Tumor-like Lesions of Bones

Introduction and Conceptual Framework

The category of “tumor-like lesions of bone” comprises a heterogeneous group of non- neoplastic or pseudo-neoplastic conditions whose radiographic and sometimes clinical appearance mimics that of true bone tumors. Although none of these entities meets the strict pathological definition of a neoplasm — that is, an autonomous, clonal proliferation of cells with unlimited growth potential — they all share the capacity to produce focal bony lesions that can pose diagnostic dilemmas, generate symptoms of pain, swelling, or pathological fracture, and demand a working differential which encompasses every primary and secondary tumor of bone. Apley and Solomon, summarizing decades of orthopedic oncologic experience, emphasize that an inadequate distinction between a tumor-like lesion and a malignant neoplasm is the most common error in bone-tumor management; an osteosarcoma curetted as a “cyst” is an essentially irretrievable disaster, while a non-ossifying fibroma resected as a sarcoma is an unnecessary mutilation. The principles that emerge from this consideration are: (a) always interpret a focal bone lesion in the context of patient age, exact bone, and exact location within the bone; (b) consider the radiographic appearance using the standard checklist of Lodwick and Enneking — pattern of bone destruction, zone of transition, periosteal reaction, presence of a soft-tissue mass, matrix; (c) obtain definitive histology before definitive surgery whenever the diagnosis is not unequivocal. The entities classically grouped as tumor-like lesions, and addressed in this chapter as a synthesis of Miller’s Review of Orthopaedics, Apley & Solomon’s System of Orthopaedics and Trauma, Tachdjian’s Pediatric Orthopaedics, Netter’s Concise Orthopaedic Anatomy, Rockwood and Green’s Fractures in Adults, and Rothman-Simeone The Spine, are the following: simple (unicameral) bone cyst; aneurysmal bone cyst; fibrous dysplasia (monostotic, polyostotic, and the McCune-Albright and Mazabraud syndromes); non- ossifying fibroma and fibrous cortical defect (including the Jaffe-Campanacci association); Langerhans cell histiocytosis, of which eosinophilic granuloma is the localized form; the brown tumor of hyperparathyroidism; the sclerotic lesions of bone islands (enostosis), osteopoikilosis, osteopathia striata, and melorheostosis; myositis ossificans and post- traumatic heterotopic ossification; and finally a small number of conditions — Brodie’s abscess, intraosseous lipoma, intraosseous ganglion, osteofibrous dysplasia (Campanacci disease) — that are often encountered in the same radiographic differential. Each will be discussed with attention to epidemiology, pathology, clinical features, radiographic findings, differential diagnosis, treatment, and complications.

Simple Bone Cyst (Unicameral Bone Cyst, Solitary Bone Cyst)

Epidemiology and Etiology The simple bone cyst (SBC), also known as unicameral bone cyst (UBC) or solitary bone cyst, is one of the commonest tumor-like conditions of childhood and adolescence. Approximately two-thirds of patients are under the age of 14, and males are affected approximately twice as often as females. Although it can arise at any location, the vast

majority — well over 80% — occur in the proximal humerus or the proximal femur, with the proximal tibia, the os calcis, and the iliac bone forming the next tier of common sites. In the adult, when an SBC is encountered, it is most commonly in the calcaneus or the ilium, where the lesion may remain dormant for decades. The pathogenesis is not entirely established. The most enduring theory, proposed by Cohen and refined over decades, holds that the cyst represents a focal disturbance of venous drainage from the metaphyseal bone such that interstitial pressure rises above arterial inflow pressure, the marrow undergoes infarction and serous transformation, and a cavity forms. Elevated intracystic concentrations of prostaglandins, interleukin-1β, and matrix metalloproteinases support an additional inflammatory component to the bone resorption observed at the cyst wall. Pathology Macroscopically the SBC is a solitary, unilocular cavity lined by a thin fibrous membrane and filled with serous or serosanguineous fluid. Bony septa may project incompletely from the wall, giving the impression on imaging of a multiloculated structure, but pathologically these septa are best understood as cortical ridges rather than true compartmentalization. Histologically the lining membrane is composed of flattened fibroblastic cells supported by a thin layer of collagen; scattered osteoclast-type giant cells, hemosiderin, and occasional calcified material resembling cementum (so-called “cementicles”) may be present. There is no atypia, mitotic activity, or pleomorphism. The differentiation from aneurysmal bone cyst, with which it can coexist, depends chiefly on the absence of blood-filled cavernous spaces lined by fibroblast-like cells. Clinical Features The lesion is almost always asymptomatic until a pathological fracture occurs, which is the presenting feature in approximately two-thirds of cases. The fracture is typically through the proximal humeral metadiaphysis in a child of eight to fourteen years, often after a relatively minor athletic trauma. Pain or a slight swelling without fracture is occasionally noticed, and rare cases of presentation with limb-length discrepancy or premature physeal closure related to extensive cyst involvement are reported.

Radiographic Findings The plain radiograph is usually diagnostic. The cyst appears as a centrally placed, well- circumscribed, expansile lytic lesion of the metaphysis, abutting but not crossing the physis. The cortex is thinned but a thin rim of intact cortex usually persists, and the lesion typically respects the contour of the bone rather than producing an extracortical soft-tissue mass. The diameter of the lesion is approximately equal to or less than the diameter of the adjacent physis; this morphology distinguishes the cyst from aneurysmal bone cyst, which is typically wider than the physis it abuts. As the bone grows, the cyst migrates away from the physis toward the diaphysis, and it is therefore classified as “active” when in contact with the physis or as “latent” when separated from the physis by a margin of normal bone. After a pathological fracture, a small cortical fragment may settle dependently within the fluid-filled cavity — the classic “fallen-leaf” or “fallen-fragment” sign of Reynolds, pathognomonic of an SBC and a useful feature that distinguishes the lesion from solid

tumors. Magnetic resonance imaging shows a homogeneous, fluid-signal lesion (low T1, high T2) without enhancement of the contents and only a thin enhancing rim. Treatment Many simple bone cysts heal spontaneously following pathological fracture, and the management of an asymptomatic cyst is generally observation. When intervention is needed, the historical mainstays have evolved over several decades. Scaglietti’s percutaneous injection of methylprednisolone acetate (Depo-Medrol) into the cyst, popularized in the 1970s, achieves complete healing in roughly half of cases and partial healing in another quarter; multiple injections are often required, and the cumulative response can be improved by aspiration of cyst fluid prior to instillation. The combination of cyst decompression by multiple drilling, with or without bone marrow injection and demineralized bone matrix, has been advocated by several groups with healing rates approaching 80%. For lesions at risk of pathological fracture in mechanically critical locations — particularly the proximal femur — curettage and bone grafting, with or without internal fixation by an elastic intramedullary nail or a paediatric hip plate, is the most reliable method of achieving definitive healing. Pathologic fractures of the proximal femur in particular almost always require fixation and stabilization because of the high risks of avascular necrosis, coxa vara, and shortening if conservative management fails. Spontaneous resolution following uncomplicated fracture through the lesion in the proximal humerus has been reported in many series and, in the absence of further symptoms, may justify simple sling immobilization for these less mechanically critical fractures.

Aneurysmal Bone Cyst (ABC)

Epidemiology and Pathogenesis The aneurysmal bone cyst is a benign but locally aggressive osteolytic lesion characterized by blood-filled cavernous spaces. Three-quarters of cases occur in the first two decades of life with an even sex distribution. Long-bone metaphyses of the femur, tibia and humerus account for the majority of cases, but ABCs may arise in any bone and have a particular predilection for the posterior elements of the spine and the pelvic flat bones. Historically debated between “primary” and “secondary” categories, the modern molecular understanding distinguishes the two: the primary ABC harbors a characteristic translocation involving the USP6 gene on chromosome 17p13, demonstrating its clonality and supporting its classification as a true neoplasm despite the long-standing inclusion in tumor-like lesions; the secondary ABC arises within another pre-existing lesion — most commonly giant cell tumor of bone, chondroblastoma, osteoblastoma, fibrous dysplasia, or osteosarcoma — and represents a reactive vascular cystic change without the USP6 rearrangement. The clinical importance of this distinction is that an apparent ABC must be examined histologically with care to exclude an underlying neoplasm, particularly telangiectatic osteosarcoma, which can mimic ABC radiographically and histologically.

Pathology The lesion consists of cavernous, blood-filled spaces separated by fibrous septa containing fibroblasts, multinucleated osteoclast-type giant cells, scattered foci of woven bone (often with a basophilic mineralization pattern described as “blue bone”), and hemosiderin. The cavernous spaces lack endothelial lining — a critical feature, since true vascular lesions do have endothelium and the absence of endothelium is what distinguishes ABC histologically from a hemangioma. In a small but important fraction of cases, areas of solid ABC predominate over cystic areas, producing the so-called “solid aneurysmal bone cyst,” which can be radiographically confused with malignant tumor and histologically with giant cell reparative granuloma.

Clinical Features and Radiographic Findings Patients typically present with pain and progressive swelling at the affected site, often of several months’ duration; spinal lesions can produce nerve-root or cord compression. Radiographically the ABC appears as an eccentric, expansile, osteolytic lesion that elevates and progressively erodes the cortex, often with a characteristic “egg-shell” rim of periosteal new bone. The transverse diameter of the lesion typically exceeds the diameter of the adjacent physis — a feature that distinguishes ABC from simple bone cyst — and the lesion may break through the cortex into the surrounding soft tissues. MRI demonstrates the cavernous spaces as multiple fluid-filled compartments with characteristic fluid-fluid levels representing the layering of cellular debris and blood products at different stages of degradation; these fluid-fluid levels, although not entirely pathognomonic, are highly suggestive of ABC and are best seen on T2-weighted images. Contrast-enhanced sequences show enhancement of the septa but not the cyst contents. Treatment The treatment of an ABC must accomplish three goals: confirmation of the diagnosis (with particular attention to excluding telangiectatic osteosarcoma or an underlying neoplasm), eradication of the lesion with an acceptable rate of local recurrence, and preservation of function. Curettage of the lesion, with adjunct measures such as high-speed burring of the cyst wall, phenol cauterization, argon-beam coagulation, or local cryotherapy, followed by bone grafting or polymethylmethacrylate cement, is the standard surgical treatment. Local recurrence after intralesional surgery may approach 20%, and is more common in young children with metaphyseal lesions close to the physis. Selective arterial embolization, either as a definitive treatment for surgically inaccessible spinal or pelvic lesions or as a preoperative adjunct to reduce intraoperative blood loss, has gained importance over recent decades. Sclerotherapy, particularly with intralesional polidocanol or with doxycycline foam, has been reported with encouraging results in several pediatric centers. Radiation therapy was historically effective at stimulating cyst calcification but is now reserved for the rare unresectable lesion because of the risk of secondary sarcoma — particularly in the long-lived pediatric population — and growth-plate damage. Spinal aneurysmal cysts may require posterior decompression and stabilization in addition to lesion treatment.

Fibrous Dysplasia

Definition, Genetics, and Classification Fibrous dysplasia is a benign, medullary fibro-osseous lesion in which normal bone is replaced by abnormal fibrous tissue containing scattered, poorly mineralized, immature woven bone trabeculae. The lesion has been understood since the 1990s as a developmental disorder caused by a post-zygotic activating mutation in the GNAS1 gene, which encodes the α-subunit of the stimulatory G-protein coupling membrane receptors to adenylyl cyclase. The somatic mosaicism produced by this mutation explains the highly variable phenotype, ranging from a single asymptomatic monostotic lesion to widespread polyostotic disease with severe skeletal deformity. Three principal clinical phenotypes are recognized: monostotic fibrous dysplasia, by far the commonest form and accounting for perhaps 80% of cases; polyostotic fibrous dysplasia, in which multiple bones are involved; and McCune-Albright syndrome, the classic triad of polyostotic fibrous dysplasia, café-au- lait skin pigmentation (with characteristically irregular “coast-of-Maine” borders, in contrast to the smooth “coast-of-California” borders of neurofibromatosis), and endocrine hyperfunction — most commonly precocious puberty, but also hyperthyroidism, growth- hormone-secreting pituitary adenomas, and adrenal hyperplasia. The Mazabraud syndrome describes the association of fibrous dysplasia with intramuscular myxomas, and is a rare but well-documented entity. Pathology The dysplastic medulla shows a characteristic background of bland, whorled fibrous stroma supporting irregular, curvilinear trabeculae of woven bone that have been likened to “alphabet soup” or “Chinese characters” in their tortuous outline. Critically, these trabeculae lack an osteoblastic rimming — unlike the trabeculae of an ossifying fibroma, which they superficially resemble. The interface between dysplastic bone and adjacent normal bone is often well demarcated but may permeate the host trabecular bone. In approximately 1% of monostotic and up to 4% of polyostotic cases, malignant transformation occurs, typically to osteosarcoma, fibrosarcoma, or malignant fibrous histiocytoma; the risk is highest in irradiated lesions, which is one of several reasons radiotherapy is contraindicated. Clinical Features and Radiographic Findings Monostotic disease is often discovered incidentally on imaging performed for unrelated reasons. When symptomatic, presentation is with pain, deformity, or pathological fracture. The femur is the most frequently involved long bone, and the characteristic “shepherd’s- crook” varus deformity of the proximal femur — caused by repeated stress fractures with secondary plastic deformation through the weakened bone — is a clinical hallmark of severe polyostotic disease. The craniofacial skeleton, ribs, tibia, and pelvis are also commonly involved. In the spine, fibrous dysplasia can produce vertebral deformity, scoliosis, and rarely nerve-root compression. Radiographically the lesion is intramedullary, well demarcated, with a characteristic “ground-glass” or “smoky” matrix density that reflects the mineralization of the woven trabeculae; expansion of the bone, cortical

thinning, and a sclerotic rim are typical. Long-standing lesions may contain cystic or cartilaginous areas. The bone scan is hot, which is useful in mapping the extent of polyostotic disease. Treatment Asymptomatic and small lesions are observed. Symptomatic monostotic lesions can be managed with curettage and bone grafting, although the propensity of the dysplastic process to incorporate or replace any bone graft means that allograft, particularly cortical structural graft, is preferred over autograft, which is converted back to dysplastic bone over time. Internal fixation with intramedullary devices is generally preferred over plate fixation for diaphyseal lesions in long bones, both because the diseased cortex is poor for screw purchase and because the entire length of the bone may be affected by lesional or dysplastic tissue. The shepherd’s-crook deformity of the proximal femur is one of the classic orthopedic indications for valgus subtrochanteric osteotomy and intramedullary fixation. Bisphosphonates, particularly intravenous pamidronate or zoledronate, reduce bone pain and biochemical markers of bone turnover in polyostotic disease and McCune- Albright syndrome, although their effect on the radiographic appearance of the lesion is modest. Endocrine manifestations of McCune-Albright syndrome require dedicated medical management. Radiotherapy is contraindicated because of the risk of inducing sarcomatous transformation in the diseased bone.

Non-ossifying Fibroma and Fibrous Cortical Defect

Concept and Nomenclature The terms “fibrous cortical defect” (FCD) and “non-ossifying fibroma” (NOF) refer to a single histopathological entity at different sizes and stages: the small, asymptomatic, cortical-based lesion of a growing child is termed a fibrous cortical defect, and as it enlarges and migrates into the medullary cavity it acquires the designation non-ossifying fibroma. The lesion is best understood as a developmental anomaly rather than a true neoplasm, in which a focus of fibrous tissue persists within the bone before eventually ossifying as the child approaches skeletal maturity. As Apley puts it, the lesion is “asymptomatic and almost always encountered in children as an incidental X-ray finding.” Estimates suggest that as many as 30% of normal children have a fibrous cortical defect at some point in their growth, making this by some margin the commonest benign bony lesion of childhood. Pathology, Clinical Features, and Imaging Histologically the lesion is composed of unremarkable, bland fibrous tissue with scattered giant cells, foam cells, and hemosiderin-laden macrophages — a pattern that has historically led to the term “fibroxanthoma” being applied. There is no atypia. The classical site is the metaphysis of a long bone, particularly the distal femur, distal or proximal tibia, and proximal humerus. The lesion is eccentric, abutting or based on the cortex, and on plain radiographs appears as a well-circumscribed, oval, multiloculated lytic area with a

thin sclerotic margin and a long axis parallel to the long axis of the bone. Larger NOFs may attain a “soap-bubble” appearance. Jaffe-Campanacci Syndrome The association of multiple non-ossifying fibromas with café-au-lait spots, mental retardation, hypogonadism, cardiovascular and ocular anomalies constitutes the rare Jaffe- Campanacci syndrome. Its inclusion in the differential of polyostotic café-au-lait associations (alongside neurofibromatosis and McCune-Albright syndrome) is an exam- classic point. Treatment The natural history of the NOF is one of spontaneous resolution as the child matures, and the great majority require no treatment. Indications for intervention are limited to large lesions in mechanically loaded bones — typically those exceeding 50% of the bone diameter, or 33 mm in absolute size, especially in the distal femur or distal tibia — where the risk of pathological fracture justifies prophylactic curettage and bone grafting. Internal fixation is rarely required.

Langerhans Cell Histiocytosis (Histiocytosis X) and Eosinophilic Granuloma

Spectrum of Disease Langerhans cell histiocytosis (LCH), formerly named histiocytosis X and Letterer-Siwe / Hand-Schüller-Christian / eosinophilic granuloma, comprises a spectrum of disorders characterized by clonal proliferation of bone-marrow-derived Langerhans cells. The spectrum extends from the localized monostotic bone lesion (eosinophilic granuloma) at one pole, through multifocal disease without systemic involvement and the multisystem disease of Hand-Schüller-Christian (with the classical triad of diabetes insipidus, exophthalmos, and lytic skull lesions), to the acute disseminated Letterer-Siwe disease of infancy. Although strictly classified as a clonal disorder, the bone manifestations are frequently included among the tumor-like lesions because their imaging features, behavior in single-system disease, and treatment principles share much with the conditions discussed in this chapter. Pathology, Clinical Features, and Imaging The pathognomonic Langerhans cell, identified by S-100 and CD1a positivity and by the cytoplasmic Birbeck granules on electron microscopy, is admixed with an inflammatory infiltrate dominated by eosinophils — the source of the historical name “eosinophilic granuloma.” Children under 10 years of age account for the majority of cases. Bone pain, tenderness, and a soft-tissue mass are usual; spinal involvement, particularly of a thoracic vertebra, can produce the classical vertebra plana, the symmetrical collapse of a vertebral body to a wafer-thin disc of dense bone. Other characteristic sites include the skull (lytic lesions with sharp, beveled edges), the proximal femur, and the ribs. On plain radiographs the lesion is a punched-out, geographic lytic area, sometimes with a sclerotic margin and

adjacent periosteal reaction that can be confused with osteosarcoma or Ewing sarcoma — both of which are critical differentials. Treatment The natural history of single-system bony LCH is one of spontaneous resolution over months to years, and the majority of solitary lesions require nothing more than confirmation by biopsy or fine-needle aspiration and observation. Symptomatic lesions can be treated by curettage, intralesional corticosteroid injection (with methylprednisolone), low-dose radiotherapy, or systemic chemotherapy in multifocal or systemic disease. Vertebra plana usually reconstitutes a substantial proportion of normal vertebral height with growth, and spinal fusion is rarely required in the absence of neurological deficit.

Brown Tumor of Hyperparathyroidism

The brown tumor — so named because of its macroscopic hemosiderin-tinged color — is a focal lytic lesion of bone occurring in the setting of severe hyperparathyroidism. Although now uncommon in regions of routine biochemical screening, brown tumors remain a significant differential in patients with end-stage renal disease, in untreated primary hyperparathyroidism, and in patients from settings where parathyroid disease is not detected until advanced. Pathophysiologically the lesion is the localized expression of the diffuse process of osteitis fibrosa cystica: persistent parathyroid hormone elevation drives osteoclastic bone resorption, with replacement of marrow spaces by vascular fibrous tissue, hemorrhage, and reactive giant cells. Apley summarizes the appearance: “Haemorrhage and giant-cell reaction within the fibrous stroma may give rise to brownish, tumour-like masses, whose liquefaction leads to fluid-filled cysts.” Radiographically the lesion is an expansile, lytic area, often multiple and often associated with the characteristic features of hyperparathyroidism such as subperiosteal resorption of the radial aspects of the middle phalanges, terminal phalangeal tuft resorption, a “salt-and-pepper” granular skull, and erosion of the distal clavicle. Histologically the brown tumor is indistinguishable on its own from a giant cell tumor of bone, and the diagnosis is suggested only by the biochemical context: an elevated serum calcium, an elevated parathyroid hormone, and a relative or absolute hypophosphatemia. Treatment is overwhelmingly that of the underlying hyperparathyroidism, with most brown tumors regressing after parathyroidectomy or after correction of secondary or tertiary hyperparathyroidism in renal disease. Local treatment, with curettage or internal fixation for pathological fracture, is rarely required and is undertaken only after metabolic stabilization.

Sclerotic Tumor-like Lesions: Bone Island, Osteopoikilosis, Osteopathia

Striata, and Melorheostosis Bone Island (Enostosis) The bone island is a focus of compact, mature, lamellar bone within the cancellous bone of an otherwise normal medullary cavity. It is asymptomatic, an incidental finding on radiographs, and presents typically as a small, dense, round to oval focus of compact bone in the pelvis, proximal femur, ribs, or vertebrae. On bone scan the lesion is usually cold or

normal — a useful feature in distinguishing it from a sclerotic metastasis, which is typically hot. Bone islands greater than 2 cm have been termed “giant bone islands” and may require radiological follow-up to confirm stability, but malignant transformation is not described and they are best regarded as a normal anatomical variant. Osteopoikilosis (Spotted Bone Disease) Osteopoikilosis is an inherited autosomal-dominant condition in which multiple small foci of compact lamellar bone — essentially, multiple miniature bone islands — are distributed symmetrically through the cancellous bone of the long-bone metaphyses, the carpus, tarsus, pelvis, and proximal phalanges of the hands and feet. The condition is asymptomatic, of no clinical consequence, and is identified by its characteristic distribution: small (2-10 mm) round or oval foci of dense bone clustered around the joints. The chief clinical importance is the avoidance of mistaking the appearance for blastic metastases, which it can mimic — although the symmetrical, juxta-articular distribution, the small size, and the negative bone scan are reassuring. Association with collagenous skin nodules (the Buschke-Ollendorff syndrome) is well described. Osteopathia Striata Osteopathia striata, also termed Voorhoeve’s disease, is characterized by linear vertical striations of dense bone radiating from the physis along the long axis of the long bones, particularly in the metaphyses of the distal femur and proximal tibia. It is usually asymptomatic and incidental, autosomal-dominant inheritance has been described, and the condition is sometimes seen in combination with osteopoikilosis or with cranial sclerosis. Distinction from melorheostosis is by the radiographic morphology: striae rather than the candle-wax appearance of melorheostosis. Melorheostosis Melorheostosis is a sporadic, often unilateral developmental disorder of bone in which hyperostotic, irregular new bone is deposited along the cortex of long bones in a pattern reminiscent of melting wax dripping down a candle (“flowing candle wax” or “melted-wax” appearance). The condition typically follows a sclerotomal distribution, supporting a hypothesis of segmental developmental error. Pain, stiffness, and limited joint motion are common; soft-tissue contractures and limb-length discrepancy may develop in childhood. Histologically the bone is dense, lamellar, and abnormally organized; the periosteum and adjacent soft tissues are often involved. Treatment is symptomatic: physiotherapy, analgesia, sympathetic blocks, and occasionally surgical release of contracted soft tissues. Bisphosphonates have been tried with variable response. Osteotomies for limb-length discrepancy are sometimes required.

Myositis Ossificans and Post-traumatic Heterotopic Ossification

Myositis ossificans, more accurately described as post-traumatic heterotopic ossification, is the formation of mature lamellar bone within skeletal muscle and soft tissue following a usually clearly identifiable injury — a direct contusion, an athletic blunt-force injury, or a tear with deep hematoma. The condition is included among the tumor-like lesions because

both clinically and radiographically it can mimic a soft-tissue sarcoma, particularly extraosseous osteosarcoma. The early phases of the process are characterized by a painful, tender, swollen mass without radiographic abnormality; by two to four weeks, ill-defined flocculent calcification begins to appear within the lesion, and over the subsequent two to four months the lesion matures from the periphery inward, ultimately forming a well- circumscribed mass of mature lamellar bone separated from the underlying skeleton — the radiographic feature of “peripheral mineralization” with a more lucent center being characteristic. This pattern of maturation from the periphery inward, sometimes called the “zone phenomenon” of Ackerman, is the diagnostic mirror image of the extraosseous osteosarcoma, where mineralization is densest centrally. Histologically the same zonal phenomenon is found: mature trabecular bone at the periphery, immature osteoid in the middle, and a central zone of fibroblastic proliferation that can be highly cellular and atypical-appearing in the early phase — a critical pitfall, as a biopsy taken too early from this central zone can be misdiagnosed as sarcoma. Treatment is symptomatic in the early phase: ice, compression, gentle range-of-motion exercise, and avoidance of further trauma. Once mature, the heterotopic bone can be excised if it produces persistent pain or restricts joint motion; excision is best deferred for at least six months, ideally until the lesion is metabolically inactive on bone scan and serum alkaline phosphatase has normalized, to minimize the risk of recurrence. Single-dose perioperative radiotherapy (700-800 cGy within 24-48 hours of surgery) and oral indomethacin are the two principal adjuvants used to prevent recurrence after excision and to prevent heterotopic ossification after high-risk operations such as acetabular fracture fixation and total hip arthroplasty.

Additional Lesions in the Differential

A small number of additional lesions deserve mention because they are often considered alongside the conditions above when interpreting a focal bone lesion. The Brodie’s abscess, a subacute intraosseous focus of indolent staphylococcal infection, typically affects the tibial metaphysis in children and presents as a well-circumscribed lytic lesion surrounded by a halo of sclerotic bone; the “double-line” sign on MRI and elevated inflammatory markers usually distinguish it from a tumor-like lesion, but the clinical and imaging overlap is enough that infection should always be considered. The intraosseous lipoma, a rare benign fatty lesion most commonly of the calcaneus and the intertrochanteric region of the femur, can mimic a simple bone cyst on plain films; MRI is diagnostic because of the unique fat signal. The intraosseous ganglion is a benign cystic lesion typically subchondral in location at major joints, often associated with degenerative joint disease. Osteofibrous dysplasia of the tibia and fibula (Campanacci disease) is a fibro-osseous lesion of the anterior tibial cortex in infants and young children that is closely related histologically to fibrous dysplasia, but differs in its restricted location, its rim of osteoblasts, and its association with adamantinoma — for which it has been proposed as a precursor in a small subset of cases.

Principles of Diagnosis and Differential

Across all of these conditions, the synthesis of clinical history, age, exact anatomical site, and the radiographic features described by Lodwick (pattern of bone destruction:

geographic, moth-eaten, or permeative; zone of transition: narrow or wide), Enneking (presence and character of periosteal reaction and soft-tissue mass), and Mirels (in the assessment of pathological fracture risk) usually permits a confident differential and often a working diagnosis. The age of the patient is the single most important determinant: lesions of the first decade are dominated by simple bone cysts, eosinophilic granuloma, and Ewing sarcoma; the second decade adds non-ossifying fibroma, osteoblastoma, fibrous dysplasia, aneurysmal bone cyst, chondroblastoma, and chondromyxoid fibroma; the third and fourth decades emphasize giant cell tumor, enchondroma, brown tumor, and osteoblastoma; and lesions in patients over 40 are predominantly metastatic, myeloma, geode, or high-grade chondrosarcoma. The location within the bone provides further refinement: diaphyseal lesions classically include osteoid osteoma, Ewing sarcoma, adamantinoma, and fibrous dysplasia; metaphyseal lesions include almost all of the tumor- like lesions of this chapter, plus osteosarcoma; epiphyseal lesions are essentially restricted to chondroblastoma, giant cell tumor (after physeal closure), and geode. The matrix character on radiographs — chondroid (rings and arcs of calcification), osteoid (cloud-like dense mineralization), ground-glass (fibrous dysplasia), or fluid-fluid level (ABC) — supplies the final piece of imaging information. When the working differential remains uncertain, biopsy is required. The biopsy must be planned in consultation with the definitive treating surgeon, must follow the anatomical principles of placing the biopsy tract in the line of the future definitive resection so that the tract can be excised en bloc, must avoid contamination of multiple anatomical compartments, must obtain a sufficient quantity of representative tissue, and must include a sample for culture as well as histology. The Enneking and Mankin studies remain the touchstone of biopsy planning: a poorly planned biopsy compromises the eventual oncological outcome in a disturbingly high proportion of cases, and the principle that “the biopsy should be planned with as much care as the definitive surgery” should govern every case in which a malignant lesion is in the differential.

Summary and Take-Home Points

The tumor-like lesions of bone are a heterogeneous family that share, with one another and with true bone tumors, the capacity to produce a focal lytic, sclerotic, or expansile bone lesion. The simple bone cyst of the proximal humerus in a child; the eccentric, expansile aneurysmal bone cyst with its fluid-fluid levels; the ground-glass shepherd’s-crook deformity of fibrous dysplasia; the asymptomatic eccentric non-ossifying fibroma of the distal femoral metaphysis; the punched-out skull lesion or vertebra plana of Langerhans cell histiocytosis; the brown tumor of hyperparathyroidism in a patient with elevated serum calcium and PTH; the asymptomatic bone island; the symmetrical small foci of osteopoikilosis; the candle-wax cortical hyperostosis of melorheostosis; the maturing peripheral mineralization of myositis ossificans — each has a characteristic clinical, radiographic, and sometimes histological signature that, taken together, permits accurate diagnosis. The unifying principles of management are: confirm the diagnosis with appropriate clinical, radiographic, biochemical, and where necessary biopsy information; observe asymptomatic and self-limiting lesions; intervene with curettage and bone grafting when pathological fracture risk, persistent symptoms, or progressive deformity demand;

reserve radiotherapy for surgically unresectable lesions because of the risk of secondary sarcoma; and treat any underlying systemic disorder — hyperparathyroidism, McCune- Albright endocrinopathy, end-stage renal disease — appropriately. The single greatest pitfall is the failure to consider, and to exclude, an underlying malignancy in a lesion that appears straightforward on first inspection; this is the consideration that justifies the time, the imaging, the biochemistry, and the biopsy planning that the responsible orthopedic surgeon brings to every case.