ABSTRACT Background Fibrous hamartoma of infancy (FHI) is a rare soft tissue lesion arising as a subcutaneous mass involving the axilla, trunk, and upper arm in infants and children <2yrs. Sarcomatous transformation in FHI is described in anecdotal cases in the literature. Case Report We describe one such example arising as a mass in the lower back in a 3-month-old infant. On histology, the tumor contained classic triphasic morphology; however, brisk mitotic activity noted at multiple foci was diagnostically challenging to categorize. The tumor was evaluated for ETV6-NTRK3 fusion to exclude other common differentials. Conclusion While FHI may be frequently encountered in infants, rare sarcomatous transformation are known to occur and merits special attention as it can be misdiagnosed. Also, a close follow-up is warranted as the lesion is known to recur locally.

ABSTRACT Wilms tumor (WT) can occur at various extrarenal sites; however, the urinary bladder as the primary site is occasional. A 4-year-old-female child presented with difficulty in micturition for the past month. The contrast-enhanced magnetic resonance imaging with magnetic resonance (MR) urography revealed a polypoidal, heterogeneous mass in the urinary bladder with no abnormality in the kidneys. Cystoscopy-guided biopsy was reported as an extrarenal Wilms tumor (ERWT) with triphasic components. Post-chemotherapy, a computed tomography scan revealed a residual tumor for which she underwent partial cystectomy. The diagnosis of ERWT was confirmed. She received adjuvant chemotherapy and remained well at the 9th month post completion of chemotherapy. The primary bladder WT must be considered in the differential of a small blue round cell tumor at an extrarenal site in the pediatric age group. The diagnosis is especially challenging in small biopsy material, although it has immense significance in management and prognosis.