Case 7

A 5-year-old boy presented with a hard fixed mass on the flexor aspect of the thumb.
    A: A lateral radiograph of the thumb at presentation. The lateral radiograph demonstrated a faintly ossified arising from the surface of the phalangeal cortex without involvement of the inner aspect of the phalanx.
    B: An isotope scan of the hands. The intense focal uptake at the site of the lesion is suggestive of active ossification within the lesion.

An excisional biopsy was done.
    C: A magnified axial CT of the hands.CT imaging show a faintly-radiodense lesion bounded by a shell of reactive bone that had destroyed the volar cortex of the phalanx.
    D: A portion of the tissue from the excisional biopsy. The tissue had a meaty feel with fine gritty granules within it. This is the gross texture of a lesion containing immature spicules of bone. The tissue also has a very vascular appearance.
    E: A panoramic field from the interior of the lesion. The field is composd of a network of immature trabeculae with no stress orientation. The stromal cells are admixed with many vascular channels containing red cells.
    F: A low power field from the prior field. This field again is rich in vascular channels. Here nad there a few large giant cells are present adjacent to the trabeculae.
    G: A high power field from the panoramic field. At this magnification large plump osteoblasts are lining the edges of the immature trabeculae. The stromal cells have the same appeaance as the osteoblasts. Neither have and cytologic atypia and mitoses are not evident.
    H: An adjacent high power field. The same features are seen. In addition there are two large osteoclasts in the lower right corner.

Diagnosis: Osteoblastoma

Osteoblastoma is a benign bone-forming neoplasm featured by repetitous bars of poorly-mineralized neoplastic osteoid. The age peak incidence is between 15 and 20 years old, and there is a predilection for the posteriors elements of the spine. The lesions more often present as a symptomatic slowly enlarging benign process. The radiographic features are dependent upon the degree of mineralization of the predominantly osteoid stroma.
Stage 2 lesions present as relatively radiolucent processes with a fine reticulated subtle radiodense pattern reflecting the homogeneous pattern of the poorly-mineralized immature bars of neoplastic osteoid. Although they may reach considerable size, lesions are crisply marginated by a thin ream of cortical bone that continues to contain the lesion. When they deform the overlying bone and cause thinning of the cortex, they are often described as "expansile". The differential diagnosis with stage 2 lesions is often fibrous dysplasia.
Stage 3 lesions present as radiolucent lesions with a permeative or infiltrative margin.They broach the overlying expanded or deformed cortex with significant soft tissue extension and are often associated with "blow out" aneurysmal cyst-like components.
Treatment of stage 2 lesions consists in careful curettage which is associated with a modest (circa 20%) recurrence rate. Marginal en block extracapsular excision of the intact lesion has a negligible recurrance rate, and when it does not significantly increase postoperative disability or risk of complications, is the more prudent procedure.
Stage 3 aggressive lesions have a significant recurrence rate after meticulous curettage (circa 30 to 50 %), which is indicated only when the anatomic setting makes marginal or wide excision impratical. En block marginal excision through the extracapsular reactive zone carries a lower but not insignificant risk of recurrence (circa 15 to 20 %) while wide en block excision beyond the reactive zone has a neglegible risk of recurrence.
In stage 3 lesions in difficult anatomic settings radiation therapy is worth consideration.