PLEOMORPHIC ADENOMA: CLINICAL FEATURES, HISTOPATHOLOGY & TREATMENT
Author: Sanketh DS, MDS
INTRODUCTION & ETIOLOGY/HISTOGENESIS
Pleomorphic adenoma(PA) is the most common benign salivary gland tumor that most commonly affects the parotid glands. It is basically a tumor comprising of epithelial and myoepithelial cells in a stroma showing a variety of mesenchymal components. To understand this better let’s recap the basic structure of salivary glands.
The secretory cells present in salivary glands are of two types, called serous and mucous cells. These secretory cells are housed in secretory end pieces. The serous and mucous cells line the lumen of the secretory end pieces and hence are also called luminal cells. The secretory end pieces are continuous with the main excretory duct of the salivary gland through intermediary ducts.
The lumen of the end pieces open into the intercalated duct, which is continuous with the striated duct, which in turn opens into the excretory duct of the salivary gland. Also note that the ducts are lined by ductal cells. Now, the secretory end pieces and intercalated ducts are associated with stellate shaped contractile cells called the myoepithelial cells. Myoepithelial cell is an epithelial cell that also has smooth muscle like contractile properties. One among the many functions of this myoepithelial cell is to contract and help secrete saliva from the end pieces to the ducts.
So PA comprises of neoplastic epithelial/ductal and myoepithelial cells. It has been reported by studies that, these neoplastic myoepithelial cells, ductal cells or both may may undergo mesenchymal metaplasia to give rise to varied tissue components like chondroid, myxoid, chondromyxoid and very rarely, osseous and adipose tissue which comprises the connective tissue or stroma.
The term “pleomorphic” in PA should not be confused with “pleomorphism” used to describe atypical cells in dysplastic lesions. “Pleomorphic” in this context, is used to describe the mixture of epithelial cells as well as the variety of mesenchymal tissue types under the microscope. This tumor was also referred to as a “mixed tumor” for the same reason and was thought to arise from multiple germ layers. This however is not the case, as the varied stromal components are derived as a result of mesenchymal metaplasia of the neoplastically altered myoepithelial and epithelial cells.
Cytogenetic studies demonstrate gene alterations in the PLAG1 and the HMGA2 genes in the long arms of chromosomes 8 and 12 respectively.
PA is the most common benign salivary gland tumor affecting the major and minor salivary glands. Parotid gland is the most common site (85%) followed by the submandibular gland (8%) and the minor salivary glands (7%). Sublingual glands are rarely affected. The palatal glands are the most commonly affected minor salivary glands. It has a female predilection and occurs in the elderly, between the 4th to 6th decade. The tumor presents as a slow growing, painless swelling. The superficial lobe of parotid gland is usually affected with the swelling occurring just in front and below the ear. The tumor can grow to large sizes if left unattended to.
Under the microscope, PA shows two kinds of tumor cells, the ductal epithelial cells and the myoepithelial cells. Now the ductal epithelial cells are cuboidal and usually proliferate in the form of ducts, tubules or solid sheets. Eosinophilic secretions can be found in the duct lumen. The ductal cells may also show squamous, oncocytic and mucous metaplasia. Spindle shaped myoepithelial cells can be seen surrounding the ductal or tubular elements. Also, the myoepithelial cells can be seen proliferating separately as sheets. Myoepithelial cells may lose their spindle shape and become polygonal to oval shaped. The polygonal or oval shaped myoepithelial cells have an eosinophilic cytoplasm and an eccentrically placed nucleus and are called plasmacytoid cells. They are called so because of their resemblance to plasma cells, which are oval and have an eccentric nucleus. The stroma varies from being myxoid (loose stroma) to fibrous and also hyalinised. Various other mesenchymal elements like chondroid, osseous and adipose tissue can also be seen. Most importantly, PA is encapsulated and has a surrounding fibrous capsule.
TREATMENT AND PROGNOSIS
Surgical excision of the tumor is the treatment of choice in both the major and minor salivary glands. This is accomplished by local resection with minimal surrounding tissue. In case the parotid gland is involved, surgical removal should be accomplished without damage to the facial nerve.
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Slide illustrating epithelial cell population, with ductal,tubular and solid growth patterns:
Histopathologic image of pleomorohic adenoma of the parotid gland. H & E stain /CC-BY-SA-3.0
Slide illustrating mixed epithelial and mesenchymal cell components:
Histopathologic image of pleomorphic adenoma of the parotid gland. H & E stain/ CC-BY-SA-3.0
Slide illustrating cellular area interspersed with islands of lighter stained bluish myxoid tissue:
Micrograph of Pleomorphic Adenoma of Salivary gland/Department of Pathology, Calicut Medical College/Wikimedia Commons/ CC-BY-SA-4.0
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