"Mesenchymal tumor" or "decidual-like reaction"?

For more than 40 yr, an unusual urinary bladder lesion has been known to oc cur in certain strains of mice. but no consensus has been obtained regardin g its etiology, pathogenesis, biology, or classification. The lesion was fi rst assumed to be epithelial and nonneoplastic, then it was called a smooth muscle cell tumor or leiomyosarcoma because of ultrastructural characteris tics for smooth muscle cells. Later, the nonspecific term "mesenchymal tumo r" was introduced due ro histomorphologic differences from all smooth muscl e tumors known. Recently, a proposal was made to name it "decidual-like rea ction" because of the histomorphologic similarity to the rare spontaneous d ecidual reaction in the uterus of aging mice. Both lesions are characterize d by spindle and large pleomorphic epithelioid cells with large bizarre nuc lei; these characteristics mimic anaplasia of malignant tumors and led path ologists to assume a neoplastic nature. The decidual hypothesis is supporte d by the regular presence of nuclear progesterone receptors, the occasional occurrence of eosinophilic cytoplasmic granules, the rare finding of cells morphologically resembling granulated metrial gland cells tall also observ ed in the uterine decidual reaction, and the reproducibility through long-t erm feeding of combinations of estrogens and progestogens. It appears that the new decidual hypothesis can explain many derailed facets of the lesion, with the exception of the reported smooth muscle cell characteristics. The controversy of "mesenchymal tumor versus decidual-like reaction" should be resolved soon, not only as a scientific issue, but also because of consequ ences for risk assessment.