Histopathological risk factors for malignancy in dermatomyositis

Aims: To identify possible histopathological risk factors for malignancy in skin biopsies of dermatomyositis patients. Methods and results: We analysed clinical metadata and studied 30 skin biopsies of 11 patients with and 12 patients without associated malignancy, who were treated in one secondary and one tertiary German medical centre between 2009 and 2022 and fulfilling the EULAR/ACR classification criteria for dermatomyositis. Specimens were categorized by malignancy status and evaluated based on haematoxylin and eosin (H&E), periodic acid Schiff (PAS), Alcian Blue, and anti-CD123 immunohistochemistry stains. After correcting for multiple testing, biopsies of patients with cancer exhibited more severe basement membrane thickening (P < 0.05) and pigment incontinence (P < 0.05) compared to patients without tumour burden. Patients with numerous subepidermal melanophages had a more than 5-times increased odds ratio to suffer from an internal malignancy (odds ratio [OR] 5.3, 95% confidence interval [CI] 1.3-54.2, P < 0.05). Furthermore, specimens of the malignancy group presented a distinct superficial distribution pattern of CD123+ plasmacytoid dendritic cells (PDCs, P < 0.01). Extravascular eosinophils were absent in all cases. Conclusion: Severity of basement membrane thickening, extent of pigment incontinence, and superficial distribution pattern of CD123+ PDCs could serve as useful histopathological indicators of risk for malignancy in dermatomyositis.