Soft Tissue and Bone Tumors

Chapter 139


Soft Tissue and Bone Tumors




Soft Tissue Tumors



Overview


In general, clinical presentation and findings guide diagnostic imaging in the assessment of soft tissue neoplasms. In children with subtle masses, the primary objective of imaging is to confirm or exclude the presence of a mass. In children with obvious masses, imaging is performed to assess the local extent of the tumor, to provide a differential diagnosis, and to guide potential biopsy.


Most soft tissue tumors in children are benign.1 When the mass is hard and fixed, conventional radiographs are often performed to determine whether an underlying bone lesion is present that is simulating, or is associated with, a soft tissue mass. Conventional radiographs may show secondary changes in the configuration of adjacent bones and may also reveal the presence of cortical bone erosion, periosteal reaction, and soft tissue calcifications. Occasionally, radiolucency indicates the presence of a fat-containing tumor.1


Superficial masses are often assessed by ultrasonography (US), a modality that is easily accessible, lacks ionizing radiation, and rarely requires sedation.2 In addition, color and spectral Doppler analysis can assess and characterize vascularity.


Nuclear medicine also plays a significant role in the assessment of soft tissue tumors. Many soft tissue sarcomas are detected by gallium-67 and thallium-201 scintigraphy.3,4 Technetium-99m–methylene diphosphonate (99mTc-MDP) bone scintigraphy may be performed to identify local bone involvement and distant skeletal metastases. In addition, positron emission tomography (PET) and PET computed tomography (CT) are playing an increasing role in initial tumor assessment and evaluation of tumor response and recurrence.


With continuing advances in magnetic resonance imaging (MRI), the role of CT has become limited. CT offers similar but more detailed information than that provided by conventional radiographs. Two areas in which CT has a clear advantage over MRI are in the detection of calcification, such as in myositis ossificans, and when lesions are present along the anterior abdominal wall or chest, where artifact may significantly degrade MRI quality.1 Also, CT of the lungs is usually performed to assess for metastatic disease.


MRI is the imaging study of choice for assessment of most soft tissue tumors. The superb soft tissue contrast and multiplanar images provide details on the tumor’s extent and relationship to underlying anatomic structures.


Although MRI technology continues to evolve, the basic tenets of the MRI examination of a child with a soft tissue tumor have remained constant. First, images should cover the entire tumor, including its margins, and they should include any needle biopsy tracts that may have to be excised at the time of surgical resection. Care must be taken to ensure quality images and coverage of the entire lesion and important adjacent structures. The area that contains the tumor should be imaged in at least two orthogonal planes. T1- and T2-weighted images should be obtained. Fat saturation is often added to confirm the presence of fat in a lesion and/or to highlight the presence of enhancement after intravenous gadolinium administration. Short tau inversion recovery (STIR) sequences are also useful, because they inherently suppress signal from fat and do not require a homogeneous magnetic field. Gadolinium-enhanced T1-weighted images add information about tumor vascularity and are helpful in guiding percutaneous biopsy in that viable tumor can be differentiated from areas of necrosis.5 Cystic lesions may be identified by their lack of central enhancement.6 MR angiography (MRA) is useful for detecting the presence of blood flow within the tumor and for planning the optimal surgical approach when large vessels may be involved. Most soft tissue tumors have prolonged T1 and T2 signal characteristics.7 The MRI signal characteristics of most soft tissue tumors are nonspecific and usually cannot predict histology, nor can they differentiate between benign and malignant neoplasms. Biopsy is necessary for histologic diagnosis.



Benign Soft Tissue Tumors



Desmoid Tumors




Overview: Desmoid tumors, representing deep or aggressive fibromatosis, are rare mesenchymal neoplasms with a fibrotic texture.8,9 Although distant metastases do not occur, these tumors are locally aggressive and can lead to significant morbidity and mortality. Desmoid tumors are slightly more common in females, with an incidence of 2 to 4 cases per million per year. The peak incidence is in the third and fourth decades of life.10 When these tumors occur in younger patients, they tend to be more aggressive, with recurrence rates up to 87%.11



Etiology: Desmoid tumors are classified into superficial and deep groups. Superficial tumors are usually small and slow growing, and deep lesions may occur in the abdomen or extraabdominally. In children, extraabdominal desmoid tumors are more common than the abdominal type. Most desmoid tumors in patients with Gardner syndrome are found in the abdomen. Extraabdominal desmoid tumors, also referred to as aggressive fibromatosis, are usually solitary and arise from the fascial sheaths and aponeuroses of striated muscle. In a series of tumors studied at St. Jude Children’s Research Hospital, head and neck, trunk, and extremity involvement was seen with approximately equal frequency.12 Histologically, desmoid tumors consist of benign fibrous tissues that contain spindle cells and abundant collagen. Although they do not metastasize, desmoid tumors can infiltrate contiguous structures, including bone.



Imaging: Desmoid tumors are variably echogenic on US, and their borders may be smooth or irregular. On contrast-enhanced CT, most appear more attenuated than striated muscle (Fig. 139-1). On MRI, these tumors may be nodular with infiltrative or well-defined margins. Tumors may be homogeneous or heterogeneous with varying signal characteristics. Some lesions are low signal on T1- and T2-weighted images, but more often tumors are heterogeneous and contain areas that are hyperintense to “fat” on T2-weighted images. On T1-weighted images, these masses contain areas that are hypointense, isointense, or slightly hyperintense when compared with muscle. This variability in signal reflects differences in the relative proportions of collagen, spindle cells, and mucopolysaccharides within the lesion (e-Fig. 139-2). On T2-weighted images, low signal generally reflects collagen, and high signal reflects a greater quantity of cellular tissue.13 Tumors with high signal on T1-weighted images have been found to contain fat or myxoid material. Contrast enhancement may be homogeneous, heterogeneous, or absent and does not correlate with clinical outcome.14





A desmoid tumor may be prospectively suggested by the presence of areas of T2 hypointensity suggestive of collagen stroma within the mass, but biopsy is ultimately required for definitive diagnosis.



Treatment: Stable asymptomatic desmoid tumors are often treated conservatively and observed for changes.15 Symptomatic desmoids must be treated, and therapy often depends on anatomic location. If possible, surgical resection with wide margins is the treatment of choice.10 However, recurrence is common (19% to 77%) and is more frequent with extraabdominal desmoids (30% to 50%) than with intraabdominal desmoids (15% to 30%).8,9,16 If surgery is not possible, systemic therapy should be considered. This is often the case when desmoids are associated with familial adenomatous polyposis or Gardner syndrome. These conditions increase the likelihood of postoperative complications such as hemorrhage, short-bowel syndrome, intestinal ischemia, obstruction, or fistula formation.9,15 Nonsurgical treatment options that include radiation and systemic therapy are also available. Patients treated with chemotherapy have a lower incidence of tumor recurrence compared with those treated with radiation.13



Infantile Myofibromatosis


Infantile myofibromatosis is the most common fibrous tumor of infancy. Tumors may involve skin, muscle, bone, or viscera and may be solitary (myofibroma) or multiple (myofibromatosis). Myofibromatosis occurs in children younger than 2 years of age. The prognosis of musculoskeletal lesions is excellent, and spontaneous resolution usually occurs, although visceral involvement may portend a poorer prognosis. Lesions have a variable appearance on US and range from solid to anechoic centrally with a thick wall. On CT, myofibromas enhance to a lesser degree or similarly to muscle and often exhibit a peripheral rim of enhancement. On MRI, lesions are low signal on T1 weighting and usually high signal on T2 weighting. Some masses show decreased central signal on T2 weighting, because these masses are composed of collagen and have cellular elements. Enhancement of the fibrous and cellular components is seen with gadolinium administration (Fig. 139-3).12,1721 Osseous lesions tend to occur in the metaphysis and have a lytic appearance (Fig. 139-4).20





Benign Peripheral Nerve Sheath Tumors





Etiologies and Clinical Presentation: Schwannomas account for approximately 5% of all benign soft tissue neoplasms, and they are usually found as a solitary lesion in individuals between 20 and 50 years of age. Although they may occur virtually anywhere in the body, the head and neck, flexor surfaces of extremities (notably the ulnar and peroneal nerves), mediastinum, and retroperitoneum are the most commonly involved sites.22 If the lesion is large, pain and neurologic symptoms may be clinical manifestations; otherwise, patients are usually asymptomatic.23,24


Neurofibromas may be localized, plexiform, or diffuse, and the localized form accounts for 90% of cases and is not usually associated with NF1, unlike the diffuse and plexiform subtypes.22 Localized neurofibromas account for approximately 5% of all benign soft tissue tumors and are typically seen in younger patients between 20 and 30 years of age.22 They often come to medical attention as a painless, slowly growing mass.23


Schwannomas and localized neurofibromas are similar histologically and are composed primarily of Schwann cells, therefore they exhibit similar imaging characteristics. Microscopic examination reveals a dense central core of Schwann cells surrounded by a peripheral zone of myxoid tissue. Peripheral nerve sheath tumors have a low incidence of malignant degeneration.22



Imaging: A tumor can be suspected to have a neurogenic origin if it is located along the distribution of a peripheral nerve. Calcification in degenerating (“ancient”) schwannomas may be visible radiographically; on bone scintigrams, these tumors have been observed to take up 99mTc-MDP.25 Peripheral nerve sheath tumors may be accompanied by subtle atrophy of surrounding or distally innervated muscle. Most of these tumors are well-defined spherical or fusiform masses. On CT images, they tend to be hypoattenuated, possibly because of lipids in their Schwann cells, adipocytes, and perineural tissues.26 On MRI, the bulk of the tumor shows low intensity on T1-weighted sequences and high intensity on T2-weighted sequences. Typically, the central zone consists of collagen and neurofibroma cells and is hypointense on T2-weighted images, lending a “target” appearance to the lesion that can also be seen on contrast-enhanced T1-weighted images; it is more easily appreciated when wide window settings are used to view the images. The presence of a target sign helps to distinguish these benign tumors from their less well-organized malignant counterparts (Fig. 139-5).2730



On MRI, differentiating a schwannoma from a neurofibroma can be difficult; however, schwannomas may contain more prominent areas of hemorrhage, cystic change, and necrosis with resultant heterogeneous signal intensities. In addition, neurofibromas intimately involve and are inseparable from the normal nerve. Schwannomas are eccentric to the nerve, and this may be apparent on MRI in tumors that involve larger nerves. When smaller nerves are involved, schwannomas may obliterate the nerve of origin, and the MRI appearance is indistinguishable from that of a neurofibroma.31


Plexiform neurofibromas arise from the axis of a primary nerve and form tortuous, cordlike tumors along its axis. They are regarded as indicators of neurofibromatosis, even when they are the sole manifestation of the disease. Tumors tend to appear as lobulated, amorphous masses with ill-defined borders. Similar to solitary neurofibromas, these tumors are usually hyperintense on T2-weighted imaging and often have well-defined, central, tubular, hypointense structures, or they may form large masses that resemble a “bag of worms” on transverse images.32


Diffuse neurofibromas are particularly uncommon and are often associated with NF1 (7 of 10 patients in one series).33 These lesions occur primarily in children and young adults and most commonly involve the head and neck regions. These ill-defined, infiltrative lesions tend to be located in the skin and subcutaneous tissues. They appear as linear or reticular strands of intermediate signal on T1-weighted images and are of high signal intensity on T2-weighted images, with linear areas of enhancement after gadolinium administration.



Benign Fatty Tumors




Overview: The 2002 World Health Organization (WHO) Soft Tissue Tumor Classification includes nine types of benign fatty tumor: lipoma, lipomatosis, lipomatosis of nerve, lipoblastoma/lipoblastomatosis, angiolipoma, myolipoma of soft tissue, chondroid lipoma, spindle cell/pleomorphic lipoma, and hibernoma. Except for lipoblastoma, all of these tumors are more common in adults than in children.24


Lipoblastoma and lipoblastomatosis are benign mesenchymal tumors of immature fat that occur primarily in infants and young children, most often in boys younger than 8 years of age. The average age of patients at the time of presentation is 3.6 years.34 Lipoblastomas are usually painless superficial tumors most commonly located in the extremities and the head and neck region, but they may also involve the trunk and deeper structures, such as the mediastinum and retroperitoneum. The tumor consists of lobules of immature adipose tissue with a variable amount of myxoid stroma separated by richly vascularized septa composed of connective tissue. The discrete form, lipoblastoma, is a well-circumscribed lesion that occurs in approximately 70% of cases and involves the superficial soft tissues. Lipoblastomas eventually evolve into mature lipomas. The term lipoblastomatosis refers to the diffuse type that often infiltrates adjacent deeper tissues, such as muscle, and has a tendency to recur locally35; spontaneous resolution has also been reported.36,37



Imaging: Imaging features reflect the amount of fatty tissue present. On US images, hyperechoic fat can be clearly delineated from the myxoid component, and CT images show a similar combination of hypoattenuated fat and denser myxoid tissue. On MRI, the signal is often heterogeneous (e-Fig. 139-6), and lipomatous elements appear hyperintense to muscle on T1-weighted images and isointense to subcutaneous fat on T2-weighted images; nonfatty tissues produce lower signal intensity than fat on T1-weighted images and are more intense than fat on T2-weighted images. In addition to low-signal myxoid components on T1 imaging, the fatty mass may contain low-signal fibrous septa that show contrast enhancement.3840




The differential diagnosis includes lipoma, which is much more prevalent in adults than in children; hibernoma, a rare tumor analogous to brown fat; and myxoid liposarcoma. Liposarcoma is rare in children younger than 5 years of age.41 Thus, a fat-containing tumor that occurs in a child younger than 2 years of age, even with nonlipomatous components, is almost invariably a lipoblastoma.



Treatment: The majority of fat-containing masses in children are benign. Although complete surgical resection is the treatment of choice for focal lesions,34 subtotal resection may therefore be considered, particularly when surgical resection may involve critical neurovascular structures, or when it may lead to significant cosmetic deformity.



Malignant Soft Tissue Tumors



Rhabdomyosarcoma




Overview: The head and neck and genitourinary tract are the most frequent locations of rhabdomyosarcoma (RMS). About 20% of these tumors occur in the extremities. Most extremity RMS is of the alveolar or undifferentiated histologic types, not the embryonal or botryoid types found in the face and neck and in the genitourinary system.42 Prognosis is less favorable for patients with RMS of the extremities than for those with tumors that arise from the genitourinary system or the head and neck region.43 In the extremities, tumors tend to be deep and tend to spread along fascial planes, and RMS may cause erosion of adjacent bone.




Imaging: Imaging is usually nonspecific but is essential for staging and surgical management. MRI has superseded CT and US in this clinical setting, because it can define the anatomic location of the tumor (unicompartmental vs. multicompartmental), indicate its relationship to important nerves and blood vessels, and reveal local involvement of bone or lymph nodes. MRI characteristics are nonspecific, and most tumors are predominantly low signal on T1-weighted images, high signal on T2-weighted images, and demonstrate central tumoral enhancement (Fig. 139-7). Only 15% to 20% of patients with RMS have clinically detectable metastases at presentation. The lungs, bone marrow, and bone are the most common sites of distant metastases. Lymph nodes may also be involved. Bone metastases resemble those that occur with neuroblastoma; they have been reported even in the absence of detectable primary tumor.





Primitive Peripheral Neuroectodermal Tumor and Extraosseous Ewing Sarcoma






Imaging: On MRI, these tumors have a nonspecific imaging appearance. PPNET and EOES are typically isointense to muscle on T1-weighted images and inhomogeneously hyperintense on T2-weighted and STIR images; they show variable contrast enhancement. When these lesions outgrow their blood supply or, alternatively, when masses hemorrhage spontaneously or after minor trauma, cystic components may be seen; these lesions may superficially mimic a hematoma (Fig. 139-8). Postcontrast imaging is helpful in these circumstances to help identify solid soft tissue components and guide biopsy of viable tumor cells. Distant spread occurs to bone, lung, liver, and brain.49,50





Synovial Sarcoma





Imaging: Radiographically visible calcifications are present in 30% of cases. On MRI, synovial sarcomas are often lobulated, well-defined, deep-seated lesions, although they may be infiltrative and can encase major blood vessels. Femoral vein invasion has been described, and erosion of the cortex of adjacent bones is present in up to 20% of patients.


MRI signal characteristics are nonspecific. Synovial sarcomas are usually isointense to muscle on T1-weighted images. Foci of high T1-weighted signal may be present, and fluid-fluid levels are caused by hemorrhage; tumors generally have heterogeneous signal with areas of high intensity on T2-weighted images (Fig. 139-9). In one large series, 35% of tumors showed a triple-signal pattern on T2-weighted images; these findings are consistent with high (fluid) signal intensity, intermediate signal intensity similar to that of fat, and low signal intensity resembling that of fibrous tissue (see Fig. 139-9, B).54 Some synovial sarcomas superficially mimic ganglion cysts (e-Fig. 139-10). A multilocular appearance with fluid-fluid levels may be seen in 18% to 25% of these tumors. As a rule of thumb, a diagnosis of ganglion cyst should only be considered when a fluid-filled neck can be seen extending from the cyst to a joint or tendon sheath, and no solid components lie within the cystlike lesion. Otherwise, a diagnosis of neoplasm should be favored, with biopsy recommended.5457







Malignant Peripheral Nerve Sheath Tumor






Imaging: Similar to benign neurofibromas, MPNSTs are deep soft tissue lesions that are often associated with primary nerves, especially those of the thigh and lower extremities. CT and MRI appearance is nonspecific. Tumors may be well or poorly defined, homogeneous or inhomogeneous, and they occasionally erode bone. Malignant transformation of benign neurofibromas should be considered in patients in whom the mass is painful or enlarging, or when the typical target appearance of a benign neurofibroma is absent (Fig. 139-11). Tumor uptake on gallium scintigraphy may indicate malignant transformation or progressive growth of neurofibromas; however, biopsy is usually necessary to confirm malignancy.23,24,26,2830





Infantile Fibrosarcoma




Overview, Etiologies, Pathophysiology, and Clinical Presentation: Infantile fibrosarcoma is an uncommon tumor that contains fibroblasts and myoblasts and occurs in young children, especially in the first 3 months of life.59 This tumor is now considered a low-grade malignancy, in distinction from an adult-type fibrosarcoma that occurs in older children (10 to 15 years of age) and is more aggressive. Clinically, infantile fibrosarcomas present as enlarging, sometimes painful masses. Tumors often occur in the distal ends of the extremities and occasionally in the head, neck, and trunk. Because of their high degree of vascularization, they may be confused with hemangiomas on physical and imaging examinations.60 These tumors may rarely erode adjacent bone, and angiographic studies may reveal tumor vasculature.61,62




Treatment: These tumors recur in up to 30% of cases, and metastases occur in about 5%.64,65 Unlike congenital RMS, infantile fibrosarcoma has an excellent prognosis, with 5-year survival up to 84%.66



Dermatofibrosarcoma Protuberans








Disseminated Disease of Soft Tissues



Lymphoma





Imaging: Muscle involvement results in solitary or multiple masses detectable on CT and MRI and with the use of gallium-67 scintigraphy and fluorodeoxyglucose (FDG)-PET. On CT, muscles affected by lymphoma appear diffusely enlarged with or without obliteration of normal fat planes. The tumor may be poorly defined, and its attenuation is equal to or slightly less than that of normal muscle on contrast-enhanced and noncontrast CT images. On MRI, masses are isointense or slightly hypointense to normal muscle on T1-weighted images; they are hyperintense on T2-weighted images and markedly hyperintense on STIR images, and they enhance homogeneously with gadolinium (e-Fig. 139-13). Abnormal activity on gallium-67 and FDG-PET scans correlates well with MRI findings.7577





Granulocytic Sarcoma







Bone Tumors



Radiographic Evaluation of Bone Tumors


Regardless of the site and aggressiveness of a bone lesion, the initial mode of imaging of bone tumors is radiography. Radiographs serve to confirm the presence and site of a tumor, assist in the formulation of differential diagnoses, characterize the tumor, and guide in the selection of further imaging.84,85 The decision to obtain radiographs is a clinical one, based on presentation, history, physical examination, and occasionally on laboratory tests. The differential diagnosis of a pediatric bone tumor can be narrowed down by asking a few simple questions.



How Old Is the Child?


Most bone tumors have a proclivity to occur within a certain age range. The differential diagnosis of a bone tumor in a 1-year-old infant is much different than that of a 16-year-old teenager or a 5-year-old child. Box 139-1 lists common pediatric bone tumors in accordance with the peak age at which they most commonly occur.8688




What Is the Location of the Lesion? What Bone? What Part of the Bone?


Many bone tumors have a proclivity to affect certain bones within the skeleton or to occur at certain locations within a bone. Box 139-2 lists common pediatric bone tumors according to their preferred location in growing long bones. Lesions also vary in their centricity relative to the involved bone. Some tumors, such as a simple bone cyst, are central, whereas others are eccentric within bone (nonossifying fibroma) or juxtacortical (osteochondroma, periosteal osteosarcoma).86,87,89




Is the Lesion Unifocal or Multifocal?


Some lesions are always solitary, and others are usually multifocal. Some may be solitary or multifocal, although multifocal disease often implies a systemic disease or an underlying syndrome predisposing the patient to the development of a particular type of bone tumor. Box 139-3 lists common pediatric bone tumors that are multifocal.




Is the Lesion Aggressive or Nonaggressive in Appearance?


In general, unlike malignant lesions, benign lesions have a nonaggressive radiographic appearance. Exceptions are common, and some lesions may have both aggressive and nonaggressive features. Box 139-4 lists lesions covered in this chapter by their characteristic radiographic appearance—aggressive, nonaggressive, or indeterminate.



Radiographic features of a nonaggressive and usually benign bone tumor are well-defined margins with a narrow zone of transition, particularly with sclerosis; expansion of bone contour from slow growth; smooth, single-layered periosteal new bone; and absence of an associated soft tissue mass. Radiographic features of an aggressive and usually malignant bone tumor are poorly defined margins with a wide zone of transition, permeative or “moth-eaten” bone destruction, frank destruction of bone without remodeling, aggressive forms of periosteal new bone, interrupted periosteal new bone, and the presence of an associated soft tissue mass. Aggressive forms of periosteal reaction include layering, or “onionskin,” and “hair-on-end” periosteal new bone. Interrupted periosteal new bone may take the form of a Codman triangle, a sign of an aggressive process.90


By answering the aforementioned four questions, the radiologist can often arrive at a single diagnosis or can at least narrow the differential to a few lesions. Most significant is the distinction between an aggressive and nonaggressive lesion; this is of major importance in guiding further imaging and initial therapy.


Some benign bone tumors are adequately defined by radiography and do not require any further imaging for diagnosis or treatment. Most bone tumors, however, do require additional imaging; this may take the form of CT, MRI, scintigraphy, PET scanning, and rarely even US. The choice of imaging for a given tumor depends on the differential diagnostic considerations, possible treatment options, and whether the lesion is aggressive or nonaggressive. MRI is usually the preferred modality in the delineation of aggressive and suspected malignant lesions, and radiographs followed by MRI adequately define most bone lesions. As opposed to radiography, on MRI an aggressive lesion may have a well-defined margin, particularly with T1 weighting. Although MR is also very good at delineating nonaggressive and likely benign lesions, CT is better able to delineate ossified bone and thus occasionally may better define the characteristics and anatomy of many benign lesions. Image-guided biopsy has become a viable option to determine or confirm the diagnosis of many bone lesions. CT, US, fluoroscopy, and even MRI may be used for guidance.91–95



Benign Bone Tumors


The vast majority of pediatric bone tumors are benign. It is important that radiologists recognize the typical benign tumors to avoid unnecessary diagnostic procedures. The differentiation of a nonaggressive lesion from an aggressive lesion will significantly affect the course of subsequent imaging evaluations, the approach to biopsy, and the preliminary choice of definitive treatment.



Cartilaginous Tumors



Osteochondroma (Exostosis):



Overview, Etiologies, Pathophysiology, and Clinical Presentation: In pathologic series, osteochondroma is the most common pediatric bone tumor. Rather than a true tumor, osteochondromas are thought to be a developmental defect of growing bone in which an injury to the perichondrium causes bone growth in an aberrant direction. It is theorized that islets of cartilage from the physis are displaced along the metaphyseal surface and then grow. Osteochondromas occur in approximately 1% of the general population. Solitary osteochondroma is slightly more common in boys than girls. Growth ceases at skeletal maturity, and lesions are usually detected in the second decade of life as they grow. Most osteochondromas are asymptomatic and are discovered incidentally. However, a host of complications can occur (Box 139-5). When one of a pair of adjacent bones is affected, the osteochondroma can cause pressure deformity of the other bone. Symptomatic presentations of osteochondroma are usually due to irritation of adjacent muscles, tendons, nerves, or rarely blood vessels.96,97 Pseudoaneurysm is a rare complication. A bursa may develop over an osteochondroma as a result of inflammation, and a pedunculated osteochondroma may fracture. Presentation with pain as a result of malignant transformation of a solitary osteochondroma in a child is extraordinarily rare.98,99



Osteochondromas develop in 6% to 12% of patients who received radiation at a young age. Latent periods vary from 3 to 16 years. Osteochondromas can occur even after low doses of radiation therapy and often occur in bones that were in the periphery of the radiation field. Multiple osteochondromas have been found in patients who received total-body irradiation as preparation for bone marrow transplantation at a young age. Sarcomatous degeneration of radiation-induced osteochondroma is very rare and of no greater incidence than with other osteochondromas.100


Patients with hereditary osteochondromatosis (multiple exostoses, diaphyseal aclasis) develop multiple osteochondromas throughout the skeleton. The disorder is autosomal dominant, and 10% of cases arise spontaneously. Patients have a mutation of the EXT1 gene family that results in an error in regulation of normal chondrocyte proliferation and maturation.101,102 In most patients, the disorder becomes manifest by 10 years of age. The multiplicity of lesions in these patients may lead to substantial deformity. Axial osteochondromas are frequently seen and may cause complications. Small lesions are common on tubular the bones of the hand. Most notable is a pseudo-Madelung deformity of the wrist because of forearm exostoses that cause ulnar shortening and angular deformity of the distal radius. Multiple metaphyseal lesions may interfere with normal modeling of the metaphyses.103


Malignant transformation of solitary osteochondromas, even those that are radiation induced, is exceedingly rare and probably occurs in fewer than 1% of patients.100,104 However, the reported incidence of transformation to chondrosarcoma in patients with hereditary osteochondromatosis is 0.5% to 25%. Wide variation reflects patient selection biases; the actual incidence is probably less than 5%. Malignant transformation does not occur until well after skeletal maturity. Clinical and imaging findings suggestive of malignant transformation include an osteochondroma that grows or begins to produce symptoms after physeal closure, a cartilaginous cap greater than 1.5 to 2 cm thick, indistinct lesion margins, new lucency within an osteochondroma, and an associated soft tissue mass. Chondrosarcomas tend to arise at the periphery of an osteochondroma and are usually of low histologic grade. If a diagnosis of chondrosarcoma is suggested based on pathology in someone in the first or second decade of life, it usually is misdiagnosed as chondroblastic conventional osteosarcoma or periosteal osteosarcoma, which are invariably chondroblastic.105,106


Dysplasia epiphysealis hemimelica (DEH), also known as Trevor disease, may be a manifestation of epiphyseal osteochondroma. Patients come to medical attention before 15 years of age, and 75% of patients are boys. Patients are seen initially with deformity, swelling, and pain. These patients form osteochondroma-like protuberances from the epiphyses. The lesions are usually confined to one side of the joint (medial more than lateral) and may occasionally involve contiguous joints in one extremity. The lower extremity (femur, tibia, talus) is usually affected, and the most commonly affected region is the ankle and hindfoot.107 Radiographs show deformity with irregular enlargement of one side of the epiphysis, and MRI is necessary to define the abnormality in younger children, because the lesions may be predominantly cartilaginous. With further ossification in older children, CT is preferred.


Subungual exostosis is a broad-based irregular osteochondroma of the tuft of the finger under the nail bed. The lesion is most common in males in the second decade of life and most commonly affects the great toe. Unlike conventional osteochondroma, there is no medullary continuity of the exostosis with the underlying bone.108,109



Imaging: On radiography, CT, and MRI, a hallmark of osteochondroma is continuity of the cortex and medullary space from the underlying bone into the lesion. On radiography, osteochondromas are most often found on the long-bone metaphyses, and 35% occur at the knee. Lesions in younger patients tend to be closer to the growth plates. Osteochondromas may also form on the pelvis, ribs, and scapulae; spinal lesions are rare. Underlying metaphyses are broadened as a result of disturbance of normal modeling. The shape of osteochondromas varies from sessile, plaquelike lesions (Fig. 139-15) to pedunculated lesions (Fig. 139-16) with a long stalk. The stalk of a pedunculated lesion is directed away from the adjacent joint. En face, osteochondromas may be mistaken for sclerotic intramedullary lesions.




CT nicely demonstrates the morphology of the lesions and usually confirms the diagnosis; however, MRI much better demonstrates the cartilaginous cap characteristic of the lesion (Fig. 139-17). On T2-weighted images and with other cartilage-sensitive sequences, the cartilaginous cap is seen as a well-defined, thin, high-signal crescent that caps the osteochondroma. The role of MRI is not to define the presence or absence of malignancy but to identify pathologic fractures or overlying soft tissue impingement. Pedunculated osteochondromas tend to fracture and cause soft tissue impingement, whereas sessile osteochondromas tend to only cause soft tissue impingement.110



In DEH, radiographs show deformity with irregular enlargement of one side of the epiphysis (Fig. 139-18). MRI is necessary to define the abnormality in younger children, because the lesions may be predominantly cartilaginous. With further ossification in older children, CT is preferred.

Dec 20, 2015 | Posted by in PEDIATRIC IMAGING | Comments Off on Soft Tissue and Bone Tumors
Premium Wordpress Themes by UFO Themes