Blastemal-Predominant Primary Extrarenal Nephroblastoma Associated with Lumbosacral Lipomeningocele in an Infant: A Case Report and Focused Review

Primary extrarenal nephroblastoma, or extrarenal Wilms tumor, is an exceptionally rare embryonal tumor outside the kidney. Spinal/lumbosacral involvement is particularly uncommon and may mimic congenital dysraphic lesions. We report a 10-month-old female infant followed since birth for presumed sacral lipoma and operated on for suspected meningomyelocele/lipomeningocele. Imaging demonstrated a solid S1 to S3 lesion adjacent to a thin lipomatous lesion near the conus medullaris. Histopathology unexpectedly showed a blastemal-predominant triphasic nephroblastoma with epithelial differentiation and limited stromal component. Tumor cells were diffusely positive for nuclear WT1 and PAX8. Normal bilateral kidneys, absence of teratomatous component, and negative metastatic work-up supported primary localized extrarenal disease. Complete excision was followed by vincristine/actinomycin-D-based chemotherapy; 6-month imaging showed no residual or recurrent lesion.