Mesothelial Lesions of the Testis: A Review

Review maps how BAP1 and MTAP immunostaining now separate benign from malignant mesothelial lesions of the tunica vaginalis

Journal: Advances in Anatomic Pathology | Published: 2026-01-26 | Type: Narrative Review | PMID: 41582881 Authors: Rajaram A, Fiset PO, Brimo F (McGill University, Montreal — a tertiary academic pathology center) Funding/COI: None disclosed

Summary

Mesothelial lesions of the testis and paratestis — ranging from reactive hyperplasia to malignant mesothelioma of the tunica vaginalis — share enough morphologic overlap with Sertoli cell tumors, yolk sac tumors, and Müllerian-type neoplasms to make diagnosis genuinely difficult. For decades the main tool was immunohistochemistry to confirm mesothelial lineage; distinguishing benign from malignant within that lineage was harder. Loss of BAP1 or MTAP protein expression on immunostaining, alongside molecular testing, has recently shifted the diagnostic floor. The review also highlights a newly formalized middle category — well-differentiated papillary mesothelial tumors of "uncertain malignant potential" — driven by TRAF7 or CDC42 mutations.

Claims

• Malignant mesothelioma of the tunica vaginalis is an uncommon but aggressive tumor; loss of BAP1 or MTAP immunostaining supports a distinct molecular pathogenesis and helps distinguish it from benign lesions
• Well-differentiated papillary mesothelial tumors carry TRAF7 or CDC42 mutations and behave indolently, but architecturally complex variants exist and are classified as having "uncertain malignant potential"
• Florid reactive mesothelial hyperplasia and infarcted adenomatoid tumors represent the most diagnostically treacherous benign lesions — routine histopathology alone can be insufficient
• Getting the diagnosis right has direct treatment consequences: misclassifying a benign reactive lesion as malignant, or vice versa, changes management substantially

Study Quality

This is a narrative review, not a systematic review or meta-analysis. There is no PRISMA flowchart, no formal search strategy disclosed, and no pooled data. The authors draw on published case series, molecular studies, and the evolving WHO classification framework, but the synthesis is expert opinion shaped by their own pathology experience at a single academic center. That is exactly what this type of anatomic pathology literature does — it is a teaching and reference document, not an evidence synthesis in the epidemiological sense.

Within that genre, the review appears competent. McGill's pathology group has published substantively in genitourinary pathology, and the three-part framework (reactive / borderline / malignant) aligns with current WHO and expert consensus. But readers should note this is a review of other people's data, not original research.

Red Flags

• No systematic search strategy — selection of included literature is opaque and potentially biased toward cases and series that support the authors' diagnostic framework
• No quantitative data: sample sizes, sensitivity/specificity of BAP1/MTAP staining, or false-positive rates are not reported in the abstract; whether the full paper supplies these numbers is unclear
• "Uncertain malignant potential" as a diagnostic category is clinically frustrating — the review acknowledges this tier exists without apparently resolving what it means for management
• Malignant mesothelioma of the tunica vaginalis is genuinely rare; the evidence base for any diagnostic framework here consists of small case series, which the review does not appear to critically quantify
• No funding means no industry bias, but also no formal peer-reviewed data collection infrastructure

Strengths

• Clean three-part classification (reactive / borderline / malignant) with molecular anchors (BAP1, MTAP, TRAF7, CDC42) gives practicing pathologists actionable criteria
• Explicitly addresses diagnostic pitfalls — florid reactive hyperplasia and infarcted adenomatoid tumors — that trip up generalist pathologists
• No COI is genuinely notable for a review that touches on immunohistochemistry panels, where reagent manufacturers often have relationships with academic authors
• Published in a specialist anatomic pathology journal with appropriate scope; this is not overselling findings to a general audience

Verdict

A narrow but useful pathology reference for a rare diagnostic problem. This review will not change the practice of anyone except the pathologist holding a biopsy from a paratesticular mass — but for that person, the BAP1/MTAP framework and the "uncertain malignant potential" category are the current state of the field. Not a paper to cite when discussing disease prevalence or treatment outcomes, and not a study at all in the traditional sense. Worth attention if you're writing about how genitourinary pathology has integrated molecular markers into routine diagnosis; otherwise a specialist document.