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Background: While patients with Wilms tumor (WT) have an overall survival greater than 90%, tumor rupture is associated with worse outcomes and can increase risk of abdominal recurrence to 20%1. Because our institutional rate of rupture is at least 2.5x higher than that found in literature, we conducted a retrospective chart review to understand the differences between our patient population and the population in literature2. Our aims were to 1) compare our patient demographics and outcomes, 2) describe correlations between preoperative, intraoperative, and pathologic diagnosis of rupture, and 3) compare our outcomes between upfront resection and neoadjuvant chemotherapy. Procedures: We reviewed charts from 2015 to 2019 and assessed: details of presentation, chemotherapy, operative and pathological findings, complications, and outcomes between patients with and without rupture. Results: Of 29 patients with WT, 9 had preoperative rupture by imaging and 8, all females, were confirmed surgically. Our rupture prevalence of 27.6% is higher than that in literature (11-13%.) Despite patients with rupture having more positive margins (50% vs. 19% non-ruptured), they had lower recurrence rates at one year (0 vs 19% non-ruptured) and improved survival mortality (100% vs. 90.5% non-ruptured). Neoadjuvant therapy and flank radiation compared to whole abdominal radiation did not change complication rates or outcomes. Conclusions: The high percentage of ruptured WT at our institution allowed for comprehensive review and revealed variations from literature. Patients with rupture had better outcomes than patients without rupture, with no difference comparing neoadjuvant chemotherapy to upfront resection and flank vs whole abdomen radiation.

Aims The classification of paediatric spindle mesenchymal tumours is evolving, and the spectrum of so-called 'infantile fibrosarcoma' has expanded to include tumours with NTRK, BRAF and MET gene fusions. RET-rearranged paediatric spindle cell neoplasms are an emerging group; there is sparse literature on their clinical, pathological and genetic features, and their nosological place in the canon of soft tissue tumours is uncertain. In this study, we report five RET-rearranged paediatric spindle cell tumours with fusion partners MYH10, KIAA1217 and CLIP2. Methods and results The tumours occurred in the pelvic region, paraspinal region, kidney and subcutaneous tissue of hand and abdomen. The patients' ages ranged from 6 months to 13 years (median 1 year). The tumours were composed of monomorphic spindle cells arranged in a fascicular pattern. Lesional cells had minimally atypical ovoid or tapered nuclei and pale cytoplasm with indistinct borders. Necrosis was not identified. Mitoses numbered three to 12 per 10 high-power field. Cases showed inconsistent and variable expression of S100, CD34 and SMA. Clinical behaviour ranged from small lesions potentially cured by simple resection to large lesions exhibiting metastasis, but responsive to kinase inhibitor therapy. Conclusions Our findings help to define RET-rearranged spindle cell tumours. Although it is likely that these tumours comprise part of the morphological and clinical spectrum of infantile fibrosarcoma (IFS), identification of RET gene alteration is important for its unique therapeutic implications.