Benign mixed tumour, also referred to as pleomorphic adenoma, is the most common benign salivary gland tumour, of which approximately 80% occur in the parotid gland, 10% in the submandibular glands and 10% in the minor salivary gland of the oral cavity. This tumour is most often observed in 30 to 60 years old patients and is more frequent in women than in men.2
It is usually a solitary slow growing painless mass.
Pleomorphic adenomas are known for their morphologic and architectural variability. They share common features of epithelial, myoepithelial and mesenchymal components. The proportion of each of these elements can vary widely.1
The stromal component of these tumours is most often predominantly myxoid with focal chondroid or fibrous aspects. Modified myoepithelial cells are thought to play an important role in the histopathological changes of the stroma. Sometimes squamous or osseous metaplasia is found in pleomorphic adenoma but is encountered less frequently. On a retrospective study on 83 pleiomorphic adenomas over a period of five years, we found an overall prevalence of 5% of metaplasia (unpublished data
Squamous cell metaplasia in pleomorphic adenoma is an uncommon and most often accidental finding.3–6
Squamous cell metaplasia has been reported as a potential pitfall in fine-needle aspiration cytology (FNAC) of pleomorphic adenoma, which can be confused with a squamous cell or mucoepidermoid carcinoma. Different hypotheses have been advanced for its etiopathology. Firstly, it may be a repair process following FNAC, as reported by Li et al.7
In a series of 10 benign parotid lesions following FNAC, they found 8 cases of squamous cell metaplasia. In our case, we did not have any proof of a previous FNAC. A second hypothesis could be intra-lesional ischemia as it can be found in chronic sialadenitis or necrotizing sialo-metaplasia. Experimental data support this hypothesis, squamous metaplasia can be induced in rat salivary glands by arterial ligation.8
FNAC can also induce necrosis and then ischemia, which may support this last explanation.
Osseous metaplasia can be found in all salivary gland locations such as the parotid, submandibular and minor salivary glands. In some cases an important chondroid matrix was present with the formation of enchondral ossification at the borders.9
Others described the formation of osteoid indicating a possible direct differentiation from myoepithelial cells10,11
Osseous metaplasia can also be found in carcinoma ex pleomorphic adenoma12,13
and mixed tumour of the skin.14,15
To our knowledge only five cases of schwannoma-like pleomorphic adenoma were reported in the English literature.16–18
Previous cases included four women and one man, aged from 39 to 75 years (). The majority of the lesions, as ours, were located in the parotid gland with one exception in the hard palate. All cases including this one had palisading areas of spindle-shaped cells in an otherwise classical pleomorphic adenoma.
Schwannoma-like features in pleomorphic addenoma. Review of the literature.
Spindle cell tumours are rare in the salivary glands, representing from 1.9% to 5% of parotid neoplasms.1,19,20
Differential diagnoses of benign spindle cell tumours in salivary glands include neurogenic tumours: schwan-noma, composed of areas with Verocay bodies and neurofibroma, as well as smooth muscle proliferations: leiomyoma.
Immunohistochemistry (IHC) may solve this dilemma. Leiomyomas are negative for cytokeratin positive for smooth muscle markers including alpha-smooth muscle actin, desmin and caldesmon, whereas benign neurogenic tumours, also negative for cytokeratine, express neurogenic marker (S100-Protein, CD57 and neurofilament).
Shwannoma-like pleomorphic adenomas are composed of modified myoepithelial cells expressing p63, CD10 and cytokeratin by IHC.16
Merino et al.
demonstrated the presence of desmosomes in these spindle-shaped cells by electron microscopy confirming a myoepithelial origin.17
The pathogenesis of this phenomenon could be a result of plasticity of myoepithelial cells.
Fine needle aspiration cytology is an important diagnostic procedure used to evaluate salivary gland lesions and to help in their pre-operative management. Typically, FNAC of pleomorphic adenoma shows a combination of cohesive epithelial cells in a pale myxoid matrix. When the cellularity is abundant and no matrix is identified this diagnosis is challenging. In the presence of spindle-shaped cells immunohistochemical analysis may be used to identify the myoepithelial phenotype.
Pleomorphic adenoma with schwanoma-like feature is a rare variant that could be confused with a schwannoma or a leiomyoma, but immunohistochemical study is helpful to differentiate these entities.