examination revealed a well-defined, broad-based, fibrous lesion measuring 52 × 34 × 20 mm with no signs of ulceration, infection, or bone involvement. Radiography and contrast-enhanced computed tomography confirmed a solid, uniformly enhanced mass without adjacent tissue invasion. Biopsy under local anesthesia led to a diagnosis of irritation fibroma. The lesion was completely excised under general anesthesia, including the periosteum at its base. Histopathological analysis revealed thick collagen fiber proliferation and numerous capillaries. Immunohistochemically, the lesion was positive for vimentin and negative for CD34 and α-SMA. Ki-67 labeling index was less than 1%, indicating low proliferative activity. No recurrence or functional impairment was observed during follow-up. Large irritation fibromas exceeding 30 mm are extremely rare, with only six cases, including the present one, identified in the available literature. Five of these cases occurred in the palate, suggesting that the anatomical characteristics of the hard palate and repetitive compressive forces during mastication may contribute to excessive enlargement. All reported cases exhibited a prolonged disease duration, and the paucity of symptoms likely allowed the lesions to remain untreated and progressively increase in size. In addition to the accumulation of chronic mechanical irritation with aging, the present case also exhibited periodontitis, which may have acted as an endogenous factor promoting proliferative responses. These observations suggest that the development of large irritation fibromas may be influenced by multiple factors, including chronic irritation, aging, mucosal structural characteristics, and bacteriological factors. This case represents one of the largest lesions ever reported and provides valuable clinical and immunohistopathological insights.