Lipomatous pleomorphic adenoma in the hard palate: Report of a rare case with cyto-histo correlation and review

Abstract

Pleomorphic adenoma is the most common benign tumor of the salivary glands. They are usually composed of epithelial/myoepithelial cells and chondromyxoid stroma. Extensive lipomatous differentiation is very rare. We report a case of lipomatous pleomorphic adenoma (LPA) that presented with a mass in the hard palate of a 32-year-old woman. The fine-needle aspiration cytology material was reported as benign cytology consistent with adenoma with major adipocytic component. Histopathological examination of the excision material displayed that more than 90% of the tumor was adipocytic in texture, containing scant epithelial and myoepithelial cells and chondromyxoid stromal fragments. Preoperative cytodiagnosis of lipomatous pleomorphic adenoma on FNA is based on cytomorphology intimately associated pleomorphic adenomatous and lipomatous tissue elements. LPA should be on the mental list of the (cyto)pathologist in differential diagnosis of lipomatous tumors or non-tumorous lipomatosis or carcinoma invasion in the adipose tissue of the minor salivary gland of the hard palate.

Introduction

Pleomorphic adenoma (PA) is a benign tumor that is commonly observed in the parotid and less frequently seen in minor salivary glands. PA which is composed of chondromyxoid fibrillary stroma and epithelial/myoepithelial cells may show metaplastic changes in both stromal and epithelial components; however extensive degree of lipomatous differentiation is rarely seen. Lipomatous pleomorphic adenoma (LPA) is described as a PA which consists of more than 90% lipomatous stromal component.

In this report, a case of a hard-palate-located PA which originated from minor salivary gland and showed extensive lipomatous differentiation was presented. To the best of our knowledge, our case is the second LPA diagnosed by fine needle aspiration (FNA) in the English literature.

Case Report

An oral, asymptomatic, slowly growing mass was detected in the submucosa of the hard palate of a 32-year-old woman who was admitted to the Department of Oral and Maxillofacial Surgery of Azerbaijan Medical University [Figure 1]. FNA was performed by using 24-gauge needle and 10-mL syringe attached to the syringe-holder and two direct smears were prepared by the (cyto)pathologist. One smear was air-dried for May-Grünwald-Giemsa (MGG) staining while the other one was alcohol-fixed for Papanicolaou dye. Abundant mature adipose cells, and some epithelial/myoepithelial cells and metachromatically staining fibrillary stromal fragments were observed [Figure 2a,b]. FNA material was reported as benign cytology, being compatible with pleomorphic adenoma with adipose component, and the mass was excised. Macroscopically, the specimen was 20 × 18 × 14 mm in size, well-circumscribed, unilaterally mucosa-covered, yellow-colored on sectioning and nodular in appearance. In histopathological examination of completely sampled material, more than 90% of the tumor was composed of mature adipose tissue consisting of univacuolar adipocytes [Figure 2c,d]. Local islands and septa-forming epithelial/myoepithelial cells were identified within adipose component. No mitosis, cellular atypia or necrosis were encountered in either adipose component or epithelial/myoepithelial cells. No recurrence was observed during the 12-month follow-up of the well-circumscribed and totally excised mass.

Figure 1.

Lesion manifested as a mass in hard palate

Figure 2.

Epithelial/myoepithelial cells (a) and metachromatic stromal fragment (b) accompanying adipose component on FNA (Pap, ×40 and ×100). Adipose component involving more than 90% of the tumor, septa-forming epithelial/myoepithelial groups and chondromyxoid stroma (c,d), (H and E, ×40)

On immunohistochemical investigation, cells in lipomatous component displayed no cytoplasmic reactivity with calponin, CD10 and CK14, or nuclear positivity with p63 while some immunoreactivity was detected in epithelial/myoepithelial cells.

Discussion

PAs may show differentiations in both stromal and epithelial components such as sebaceous, lipocytic, oncocytic and squamous metaplasia; extensive lipomatous differentiation is a rare characteristic for PA. Eleven cases of LPA of salivary gland are reported in English literature. Clinico-pathological features of these cases are summarized in Table 1. The first LPA case in literature was reported in 1995. The more precise characteristics of the tumor were identified by Seifert et al.[1] Based on this identification, LPA is a pleomorphic adenoma which consists of more than 90% lipomatous stromal component.[1]

Table 1.

Clinico-pathological characteristics of salivary gland lipomatous pleomorphic adenoma (LPA) cases reported in English literature

The mean age for reported cases including present case is 37.83 [range 14-74. Women to men ratio is 2:1. Parotid gland was the most common localization of LPA (7/11, 58.3%). The less frequent localizations were minor salivary glands (4/11, 33.3%) and submandibular gland in one case (8.4%).[1,2,3,4,5] Three LPA cases, located out of the salivary glands, were also reported in the literature; external auditory canal and skin of the scalp and breast were the sites in these cases. These cases were seen in elderly (65-74 yrs.).[6]

LPA cases are well-circumscribed lesions and tumor-diameter ranges between 1.0 and 4.0 cm. Computed tomography and ultrasonography were commonly used during diagnostic period for the literature cases. Of these, FNA practice was reported in two cases and cytopathological image was available in only one case.[3,5] In our case, radiological findings were not available and FNA was the first-step diagnostic method.

FNA has an important role as the first tool in diagnostic process and to triage the patient for the proper management of salivary gland lesions. Indications for FNA of salivary glands are mostly swellings and/or pain in the salivary glands.[7,8]

As in classical PA, chondromyxoid stroma, solid areas and cords which are formed by epithelial/myoepithelial cells are the main microscopic features in LPA cases. In addition, tumor is largely constituted by a lipomatous component consisting of mature adipocytes. In some immunohistochemical and electron microscopy studies, the mentioned lipomatous component was composed of fusiform-shaped modified myoepithelial cells.[9] However in our study, CK14, CD10, calponin and p63 did not show any positivity in the lipomatous component.

Recurrence was reported in none of the reported LPA cases. Only in one case, lipomatous metaplasia was observed in recurrent tumor; although, no such feature was detected in primary tumor.[10] Our case was a primary tumor and no recurrence was observed during 12-month follow-up. Actually, recurrence was not expected since the well-circumscribed lesion was totally excised; however, the patient will be kept under follow-up process for longer period.

In differential diagnosis, lipoma, atypical and spindle cell lipoma, lipoadenoma, interstitial lipomatosis, lipomatous atrophy, sialolipoma, well-differentiated low-grade liposarcoma should be considered.[1,5] The cytopathological and histopathological presence of metachromatically stained chondromyxoid stroma and epithelial/myoepithelial component should always be suggestive of PA. Multiple aspirations and different stainings, consisting of both Romanowsky type and alcohol-fixed dyes, could enhance the differential diagnosis.

Tubules, formed by columnar/basal-type epithelial cells within adipose tissue without metachromatic stroma are characteristic for lipoadenoma. In lesions such as non-neoplastic interstitial lipomatosis and lipomatous atrophy, adipocytes fill the stroma in irregular fashion and in different ratios; and in sialolipoma the similar tissue combination form the mass. In spindle cell lipoma and liposarcoma, spindle cells accompany the adipose component. The atypical cells, mitosis in the spindle component and necrosis are significant clues regarding malignancy. Besides, one should be aware that LPA could mimic carcinoma invasion in the adipose tissue.

Conclusion

Elimination of malignancy and determination of specific diagnosis are possible by FNA for LPA that is rarely encountered in minor salivary glands of hard-palate. Multiple aspirations and different stainings could enhance the differential diagnosis. LPA should be on the mental list of the (cyto)pathologist in differential diagnosis of lipomatous masses in the hard-palate with distinctive cytomorphologic features of intimately related lipomatous and pleomorphic adenomatous cellular elements.