MIXED TUMOR OF THE LACRIMAL GLAND (Pleomorphic Adenoma)
Definition: benign primary lacrimal gland neoplasm composed of both stromal and epithelial components.
Incidence/Prevalence: the most common primary neoplasm of the lacrimal gland accounting for about 40-50% of neoplasms in some series.
Etiology: The tumors presumably arise from ductular epithelium based on the ultrastructural characteristics that recapitulate the lacrimal ducts. The stromal cells are myoepithelially derived and the epithelial cells take origin from basal germinal epithelium. Recurrent chromosomal abnormalities involving chromosomes 3, 8, 9, and 12 have been reported similar to benign and malignant salivary gland tumors. There is evidence that mutations in TP53 gene are related to the pathogenesis of salivary gland neoplasms and that exons 5 and 8 are most frequently involved. A similar situation is suspected but not proven for lacrimal gland neoplasms. The tumors have been reported to arise from the glands of Wolfring, Krauss in the eyelid or even Popoff's glands in the caruncle.
Clinical Findings: The mean age is given as 35- 39 years with a range of about 7-77 years. The tumors occur with a male predominance up to about a 2:1 ratio of male:female in some series. Patients present with painless proptosis that has been slowly progressive. Examination shows proptosis, downward and inward displacement of the eye, ptosis, and a palpable mass. If the tumor is large the eye may be indented and retinal striae may be visible.
Radiologic Findings: CT and MRI show a rounded tumor that is generally centered over the region of the orbital portion of the lacrimal gland. There may be pressure induced scalloping of the adjacent zygoma. The sclera may be flattened slightly.
Histopathology: Pleomorphic adenomas are encapsulated tumors (number 2 in the figure shows the capsule) in which the normal lacrimal gland is usually pushed to one side (number 1).
The neoplasms are composed of both epithelial and stromal components, hence mixed tumor.
The epithelial component usually features strands and cords of epithelium that form ductular structures (arrows 3 and 10). There is often PAS positive hyaline or basement membrane material produced by the ducts (arrow 7 and number 9). There may be calcification in the stroma (number 4). Alternatively squamous epithelium may be present and produce keratin. Associated chronic inflammation is common at the interface between the capsule of the tumor and the lacrimal gland (arrow 5). In addition there may be seromucinous material from more dilated ductular structures (number 8). The stromal component may have a myxoid or loose stromal character with wavy spindle cells (arrow 6).
Sometimes the stroma component has a chondroid or cartilaginous appearance (number 8). The epithelial cells are usually enlarged and may vary somewhat in size but they are generally well spaced and there is no back to back gland formation without intervening stroma (a key sign of adenocarcinoma). Mitotic activity is usually very low in these slowly growing tumors.
Treatment: The usual treatment is complete excision by lateral orbitotomy. Some prefer to verify the diagnosis prior to surgery with fine needle aspiration. If this procedure changes the management such as in the case of an uncertain diagnosis or confusion of an epithelial lesion with a lymphoproliferative disorder then FNA seems warranted. Some claim that there may be seeding along the needle tract, and cite a reference to Fechner in the orbit chapter in Spencer's Ophthalmic Pathology. However this reference presents 2 cases, neither of which were mixed tumors or in the orbit or lacrimal gland. However, it is important during surgery to remove the entire tumor with the capsule intact. Frozen sections are generally not necessary and may be inaccurate.
Prognosis: Although benign, long standing mixed tumors (>15 years) are thought to degenerate into malignant lesions such as adenoid cystic carcinoma, malignant mixed tumor and adenocarcinoma.
