MRI findings of a dermoid cyst in the floor of the mouth with “sac of marbles” sign: An immunohistopathological study

A dermoid cyst is lined by an epidermis-like epithelium with dermal adnexal structures contained within the cyst wall. The “sac of marbles” sign is a characteristic image finding of a dermoid cyst but its pathogenesis is unknown. In this report, we aim to elucidate the cause of the sac of marbles using immunohistological studies. We report the case of a 39-year-old man with a dermoid cyst in the floor of the mouth with a “sac of marbles” sign that was revealed by magnetic resonance imaging. We analyzed the histology of the sac of marbles in dermoid cysts and performed an immunohistological analysis of keratin subtypes. Histopathologically, the sac of marbles contained a spherical accumulation of anucleated epithelial cells; immunohistochemistry revealed that it was positive for keratin1 and keratin 10 and was negative for keratin 15 and keratin 17, demonstrating the same direction of differentiation as the cyst-lining epithelium. The results of this study show that the sac of marbles originates from denatured keratin derived from the lining epithelium.