Discussion

A high level of clinical suspicion for malignancy should be maintained for patients presenting with a unilateral polyp. The first step is to perform a thorough history and examination, including full rhinological and endoscopic examinations of the rest of the upper aerodigestive tract, as well as a neck examination. The history and examination findings of our patient were in keeping with a sinonasal benign inverted papilloma. Benign papilloma is the commonest cause of unilateral polyps, especially in the younger age group. Benign sinonasal lesions are related to allergens, air pollution, and human papillomavirus types 6 and 11, whilst malignant lesions tend to be associated with tobacco, alcohol and occupational exposure to heavy metal particles. Nevertheless, it is impossible to separate benign from malignant lesions by history and clinical examination alone.^{Reference Lund, Clarke, Swift, McGarry, Kerawala and Carnell3} Furthermore, benign inverted papilloma carries a 9 per cent risk of malignant transformation and so requires further investigation.^{Reference Nygren, Kiss, von Buchwald and Bilde4}

Following history-taking and clinical examination, the next investigation recommended in the UK National Multidisciplinary Guidelines for Head and Neck Cancer is CT or MRI.^{Reference Lund, Clarke, Swift, McGarry, Kerawala and Carnell3,Reference Dragonetti, Gera, Sciuto, Scotti, Bigoni and Barbaro5} The CT scan of the sinuses showed an expansile left-sided sinonasal mass in the left inferior turbinate that was associated with bone remodelling, but there was no infiltration of adjacent structures. This would be in keeping with a more benign diagnosis, but is not characteristic of a specific condition. There are few reports of CT or MRI findings for myoepithelioma given the rarity of disease. One study reported that a non-enhanced CT scan of myoepithelioma typically shows a well-circumscribed lesion with homogeneous attenuation.^{Reference Yue, Feng, Ning, Han and YaHong6} Magnetic resonance imaging findings demonstrate high intensity in T2-weighted and low intensity in T1-weighted images.^{Reference Hiwatashi, Matsumoto, Kamoi, Yamashita and Nakashima7,Reference Monzen, Fukushima and Fukuhara8} The MRI scans of the sinus and neck demonstrated a sinonasal tumour extending into the left maxillary sinus and ethmoid cells, with an area of high T2-weighted signal in the mandible and nasolacrimal sac. However, neither the CT nor MRI findings are specific to myoepithelioma, and they are also in keeping with benign cartilaginous tumours.

A biopsy was then taken for histopathological analysis, to help characterise the nature of the lesion. Microscopic analysis showed a chondroid matrix with atypical nuclei, which was initially thought to be a grade 2 chondrosarcoma. Interestingly, the tumour was also positive for chondrosarcoma in the posterior and anterior margins. Chondrosarcomas are most often found in the long bones, pelvis and ribs. They constitute 15 per cent of all primary malignant bone tumours, but are extremely rare in the sinonasal region. Previous cases have recognised that cartilaginous tumours are difficult to differentiate prior to surgery. Often chondrosarcoma is misdiagnosed as chondromyxoid fibroma or chordoma.^{Reference Lee, Jung, Yoon, Lee, Joo and Lim9}

Chondromyxoid fibroma is also a rare tumour; it constitutes 0.5 per cent of all primary lesions, and occurs in the head and neck region in less than 5 per cent of cases. Similar to chondrosarcoma, chondromyxoid fibroma is most typically found in the metaphysis of long bones, in particular the femur and tibia.^{Reference Rahimi, Beabout, Ivins and Dahlin10}

Immunochemistry analysis of chondromyxoid fibroma is usually positive for vimentin, GFAP, SOX9 and S100. However, the tumour in the case reported here was only positive for GFAP, EMA, S100 and AE1/AE3, which is more commonly seen in myoepithelial tumours.^{Reference Veras, Santamaria and Luna11} Myoepithelioma is a rare benign lesion that accounts for 1 per cent of salivary gland neoplasias, and most commonly affects the parotid. Myoepithelial cells are found in both acini and intercalated ducts, and play an important role in glandular excretion.

A recent review of English-language literature, published in 2019, revealed only 12 previous cases of sinonasal myoepithelioma,^{Reference Kang, Im, Song and Jung12} and there have been no subsequent publications of case reports in the literature since this review. Patients present with chronic nasal obstruction without secondary symptoms and with a painless mass on examination. Diagnosis requires immunohistochemistry analysis; tumours are typically positive for S100, calponin, GFAP, vimentin, EMA and carcinoembryonic antigen (CEA).^{Reference Sayed, Kazi, Jagade, Palav, Shinde and Pawar13} The mainstay treatment is surgical resection, performed either as an open or endoscopic procedure.^{Reference Cunha, Camurugy, Cavalcante Ribeiro, Costa, Azevedo and Vinhaes14} The risk of recurrence is extremely uncommon in most cases. However, cases with positive excision margins have been reported to have disease recurrence on follow up. Therefore, wide healthy margins on excision are necessary to avoid recurrence where possible, and close follow up is advised.^{Reference Jafarian, Omidi, Roshan, Montazer and Joushan15}

A study focusing on the clinicopathological profile of sinonasal masses found variations between clinical, radiological and pathological findings in 3.63 per cent of cases.^{Reference Bist, Varshney, Baunthiyal, Bhagat and Kusum16} In our case, the patient presented with a unilateral nasal polyp that clinically behaved in a similar manner to a benign inverted papilloma. Initially, histology suggested a grade 2 chondrosarcoma, but, on further specialist review, a chondromyxoid fibroma or myoepithelioma was thought to be more likely. On balance, after further immunohistochemical analysis, myoepithelioma with diffuse chondroid differentiation was the preferred diagnosis. This uncertainty in diagnosis can lead to mismanagement in terms of patient care. Knowledge of the tumour type is important for calculating the risk of recurrence, the extent of resection and the plans for post-operative management. For example, adjuvant proton radiotherapy may play a role in treating small volume recurrent or residual chondrosarcomas in high-risk areas such as the brainstem or cranial nerves, whereas myoepitheliomas are both radio- and chemo-insensitive.^{Reference Lund, Clarke, Swift, McGarry, Kerawala and Carnell3,Reference Bist, Varshney, Baunthiyal, Bhagat and Kusum16}