Calcifying aponeurotic fibroma (CAF) is a very rare entity that was first described in 1953 [1]. It is a benign, solitary, and nonmetastasizing lesion [2], [3]. However, malignant transformations have been reported in individual cases [4], [5]. The pathogenesis remains uncertain although a fibroblastic/myofibroblastic origin has been suggested [6].

CAF clinical presentations are nonspecific. It is typically a slow-growing solitary lesion and a painless mass [7], although painful cases or multiple tumors have also been reported [8], [9], [10]. It is usually poorly circumscribed and may cause limitations to movement or discomfort [11]. CAF typically affects children and adolescents [12] but cases in adult patients have also been reported [8], [13]. It is more frequent in males [11]. CAF principally affects the volar fascia, tendons, and aponeuroses of the hands and feet [2], [14]. It has also been described in a wide variety of regions such as the neck, lumbosacral region, forearm, wrist, elbow, arm, thigh, popliteal fossa, chest wall, and abdominal wall [2], [12], [13], [15], [16], [17], [18]. More than 50 % of cases have been reported to recur locally, being more frequent in patients younger than 5 years old and during the first three postoperative years [7]. The recommended treatment is marginal resection [7].

CAF radiological findings are nonspecific. On radiographs, it appears as a soft tissue mass with or without fine calcification [19], and erosion of the adjacent bone or scalloping of the cortex may be observed [19]. Through ultrasound imaging (US) examination, CAF may be seen as well-defined or irregular masses, being either hypoechoic or isoechoic, and may have hyperechoic foci if there are calcifications [20]. Magnetic resonance imaging (MRI) is the best option for evaluating these lesions [19]. On the MRI, the calcification of CAF does not appear during the early stage and radiologists should be aware of this fact when differentiating CAF from other soft tissue tumors [20], [21]. On the MRI at the later stage, fat-like signals can appear at the site of calcification [21]. CAF may mimic malignancy, particularly with synovial sarcoma or undifferentiated pleomorphic sarcomas [2], [19], on preoperative MRI due to its ill-defined appearance. Computerized tomography is the best modality for the characterization of the calcifications located in an otherwise nonspecific soft tissue mass [22].

Macroscopically, most CAFs are poorly defined, gray-white, with or without calcifications, and measure less than 3 cm [11]. In terms of histology, various evolution phases have been suggested [16]. In the first phase, there is high cellularity composed of spindle mesenchymal cells with round or ovoid nuclei without atypia, which are arranged in loose infiltrative fascicles and small nests. They present scanty mitoses. Nodules with cartilaginous foci and no calcifications or focally calcified nodules may be present [11]. In the second phase, the presence of granular calcifications in the nodular component predominates. These are bordered by epithelioid cells arranged in short radiating columns with occasionally scattered osteoclast-like giant cells [11]. In the final phase, the tumor is predominantly hypocellular with a diffusely fibrotic and calcified stroma [11]. The lesion often extends into the adjacent tissue [11]. The main features seen in fine needle aspiration procedures are benign-appearing spindle cells, chondroid cells, multinucleated giant cells, hyaline, myxoid stroma, and calcified debris [23].

In terms of immunohistochemistry, CAF is positive to vimentin, CD99, CD68, muscle-specific actin (50 % of the cases), smooth muscle actin (50 % of the cases), S100 and WT1; and is negative to CD34 and desmin [11], [16], [23]. Positivity to EGF is also described [24]. The FN1 – EGF gene fusion appears to be the driver mutation of CAF and has not been observed in any other neoplasm [24].

In this study, we report ten cases of calcifying aponeurotic fibroma diagnosed in the La Paz University Hospital in Madrid with correlations between the clinical, radiological, and pathological features of this unusual mesenchymal neoplasm.