Tenosynovial giant cell tumour, localized type, is a benign tumour of the tendon or joint synovial membranes most commonly affecting the hands, typically proximal to an interphalangeal joint. It can appear at any age; however, it is more frequent between 30 and 50 years of age [5
]. Synovial origin seems to be confirmed by positivity to immunohistological markers of the normal synoviocytes, although these markers are equally expressed by the cells of the monocyte-macrophagic system [7
The neoformation consists of a proliferating focus of cells with an elliptical nucleus and a broad circular cytoplasm, which mirrors the morphology of synoviocytes and bizarre giant cells, being intensely eosinophilic and provided with numerous nuclei, scattered or clustered centrally or peripherally, or along the periphery. A variable number of siderophages is associated, together with inflammatory cells and, sometimes, xanthomatous cells.
Unlike giant cell tumour of the tendon sheath, diffuse type, which originates from the large joints and their bursae, and propagates freely into the soft tissues, the localized type is, for the most part, contained in a dense connective capsule, in continuity with the tendon's sheath, from which septa depart, penetrating the newly formed tissue. The existence of a rare malignancy, so much debated and contested, is nowadays accepted as long as this definition is restricted to the rare lesions characterized by declaredly malignant areas in the framework of benign tenosynovial giant cell tumour, or to histologically and biologically malignant areas of originally typical lesions [1
]. This allows the ruling out of a variety of sarcomas with a giant-cell component which, in their infiltration process, can aggress tendons and their sheaths. Among them, namely, the malignant fibrohistiocytoma.
In our case, which, among other reasons, stands out for the early age at onset, the histopathology of the original lesion can be related to the benign form of neoplasia and must therefore be differentiated from its mimics that can accidentally be found in a tendon. If the quota of foamy cells is relevant, the picture might recall that of tendinous xanthoma, which nevertheless is lodged in the tendon's body, rather than in the synovial space, and contains numerous cholesterol clefts along with a more conspicuous inflammatory component. The problem of histopathological distinction with this lesion is merely theoretical, since tendinous xanthoma is associated with hereditary hyperlipemia type IIa (hypercholesterolemia with LDL increase), and is typically multiple.
Unlike tenosynovial tumour, the solitary reticulohistiocytoma appears almost exclusively in adults, and it primarily originates in the dermis. From a histological point of view, it differs in the intense eosinophilia of its mononucleated elements (oncocytic histiocytes), and in its polykaryocytes with the typical ground-glass cytoplasm. Fibroma of the tendon sheath, which, like giant cell tumour, shows preferential localization in the hands, can also show some similarity only to the hyalinized forms of giant cell tumor, since it consists of a sparse population of fusiform cells (rather than circular), lodged in an abundant eosinophilic and hyalinized stroma.
Finally, giant cell tumour of soft tissue with low malignant potential has to be considered; today it is believed to be the counterpart of giant cell bone tumour of soft tissue, which typically localizes in the arm or hand, and that can manifest at any age. Cytologically, though very similar to tenosynovial tumour, it differs from it because it diffusedly infiltrates the host tissues and does not contain siderophages or xanthomatous cells, whilst it can present with areas of bone metaplasia [13
Having ruled out mimics, the diagnosis of tenosynovial giant cell tumour could thus be confirmed with postsurgical paracicatricial cutaneous satellitosis. The latter represents the outcome of a locoregional dissemination, which is the expression of a transformation of the lesion from a localized type to a diffuse type, despite the innocent histology of both lesions. The acquisition of this competence is a landmark in the evolution of the neoplasia and it is matched by a major change in the histological picture observed in the foci of cutaneous colonization. Indeed, these lesions (cutaneous foci) show evident signs of transformation, represented by the lack of stroma, by the presence of pleomorphism and bulging nucleoli, and by thick vascularization.
Nevertheless, the degree of atypia is low, and the cells preserve their normal immunohistological positivity, whilst the rare – and not atypical – mitotic figures observed have a poor prognostic value, since in the benign form a good 3 per 10 high-power fields have been described [14
]. Lastly, no necrotic areas are observed. The picture, therefore, only partly seems to overlap with that of malignant tenosynovial giant cell tumour, in which the neoplastic progression was likely identified at its onset.
From a practical point of view, these lesions (cutaneous satellitosis) should be regarded as locally aggressive but non-metastasizing lesions, and the absence of signs of relapse 1 year later is indicative of that.