Dermatology Online Journal

Fibro-osseous pseudotumor of the digit: Presentation as an enlarging erythematous cutaneous nodule
Kong-Bing Tan MBBS FRCPA FRCPath¹, Sze-Hwa Tan MB BCh BAO¹, Derrick Chen-Wee Aw MRCP (UK) MMed (Int Med) FAMS (Dermatology)², Yoke-Sun Lee MRCP (UK) FRCPA MD¹
Dermatology Online Journal 16 (12): 7

1. Department of Pathology
2. Department of Medicine
Yong Loo Lin School of Medicine, National University Health System, Singapore. pattankb@nus.edu.sg

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Abstract

Fibro-osseous pseudotumor of the digit is an unusual ossifying fibro-proliferative lesion. The current case is a 54-year-old woman who presented with an enlarging reddish nodule on the distal aspect of her left big toe. Excision biopsy was performed and the histopathology featured a dermal-centered lesion composed of osteoblast-rimmed bony trabculae lying amidst fibrovascular stroma. This entity resembles and may be within the spectrum of myositis ossificans, the prototype post-traumatic reactive fibro-osseous proliferation, although the latter is typically more proximally located and features a zonation pattern on histopathology. The distinguishing features from other pathological differential diagnoses, such as subungual exostosis and extraskeletal osteosarcoma, are discussed. The growth is considered benign and local recurrence following complete excision is unusual.

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Case report

Figure 1
Figure 1. Nodular reddish lesion on the left big toe

The patient, a 54-year-old woman, presented with a tender nodule on the distal aspect of the left big toe, slowly growing in size over a 2-month period. Examination revealed an erythematous, ulcerated, and mobile nodule measuring 1 x 0.5 x 0.5 cm in size (Figure 1). In view of the short history and a richly vascular appearance, a clinical diagnosis of pyogenic granuloma was made. Another diagnosis considered was glomus tumor. A complete excisional biopsy was performed, followed by electro-cautery of the lesion base.

Figure 2	Figure 3
Figure 2. Scanning magnification view showing a dermal-centered fibro-osseous lesion with surface acanthotic epidermis with focal ulceration. Figure 3. The dermis shows osteoblast-rimmed bony trabeculae lying within fibrovascular tissue.

The tissue was entirely sampled for histopathological examination. The nodule was covered by an acanthotic and hyperkeratotic epidermis with an area of ulceration (Figure 2). In the dermis, bony trabeculae, rimmed by bland-appearing osteoblasts, were observed lying within a fibrovascular stroma (Figures 2, 3, and 4). Stromal blood vessels were prominent and variably dilated (Figure 4). In areas, the bony trabeculae were of varying maturity and the background fibrous stroma was moderately cellular (Figure 5). Scattered stromal mitoses and lymphocytes were seen. A zonation pattern for the fibro-osseous lesional tissue was not evident. An overlying fibro-cartilaginous cap was not identified. A diagnosis of fibro-osseous pseudotumor of the digit (FOPD) was made.

Figure 4	Figure 5
Figure 4. Higher magnification view showing osteoblast-rimmed bony trabeculae and intervening richly vascular stroma. Figure 5. Higher magnification view of another area showing osseous tissue of varying maturity (arrows), with intervening fibroblastic stroma.

Discussion

Fibro-osseous pseudotumor of the digit is a rare and benign fibro-osseous proliferative lesion of the skin and superficial soft tissue [1, 2, 3, 4, 5]. Synonymous reported entities include florid reactive periostitis, parosteal fasciitis and fasciitis ossificans [6, 7, 8]. Clinically, it presents as a non-specific nodule that is slightly mobile. X-ray study reveals the presence of a soft tissue density with specks of calcification/ossification, with no connection with the underlying bone.

The clinical history of the current case epitomizes the known epidemiology of FOPD. The growth is slightly more common in women than men with a predilection for occurrence in fingers and toes of adults. Fibro-osseous pseudotumor of the digit is regarded as a reactive process and trauma is thought to play some role in its pathogenesis, being reported as a preceding event in between 11 to 43 percent of cases [1, 2, 3]. The variably florid spindle cell proliferation is myofibroblastic in nature as evidenced by immunohistochemical features of smooth muscle actin positivity and desmin negativity, whereas the osseous component represents a metaplastic phenomenon.

An unusual feature in the current case is the prominent stromal vascularity which was evident on the histopathology (Figure 4). Clinically, this correlates with the reddish appearance, prompting the diagnostic consideration of a cutaneous vascular neoplasm.

The histopathological nature of the lesion may be mimicked by several other entities, both benign and malignant. Myositis ossificans (MO) is most similar to FOPD and also features a fibroblastic proliferation with osseous metaplasia, often arising following trauma [2, 3]. Some authors consider MO and FOPD as closely related entities while others regard them as being within the same disease spectrum but with occurrence in different locations [1, 2, 3, 4]. Myositis ossificans typically occurs in the more proximal aspect of the limbs and involves the deeper soft tissue compartment. Histopathologically, it is characterized by a zonation pattern, featuring a fibroblastic tissue core or base and a superficial rim of osseous tissue. Such a zonal organization may or may not be present in FOPD [1, 4]. Subungual exostosis can appear clinically and histopathologically very similar to FOPD, except for the presence of connection to the underlying phalangeal bone, the presence of bone marrow tissue, and the feature of an overlying fibro-cartilaginous cap [9]. Extra-skeletal osteosarcoma is a rare malignant tumor that may present a diagnostic pitfall [2]. It is typified by being destructive and more often localized to the deep soft tissue. The neoplastic spindle cells in between the lesional osseous tissue show marked nuclear atypia with mitotic activity.

Fibro-osseous pseudotumor of the digit has an excellent prognosis following complete excision with low risk of recurrence (between 0-14% in various series) [1, 3]. No cases of malignant transformation or reports of metastases are on record [1-8].

ACKNOWLEDGEMENTS: The authors thank Mr. TC Tan for the photomicrography.

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