Myoid gonadal stromal tumours (MGST) represent a rare type of testicular sex cord-stromal tumour that has recently been recognised as a distinct entity by the World Health Organization (WHO) classification of genitourinary tumours. MGSTs affect adult men and have been reported to behave in an indolent fashion. Histologically, MGSTs are pure spindle cell neoplasms that coexpress SMA and S100 protein. Given that the molecular features of these neoplasms remain largely undescribed, we evaluated a multi-institutional series of MGSTs using DNA and RNA sequencing. This study included 12 tumours from 12 patients aged 28 to 57 years. Tumour sizes ranged from 0.6 to 4.3 cm. Aggressive histologic features, such as vascular invasion, necrosis, invasive growth, and atypical mitoses were invariably absent. Mitotic activity was low, with a median of less than 1 mitosis per 10 high power fields (HPF; maximum: 3 mitoses per 10 HPF). Molecular analyses did not identify recurrent mutations or gene fusions. All cases with interpretable copy number variant data (9/10 cases sequenced successfully) demonstrated a consistent pattern of chromosome arm-level and whole-chromosome-level copy number gains indicative of ploidy shifts, with recurrent gains involving chromosomes 3, 6, 7, 8, 9, 11, 12, 14q, 15q, 17, 18q, 20, and 21q. Similar findings have also been recognised in pure spindle cell and spindle-cell predominant sex cord-stromal tumours without S100 protein expression. MGSTs are characterised by ploidy shifts and may be part of a larger spectrum of spindle cell-predominant sex cord-stromal tumours, including cases without S100 protein expression.

Departments of Pathology of Alberta Precision Laboratories and University of Calgary Calgary Alberta Canada

Departments of Pathology of Brigham and Women's Hospital and Harvard Medical School Boston MA USA

Departments of Pathology of Douglass Hanly Moir Pathology and Macquarie University Sydney Australia

Departments of Pathology of Erasmus University Medical Center Rotterdam The Netherlands

Departments of Pathology of Fondazione IRCCS Istituto Nazionale dei Tumori Milan Italy

Departments of Pathology of Indiana University School of Medicine Indianapolis IN USA

Departments of Pathology of Mount Sinai Hospital University of Toronto Toronto ON Canada

Departments of Pathology of Polytechnic University of The Marche Region Ancona Italy

Departments of Pathology of Stanford University Stanford CA USA

Departments of Pathology of Tenon Hospital and Sorbonne University Paris France

Departments of Pathology of The Johns Hopkins Medical Institutions Baltimore MD USA

Departments of Pathology of University of Illinois at Chicago Chicago IL USA

Departments of Pathology of Univerzita Karlova Plzen Czech Republic

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