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

Overview: Benign peripheral nerve sheath tumors are divided into schwannomas, also known as neurinomas and neurilemomas, and neurofibromas. Benign neurofibromas and, less commonly, schwannomas are frequently multiple and are associated with neurofibromatosis type 1 (NF1). However, both tumors may be solitary and may occur sporadically.

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).^27–30

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.

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.^38–40

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.

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.

Etiologies, Pathophysiology, and Clinical Presentation: RMS contains a mixture of rhabdomyoblasts, which are recognized by their typical cross striations, and undifferentiated cells. These tumors are thought to arise from progenitor cells for striated muscle and can occur anywhere in the body, even in areas with no striated muscle. RMS accounts for more than 60% of soft tissue sarcoma (STS) in children younger than 5 years of age but only 25% of STS in those aged 15 to 19 years.⁴⁴

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.

Treatment: Even if not clinically detectable, all patients are considered to have micrometastatic disease; this has resulted in the universal use of chemotherapy. The 5-year survival rate for children with RMS has improved from 55% in the 1970s to more than 70% in modern times.⁴⁵

Overview: Primitive peripheral neuroectodermal tumor (PPNET) and extraosseous Ewing sarcoma (EOES) are small round cell neoplasms that belong to the Ewing sarcoma family of tumors; they can arise in both soft tissue and bone. These neoplasms are related histogenically and share a common cytogenetic characteristic, the translocation of bands 24 and 12 of the short arms of chromosomes 11 and 22, but they are often indistinguishable histologically. Also known as peripheral neuroepithelioma, PPNETs have a higher degree of neural differentiation than is seen with Ewing sarcoma; thus these two tumors can be distinguished on the basis of immunohistochemical markers. This distinction is important, because disease-free survival is poorer for patients with PPNETs than for those with EOES.46,47

Etiologies, Pathophysiology, and Clinical Presentation: Both tumors occur most commonly in truncal and paravertebral soft tissues (50% to 60% of cases) and in the extremities (25% of cases), although PPNET occurs less commonly in the extremities than does EOES, and patients with EOES are generally younger. Askin tumors are thoracic PPNETs that involve the chest wall.⁴⁸ These tumors can be very large at the time of presentation and tend to be poorly circumscribed. The soft tissue mass does not calcify but may erode adjacent bone.

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

Treatment: Treatment is dictated by location and extent of disease and may involve surgery for localized disease with or without adjuvant/neoadjuvant chemoradiotherapy.⁵¹

Overview, Etiologies, Pathophysiology, and Clinical Presentation: Synovial sarcomas predominantly occur in adults younger than 50 years of age, but they account for about 10% of pediatric STS. These tumors arise from undifferentiated mesenchymal cells, not from true synovial cells. A monophasic variety comprises spindle cells, and a biphasic variety consists of spindle cells and epithelial elements. Although synovial sarcomas often occur close to joints, tendons, and bursae, they are rarely intraarticular.⁵² About 80% of synovial sarcomas occur in the extremities, and considerably more lower than upper extremity involvement has been noted. Synovial sarcomas may spread to regional lymph nodes, and the lungs are the most common sites of distant metastasis.⁵³

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.^54–57

Treatment: The treatment of choice is surgery with or without adjuvant and neoadjuvant chemotherapy. A recent multicenter study of patients under 20 years of age with a minimum of 10 years of follow-up found that wide surgical resection is the most efficacious method of treatment. Presence of tumor in the trunk and high histologic grade were negative factors for recurrence-free and metastasis-free survival.⁵⁸

Overview: Malignant peripheral nerve sheath tumor (MPNST) is the accepted name for a spindle cell sarcoma that arises from a nerve or a neurofibroma. In contrast to other STS that is of mesenchymal cell origin, MPNSTs are of neuroectodermal origin. The MPNST designation has replaced many formerly used terms, including malignant schwannoma, neurofibrosarcoma, neurogenic sarcoma, and malignant neural neoplasm. A variant of MPNST is the so-called triton tumor (named after a salamander) that contains neural and rhabdomyosarcomatous elements.²²

Etiologies, Pathophysiology, and Clinical Presentation: MPNST accounts for about 4% to 10% of STS, and they are the most common malignancy associated with NF1. Half of these tumors occur in patients with NF1; conversely, 2% to 29% of patients with NF1 develop MPNSTs—a much higher incidence than is seen in the general population. MPNST patients with NF1 are usually younger than those without associated NF1. Furthermore, in patients with NF1, MPNSTs tend to arise in preexisting benign neurofibromas; they are high-grade tumors that have a tendency to recur locally and metastasize. MPNSTs may also arise at previously irradiated sites.³¹

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.^{23,24,26,28–30}

Treatment: Surgical resection with or without adjuvant and neoadjuvant chemoradiotherapy is used for treatment, depending on the tumor’s location.

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

Imaging: Because these masses are often present during fetal life, extensive remodeling of adjacent bones may occur (e-Fig. 139-12). MRI characteristics are nonspecific. Fibrosarcomas are usually isointense to muscle on T1-weighted images and are hyperintense on T2-weighted images. They may contain hypointense foci, which correlate with fibrosis.⁶³

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

Overview, Etiologies, Pathophysiology, and Clinical Presentation: Dermatofibrosarcoma protuberans is an intermediate-grade malignancy that involves the dermis. This tumor is most often seen in adults; it occurs rarely in children but may be seen at birth. The tumor most commonly appears as a red-blue or pink plaque that grows slowly and may become nodular. A less common atrophic variant occurs as a depressed plaque. Lesions are typically fixed to the dermis but may extend into the underlying tissues. Lesions frequently recur locally, and metastasis occurs in 1% to 6% of patients; 75% of metastases occur in the lungs.67,68

Imaging: MRI is useful for determining the extent of disease, especially with deep tumor invasion. MRI characteristics are nonspecific. On T1-weighted images, lesions are hypointense to fat and may be isointense, hyperintense, or slightly hypointense to muscle. On T2-weighted images, lesions are isointense or hyperintense to fat. Lesions can enhance after gadolinium administration. On MRI, these tumors may be confused with subcutaneous granuloma annulare, a benign localized inflammatory dermatosis that occurs in children.⁶⁹

Therapy: Treatment involves surgical resection with or without adjuvant and neoadjuvant chemoradiotherapy. One review found that only the presence of metastasis had an overall decreased patient survival rate, and those with fibrosarcomatous change or acral location had a decreased disease-free interval despite wide local resection.⁷⁰

Overview, Etiologies, Pathophysiology, and Clinical Presentation: Muscle involvement by non-Hodgkin lymphoma (NHL) is usually due to metastatic spread via lymphatic and hematogeneous routes; however, it may be the result of direct extension from primary bone lymphoma. Much less commonly, muscle lymphoma occurs as a primary extranodal tumor. The disease can cross compartmental boundaries, or it may invade subcutaneous tissues. Involvement of adjacent bone and bone marrow may also be noted. Primary T-cell lymphoma of the skin is referred to as mycosis fungoides. Typical findings include focal thickening caused by dermal and epidural infiltrates and lymphadenopathy in advanced-stage disease.⁷⁴

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.^75–77

Overview, Etiologies, Pathophysiology, and Clinical Presentation: Granulocytic sarcoma is a rare solid tumor of primitive precursors of white blood cells and is also known as chloroma and extramedullary myeloblastoma. It occurs in patients with acute and chronic myelogenous leukemia and other myeloproliferative diseases. Children are more often affected than adults, and 60% of these tumors occur in children younger than 15 years of age.⁷⁸

Imaging: Chloromas may be solitary or multiple, and they can involve any part of the body, including the brain and muscles. Orbits and subcutaneous tissues, however, are the most common sites (see Chapter 7). These tumors are typically isoattenuating to slightly hyperattenuating to muscle on noncontrast CT and hyperattenuating to muscle on contrast CT. On MRI, lesions are typically isointense to muscle on T1-weighted images and hyperintense to muscle on T2-weighted images. Tumors usually enhance after gadolinium administration.^79–81

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.