MUC5AC and cyst: On the other hand, the primary pulmonary origin of this neoplasia is supported by the gradual histological changes of the mucinous lining epithelium, slowly progressing from a benign to a malignant phenotype and by the presence of the smaller cyst, which lacked any epithelial lining, but which contained mucous and was surrounded by a foreign body-type giant cell reaction as described in many mucinous tumors found at other sites, for example the ovary, in relation to focal mucin spillage from ruptured or degenerating cysts or glands [11].