Lumbosacral ectopic nephrogenic rest unassociated with spinal dysraphism.

Nephrogenic rests (NRs) are thought to originate from persistent nephrogenic blastema and are considered precursor lesions of Wilms' tumor (WT). These rests usually occur as perilobar and intralobar lesions in the kidney and, rarely, in ectopic sites. We report a midline lumbosacral ectopic NR in a healthy full-term newborn male with no family history of WT or WT-associated syndromes. The NR presented as a soft polypoid mass covered by normal skin. An MRI study revealed no lumbosacral spine abnormalities and no communication with the vertebral canal. The resected mass measured 3 cm and contained fat and had a central 1.2-cm solid nodule. The nodule was composed of blastema, epithelial elements (mature tubules and nephrons), and abundant stroma. No other somatic tissue elements were identified after complete microscopic examination. There are 4 cases of NRs reported in the lumbosacral area associated with spinal dysraphism, and only 2 cases, in addition to our report, unassociated with spinal abnormalities. The pathogenesis of heterotopic immature nephrogenic tissue remains a source of conjecture and speculation. If these lesions are heterotopic rests, their potential for neoplastic progression is probably quite limited, but if a monodermal teratoma, then more scrupulous clinical follow-up is warranted.