Benign growing mass of the digit presenting as an ulcerated mass – case report and review of the literature

Abstract A 68 year-old female presents with an ulcerated mass of the 5th digit, with rapid growth during the previous month to surgery. The mass was excised and covered with a 4th dorsal metacarpal artery perforator flap. The histologic analysis was compatible with the diagnosis of fibro-osseous pseudotumor of the digit.


Introduction
The hand surgeon acts both as an oncological surgeon, ablating all the tumor mass, potentially compromising aesthetics and function, and as a reconstructive surgeon, trying to optimize hand function. Balancing these goals may be a demanding task [1].
Even though the majority of hand tumors are benign, especially if they do not involve the skin, some malignant tumors arise in the hand. In the latter, a more aggressive approach would be justified [2].
Fibro-osseous Pseudotumor of the Digits (FOPD) is a rare benign tumor with clinical characteristics that can mimic a malignant tumor. We present a case of FOPD and a review of the literature.

Case description
A 68 year-old female, smoker, presented to the ER with an ulcerated mass of the dorsum of the 5th finger involving the proximal phalanx to the DIP joint ( Figure 1). It had been progressively enlarging for the past year with substantial growth in the previous month. It was presented as a painless mass that limited PIP flexion. The neurovascular examination was normal. The ultrasound showed a 4.4 Â 2.6 cm hypoechogenic mass with a significant doppler sign.
Excision with the overlying skin was performed down to a tumor less surgical plane, preserving the extensor apparatus ( Figure 2). The resulting defect was covered with a 4th dorsal metacarpal artery perforator flap and the donor area primarily closed ( Figure 3). Histopathological study revealed a lobulated tumor, self-limited and centered in the dermis, causing epidermal ulceration. Histologically, it is composed of fascicles of uniform spindle cells, admixed woven bone without zonation. There's a mixture of fibroblasts and myofibroblasts, arranged in hyper and hypocellular hyalinized areas and deposits of osteoid rimmed by uniform osteoblasts. Cells have bland cytology and no necrosis or mitotic figures are seen, compatible with the diagnosis of FOPD (Figure 4 and 5). The patient was followed for 22 months with no evidence of recurrence ( Figure 6).

Discussion
FOPD is a rare entity. There are 173 cases documented in the literature , totaling 174 with the present case report ( Table 1). The first case dates back to 1931. There have been several terms to classify this entity including: 'pseudo-malignant osseous tumor of soft tissue' [36], 'parosteal fasciitis' [49], and 'florid reactive periostitis' [3,4,7,13]. This disease was unified under the term 'fibro-osseous pseudotumor of the digits' in 1986 by Dupree and Enzinger [50]. The tumor is characterized by reactive fibroblastic proliferation and focal bone formation limited to the skin and subcutaneous tissue. The tumor is not of bone origin, though [52]. It presents as an enlarging mass, which can be tender or painless. Even though a history of trauma has been described in as many as 40% of patients, it is hard to assess the relevance and prevalence of injury since minor trauma of the extremities is common. Skin ulceration may occur; the tumor mimics pyogenic granuloma, especially if it affects the toes [7,31,38,48]. Due to its growth and local aggressiveness, it can simulate malignancies. When mentioned, we found a high rate of suspicion of sarcoma [14,19,50,51], namely osteogenic sarcoma. Some case series did not report the individual patient data regarding age [19,46,50]. The age ranged from 5 to

81
, with a mean age of 36 and a median of 30 years old. The majority of patients were female (56.9%, 99 cases). It is more frequent in the upper limb (82.6%) than in the lower limb (16.9%). There is a sole case of FOPD outside the extremities [28]. In the upper limb, the most common segment affected is the finger (76.8%), more precisely the 2nd finger (32%) and the proximal phalanx (62.3%), followed by the hand (21.1%) and wrist (2.1%).
Radiologically, FOPD usually presents as a soft tissue mass with ill-defined margins and might reveal extraosseous ossification and bone formation. The periosteal reaction has been described [42]. Cortical involvement or bony destruction seldom occurs. MRI findings are commonly non-specific and frequently demonstrate a non-invasive soft tissue mass.
Histologically, FOPD presents as a multi-nodular lesion with irregular margins and it is localized in the dermis and subcutaneous tissues. Muscular and cartilage involvement is absent.
Immunohistochemically, the majority of studies has demonstrated positive staining for vimentin 14 [11,21,25,26,34], and focal reactivity to actin [18,21,26,27,40,51,53], which is in favor of myofibroblastic differentiation [18]. There is, however, controversial data, since Chan et al. found no reactivity to actin, myosin, myoglobin, desmin, and no dense bodies on electron micrograph, which is against the involvement       of myofibroblasts [11], and Sayar et al. found no evidence of immune-reactivity to actin and desmin [34]. The majority of cases demonstrated negative staining to S -100, CD 34, cytokeratin (MAK-6 and CAM 5.2), desmin, and epithelial membrane antigen (EMA) [3,11,18,[25][26][27]. Diagnosis is challenging -FOPD is a rare disease with only 174 cases documented, and it shares many histological and clinical features with malignant disease. This might lead to incorrect diagnoses, such as extraskeletal osteosarcoma, resulting in unnecessary procedures, including amputation. It should, however, be considered as a potential diagnosis of fast-growing tumors of the hand or feet with no previous history of trauma [42]. Osteosarcoma is usually diagnosed in older people (scarcely under 40) and is rarely found in the fingers, being more common in the large bones of the upper and lower extremities [42]. Myositis ossificans has been associated with FODP, being the latter considered by some authors as a superficial variant of the first. However, in myositis ossificans there is commonly a history of trauma [38,42,51]. Other differential diagnosis include bizarre parosteal osteochondromatous proliferation (Nora's lesion), ossifying plexiform tumor, acral osteoma cutis, and subungual exostosis [45,50].
FOPD might be included in a group of USP-6-rearranged myofibroblastic neoplasms that share a genetic rearrangement of the USP-6 gene and are known for their rapid yet self-limited growth and low recurrence rate [52,55]. Recent studies have identified COL1A1-USP6 fusions in the majority of their FOPD cases [50,52,56]. Molecular and genetic studies can therefore help diagnosis and avoid over-treatment, particularly in cases where biopsy is difficult to obtain [52,55], or clinical history is insufficient [52] since these lesions frequently mimic soft tissue sarcomas [52], but are self-limited and can be cured with a more conservative surgical excision [55].
The treatment for FOPD is surgical excision. Prognosis is excellent as recurrence has been related to incomplete excision [10,11,42]. We found 13 cases in which amputation was the treatment of choice [4][5][6][7][8][9][10][11]23,25,27,39]. Some of these cases were initially suspected or diagnosed as malignantde Silva and Reid described a 13% of malignancy assumption by radiologists and a 9% incorrect malignant diagnosis by pathologists [23], while in others the extension of the disease and bone destruction led the surgeons to decide for amputation.
Nine cases of recurrence were described in the literature [6,[8][9][10][11]15,23,31,43]. Most of them did not provide information about the reason for the recurrence. However, in three cases incomplete excision seems to be the reason for it thus reinforcing the need for complete excision. After complete removal of the tumor, no recurrences were found with a median follow up of 15  months [4,5,7-16,19,21-25,28-31,34-37, 40-46,48,53]. Even though no malignant transformation has been described in the literature, close follow-ups are recommended.

Conclusion
FOPD should be considered in the differential diagnosis of fast-growing lesions of the extremities. An excisional biopsy allows for complete tumor removal and avoids overzealous treatment. FOPD should be treated without radical excisions provided if totally removed.

Disclosure statement
The authors report no conflict of interest.