Well-differentiated acinic cell carcinoma of salivary glands associated with lymphoid stroma.

In a multicenter study, 69 acinic cell carcinomas of the salivary glands were identified, of which 12 constituted what the authors believe to be a distinct subgroup. Their most noticeable feature was a dense lymphoid stroma with well-developed germinal centers, surrounding a sometimes scanty epithelial component, which in each case had a microcystic growth pattern. All these tumors were enveloped by a thin fibrous pseudocapsule, thus mimicking an intraparotid lymph node containing a metastasis. All 12 cases showed low MIB1 proliferative activity, with a mean index of 1.7% (range, 0.5 to 3.7). All patients remained well without recurrence or metastasis in followup periods of 19 months to 14 years. A second subgroup of nine acinic cell carcinomas also possessed a heavy lymphoid stroma with germinal centers, but its distribution was more patchy than in the first subgroup, and in addition, the fibrous pseudo-capsule was incomplete or absent. In each case the epithelial growth pattern was other than microcystic. These tumors had significantly higher MIB1 indices (mean, 17%; range, 3.4 to 45). In contrast to the first subgroup, only three of nine patients remained well with no further disease. The other six patients developed recurrences or metastases, and two died of disseminated cancer. In view of the clinical and pathological data, it is speculated that the tumor foci lacking lymphoid stroma in each of the second subgroup possibly represented a clone of high-grade malignancy arising within a low-grade acinic cell carcinoma with lymphoid stroma.