Definition

 This is a rare, potentially aggressive, parotid or submandibular tumour that is usually present at birth and recapitulates the primitive salivary anlage.

 ICD-O code 8974/1

Synonyms

 Congenital basal cell adenoma, basal cell adenoma, basaloid adenocarcinoma, congenital hybrid basal cell adenoma- adenoid cystic carcinoma, embryoma {2570,2685}.

Epidemiology

Most tumours are identified at birth or shortly thereafter; occasional children may be diagnosed after the age of two years. The male to female ratio is 2:1. Sialoblastomas are extremely rare; 23 such cases have been reported {48,156, 251,867,945,1016,1574,1688,1786, 1952,2353,2570,2685}.

Localization

The ratio of parotid to submandibular gland involvement is approximately 3:1.

Clinical features

 Most babies present with a mass of the cheek or submandibular region. Occasional tumours may reach massive proportions and ulcerate skin. One baby presented with a concomitant hepatoblastoma {2353}, and two other children both had congenital nevi associated with their tumours {251,945}. Some babies have been diagnosed by prenatal sonography. Radiographically, these tumours appear as expansile, lobulated masses. True-cut preoperative biopsy can be diagnostic, and is useful in ruling out neoplasia that require neoadjuvant chemotherapy, such as rhabdomyosarcoma.

Histopathology

Sialoblastomas are composed of basaloid epithelial cells, with scanty cytoplasm, round to oval nuclei, single or few nucleoli, and relatively fine chromatin pattern. More mature cuboidal epithelial cells with pink cytoplasm can also be seen. These cells form ductules, bud-like structures and solid organoid nests, and may demonstrate peripheral palisading. The intervening stroma may appear loose and immature. Myoepithelial cells can be identified, and have been confirmed by ultrastructural study. More familiar salivary patterns such as adenoid cystic-like cribriform areas can be seen. The mitotic rate within sialoblastomas is highly variable, and may increase with subsequent recurrences {251}, as may necrosis, nuclear pleomorphism and MIB1 proliferative index. It has been suggested that these tumours be separated into benign and malignant based on the absence or presence of invasion of nerves or vascular spaces, necrosis and cytologic anaplasia {251,1574}.

 Immunoprofile

These tumours express S-100 and vimentin diffusely. Cytokeratin accentuates the ductal structures.

Histogenesis

It has been suggested that these tumours originate from retained blastemous cells rather than basal reserve cells {2570}. Dysembryogenic parotid changes have been described adjacent to the tumour, with proliferation of the terminal ductal epithelial bulbs {1952}.

Prognosis and predictive factors

Sialoblastomas have the potential to recur (22%), and can occasionally metastasize regionally (9%), and one fatality has been reported {251,1688}. Most of these children are cured by primary surgical resection.