Clinical Aspects: Mesoblastic nephroma is the predominant renal
neoplasm in the first three months of life, and is uncommon after 6 months. An
abdominal mass is almost always the presenting finding. The vast majority of
patients are cured by surgical resection. A few recurrences and adverse outcomes
have been recorded, principally in patients older than three months at presentation.
This tumor is very rare in adults. Mesoblastic nephroma has infiltrative borders
which the surgical pathologist must study carefully because the risk of recurrence
appears to be dependent upon the completeness of the resection. Metastasis is rare.
Gross Features: Mesoblastic nephroma is usually large relative to the
infant's kidney. Externally, the surface of the tumor and kidney is smooth and the
renal capsule and calyceal systems are stretched over the tumor. The cut surface
resembles that of a leiomyoma: firm, whorled or trabeculated, and light colored.
The tumor is not encapsulated and typically interdigitates with the surrounding
kidney, and may extend into surrounding tissues. Renal vein invasion also occurs
occasionally. Cysts, hemorrhage, and necrosis are present in a minority of cases,
particularly those that are cellular on microscopic examination.
Microscopic features: The classical pattern of mesoblastic nephroma
described by Bolande et al. is a moderately cellular proliferation of thick
interlacing bundles of spindle cells with elongate nuclei which usually infiltrate
renal and perirenal tissues. Glomeruli and renal tubules are commonly entrapped.
Mitotic figures are usually in the range of O to 1 per 10 high power fields. Islands
of cartilage and foci of extramedullary hematopoiesis are present in some tumors.
Another, more common, pattern was recognized later and consists of a densely cellular
proliferation of polygonal cells with mitotic figures in the range of 8-30 per 10
high power fields, and often pushing borders. This pattern has been called
"cellular" mesoblastic nephroma. Cysts are common in this pattern. The classical
and cellular patterns often are mixed in the same tumor. In view of the generally
favorable outcome for patients with mesoblastic nephroma and the predominance of
lesions containing the cellular pattern, the histological pattern should not be a
primary indication for therapy beyond adequate surgical resection. In adults,
mesoblastic nephroma is virtually always of the classical pattern.
Differential Diagnosis: Mesoblastic nephroma usually is easily
diagnosed when the histology and patient age are considered. Wilms' tumor with
stromal predominance may be confused with mesoblastic nephroma, particularly Wilms'
tumor which has been treated preoperatively. This problem can usually be resolved by
the identification of blastema which is not found in mesoblastic nephroma; also,
Wilms' tumor usually has sharply circumscribed borders, whereas those of mesoblastic
nephroma often are infiltrative. Age assists in making the correct diagnosis, and
bilaterality favors Wilms' tumor. Although both occur in the same age group,
mesoblastic nephroma, even the cellular variant, and rhabdoid tumor are usually
easily distinguished.
