Although focal balloon cell changes of the melanocytes are relatively frequent, pure balloon cell lesions, defined as lesions mostly (>50%) composed of balloon cells, are exceedingly rare. In the literature, they represent about 1.7% of skin nevi and 1% of skin melanomas. Nevertheless, ballooning has been reported in a wide spectrum of benign and malignant melanocytic lesions in the skin and in the mucosa. In the skin, ballooning of the melanocytes can occur in blue, Spitz, halo, combined, and dysplastic nevi, as well as in melanomas.
Balloon cell melanomas can be both superficial spreading and nodular but are invariably invasive. In addition, the growth at the epidermal junction is very rare, and the neoplastic proliferation is mainly dermal based.
We report the case of an otherwise healthy 55-year-old woman who presented with a 1-cm melanocytic lesion on her back. The lesion was completely excised. At histology, a broad proliferation of small nests and single atypical clear cells was present, limited to the epidermis. All the cells showed large ballooned cytoplasm and centrally located oval nuclei, providing a sebocyte-like appearance (Figs. 1 and 2). Pagetoid spread till the stratum corneum was prominent. No neoplastic cells were visible in the dermis. Immunohistochemical staining with HMB45 (Fig. 3) was positive, and a diagnosis of balloon cell in situ melanoma was rendered.
In the surgical re-excision, no residual melanoma was present.
A balloon melanocyte is a large cell with abundant vacuolized cytoplasm and a centrally located oval nucleus. Recently, balloon melanocytes have been distinguished from sebocyte-like melanocytes by the shape of the nucleus, which is mostly oval in the former and scalloped in the latter. Regardless of this subtle distinction, however, balloon and sebocyte-like melanocytes are always intermingled in the same lesion and could be considered a morphologic spectrum.
The peculiar ballooning appearance is supposed to be due to progressive vacuolization of melanosomal organelles caused by intrinsic apoptotic processes of the melanocytes. This apparently does not alter immunohistochemical characteristics of balloon cell melanocytes, being immunoreactive with S100, HMB45, and Melan-A, as usual melanocytes.
The ballooning phenomenon does not show a correlation with prognosis, as well. In the largest series of balloon cell melanoma, as expected, the prognosis is related to Breslow thickness, and it is not dependent on cytological characteristics.
A common feature of balloon cell melanoma is the massive involvement of the dermis with a vertical growth phase or a nodular pattern, being the spread to the epidermis very rare, and the exclusively intraepidermal (in situ) localization has never been described so far.
In the present case, balloon melanocytes show an unequivocal malignant feature, that is, prominent pagetoid ascent in the epidermis, but maintain an exclusively in situ pattern of growth and can be considered the first reported example of balloon cell in situ melanoma.