Perinephric myxoid pseudotumor of fat: a distinctive pseudoneoplasm most often associated with non-neoplastic renal disease

Perinephric myxoid pseudotumour of fat – first described case in allograft kidney, a mimicker of malignancy

“…It is hypothesised that the tumour develops as a mesenchymal reaction to nearby inflammation or neoplasia within the kidney. 2 Similarly, a link to early/developing IgG4 disease has been suggested, as similar cases have shown IgG4 features. 2,6 In the current case, progressive kidney graft failure was developing.…”

“…The few studies describing the spectrum of these lesions lack specific clinical evaluation guidelines. Perinephric myxoid pseudotumour of fat was recently described by Dashti et al 2 The authors reviewed 11 such cases, noting their common association with non-neoplastic renal disease. Herewith we present the first case, to our knowledge, of perinephric myxoid pseudotumour of fat described in a renal transplant.…”

‘I Can’t Keep Up!’: an update on advances in soft tissue pathology occurring after the publication of the 2020 World Health Organization classification of soft tissue and bone tumours

“…16 To this list have recently been added 'dysplastic lipoma' 79 and 'perinephric myxoid pseudotumour of fat'. 80 Although similar cases had been previously described as 'anisometric cell lipoma', 81 the term 'dysplastic lipoma' was more formally codified by Michal et al in a series of 66 unusual lipomatous tumours. 79 These lesions presented as subcutaneous masses of variable size, with a striking predilection for the upper back, shoulder and neck region of older males.…”

“…Although the term 'perinephric myxoid pseudotumour of fat' was first coined by Dashti et al in 2019, 80 identical lesions had been reported several years previously by Tanas et al, as 'pseudosarcomatous fibroblastic/myofibroblastic proliferation in perinephric adipose tissue adjacent to renal cell carcinoma'. 86 Although this earlier study had emphasised the relationship between this unusual simulant of WDL and renal cell carcinoma, perinephric myxoid pseudotumours of fat appear to much more commonly occur in patients with non-neoplastic kidney disease, usually diabetes mellitus-related (presumably because diabetes is far more common than renal cell carcinoma).…”

“…Because problematic lipomas (spindle cell/pleomorphic lipoma, hibernoma, and others) and non‐neoplastic changes in normal fat (fat necrosis, exogenous lipid reaction, fat atrophy, and others) are overwhelmingly more common than ALT/WDL, it is hardly surprising that a vast literature exists on potential mimics of ALT/WDL 16 . To this list have recently been added ‘dysplastic lipoma’ 79 and ‘perinephric myxoid pseudotumour of fat’ 80 …”