Abstract
A 60-year-old man presented with increasing swelling of his right thumb, duration one year. Imaging studies demonstrated a bone-forming lesion extending from the dorsal cortex of the thumb metacarpal and involving the underlying medullary canal. Incisional biopsy yielded the diagnosis of parosteal osteosarcoma. The differential diagnosis for and rarity of parosteal osteosarcoma arising in the tubular bones of the hand are discussed.
INTRODUCTION
Osteosarcomas are rare in the distal extents of the appendicular skeleton. Parosteal osteosarcoma, a typically well-differentiated subtype of osteosarcoma, is exceedingly rare in the hand.1–4 More commonly, benign neoplasms and pseudotumors arise in the hand and are mistaken for parosteal osteosarcoma.5–7
CASE REPORT
This 60-year-old man was referred to the orthopaedic oncology clinic due to an enlarging mass in his right thumb over the preceding year. He experienced no pain from the mass and recalled no prior injury to his thumb or hand. He had no significant medical co-morbidities or medical history.
On physical examination, diffuse swelling was palpable around his right thumb metacarpal. It was not tender. His thumb was neurovascularly intact.
Imaging demonstrated a well-circumscribed bone-forming lesion associated with the dorsal cortex of the thumb metacarpal, but also involving the medullary space (Figures 1 and 2). Incisional biopsy demonstrated parosteal osteosarcoma with woven bone in a fibrous stroma, including nuclear atypia characteristic of a low-grade osteosarcoma (Figure 3).
Figures 1A and 1B.
Anteroposterior (A) and lateral (B) plain radiographs of the right thumb showing a bone-forming lesion arising from the dorsum of the thumb metacarpal. The lateral radiograph demonstrates a "string sign" with tumor growth abutting but not disrupting periosteum, which suggests a diagnosis of parosteal osteosarcoma (black arrow).
Figures 2A and 2B.
Magnetic resonance imaging of the right thumb metacarpal, demonstrating dorsal cortical destruction and both extramedullary and intramedullary involvement. The axial image in A is T1 weighted (TE 690, TR 14) and the coronal image in B is T2 weighted (TE 3456, TE 99.)
Figures 3A and 3B.
Photomicrographs at medium (A) and high magnification (B) of the hematoxylin- and eosin-stained sections from an incisional biopsy of a parosteal osteosarcoma showing woven bone of variable degrees of maturity within a fibrous stroma that demonstrates nuclear atypia and variable cell morphologies consistent with parosteal osteosarcoma.
The lesion was widely resected and the thumb reconstructed with a vascularized fibular autograft, which continues to function well four years later (Figure 4). There has been no recurrence or metastasis after the resection.
Figures 4A and 4B.
AP and lateral radiographs of thumb at five-year follow- up after resection of low-grade osteosarcoma and fibular graft reconstruction.
DISCUSSION
Only six cases of parosteal osteosarcoma have previously been reported to arise from tubular bones in the hand.1–4 However, a number of non-malignant mimicker lesions are often confused with osteosarcomas. These include florid reactive periostitis,7 bizarre parosteal osteochondromatous proliferation or Nora's lesion,5 myositis ossificans,6 and ossified hematoma.6
Distinguishing a parosteal osteosarcoma from these alternate benign lesions prior to treatment is critical. Wide excision of parosteal osteosarcoma is often curative, but the most successful attempt at this is usually the primary excision. While two of the previously reported hand parosteal osteosarcomas arose in phalanges and were successfully treated with digital amputation,1 two of the three previously reported cases of parosteal osteosarcoma in metacarpals underwent complicated treatment courses secondary to initial misdiagnoses.3,4 One ended in hand amputation,3 and the other resulted in lung metastasis following three resections for local recurrences.4
The features that helped to distinguish this lesion from possible benign mimickers were manifold. Many of the benign entities arise following antecedent injury of some sort, but such a history was lacking in this case. The lack of a radiographically zonal organization with peripheral density and central lucency was also helpful, although this feature is less distinguishing in the hand than in juxtacortical lesions associated with other long bones. The appearance of the lesion closely associated with, but not involving, the underlying cortical bone in its peripheral regions is highly suggestive of parosteal osteosarcoma. This feature is often termed the "string sign." Medullary involvement as determined by three-dimensional imaging also makes most benign entities less likely. Histopathologically, the presence of malignant-appearing osteoblasts weaving seams of irregular osteoid is best interpreted as osteosarcoma.
Because of its rarity, parosteal osteosarcoma remains primarily a diagnosis to be excluded in the hand. However, the implications of this diagnosis and the fact that it can occur in the hand must be considered carefully in each case. This article documents the sixth reported case of parosteal osteosarcoma arising in the hand. This entity should continue to figure prominently in the differential diagnosis of surface bone-forming lesions associated with the tubular bones of the hand.
TABLE 1. Reported Cases of Parosteal Osteosarcomas Arising in the Hand.
The first two cases above are reported in reference 1, the third in reference 3, the fourth in reference 4, and the fifth in reference 2. The final case in the table is presented in this report.
| Age/Gender | Tumor Location | Treatment Course | Outcome |
|---|---|---|---|
| 40F | index middle phalanx | digital amputation | no recurrence or metastasis 2 years later |
| not reported | index middle phalanx | digital amputation | no recurrence or metastasis 3 years later |
| 26M | R. long finger metacarpal |
|
no recurrence or metastasis 4 years later |
| 53F | L. index metacarpal |
|
recurrence 6 years later with lung metastasis |
| 47F | R. ring finger metacarpal | resection, extra-corporeal radiation, and reimplantation | no recurrence or metastasis 9 months later |
| 60M | R. thumb metacarpal | resection and free-fibular reconstruction | no recurrence or metastasis 5 years later |
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