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pleomorphic adenoma ( pa ) , the most common salivary gland tumor , accounts for 54 - 65% of all salivary gland neoplasias and 80% of the benign salivary gland tumors . the incidence of it in intraoral minor salivary glands is 40 - 50% .
it is characterized by a wide spectrum of morphological patterns , including squamous cells , mucous cells , oncocytes , sebaceous cells , bone , adipose tissue , and crystalline materials .
, we present an unusual case of pa with extensive squamous metaplasia and keratin - filled cysts formation in a minor salivary gland , with special attention given to the potential sources of diagnostic pitfalls .
a 50-year - old man presented with a mass in the right retromolar region that had been slowly enlarging over the previous 2 years with no pain .
physical examination revealed a firm mass measuring 1 cm  1 cm [ figure 1 ] .
grossly tumor was well - encapsulated , grayish - white , and measured 2 cm  1 cm  1.5 cm .
the cut surface showed no cystic area , hemorrhage or necrosis [ figure 2 ] .
histological features showed an encapsulated tumor mass with approximately 70% of the tumor volume composed of superficial and deep - seated keratin - filled multicystic spaces of variable size and shape , lined by metaplastic squamous epithelium .
intraoral photograph showing well - circumscribed nodular mass in the left retromolar trigone measuring 2 cm  2 cm in diameter cut surface of well - encapsulated grayish - white tumor mass with no evidence of cystic areas , hemorrhage or necrosis benign mixed tumor composed of both epithelial and myoepithelial cells in a myxoid mesenchymal background ( 4 ) multiple keratin cysts of variable size lined by stratified squamous epithelium ( 10 ) keratin - filled multicystic spaces are lined by metaplastic squamous cells ( 40 )
tumors originating in the minor salivary glands are infrequent and represent < 20% of all salivary neoplasms .
the most common benign histological type is pa , in coincidence with different studies that report a variable incidence of between 40% and 72% of all salivary gland tumors .
pa is characterized by great histologic diversity ; however , pa presenting extensive squamous metaplasia is uncommon and can signify a potential pitfall in the histopathological diagnosis .
focal squamous metaplasia in pa can be related to ischemia , repair following infarction and necrosis of the salivary gland and may be found in about 25% of the pa . rarely extensive squamous metaplasia with cystic changes are reported . in the present case ,
many glandular cells were transformed into squamous cells through a process of squamous metaplasia , resulting in multiple squamous epithelium - lined cysts containing keratotic lamellae and some solid squamous cell islands presenting keratin pearls .
extensive squamous metaplasia with cystic changes in pa , especially in the absence of chondromyxoid stroma , can mistakenly lead to a diagnosis of benignity , such as choristoma or keratocystoma , and malignancy including mucoepidermoid carcinoma and squamous cell carcinoma on microscopy due to limited and selective samplings .
keratocystoma , previously known as choristoma , is a benign salivary gland tumor resembling a trichoadenoma .
it also exhibits solid squamous cell islands surrounded by the basement membrane within the collagenous stroma .
believe that the pa and keratocystoma may constitute related lesions , representing different stages in the evolution of a specific type of salivary gland tumor
. however , keratocystoma lacks myxochondromatous , myoepithelial or glandular components that are demonstrated in our case . regarding the differential diagnosis with squamous cell carcinoma , the absence of cytological atypia , metastasis , necrosis , invasion as well as minimal cellular proliferative activity and the presence of a fibrous capsule in the case presented here weigh against the diagnosis of malignancy .
compagno and wong reported that microscopically pa of the nasal cavity resembles mixed tumor of the major salivary glands but due to high epithelial cellularity and little stromal component , this benign tumor can be mistaken for malignant epithelial neoplasm .
microscopically , mucoepidermoid carcinoma ( mec ) presents mucous , intermediate and squamoid ( epidermoid cells ) and is usually multicystic . unlike our case
, the cystic spaces of mec are usually lined by mucous cells and prominent keratinization is rare , with scarce epidermoid cells associated with keratin production including keratin pearl formation .
there are reported cases regarding limitations of cytological procedure in the diagnosis of such salivary gland neoplasms .
this is because of lack of the architectural features and the tumor - stroma interface , which are both of critical importance in the diagnosis of this salivary gland neoplasm .
there has been recent interest in the application of antibodies to cytokeratin 7 and cytokeratin 20 for determining the site of origin of epithelial tumors .
draeger et al . have reported the expression of cytokeratin 7 in the tubules and acini of the epithelial component of pas .
strong expression of cytokeratin 7 and focal cytokeratin 20 expression in the squamous islands of the tumor .
higher proliferative index was noticed in the epithelial lining of such keratin - filled cysts compared to the conventional pa .
it may signify that the squamous metaplasia resulting in the large keratin - filled cyst in pa may be clinically significant , probably relate to an important growth potential .
to prevent the misdiagnosis of pa with exuberant squamous metaplasia as a malignancy , cautious and systematic approach in the histopathologic interpretation of the epithelial and connective tissue component is needed .
it is important to be aware of this possibility to distinguish it from malignant lesions and to avoid unnecessarily aggressive therapy .
pleomorphic adenoma ( pa ) is the most common benign tumor of major or minor salivary glands . 
 microscopically , pa exhibits a great diversity of morphological aspects . here 
 , we present an unusual case of pa with extensive squamous metaplasia and keratin - filled cysts in the left retromolar region of a 50-year - old edentulous person whose microscopic finding may represent a diagnostic dilemma for pathologists .