Teaching Case

A 22-year-old girl without fever or serological abnormalities presented with a painful swelling in her right fourth finger where she had suffered a mild trauma one month before (Fig. 1). Foreign bodies/insect bites were excluded. The nodule, increased despite the antibiotics, appeared tender, slightly mobile, erythematous without function/sensitivity deficits of the finger.

Radiographs showed swelling of the soft tissues without osseous findings (Fig. 2). A non-enhanced CT found a non-calcified, slightly hyperdense nodule (25 mm), cleavable from the bone cortex without erosion, periosteal reaction nor calcifications (Fig. 3). An MRI performed 12 days later demonstrated an increase of the nodule (30 mm) which had thin, smooth hypointense walls and an iso/hypointense matrix on T1-weighted images (even after fat suppression – Fig. 4a,d,e) with intermediate/slight hyperintensity on T2-weighted/STIR images, especially peripherally (Fig. 4b,f). It enhanced strongly with a “rim-like” pattern post gadolinium (Fig. 4c,g). No hypointense images compatible with calcifications were demonstrated and subcutis appeared hyperintense on T2-weighted images with normal appearance of bone cortex, bone marrow, muscles and joints. Displacement of the extensor tendon was noticed (Fig. 4a,b,c).

Background
The non-calcified form of fibrosseous pseudotumour of the digit (a.k.a. parosteal fasciitis, fasciitis ossificans, florid reactive periostitis) is an unusual presentation of a rare benign reactive process that affects young adults [1-3]. It can mimic infective or malignant processes (e.g. extraskeletal osteosarcoma) because of its rapid growth, hypercellularity, mitotic activity and low-grade cellular atypia. A correct diagnosis is critical to avoid amputation and radiotherapy [2, 4-7]. FOPD is a localized, non-neoplastic, heterotopic focus of ossification induced by fibroblast dysregulation and hyperstimulation of periosteal osteoblasts with formation of osseous trabeculae within a myxoid matrix. It can occur as a result of mild trauma/repeated microtrauma [8] even if in 40-60% of cases a specific trauma is not clearly referred [1-4, 6-9]. It is considered a superficial subtype of myositis ossificans (MO) that involves the subcutis of the tubular bones of hands/feet with calcifications spread within its stroma, lacking the zoning phenomenon of MO [2-4, 6, 7, 10].
Clinical perspective
The nodule presents as a hard, slightly mobile, painful, solitary, erythematous, fusiform swelling without infiltration of the adjacent muscles/tendons (no functional deficit). The size varies from 1 to several centimetres and it takes about 6 weeks to become clinically significant. Distal/middle phalanges or toes are seldom affected [6, 7]. Reported complications consist in ulceration or suppuration [8, 9].
Imaging perspective
Imaging findings should allow radiologists to locate the lesion and to describe its characteristics and its relationship with the adjacent tissues. The possibility of periosteal thickening with rare bone erosions has been reported [1]. The main imaging features are summarized in table 1 [5, 7, 10]. The first differential diagnosis was the giant cell tumour of the tendon sheath (TSGT) [7, 10], a benign neoplasm without history of trauma and onset as a painless, slow-growing nodule with functional deficit. MR examination can demonstrate its origin from the tendon sheath: such characteristics were not suitable to our case so our conclusion was a benign, extraskeletal entity arising from the subcutis.
Outcome
Histopathological examination after surgical excision revealed a mesenchymal lesion with a final diagnosis of fibrosseous pseudotumour (Fig. 5). Complete surgical excision was considered resolutive since follow-up data suggest that recurrence after appropriate conservative surgery is infrequent [1, 2, 6].
Take home messages
A rapidly growing, painful, erythematous nodule of the digit in young patients with a history of mild trauma/recurrent microtrauma complemented by imaging/histological findings can lead to the correct diagnosis, avoiding unnecessary aggressive treatments.