Pleomorphic adenoma of retrobulbar ectopic lacrimal gland: a rare occurrence

Ectopic lacrimal gland encompasses all lacrimal tissue that is outside the lacrimal gland fossa, excluding the accessory glands of Krause and of Wolfring. The bulbar conjunctiva and limbus are most commonly involved regions while retrobulbar region is unusual. This tissue may undergo neoplastic transformation, the commonest tumour being pleomorphic adenoma. A 30 year old male presented with painless loss of vision and mild proptosis of the right eye since 5 months. MRI disclosed a well-defined, lobulated contrast enhancing mass in the right retrobulbar region measuring 2 X 2 X 2 cm and a differential diagnosis of vascular or glial neoplasm was entertained. Excision biopsy revealed an encapsulated mass away from the lacrimal fossa. Histopathology showed features of pleomorphic adenoma which was confirmed on IHC.


Introduction
The lacrimal apparatus comprises of the main lacrimal gland and the two sets accessory glands (of Krause and of Wolfring). While the main lacrimal gland is situated in the orbital lacrimal fossa, its palpebral lobe is situated on the temporal side of the superior fornix [1]. Whilethe eyelids [2], tarsal plate [3] and the nasal mucosa [4] have been reported to harbor ectopic lacrimal gland tissue, the commonest sites involved are the bulbar conjunctiva and limbal area [5,6]. Orbital involvement is relatively uncommon [7]. Pleomorphic adenoma is the commonest epithelial neoplasm to beset the lacrimal gland. [8] We present a case of unilateral proptosis with diminished vision secondary to a pleomorphic adenoma in an ectopic lacrimal gland deep within the orbit.

Case Report
A 30-year-old man presented the OPD with complaints of painless loss of vision in the right eye and slight protrusion of the right eyeball since 5 months. On examination, there was mild proptosis of the right eye accompanied by visual acuity limited to counting fingers at 3 feet; his best corrective visual acuity in the left eye was 6/12. There was restriction in abduction of the right eye while the left eye showed no remarkable indispositions.Ultrasound revealed a hypoechoic, predominantly solid (with small cystic area) mass superolateral to right eyeball, suggestive of epidermoid lacrimal gland tumor. Magnetic resonance displayeda well-defined, lobulated, contrast enhancing retrobulbar mass in right orbit, separate from the lacrimal gland ( Figure-1). Based on these observations a differential diagnosis of cavernous hemangioma or glial tumour or connective tissue tumour was proposed. Lateral orbitotomy was done and the mass was completely excised. Gross examination exhibited a well circumscribed lobulated mass measuring 2X2X2cm (Figure2) Pathology Update: Tropical Journal of Pathology & Microbiology Available online at: www.pathologyreview.in 417 | P a g e with a grayish white cut surface. Histopathology unveiled an encapsulated tumor with predominantly oval to spindle cells arranged in principally in cords, hemangiopericytoma like pattern and occasional pseudo rosettes, with microcysticareas. Abundant myxoid stromainto which the spindle cells appeared to "melt" and sparse glandular elements were also present; mitoses and necrosis were absent (Figures3, 4 and 5). Pericapsular normal lacrimal gland tissue was also noted. In light of the morphological features pleomorphic adenoma was considered the principal diagnosis while hemangiopericytoma was entertained as a differential due to the unusually large areas of perivascular arrangement of the spindle cells. Confirmation was done through immunohistochemistry (IHC) which showed diffuse positivity for cytokeratin and S100 protein ( Figure 6). The postoperative recovery was uneventful and the patient was discharged.

Discussion
The orbital lobe of lacrimal gland is derived from five to six epithelial buds formed by the proliferating epithelial cells of the conjunctival fornix at around two months of gestation following which other epithelial buds commence the formation of the palpebral lobe [9][10][11]. During development, a part of the gland may get secluded and develop dissociated from the main gland [12]. 30% of lacrimal gland lesions are made up by  [13]. Some investigators have reported the proportion to be as high as 48% [14]. The tumour has a high tendency to occur in the orbital lobe of lacrimal gland [8], however it has been reported in thepalpebral lobe [15], in the upper lid [8], Krause's gland in lower lid [16], gland of Wolfring in lower tarsal conjunctiva [17] and the orbit [18].
Although cases have been reported between 6-80 years of age, the tumour usually manifests between 30-70 years [18].Perusal of literature unearthed 4 reported occurrences of pleomorphic adenoma in an ectopic lacrimal gland situated deep within the orbit [19][20][21][22]. Complete removal, with the capsule remaining intact, is the curative treatment for pleomorphic adenoma arising from lacrimal gland tissue [8].
Gradually increasing painless proptosis of the right eye were the main findings in our patient. This growth pattern seems similar to the growth pattern of pleomorphic adenoma arising from the main lacrimal gland [23,24]. The essential diagnostic feature of a pleomorphic adenoma is that it is composed of both epithelialand mesenchymal tissues.
The epithelial cells form characteristic ductal structures with surrounding myoepithelial cells, which trail out gradually into myxomatous mesenchyme [14,25]. IHC exhibits positivity for vimentin, S-100, calponin, p63, and cytokeratin [26]. The present case demonstrates an unusual site of occurrence of pleomorphic adenoma. Due to the anatomical location, tumours like hemangiopericytoma and glial tumours like astrocytoma were first considered in differential diagnosis. However, the representative morphology along with IHC proved confirmatory.

Conclusion
Pleomorphic adenoma should be considered a possibility in slowly growing retro-orbital tumours which on radiological assessment are well circumscribed and devoid of any bony or soft tissue invasion. While microscopic evidence of glandular structures surrounded by myoepithelial cells "melting" into a chondromyxoid stroma is the prototypic histology, demonstration of epithelial (cytokeratin, EMA and CEA) and myoepithelial (calponin, S100 and p63) elements on IHC clinches the diagnosis in doubtful cases.
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