14.4 DXI Tumor-like Processes

Tumor-like Processes of Bone

Quick Answer: Tumor-like lesions are conditions that produce bone changes mimicking a true neoplasm on imaging but that arise from a developmental, metabolic, or reactive process rather than uncontrolled cell proliferation. The four DXI-tested lesions are Paget disease of bone, fibrous dysplasia, unicameral (simple) bone cyst, and non-ossifying fibroma. None of these behaves like a true malignancy, but each can be radiographically confused with one, so recognizing their signature patterns prevents unnecessary anxiety and unnecessary biopsy.

What Makes a Lesion Tumor-like Rather Than a True Neoplasm

A true neoplasm arises from a single clone of cells that proliferates independent of normal regulatory control. Tumor-like lesions, by contrast, result from a localized error in normal bone remodeling (Paget disease), a developmental failure of normal bone formation (fibrous dysplasia), a self-limited fluid-filled cavity within growing bone (unicameral bone cyst), or a benign fibrous cortical overgrowth that resolves on its own (non-ossifying fibroma). Because none of them is a true clonal neoplasm, all four generally lack the aggressive radiographic hallmarks discussed earlier in this chapter - permeative destruction, a wide zone of transition, and a soft-tissue mass - even when they produce dramatic-looking bone expansion.

Paget Disease of Bone

Paget disease is a disorder of abnormally accelerated and disorganized bone remodeling that most often affects patients over age 50, favoring the pelvis, spine, skull, and femur. The disease progresses through three overlapping phases: an early osteolytic phase, a mixed osteolytic-osteoblastic phase, and a late osteoblastic (sclerotic) phase, though a given patient's imaging may show any combination of these depending on how far the disease has progressed. Classic radiographic findings include cortical thickening, coarsened and disorganized trabeculae, and bone enlargement, producing a picture-frame appearance in the vertebral body when the thickened cortical margins stand out against a coarsened, less dense center. In the skull, Paget disease produces the well-known cotton-wool appearance from patchy sclerotic and lytic areas. Alkaline phosphatase is characteristically markedly elevated, reflecting the intense bone turnover, while serum calcium and phosphate typically remain normal, a combination that helps separate Paget disease from both hyperparathyroidism and metastatic bone disease on laboratory testing. Complications include bowing deformity of weight-bearing bones, an increased risk of pathologic fracture, and, in a small percentage of long-standing cases, malignant transformation to secondary osteosarcoma, which should be suspected whenever a patient with known Paget disease develops a new, aggressive-appearing lesion or a sudden increase in pain.

Fibrous Dysplasia

Fibrous dysplasia is a developmental disorder in which normal medullary bone is replaced by abnormal fibro-osseous tissue due to a somatic mutation affecting osteoblast differentiation; it is not inherited and not a true neoplasm. It typically presents in children and young adults and can affect a single bone (monostotic, the more common form) or multiple bones (polyostotic). The proximal femur, ribs, and craniofacial bones are the most frequently involved sites. The signature radiographic finding is a well-defined, expansile lesion with a homogeneous ground-glass matrix, a hazy, smoky density that reflects the immature, disorganized woven bone replacing the normal medullary cavity. The lesion typically shows no periosteal reaction and, despite sometimes dramatic bone expansion, a well-defined margin, both of which support its benign, non-neoplastic nature. When fibrous dysplasia involves the proximal femur, progressive bowing under weight-bearing stress can produce the characteristic shepherd's crook deformity of the femoral neck. Polyostotic fibrous dysplasia associated with cafe-au-lait skin macules and endocrine abnormalities such as precocious puberty is known as McCune-Albright syndrome.

Unicameral (Simple) Bone Cyst

A unicameral bone cyst, also called a simple bone cyst, is a fluid-filled cavity within the medullary canal that occurs in growing bone, most commonly in the proximal humerus and proximal femur of children and adolescents. It is a central, well-defined lucent lesion that expands and thins the cortex from within but, in an uncomplicated cyst, does not breach it, producing a narrow zone of transition and a sclerotic rim consistent with a slow-growing, non-neoplastic process. Most unicameral bone cysts are entirely asymptomatic and are discovered only after a pathologic fracture through the thinned cortex following minor trauma. The single most distinctive sign associated with this lesion is the fallen fragment sign, in which a fractured piece of the cortex settles into the dependent portion of the lucent cavity, a finding that is only possible because the lesion is a genuinely fluid-filled space rather than a solid tissue mass; this sign essentially confirms the diagnosis and excludes solid lesions such as fibrous dysplasia or a true neoplasm.

Non-Ossifying Fibroma (Fibrous Cortical Defect)

Non-ossifying fibroma, also called a fibrous cortical defect when small, is an extremely common, entirely benign fibrous lesion found incidentally in the metaphysis of long bones, especially around the knee, in children and adolescents. It is almost always an incidental finding on imaging obtained for an unrelated reason, causing no symptoms on its own unless it is unusually large and weakens the cortex enough to permit a pathologic fracture. Radiographically, it appears as an eccentric, cortically based, well-marginated lucent lesion, often somewhat lobulated or soap-bubble in contour, surrounded by a thin sclerotic rim. Unlike the tumors discussed elsewhere in this chapter, non-ossifying fibromas are expected to spontaneously regress and often fill in with normal bone as the patient reaches skeletal maturity, which is why the standard management for an asymptomatic, radiographically typical lesion is observation rather than biopsy or excision.

Distinguishing Tumor-like Lesions from True Neoplasms

• Margin and zone of transition: all four tumor-like lesions in this section characteristically show a well-defined margin and narrow zone of transition, similar to a benign true neoplasm, but they lack the cellular proliferation that defines neoplasia
• Periosteal reaction: none of these four conditions produces the aggressive lamellated, spiculated, or Codman-triangle periosteal patterns associated with malignancy; any aggressive periosteal reaction in a patient with known Paget disease or fibrous dysplasia should prompt evaluation for secondary osteosarcoma rather than reassurance
• Natural history: non-ossifying fibroma and many unicameral bone cysts are expected to resolve or heal over time, a self-limited course that no true neoplasm follows
• Laboratory correlation: Paget disease has a distinctive lab signature (markedly elevated alkaline phosphatase with normal calcium and phosphate) that neither a true bone neoplasm nor the other tumor-like lesions typically produce

Keeping these distinguishing features in mind allows a test-taker to confidently label a described lesion as a benign, self-limited process rather than escalate it to the malignant differential covered earlier in this chapter.