Gigantic retroperitoneal pararenal ancient schwannoma masquerading as a mesenchymal malignancy: A diagnostic conundrum

Abstract

Ancient schwannoma is a rare benign variant of schwannoma with marked degenerative changes, often mimicking malignancies, particularly when retroperitoneal and pararenal. A 34-year-old woman presented with a rapidly growing 15 cm retroperitoneal pararenal mass. Imaging suggested an aggressive malignancy. Surgical resection and histopathology revealed a well-encapsulated tumor with degenerative changes, nuclear atypia, and strong S100 positivity, confirming ancient schwannoma. Complete excision achieved negative margins, and follow-up showed no recurrence. Diagnostic challenges arise from its rarity and atypical features. Histopathology is crucial for differentiation from malignancies. Ancient schwannomas should be considered for large retroperitoneal masses to ensure accurate diagnosis and management.

Highlights

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Rare Presentation and Diagnostic Challenge : A gigantic pararenal retroperitoneal ancient schwannoma mimicking a malignant mesenchymal tumor due to its size, location, and degenerative changes.
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Key Role of Histopathology and IHC : Definitive diagnosis achieved through histopathological evaluation and immunohistochemical markers, particularly S100 positivity and a low Ki-67 index.
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Clinical Relevance : Emphasizes the need to consider ancient schwannoma in the differential for large retroperitoneal masses and highlights surgical excision as curative with excellent prognosis.

1 Introduction

Schwannomas are typically well-circumscribed, encapsulated, benign peripheral nervous system tumors, arising from Schwann cells in the nerve sheath. These tumors exhibit a very low risk of malignancy and can occur across all age groups, with the highest incidence in people aged 30–60. Ancient schwannoma, a rare and benign histopathological variant, is distinguished by a long duration and degenerative changes within the tumour, often affecting older adults.

While schwannomas are usually small (1–2 cm), they can uncommonly grow larger, which, along with the presence of long-standing degenerative changes, can very well mimic a malignant neoplastic etiology. Moreover, retroperitoneal humungous mass lesions are frequently aggressive mesenchymal malignancies like dedifferentiated liposarcoma, leiomyosarcoma or undifferentiated pleomorphic sarcoma (UPS). Pararenal retroperitoneal schwannomas are exceptionally rare, with only a handful of cases reported in international literature. Herein, we elucidate the complete clinical, radiological and pathological spectrum of the present case of a gigantic pararenal retroperitoneal ancient schwannoma in a 34-year-old woman masquerading as a sarcoma. This case study describes the atypical presentation of a benign entity owing to its enormous size, occurrence at an unusual site, and exhibiting marked degenerative atypia which can pose diagnostic and management dilemmas to urologists, uroradiologists and uropathologists alike. However, arriving at an accurate diagnosis remains pertinent as the prognostic outcome is entirely different and timely diagnosis can help guide clinicians in formation of appropriate management and follow-up plans.

2 Case presentation

A 34-year-old woman presented with a progressively enlarging lump in the right lumbar region, which had grown from 4 × 4 cm to 15 × 14 cm over a span of three months. She reported intermittent pain that resolved spontaneously, with no other abdominal symptoms. On examination, the lump had a dull percussion note, was non-pulsatile, and was markedly tender on deep palpation.

Contrast-enhanced MRI (CE-MRI) revealed a large, solid-cystic mass lesion in the right anterior pararenal space measuring 15.2 cm (antero-posteriorly) x 14.7 cm (transversally) x 18.1 cm (cranio-caudally) with cystic changes, areas of haemorrhage, and fluid levels, creating a mass effect on the adjacent anatomical organs ( Fig. 1 A–C). Contrast-enhanced Computed Tomography (CECT) scan exhibited the presence of a large hypodense lesion with mild enhancement and small amorphous calcification in the right pararenal lumbar region which was displacing the right colon antero-medially, indenting the anterior surface of right kidney, abutting the inferior surface of the right lobe of the liver and the infero-medial surface of gall bladder with inferior vena cava pushed along the medial border of the lesion ( Fig. 1 D). Radiological findings suggested a primary retroperitoneal tumor, possibly arising from the right pararenal space. Radiological differential diagnoses included a large mesenchymal malignancy with degenerative cystic changes with possible differentials of a pleomorphic liposarcoma or UPS.

The patient underwent an exploratory laparotomy and the entire mass lesion was resected with adequate margins and sparing the kidney. The excised retroperitoneal mass was sent for histopathological evaluation in 10 % neutral buffered formalin. Gross examination showed a well-encapsulated, grey-brown soft tissue mass measuring 18 × 15 × 14 cm. Serosanguinous fluid oozed out on serial sectioning, with variegated solid and multiloculated cystic areas varying in diameter from 1 to 12 cm in diameter ( Fig. 2 A). Extensive representative sampling was done to assess the heterogeneity of the neoplasm adequately. Routine 5-μm thick sections were cut from the paraffin-embedded and formalin-fixed tissue and were subsequently stained with Hematoxylin and Eosin (H&E) stains.

Multiple microscopic sections examined revealed a well-encapsulated, circumscribed spindle cell neoplasm with variable alternating areas of hypercellularity and hypocellular myxoid zones ( Fig. 2 B), cystic degeneration, haemorrhage, and marked nuclear atypia. Markedly large areas of myxoid degeneration were evident ( Fig. 2 C). Individual tumor cells were spindle to elongated with a moderate amount of eosinophilic cytoplasm, and indistinct cellular membranes. They had elongated nuclei exhibiting pleomorphism with irregular nuclear contours ( Fig. 2 D). However, the chromatin was bland and homogenous with inconspicuous nucleoli. Notable features included infarction, haemorrhage ( Fig. 3 A), hyalinization, calcification ( Fig. 3 B), clusters of foamy macrophages ( Fig. 3 C), and the occasional presence of Verocay bodies ( Fig. 3 D). There was no evidence of tumor necrosis and mitotic activity was sparse. A comprehensive immunohistochemistry (IHC) panel of antibodies was applied on 2-μm thick sections on Poly-L-Lysine coated slides to clinch an accurate diagnosis. On IHC, the tumor cells showed immunopositivity for S100 ( Fig. 4 A) , Calretinin ( Fig. 4 B), CD56 ( Fig. 4 C), and Vimentin, and were negative for neurofilament, synaptophysin, and chromogranin, NSE, MDM2, CDK 4, CD 68, Desmin, Myogenin, Smooth Muscle Actin (SMA), and CD 34. Ki67 proliferation index was very low, around 1–2% ( Fig. 4 D). Based on the concordant histomorphological and immunohistochemical findings a final diagnosis of ancient schwannoma in the right pararenal region was rendered.