ABC is a type of benign lesion that may occur in any bone, but rarely in soft tissue. It occurs predominantly during the first two decades of life. An ABC is composed of numerous irregular cysts filled with blood and separated by connective tissue septa containing fibroblasts, osteoclast-type giant cells and reactive bone (1
). It usually appears on radiographs as an eccentric lytic and expansile lesion with a well-defined margin within bones (1
ABC was first described by Jaffe and Lichtenstein in 1942 (9
). Its development has been widely regarded as a reactive process since then. However, it is recognized that there is no previous lesion in the majority of cases of this disease. Panoutsakopoulos et al
) demonstrated the presence of a chromosomal translocation t(16;17)(q22;p13) in two osseous cases, suggesting a neoplastic process. These initial findings were later confirmed by several subsequent cytogenetic studies (11
). Oliveira et al
) reported that the t(16;17)(q22;p13) translocation fuses the promoter region of the osteoblast cadherin 11 gene (CHD11) on chromosome 16q22 to the entire coding sequence of the ubiquitin protease USP6 gene. Ye et al
) showed that the overexpression of USP6 in pre-osteoblastic MC3T3 cells is sufficient to drive the formation of tumors that reproduced the molecular and histological features of ABC. These authors also suggested that USP6 may play a direct role in establishing a degradative and vascularized microenvironment. Five partner genes known to upregulate USP6 expression in ABC have been identified: CDH11(16Q22), ZNF9(3Q21), OMD(9Q22), COL1A1(17Q21) and TRAP(1P34) (15
). In addition, Geiersbach et al
) reported an ABC case with an SS18 rearrangement, which has not previously been described. These findings clearly demonstrate that ABC is a neoplastic disease.
STABC is extremely rare. The number of published STABC cases does not exceed 20, with only a few epidemiological and histological reports (7
). Of these cases, only 5 are in the pediatric age group, as shown in . Our patient is a 10-year-old girl with a lesion in the posterior aspect of the left shoulder. STABC shows a rather similar appearance to its osseous counterpart on radiography and MRI (4
). In our case, plain radiographs revealed a rectangular lesion with a fuzzy image of non-uniform density in the center, when observed carefully. Although the peripheral rim was not as clear as described in previous cases, the specific features of ABC on MRI, including an expansile lesion surrounded by a thin low- signal rim, increased signal with augmented T1-weighting and a lobulated contour with fluid-fluid levels were all clearly visible in our patient. The MRI presentation of our case may be described as relatively typical.
The differential diagnosis of STABC mainly includes giant cell tumor of soft tissue, giant cell tumor of the tendon sheath, extraskeletal telangiectatic osteosarcoma (EOS) and myositis ossificans.
Giant cell tumors are often associated with ABCs, since ABCs often contain a large number of giant cells. These two diseases are easily confused, and as a result it is occasionally difficult to distinguish ABC from a giant cell tumor, especially when a giant cell tumor exhibits bleeding, necrosis and cystic changes. However, giant cell tumors usually occur in adults over the age of 20, whereas ABCs are often found in the first two decades of life. However, the results noted in show that the age of patients with STABC is not fixed. Histologically, compared with giant cell tumor of soft tissue or giant cell tumor of the tendon sheath, STABC often contains more reactive bone-like tissue and woven bone with a well-defined peripheral rim, but fewer giant cells.
Differentiation between STABC and EOS is essential as they require completely different management and have different outcomes. EOS most commonly affects individuals older than 30 and is rarely encountered during the first two decades of life (17
). EOS differs from STABC in that it usually has an ill-defined rim on X-rays and is more prone to becoming malignant. Microscopically, anaplastic tumor cells and atypical mitosis may be found in EOS, which do not exist in STABC.
Although radiography and CT may reveal certain similarities between STABC and myositis ossificans, i.e., a radiolucent lesion with a thin rim of ossification at the periphery, the presence of septa within the lesion in STABC may differentiate it from myositis ossificans (4
). Myositis ossificans usually contain a solid inner component, whereas STABC does not show the presence of any solid parts (except for the septa). Nevertheless, STABC may also be secondary to myositis ossificans. In this case, cystic spaces filled with blood and connective tissue septa are also the main points of differentiation.
In conclusion, STABC is an extremely rare type of benign soft tissue tumor, with no more than 20 cases having been reported in the English language literature. It is occasionally secondary to lesions such as giant cell tumor, myositis ossificans and conventional osteosarcoma, therefore caution should be used in the diagnosis of STABC. If STABC is correctly diagnosed, surgical treatment should be implemented with complete excision considered the most appropriate treatment.