Two primary bone tumors of the childhood will deserve special attention in this topic: osteoid osteoma (OO) and chondroblastoma. OO is the most common of the benign bone tumors, usually found in males in the second decade of life. It is a small (<1.5 cm), hypervascular osteogenic lesion, found most often in the lower limbs. Intra-articular lesions comprise roughly 13 % of all OO and are more common in the hips, in the tarsal and carpal bones, and in the spine. Classic OO is a cortical tumor of the diaphyses of the long bones, which usually does not pose an important diagnostic challenge (Fig. 1.17). However, juxta-articular/intra-articular lesions may be difficult to diagnose because they are often subperiosteal or centrally located, not infrequently found in anatomically complex regions. Patients complain of local pain (which typically gets worse at night and is relieved by salicylates), inflammatory signs, and limited range of motion that may simulate primary monoarthritis. Typical radiographic findings include a radiolucent nidus with central calcification, periosteal reaction, and peripheral sclerosis (Fig. 6.1); however, detection of articular/periarticular OO may be particularly tricky due to superimposition of osseous ­structures, especially in complex joints (Fig. 6.2). In addition, the real size of the tumoral nidus is often underestimated, as reactive sclerosis and periosteal reaction are frequently mild or even absent in intra-articular lesions. Regional osteoporosis and widening of the joint space (due to joint effusion/synovitis) are also found, as well as premature osteoarthritis (Fig. 6.1). Computed tomography (CT) is the most sensitive imaging method to demonstrate the hypodense nidus, the internal calcifications, and the peripheral bone sclerosis, being especially useful in the assessment of intra-articular OO (Figs. 6.2 and 6.3). On magnetic resonance imaging (MRI), the nidus show low to intermediate signal intensity on T1-weighted images (T1-WI) and variable signal on T2-weighted images (T2-WI), with post-gadolinium enhancement in most cases (Figs. 6.1 and 6.2); calcifications appear as areas of low signal intensity in all sequences. MRI is the best imaging method to demonstrate the inflammatory component associated with OO, represented by bone marrow edema pattern (which may be very intense, up to the point of masking the tumor itself) and edematous changes of the surrounding soft tissues (Figs. 6.1 and 6.2). Although insensitive for tumor detection, US is useful to demonstrate synovitis and hyperemia. Scintigraphies are not particularly valuable for the diagnosis of intra-articular OO, usually showing diffuse and nonspecific uptake (Fig. 6.4).

The second bone tumor to be addressed in this topic is chondroblastoma, a rare and benign neoplasm that originates from cartilage germ cells in the epiphyses of the long bones of skeletally immature patients. It is more prevalent in males in their second or third decades of life, and typical locations include the proximal epiphyses of the humerus and of the tibia and the distal epiphysis of the femur. In most cases, chondroblastomas are small lesions (<4 cm), but as much as 60 % of these tumors may cross the growth plate and extend to the metaphysis; cortical rupture and intra-articular ­dissemination may also occur. On radiographs, they appear as lytic, geographic expansile lesions, with lobulated borders. A thin peripheral rim of sclerosis may be present, as well as punctate calcifications in the tumor matrix, which are even more evident on CT (Fig. 6.5). Roughly half of the patients present solid periosteal reaction along the diaphysis of the affected bone. On MRI, the lesion is predominantly hypointense on T1-WI and heterogeneous on T2-WI, varying from hypointense to hyperintense depending on the degree of mineralization of the tumor matrix. Calcifications are hypointense in all sequences, while cartilaginous islets are hypointense on T1-WI and hyperintense on T2-WI. Extensive bone marrow edema pattern (which is often disproportionate to the size of the tumor), joint effusion, and edema of the soft tissues can also be seen on MRI, as well as post-contrast enhancement of the tumor itself and of the above-described ­edematous changes. Fluid levels may indicate coexistence of an aneurysmal bone cyst.

Other conditions may simulate bone tumors and must be included in the differential diagnosis of true neoplasms. Cortical desmoids are common lesions, more frequently found in the posterior cortex of the medial femoral condyle of preadolescent and adolescent males, a few centimeters above the growth plate. They are benign, self-limited fibro-osseous lesions that may represent a developmental variant or a response to chronic traction in the insertion of the aponeurosis of the adductor magnus or at the origin of the medial gastrocnemius. On radiographs and CT, they vary from a subtle focal irregularity (Fig. 6.6a, b) to a bone defect (Fig. 6.6c–f) along the posterior cortex of the femur, which sometimes may be ill-defined, simulating an aggressive ­condition, especially if associated with bone overgrowth or periostitis. On MRI, cortical desmoids are hypointense on T1-WI and hyperintense on T2-WI (Fig. 6.6b), and a rim of low signal intensity in all sequences may be present; bone marrow edema pattern and edema of the adjacent soft tissues are also sometimes found. Although the bone defect itself may present some degree of post-gadolinium enhancement, there is no soft-tissue mass.

At least two dysplastic lesions that may simulate tumors also deserve mention here: focal fibrocartilaginous dysplasia (FFD) and dysplasia epiphysealis hemimelica (DEH). FFD is a rare entity, probably related to abnormal development of the mesenchymal anlage at the insertion of the pes anserinus. It is usually diagnosed during the second year of life, after the child begins to walk. Affected patients present pain, limping, and, occasionally, refusal to walk, often associated with progressive tibia vara. Radiographic and CT findings are typical, demonstrating a well-delimited cortical lucency with sclerotic borders in the anteromedial portion of the proximal tibial metaphysis, extending to the diaphysis (Fig. 6.7). On MRI, the lesion presents low signal intensity in all sequences due to the presence of dense fibrous connective tissue. FFD usually follows a benign course, with spontaneous correction of the deformity and resolution of the lesion within a few years, although surgical correction may be occasionally required.

Dysplasia epiphysealis hemimelica (DEH, also referred to as Trevor’s disease) is a rare skeletal abnormality of unknown etiology characterized by unilateral overgrowth of the epiphyseal cartilage. It usually affects joints of the lower limbs of males before age 15, being considered by many as an intra-articular variant of osteochondroma. Involvement of the medial portions of the epiphyses is twice as common as lateral involvement. The distal femur and the talus are the preferred sites, followed by the proximal and the distal tibia and the tarsal bones. There is a mass in the affected area (which may be painful) that frequently leads to restricted joint mobility, limb-length discrepancy, and joint deformity. DEH may affect one or more epiphyses in the same limb and, in some cases, the whole extremity is involved. Imaging findings are characteristic and biopsy is not necessary in most cases. Radiographs and CT reveal a variably calcified/ossified lobulated mass in one of the halves of the epiphysis of a long bone, in an apophysis, or in the tarsal bones (Figs. 6.8 and 6.9). As bone maturation occurs, the lesion tends to be incorporated into the adjacent bone. Nevertheless, some lesions remain separated from the host bone and may resemble loose bodies on radiographs. On MRI, DEH usually displays intermediate signal intensity on T1-WI and high signal intensity on T2-WI; scattered hypointense foci in all sequences are related to the presence of calcifications. Ossified lesions have signal intensity similar to that of the host bone (Fig. 6.8). Post-contrast enhancement is variably found, but edematous changes of the bone marrow and of the soft tissues are absent. MRI is the method of choice to evaluate the relation between the mass and the subjacent bone and to demonstrate the real extent of the lesion, mostly in small children, because of its ability to assess both calcified and non-calcified components (Fig. 6.8).