Sialoblastoma of Intraoral Minor Salivary Gland Origin; A Case Report and Review of the Literature
Siema Hassan, D.M.D., Paul Freedman, D.D.S., Renee Reich, D.D.S., Maria Alba Greco, M.D.
Background: Sialoblastoma is a rare, congenital (or perinatal) low-grade malignant salivary gland neoplasm, which resembles fetal salivary gland tissue. Only 24 cases have been reported in the English literature. The earliest cases have been termed embryomas, congenital carcinomas and hybrid basal-cell adenoma-adenoid cystic carcinomas. Sialoblastoma has been reported in association with hamartomas and hepatoblastoma. The term sialoblastoma was first used by Taylor in 1988 and is the current name used by the WHO. Multiple recurrences and metastases have been reported but the rarity of sialoblastoma has limited the understanding of its biologic behavior. Several authors have attempted to classify sialoblastoma into benign and malignant variants based on microscopic features such as cytologic atypia and necrosis. However, because of this tumor’s unpredictable behavior; it is prudent to consider all sialoblastomas as malignant neoplasms.
Learning Objectives: The learning objective is twofold: The first is to familiarize clinicians with this rare entity. The second is to stress the unpredictable biologic behavior of this tumor and the importance of correlation between the microscopic features, clinical presentation and prognosis.
Case Report: We report a case of sialoblastoma in a newborn male that was not detected by prenatal sonography. Unlike previously reported cases that arose from major glands, our case arose from the minor salivary glands of the buccal mucosa. The mass was firm, freely mobile and non-ulcerated. Clinical impression at that time was a hemangioma. An incisional biopsy was performed. The microscopic features were consistent with a salivary gland tumor. A diagnosis of sialoblastoma was rendered. In addition to histopathologic diagnosis, tissue was also sent for cytogenetic and electron microscopic studies. An MRI was also performed. No genetic abnormalities were discovered and no other tumors were noted elsewhere in the body. The tumor did not involve the parotid gland. Histopathologic examination of the excised specimen revealed extension of the tumor to the surgical margins. A proposed method of classification that was implemented by the AFIP was used to attempt to classify this case. Due to the high proliferation index, high mitotic count, marked pleomorphism and other factors, this tumor was considered to be “unfavorable”. The patient is to be closely monitored with 3-month follow-up visits.
Discussion: Most sialoblastomas described in the literature have originated in the major salivary glands (majority in the parotid gland). The proposed pathogenesis of sialoblastoma is arrested differentiation of salivary gland tissue, with the resultant tumor resembling fetal salivary gland tissue. Most sialoblastomas present at birth or arise shortly thereafter, and present as a mass of the cheek or over the angle of the mandible. Tumors range in size from 1.5 cm in greatest diameter to as large as 15 cm. Nearly all patients were asymptomatic but facial paralysis has been reported. In the largest series of reported sialoblastomas, the AFIP has attempted to classify these neoplasms according to microscopic features, to better predict biologic behavior. These features include degree of anaplasia, mitotic count and proliferation index, amount of stroma seen and presence of necrosis. The differential diagnoses of sialoblastoma in a clinical setting include hemangioma, rhabdomyosarcoma, neuroblastoma and lipoblastoma. Histologically, the most common misdiagnoses include basal cell adenoma and adenoid cystic carcinoma.
Of 15 cases reported previously, 4 recurred and 3 of these recurred multiple times. Recurrences usually occur several months to 1.5 years after the excision. Extension into the base of the skull and metastases to regional lymph nodes has also been reported.
Thus it is concluded that surgical excision with tumor-free margins is crucial to successful therapy. The efficacy of both chemo- and radiotherapy remains difficult to assess due to the limited number of cases.