Inverted papilloma

An inverted papilloma is an uncommon sinonasal tumour with distinctive pathological and imaging features.

The term inverted papilloma is also used to describe a urothelial lesion. For a discussion of that entity please refer to inverted papilloma of the urinary tract ⁴.

Epidemiology

Inverted papillomas account for approximately 0.5-4.0% of all nasal tumours and are most frequently seen in patients 40-60 years of age ^2,6. There is a significant predilection for males (M:F of ~  3-5:1) ².

Clinical presentation

Presentation can be similar to other sinonasal masses, with nasal obstruction, sinus pain, and epistaxis.

Pathology

Macroscopically inverted papillomas appear as irregular polypoid masses of variable consistency, pink in colour, with a tendency to bleed ¹.

Histology is that of ribbons of respiratory epithelium enclosed by basement membrane which grows into the subjacent stroma (thus with an inverted pattern) with characteristic micro-mucous cysts ^1,3. Approximately 20% demonstrate some keratinisation, and 10% dysplasia ³.

Malignant transformation occurs into a variety of histologies, including keratinising and non-keratinising squamous cell carcinoma (by far the most common, seen in approximately 11% of cases) as well as much less frequently other malignant histologies including mucoepidermoid carcinoma, verrucous carcinoma and adenocarcinoma ^1-3. Tumours may be either synchronous or metachronous ⁶.

Location

Inverted papillomas most commonly occur on the lateral wall of the nasal cavity, most frequently related to middle turbinate and maxillary ostium, although they are seen elsewhere in the nasal passage. As the mass enlarges it results in bony remodelling and resorption and often extends into the maxillary antrum ¹.

Due to the location, impairment of normal drainage of the maxillary antrum is common, although mucocoele formation is rare ¹.

Radiographic features

Plain film

No longer has a significant role play in the assessment of sinonasal disease. If obtained the most common finding is that of a nasal mass with associated opacification of the adjacent maxillary antrum ¹.

CT

CT features are largely non-specific, demonstrating a soft tissue density mass with some enhancement. The location of the mass is one of the few clues toward the correct diagnosis.

Calcification is sometimes observed which is helpful, as is focal hyperostosis which tends to occur at the site of tumour origin ⁵. This is useful not only in suggesting the diagnosis, but also to aid surgical planning, as the location of tumour origin determines the extent of surgery required.

As the mass enlarges bony resorption and destruction may be present, with similar pattern to that seen in patients with squamous cell carcinoma ².

MRI

MRI often demonstrates a distinctive appearance, referred to as convoluted cerebriform pattern seen on both T2 and contrast enhanced T1 weighted images. This represents alternating lines of high and low signal intensity, the appearance of which has been likened to, albeit loosely, cerebral cortical gyrations. This sign is seen in 50-100% of cases, and is uncommon in other sinonasal tumours ⁶.

• T1: isointense to muscle
• T2:
  • generally hyperintense to muscle
  • alternating hypointense lines ⁶
• T1 C+ (Gd)
  • heterogeneous enhancement ²
  • alternating hypointense lines ⁶

DSA - angiography

Angiography has no significant role to play in the diagnosis or assessment of inverted papillomas. If performed these tumours are mostly avascular ¹.

Treatment and prognosis

Due to the high association with malignancy (see above) and their unlimited growth potential, inverted papillomas have historically been resected en bloc with the lateral nasal wall (medial maxillectomy) via an external incision ².  Increasingly advances endoscopic techniques have been used to limit the size of resection, and localising the site of tumour origin is necessary. This is often only possible at the time of surgery but can be suggested by the presence of focal hyperostosis ⁵.

Recurrence rates are nonetheless high (15-78%) and are usually attributed to incomplete local resection ^2,5.

Other than malignant transformation and recurrence, morbidity stems from local growth which can be extensive extending to adjacent spaces including the orbit and intracranial compartment.

Differential diagnosis

General imaging differential considerations include

• sinonasal carcinoma -unfortunately imaging is unable to confidently distinguish between inverted papillomas, inverted papilloma with malignancy and pure malignancy
• antrochoanal polyp - only peripheral enhancement (if any)
• inflammatory polyp - only peripheral enhancement (if any)
• juvenile nasopharyngeal angiofibroma (JNA)
• olfactory neuroblastoma
• paranasal sinus mucocoele