[Tumors of unknown origin: Case report].

IgG4 related disease is a recently recognized chronic fibrotic, inflammatory condition, caused by infiltrating IgG4 positive plasma cells that can cause tumor like disease in almost any organ in the body. Typical histopathology is lymphoplasmocytic infiltration of IgG4 positive cells, storiform fibrosis and obliterative phlebitis. Glucocorticoids alone or in combination with B-cell depletion with rituximab causes often good, lasting response. We present here a lady with recurrent lung infiltration that simulated pneumonia and later tumor of the lung. She was also earlier diagnosed with lump in the breast that was found to contain similar IgG4 positive plasma cells that was also demonstrated in the lung biopsy. She responded very well to rituximab given on 2 occasions. Three years after this treatment she is in total remission. Key words: IgG4 related disease, rituximab treatment, plasmacytoma of breast, tumor of lung Correspondence: Arni Jon Geirsson, arnijon@landspitali.is.