Clinical and pathologic features of 535 patients with congenital mesoblastic nephroma: a report from the Children’s Oncology Group

Objective: To explore clinicopathologic features of children with congenital mesoblastic nephroma (CMN) enrolled on Children‟s Oncology Group study AREN03B2 and a historical cohort of CMN patients. Background: CMN is a pediatric renal tumor of infancy, with histologic subtypes of cellular, mixed, and classic. Given its rarity, evidence-based clinical practice guidelines are unavailable. We collected clinicopathologic findings and outcomes data in 2 large cohorts of children with CMN. Methods: From 2004-2019, 6412 patients enrolled in AREN03B2 and underwent prospective central review of pathology materials, imaging studies, and operative reports. CMNs were identified and subclassified. Similar data was extracted from a historical cohort of CMNs collected by pathology reviewers between 1973-2001. Results: In total, 535 children were included (139 from AREN03B2; 396 from the historical cohort). In the ARE03B2 cohort, 137 had available follow-up data (median follow-up: 4.5 years). Ten children (7.2%) relapsed, and 4/10 children died of disease. Four of 55 (7.3%) children with local stage II cellular or mixed CMNs relapsed, and 6/37 (16.2%) children with local stage III cellular or mixed CMNs relapsed. No child with local stage I or classic CMNs (of any stage) relapsed. All relapses occurred within 1.5 years of diagnosis, and 4/10 relapses occurred within 3 months. In the historical cohort, 31 children (7.8%) relapsed; all relapses were local stage II or III cellular or mixed. Conclusions: While recurrences are uncommon, they are highly associated with cellular or mixed histologic subtypes and stage, providing key clinical information that may guide consideration of therapy and surveillance.