Gross features: Sarcomatoid urothelial carcinomas of the bladder and renal pelvis
often have a polypoid gross appearance. When more advanced, they often replace much
of the kidney and surrounding structures with a diffuse infiltration of light colored
tissue.
Microscopic features: Sarcomatoid urothelial carcinoma usually consists of
nondescript spindle cells with highly atypical nuclei and frequent mitotic figures.
Often there is a prominent inflammatory infiltrate within the tumor. It is uncommon
for the sarcomatoid cells to take on characteristics of specific mesenchymal tissues,
so they often resemble pleomorphic malignant fibrous histiocytoma.
Differential Diagnosis: Areas of sarcomatoid dedifferentiation may arise from any of
the recognized types of renal cell carcinoma and sarcomatoid renal cell carcinoma is
not a diagnostic category independent of the others. When the sarcomatoid elements
have overgrown the antecedent carcinoma and it cannot be determined what it was, it is
appropriate to consider it to be renal cell carcinoma, unclassified. Whether of renal
tubular or urothelial origin, sarcomatoid morphology indicates high grade and in many
cases is associated with a poor prognosis. In some cases, the sarcomatoid morphology
predominates and many blocks must be examined to find a focus of carcinoma which
indicates the type of origin. In some cases, the only clue is the presence of
urothelial carcinoma in situ in the mucosa of the renal pelvis. Urothelial carcinoma
in the urinary bladder may be associated with a variety of unusual stromal reactions
and these also are seen in urothelial carcinoma of the renal pelvis. These include
osteoclast-like giant cells, stromal osseous or cartilaginous metaplasia, and
pseudosarcomatous stroma.
