Clinical Aspects: Wilms' tumors comprise more than 80% of renal tumors of childhood.
Most often, they are found in children 2 to 4 years old (median ages for males and
females, respectively, are 37 and 43 months) and Wilms' tumor is relatively uncommon
in the first six months of life and in children older than 6 years, while Wilms'
tumor is rare in the neonatal period. The incidence of Wilms' tumors is about the
same in populations throughout the world. There is a slight preponderance of
females. The tumors are bilateral in 4% of cases and patients with bilateral tumors
average more than a year younger than patients with unilateral tumors. Associations
with congenital anomalies: cryptorchidism, hypospadias, other genital anomalies,
hemihypertrophy, and aniridia are well-recognized .
Gross Features: Wilms' tumors are usually large masses (especially so, considering
the size of the patients) more than 5 cm in diameter and a third or more are larger
than 10 cm3. The cut surfaces are typically solid, soft, and grayish or pinkish
resembling brain tissue. Foci of hemorrhage and necrosis are often present and cysts
are common. The tumors usually are surrounded by a prominent pseudocapsule composed
of compressed renal and perirenal tissues.
Microscopic Features: Wilms' tumors are typically composed of variable admixtures of
blastema, epithelium, and stroma, although in some tumors only two and occasionally
only one, component is present. The epithelial component usually consists of small
tubules or cysts lined by primitive columnar or cuboidal cells. Wilms' tumors'
stroma may differentiate along the lines of almost any type of soft tissue; loose
myxoid and fibroblastic spindle cell stroma are most common.
Based on the results of the National Wilms' Tumor Study (NWTS), Wilms' tumors are
divided into two categories: favorable and unfavorable histologies, based on the
absence or presence of anaplasia. Anaplasia is found in approximately 6% of Wilms'
tumors; it is rare in patients younger than 1 year and more than 80% of patients are
older than two. Even small foci of anaplasia can be associated with an adverse
outcome. Thus, it is important to sample Wilms' tumor specimens extensively.
Anaplasia has been defined by the NWTS as the combination of cells with very large
hyperchromatic nuclei and multipolar mitotic figures. The enlarged nuclei must be at
least three times as large as typical blastemal nuclei in both axes and their
hyperchromasia must be obvious. In addition to the enlarged nuclei, hyperdiploid
mitotic figures must be present.
Differential Diagnosis: Some Wilms' tumors have a monomorphous epithelial appearance
and can pose difficult diagnostic problems, especially in adolescents and adults, in
their distinction from renal cell carcinoma. Recognition of the nuclear
characteristics typical of Wilms' tumor epithelium is usually helpful in
distinguishing these Wilms' tumors from renal cell carcinoma. The epithelial nuclei
in Wilms' tumor are often elongate or ovoid with molded, sometimes wedged, shapes, a
feature helpful in distinguishing monophasic tubular Wilms' tumor from renal cell
carcinomas in which the nuclei usually are spheroidal.
