Globules are round to oval well-demarcated structures larger than 0.1mm. They can be aggregated or located along the periphery of a melanocytic lesion (Kittler et al., 2016a). On histologic evaluation brown or black globules correspond to nests of melanocytes located at the epidermis or DEJ.
Blue globules represent melanocytic nests situated in the dermis (Woltsche et al., 2017). White globules correspond to melanocytes undergoing balloon cell changes (Jaimes et al., 2011). Distribution of globules is important: globules along the periphery of a melanocytic lesion reveal a horizontal growing phase, which can occur either in a growing nevus or in a superficial spreading melanoma. In melanoma, globules vary in size, shape and color, and can be frequently found focally and irregularly at the periphery of the lesion (Woltsche et al., 2017).
However, irregular pigmented globules located eccentrically at the periphery of a raised, otherwise homogeneous melanocytic lesion may raise suspicion for a BAP1-inactivated melanocytic tumor (BIMT), also known as Wiesner nevus or “bapoma” (Yélamos et al., 2018b). BIMT are a special subset of melanocytic lesions which have two components histologically: a more banal-looking population which corresponds to the globular component located at the periphery, plus an atypical spitzoid population which has a loss of expression of BAP1 and corresponds to the homogenous area of the lesion. Multiple BIMT have been associated with a cancer syndrome with increased risk for uveal melanoma, cutaneous melanoma, mesothelioma, renal cell carcinoma, among others (Wiesner et al., 2012, 2011).