Salivary duct carcinoma arising in IgG4-related autoimmune disease of the parotid gland

Hyper IgG4 disease or IgG4-related sclerosing/autoimmune disease is a multisystem condition characterized histologically by fibrosis, lymphoplasmacytic infiltration, and abundant IgG4 plasma cells associated with raised serum IgG4 levels. We present a case of salivary duct carcinoma of the parotid gland in a background of chronic sclerosing sialadenitis that also involved the submandibular gland with associated regional lymphadenopathy. The serology showed raised total IgG levels of 16.3 g/L (reference range, 6.0-15.0) and raised IgG4 levels of 3.41 g/L (reference range, 0.07-1.70). The salivary duct carcinoma contained areas of dense fibrosis and abundant IgG4-positive plasma cells (>100 per high-power field [hpf]). The adjacent noncarcinomatous areas, submandibular gland, and regional lymph nodes also contained plasma cells immunoreactive to IgG4 with densities higher than 100/hpf. To the best of our knowledge, this case is the first documentation of malignancy occurring in a background of IgG4-related autoimmune disease of the salivary gland.