A case of acinic cell tumor of the parotid gland is reported. This rare tumor may be clinically misdiagnosed as a mixed tumor. Ordinarily the tumor is histologically distinct but, occasionally, may be mistaken for a clear cell adenoma, a clear cell variant of mucoepidermoid carcinoma or even a renal cell carcinoma. The behavior and management of the tumor are discussed in this report.

 
Acinic cell tumor is a rare tumor of the salivary gland accounting for 1 to 3% of all the salivary gland tumors. Although, the parotid gland is more frequently involved, cases have been reported in the minor salivary glands. A number of bilateral examples have also been reported. The females are more often involved with a ratio of 2 or 3 to 1 in males. The peak incidence is in the 5th decade.1-4 The usual presentation is a lump which may occasionally be painful. The tumor grows slowly and often diagnosed clinically as pleomorphic adenoma.

Case Report

An 18-year-old girl presented with a swelling of one-year duration in the left parotid region which gradually increased in size without pain. The patient was taken to the theater for exploration. The lesion, variably firm and soft, had neither definite capsule nor did it extend to the adjacent structures. It was excised en toto with safety margins and was sent for histopathological examination.
The histopathological findings based on light microscopic examination showed two types of cells, basophilic and clear. The basophilic cells predominated and showed granular appearance. The cells were ovoid to polyhedral in shape with small rounded, eccentrically placed nuclei in basophilic or clear cytoplasm. These cells were arranged in solid sheets. No acinar, papillary or cystic changes were noted, the nuclear pleomorphism was minimal and mitosis was rare. The tumor cells were positive for PAS showing presence of glycogen, however, mucin-stain was negative. The adjoining salivary gland tissue was unremarkable [Figs. 1,2].

 
Acinic cell tumor of the salivary gland is an uncommon tumor. It is sometimes called acinic cell adenocarcinoma or acinic cell carcinoma because of its low but well documented metastatic potential. More often than not, the tumor exhibits a benign course although it cannot be predicted whether a particular acinic cell tumor will behave aggressively or otherwise.
Although it may still show considerable variation, the usual microscopic appearance of the tumor closely resembles that of the normal parotid gland except for absence of ducts and lobular pattern. When the clear cells predominate, the tumor may resemble a clear cell adenoma, a clear cell variant of mucoepidermoid carcinoma or a metastatic renal cell carcinoma. These clear cells do not contain fat or mucin but may have variable amounts of glycogen. The absence of ducts helps to differentiate an acinic cell tumor from an adenoma.
There are different opinions regarding the histogenesis of the tumor. Kay and Schatzki5 suggest that the tumor arises from the serious cells while Bhaskar6 and Abrams et al2 consider the multipotential duct cells to be the precursor of the tumor.
Adequate excision with a margin of normal parotid tissue appears to be a satisfactory treatment.7 However, on occasions, it may become necessary to sacrifice the facial nerve.
Prognosis after removal of acinic cell tumor is good with a 5-year survival of over 80% recorded by Frazel.8
In a 20-year follow up study, Eneroth et al9 found that the survival rate fell to 56% when those patients who died from other causes were excluded, however. Local recurrence is the principal threat with some patients requiring re-excision of the tumor nodules over a period of many years.
In a large-series reported by Ellis and Corio,10 there was a local recurrence rate of 12%, a metastatic rate of 7.8% and a death rate of 6.1 %. The regional nodes are the common site of metastasis, however, lungs and bones may also be involved. Neck dissection does not appear warranted unless the nodes were clinically involved.11 Radiation therapy is reported to have no appreciable therapeutic effect.7