Sinonasal inverted papilloma

Sinonasal inverted papilloma (IP) is a benign epithelial neoplasm arising from the Schneiderian membrane—the ectodermal-derived mucosa lining the nasal cavity and paranasal sinuses.[1] It is the most common sinonasal papilloma subtype and is characterized by three clinically important properties: a tendency for local bone destruction, a high recurrence rate after incomplete resection, and potential for malignant transformation to squamous cell carcinoma (SCC).[1][2][3] The etiology remains incompletely understood; human papillomavirus (HPV) infection and activating EGFR mutations have each been implicated in subsets of tumors, and these two pathways appear to be mutually exclusive.[4][5] On imaging, IP typically presents as a unilateral soft-tissue mass centered on the lateral nasal wall with focal hyperostosis at the attachment site and, on MRI, a characteristic convoluted cerebriform pattern.[6][7] Complete surgical excision with treatment of the attachment site is the cornerstone of management.[1][8]

IP belongs to the Schneiderian papilloma family, which the WHO classification divides into three established histologic subtypes: inverted (endophytic), exophytic (fungiform), and oncocytic (cylindrical cell).[3][9] The inverted subtype is the most common. The term "inverted papilloma" reflects the endophytic growth of hyperplastic epithelium into the underlying stroma, in contrast to the exophytic architecture of fungiform papillomas.[10][11] Synonyms include Schneiderian papilloma—inverted type, transitional cell papilloma, and endophytic papilloma. Recent evidence supports recognition of a condylomatous sinonasal papilloma as a potential distinct (fourth) subtype, strongly associated with low-risk HPV and carrying a higher rate of malignant transformation.[12]

• Incidence: Approximately 0.6 per 100,000 per year; IP accounts for 0.5%–4% of all primary sinonasal tumors.[2][13]
• Age and sex: Peak incidence in the fifth through seventh decades, with a mean age at presentation of approximately 55 years. Male predominance, with male-to-female ratios reported between 2:1 and 5:1.[1][2]
• Anatomic distribution: The lateral nasal wall is the most common site of origin, particularly the region of the middle meatus and middle turbinate. The maxillary sinus is the most frequent paranasal sinus involved, followed by the ethmoid, frontal, and sphenoid sinuses. Bilateral involvement is rare. Ectopic locations (middle ear, nasopharynx, lacrimal sac) are reported but exceptional.[1][14][15][16]

Symptoms

• Unilateral nasal obstruction is the most frequent presenting complaint, often accompanied by rhinorrhea, epistaxis, or chronic sinusitis with nasal discharge.[1][2]
• Hyposmia or anosmia, headache, facial pressure, and lacrimation may occur with larger or more advanced lesions.[1]

Examination findings

Nasal endoscopy typically reveals a red-grey, exophytic, polypoid, and vascular mass arising from the lateral nasal wall.[2] The lesion may prolapse through the middle meatus or extend through the choana into the nasopharynx.[17]

Tempo and complications

IP grows slowly but progressively, with local bone destruction and centrifugal extension into adjacent sinuses.[1][17] Orbital or intracranial extension occurs in advanced cases. Synchronous SCC is found in approximately 5%–10% of resected specimens, and metachronous SCC may develop years after initial treatment.[3][4][13]

Red flags for malignant transformation

Epistaxis, rapid symptom progression, orbital wall involvement, and aggressive bone erosion on imaging should raise suspicion for coexistent or transformed SCC.[18][19]

Dental radiographic imaging plays a limited role in the primary evaluation of sinonasal IP, as the tumor arises above the alveolar process. However, large lesions extending inferiorly into the maxillary sinus floor or hard palate may produce findings on panoramic or periapical radiographs.

Location and distribution

IP characteristically presents as a unilateral soft-tissue mass centered on the lateral nasal wall and middle meatus, frequently extending into the ipsilateral maxillary sinus through a widened ostium.[14][17] On panoramic radiography, a large IP may appear as unilateral opacification of the maxillary sinus, occasionally with inferior displacement or remodeling of the sinus floor toward the alveolar crest.

Periphery and shape

The mass is typically lobulated or polypoid with smooth to gently undulating margins.[17][20] On CT, the periphery is generally well-defined against adjacent sinus mucosa, though the interface with retained secretions may be indistinct without contrast enhancement.

Internal architecture

• Soft-tissue density: On CT, IP appears as a homogeneous to mildly heterogeneous soft-tissue-density mass, often indistinguishable from inflammatory polyps on unenhanced images.[6][14]
• Intralesional calcifications: Scattered punctate or linear calcifications within the mass are a recognized feature, described in early literature and attributed to bone morphogenetic protein activity. These calcifications may radiate from the attachment site in a linear pattern, a finding that can help localize the tumor origin.[6]
• Focal hyperostosis: A hallmark CT finding, focal hyperostosis (cone-shaped bone thickening) at the tumor attachment site is present in the majority of cases and serves as a reliable marker of the tumor's point of origin. Nasal cavity IPs, particularly those with septal attachment, are less likely to exhibit bony changes on CT.[6][21][22]

Effects on adjacent structures

• Bone remodeling and erosion: IP commonly causes pressure remodeling of adjacent sinus walls with smooth expansion. Bone erosion, including thinning or discontinuity of sinus walls, is seen in a subset of cases and correlates with higher Krouse stage. Aggressive, irregular bone destruction should raise concern for coexistent malignancy.[18][23][24]
• Dental effects: When the tumor extends to the maxillary sinus floor, displacement of tooth roots or loss of the lamina dura may occasionally be observed on periapical or panoramic radiographs. Frank root resorption is uncommon and, if present, should prompt consideration of a more aggressive process.

Growth pattern and extent

IP grows centrifugally from its attachment site, filling the nasal cavity and extending into contiguous sinuses through natural ostia or eroded walls.[17] Extension through the choana into the nasopharynx is characteristic of advanced lesions. Orbital or intracranial extension defines Krouse T4 disease.[1]

Recognized variants

Ossifying IP is an uncommon variant in which intralesional ossification produces oval or striped bone-density foci with a cortical rim and central fatty marrow signal on MRI.[25] This variant can mimic ossifying polyps on CT, though MRI features—particularly the cerebriform pattern and moderate enhancement—help distinguish the two entities.[25]

CT (medical)

Non-contrast CT is the standard initial modality for evaluating IP extent and bony changes. The tumor appears as a unilateral soft-tissue-density mass with variable enhancement after contrast administration.[6][17] Focal hyperostosis at the attachment site is the most diagnostically useful CT finding, present in approximately 60%–65% of cases, with concordance between hyperostosis location and the true surgical attachment point in roughly 90% of cases.[21][22] Bone erosion on CT is an independent predictor of malignant transformation.[24]

MRI

MRI is complementary to CT and superior for soft-tissue characterization, tumor mapping, and distinguishing tumor from retained secretions or inflammatory mucosa.[7][14]

• Convoluted cerebriform pattern (CCP): The signature MRI feature of IP consists of alternating curvilinear bands of high and low signal on T2-weighted and contrast-enhanced T1-weighted sequences, reflecting the histologic architecture of inverted epithelial folds separated by fibrovascular stroma. CCP is present in virtually all benign IPs and is highly specific, though it can occasionally be seen in certain malignant sinonasal tumors. Loss of CCP within a mass strongly suggests coexistent or transformed SCC.[7][18][26][27]
• Signal characteristics: IP is typically isointense to slightly hyperintense relative to muscle on T1-weighted images and intermediate signal on T2-weighted images, with solid inhomogeneous enhancement after gadolinium.[28]
• Diffusion-weighted imaging (DWI): Benign IP demonstrates higher ADC values (mean ~1.49 × 10⁻³ mm²/s) compared with IP-transformed SCC (mean ~1.12 × 10⁻³ mm²/s), providing a quantitative tool for predicting malignant transformation.[18][27]
• Dynamic contrast-enhanced MRI: A persistent or plateau-type time-intensity curve (TIC) favors benign IP, whereas a washout-type TIC is an independent predictor of malignancy.[24][29]

PET/CT

IP is FDG-avid, which limits the specificity of PET/CT for distinguishing benign IP from malignant sinonasal tumors.[30] PET/CT may have a role in staging when malignant transformation is confirmed.

The differential diagnosis of a unilateral sinonasal soft-tissue mass with bone remodeling centers on distinguishing IP from inflammatory, neoplastic, and vascular entities. The combination of focal hyperostosis on CT and the cerebriform pattern on MRI is the most discriminating feature set favoring IP.

The highest-risk diagnostic pitfall is failing to identify foci of SCC within or adjacent to an IP. Because biopsy may under-sample the tumor, imaging features—particularly loss of the cerebriform pattern, aggressive bone erosion, necrosis, and low ADC—should prompt thorough intraoperative sampling or complete excisional biopsy.[18][32]

Diagnostic microscopic features

The hallmark of IP is endophytic invagination of hyperplastic, multilayered epithelium (typically 5–10 cells thick) into the underlying edematous or fibrous stroma, enclosed by an intact basement membrane.[10][34] The surface epithelium shows squamous metaplasia with scattered residual ciliated columnar cells. The stroma contains a variably dense lymphoplasmacytic and neutrophilic inflammatory infiltrate.[34][35] Microcysts containing mucin or macrophages are commonly interspersed within the inverted epithelial nests.[11] Mitotic activity is generally low, and significant cytologic atypia is absent in benign IP.[10][34]

Immunohistochemistry

• Overexpression of p63 and CK5 reflects the extensive squamous epithelial remodeling characteristic of IP.[35]
• The CD163 "circle sign"—macrophages arranged in circular patterns within the surface epithelium—can help distinguish sinonasal papillomas from morphologically similar non-neoplastic lesions such as inflammatory polyps with squamous metaplasia, with high specificity.[36]

Dysplasia and malignant transformation

Dysplasia within IP exists on a spectrum and, when present, is associated with increased recurrence risk.[37] Transformation to SCC—most commonly keratinizing type—occurs synchronously or metachronously in approximately 5%–15% of cases.[3][4][13] Histologic features of the papilloma itself do not reliably predict which lesions will transform.[3]

Molecular findings

• EGFR mutations: Activating mutations in EGFR exon 19 or 20 are found in the majority of HPV-negative IPs and may represent a targetable pathway.[4][38]
• HPV: Low-risk HPV (types 6 and 11) RNA is detected in a subset of IPs, particularly those with condylomatous morphology; these cases are EGFR wild-type and may carry a higher risk of malignant transformation. High-risk HPV is generally absent in IP but may be found in a subset of associated SCCs.[4][5][12]
• Smoking and HPV have been identified as risk factors for malignant transformation in meta-analysis.[39]

Preoperative workup

• Nasal endoscopy with biopsy is the initial diagnostic step when IP is suspected.[19][40]
• CT defines bony anatomy, identifies focal hyperostosis to localize the attachment site, and maps tumor extent for surgical planning.[21][22]
• MRI is recommended for soft-tissue characterization, distinguishing tumor from inflammatory changes, evaluating for mal