Table 1. Comparison of histological characteristics of REAH, nasal polyps and inverted papillomas.

HPV: Human papillomavirus; REAH: Respiratory epithelial adenomatoid hamartoma.

Histological features	REAH	Nasal polyp	Inverted papilloma
Site	The majority occurs in the nasal cavity. Usually affects the nasal septum, especially in the posterior area. Usually unilateral.	Rarely show septal involvement. Usually bilateral.	Most common in nasal cavity and maxillary sinus. About a third of cases originate from multiple sites. Rarely bilateral.
Surface epithelium	Invaginates into submucosa and direct continuity with proliferating glands can be seen.	Surface invagination absent and characteristically basement membrane is thickened.	Multiple inversions of the surface epithelium into the underlying stroma.
Basement membrane thickening	Absent	Present	Absent
Glandular proliferation	Widely spaced, small- to medium-sized, round to oval glands separated by stroma (dominant feature). Glands are usually distended with mucus.	Glandular proliferation absent.	Multiple inversions of the surface epithelium into the underlying stroma with continuous, distinct and intact basement membrane.
Lining epithelium of glands	Respiratory epithelium often with admixed mucin-secreting (goblet) cells. Atrophic glands lined by single layer of flattened to cuboidal-epithelium may be present.	Glandular proliferation absent.	Proliferating squamous and/or respiratory cells with numerous microcysts (infiltration of epithelium by transmigrating neutrophils). Non-keratinizing squamous or transitional epithelium (5-30 cells thick), frequently predominates and is covered with a layer of ciliated columnar cells. Occasionally spare mitoses confined to the basal layers.
Basement membrane of glands	Hyalinization of variably thickened, eosinophilic basement membrane enveloping proliferating glands (characteristic finding).	Thickening of basement membrane of glands absent	Distinct and intact, continuous basement membrane.
Stroma	Oedematous or fibrous stroma containing mixed inflammatory cell infiltrate (plasma cells, lymphocytes)	Marked stromal oedema and mixed inflammatory cell infiltrate (eosinophils, plasma cells and lymphocytes), bland-appearing stromal fibroblasts and small- to medium-sized blood vessels	Either loose or dense, and maybe inflamed.
Possible additional findings	Co-existence with inflammatory polyps can occur. Tissues native to sinonasal tract or nasopharynx such as adipose tissue, bone, cartilage and chondromesenchymal tissue can occur rarely. Co-existence with Schneiderian papilloma (inverted type) and solitary fibrous tumour can occur rarely.	Atypical stromal cells, granulation tissue, granuloma formation, amyloid-like stroma can occur rarely.	Premalignant and malignant features: dysplasia, carcinoma in situ, invasive carcinoma can occur rarely. Thorough sampling and evidence seeking for malignant transformation should always be performed. HPV infection can be detected in a number of cases.