Cutaneous malignant glomus tumours: applicability of currently established malignancy criteria for tumours occurring in the skin

Glomus tumours (GTs) have traditionally been classified into benign GTs, GTs with uncertain malignant potential and malignant GTs, based on a combination of criteria such as size of the tumour, degree of nuclear atypia and the level of mitotic activity. Several of the proposed grading criteria are difficult, or even impossible to apply for GTs occurring in the skin. The aim of the study was to analyse the applicability of the currently established GT malignancy criteria for tumours occurring in the skin and to establish their prognostic significance. A total of 25 benign cutaneous GTs, 11 new cutaneous malignant GTs and 36 cutaneous malignant GTs previously published in the literature were studied. We analysed the following clinicopathological features and correlated them with disease outcome: age, sex, site, size, depth of invasion, degree of nuclear atypia, mitotic activity, growth pattern, vascular invasion, spindle-cell morphology and tumoural necrosis. Of all the clinicopathological parameters analysed, only tumoural necrosis was found by univariate analysis (p = 0.001) to be associated with adverse biological behaviour, and none by multivariate analysis. Multivariate statistical analysis failed to detect any clini-copathological features predictive of the disease outcome (e.g., local recurrence, development of metastatic spread and/or death of disease) in cutaneous malignant GTs. Furthermore, the currently established malignancy criteria for cutaneous GTs can be difficult to apply, mainly due to their smaller size. Likewise, counting mitotic activity per 50 high power fields can often not be accomplished in GTs occurring at superficial locations. Complete excision of these tumours coupled with long-term follow-up is the mainstay of treatment for cutaneous malignant GTs. The results of our study also suggest that cutaneous malignant GTs follow a more indolent clinical course than their deep soft tissue counterparts.