LAUSR

© BMJ Publishing Group Limited 2022. No commercial reuse. See rights and permissions. Published by BMJ. DESCRIPTION We present the case of a man in his 70s without any significant medical history who was being investigated for acute epididymoorchitis and was incidentally found to have a large pararenal mass. Of note, our patient did not have any history of cutaneous melanoma. MRI revealed a retroperitoneal complex cystic lesion anterior to the right kidney (figure 1D) which contained dependent complex material reflecting blood products (arrow). On precontrast T1 sequences, nodular high signal along the anterior aspect of this lesion (figure 1A, arrow) demonstrated enhancement of postgadolinium injection (figure 1B, arrow), indicating the presence of melanin. Enhancement of the capsule of this lesion was also be appreciated (figure 1B,C). This large, complex cystic lesion was closely related but separate, appearing to the adjacent renal parenchyma with mass affect and distortion of the adjacent anterior renal parenchyma. Following an uncomplicated radical right nephrectomy and histopathological analysis, this lesion was identified as a pararenal malignant melanotic nerve sheath tumour (MMNST). Figure 2 demonstrates the gross specimen with the large, melanin pigmented brown pararenal tumour (star) and the adjacent normal right renal parenchyma and perirenal fat. The cavity and cystic compartments of the tumour contained fresh haemorrhage as was seen as dependent complex material on MRI (figure 1D). On the histology slide beneath magnified at ×40, focal melanin (arrowhead) can be appreciated within a spindle cell area correlating with the regions of high T1 signal and enhancement seen on MRI. MMNST was previously categorised as melanotic schwannoma by the WHO prior to its most recent classification update on soft tissue and bone tumours. This reclassification reflects a better appreciation of this rare tumour’s malignant potential which is higher than previously thought. Previously, melanotic schwannomas were thought to have been generally benign entities with only occasional cases of a more aggressive course. A recent, large clinicopathological, immunohistochemical and gene expression profiling study of 40 cases found these tumours to have a local recurrence and distant metastases rates of 35% and 42%, respectively. In this series, 8 of the 11 patients with metastases developed within 48 months of diagnosis. These tumours are often associated with Carney complex (an autosomal dominant tumour predisposition syndrome with skin pigmentary abnormalities, myxomas and endocrine tumours),