Tubular Entrapment May Be Inconspicuous in Clear Cell Sarcoma of the Kidney in Early Infancy, Compared to Childhood: An In-Depth Case Comparison

Introduction Clear cell sarcoma of kidney (CCSK) is a rare malignancy accounting for <0.5% of all primary renal tumors, commonly diagnosed between 2 and 4 years of age and rarely occurring in early infancy. The challenging differentiation between CCSK and blastemal Wilms tumor is important because of the distinct clinical pattern of CCSK to recur and metastasize to bone and brain. The aim of this study is to discern subtle features that could assist pathologists in diagnosing CCSK in infancy. Method In-depth comparison of clinical, histological, and immunohistochemical findings in a case of CCSK diagnosed at 5 months of age with two cases of CCSK diagnosed at 2 and 3 years of age. Results Both groups were male, and each presented with an abdominal mass. Grossly, a single, firm, well-demarcated tumor, morphologically comprising monotonous small primitive round-to-polygonal/spindle cells, was seen in both groups. The major differences between the study groups were growth patterns and stromal reactions. In infancy, the growth pattern was diffusely uniform sheets of malignant cells with no entrapment of tubules and inconspicuous stromal changes. However, in childhood cases, the growth pattern included well-defined tubular entrapment, as well as focal microcyst formation, myxomatous stroma, palisading bodies, and anaplastic and/or rhabdoid histology. In both study groups, the immunohistochemistry showed strong immunoreactivity with cyclin D1 and nonspecific positivity for vimentin, CD99, and BAF47. Conclusion CCSK has notoriously diverse histological heterogeneity and mimics other pediatric renal tumors, making diagnosis treacherous, and commonly erroneous as Wilms tumor with unfavorable histology. Despite the advent of immunohistochemical and molecular techniques, a thorough morphologic analysis remains key in accurately diagnosing CCSK at any age, especially in early infancy. This small in-depth comparison of CCSK by age groups suggests that tubular entrapment and stromal changes may be less conspicuous in CCSK in early infancy than at older ages.