Pseudopapillary and micropapillary-like changes in classical renal angiomyolipoma: a multi-institutional series of 60 cases.

Pseudopapillary- and micropapillary-like changes in classical renal angiomyolipoma (AML) are underappreciated morphologic patterns that in some instances may cause diagnostic confusion with renal cell carcinoma or urothelial carcinoma. To further evaluate the incidence of these morphologies, a multi-institutional cohort of 60 patients with classical AML was retrospectively analyzed by genitourinary pathologists. Clinicopathologic features include tumor characteristics of fat assessment and extent of pseudopapillary/micropapillary-like changes were recorded. Chi-square test was conducted to determine associations with tumor subtype. Most cases were unilateral (56/60; 93%) and solitary (55/60; 92%), with a median tumor size of 3.1 cm (range: 0.25 - 19.0 cm). Pseudopapillary/micropapillary changes were identified in 19/60 cases (32%), significantly occurring more frequently in fat-poor AML (17/30; 57%) compared to non-fat-poor AML (2/30; 7%; p < 0.001). Pseudopapillary/micropapillary growth, mostly in mixed form, within tumors was 20% on average (range: 1 - 60%). No recurrence or tumor-related deaths were observed [44/60 patients; median follow-up = 11 months (range: 1- 96 months)]. Recognizing pseudopapillary/micropapillary features, particularly in fat-poor AML and in a biopsy setting, is critical to avoid misdiagnosis.