What are lytic bone lesions?

Lytic Bone Lesions: Definition and Characteristics

Lytic bone lesions are areas of bone destruction characterized by the breakdown of bone tissue without reactive bone formation, most commonly seen in multiple myeloma and other malignancies.

Definition and Pathophysiology

• Lytic bone lesions represent areas where bone has been destroyed due to increased osteoclastic activity coupled with decreased osteoblastic function, creating a bone remodeling imbalance 1
• These lesions occur when malignant cells (particularly plasma cells in multiple myeloma) cause an uncoupling of the normal bone remodeling process, resulting in bone loss 2
• The pathogenesis involves increased osteoclastogenesis with suppressed osteoblastic activity, leading to bone destruction without compensatory bone formation 3

Imaging Characteristics

• Lytic lesions appear as "punched-out" areas on imaging where bone has been destroyed 3
• Conventional radiography can only detect lytic lesions when 30-50% of the trabecular bone has already been lost, making it less sensitive for early detection 1
• Two distinct patterns of lytic lesions can be identified on whole-body low-dose CT (WBLD-CT):
  • Lesions with negative Hounsfield units (HU < 0), representing infiltration with fatty components
  • Lesions with positive Hounsfield units (HU > 0), indicating tissue-like infiltrative patterns 4

Imaging Modalities for Detection

• Whole-body low-dose CT (WBLD-CT) is now considered the standard procedure for detecting lytic bone lesions, as it can identify lesions with less than 5% trabecular bone destruction 1, 3
• Conventional radiography (skeletal survey) has traditionally been used but is less sensitive than newer modalities 1
• MRI is superior for detecting bone marrow infiltration and early lesions before they cause osseous response, with high sensitivity (82-100%) and specificity (73-100%) 1
• PET/CT can identify metabolically active lesions and is useful for detecting purely osteolytic lesions and marrow infiltration 1
• Bone scans using 99mTc-MDP are relatively insensitive for purely osteolytic lesions (common in multiple myeloma) but are highly sensitive for osteoblastic and mixed lesions 1

Clinical Significance

• Lytic bone lesions are present in up to 80-90% of multiple myeloma patients at diagnosis 1
• These lesions lead to skeletal-related events (SREs) including:
  • Pathological fractures
  • Need for radiation therapy or surgery
  • Spinal cord compression
  • Bone pain 1
• Lytic lesions most commonly affect the axial skeleton (spine, pelvis, ribs) and skull, but can also involve long bones 3
• The presence of lytic bone lesions is one of the CRAB criteria (hypercalcemia, renal failure, anemia, bone lesions) used to define symptomatic multiple myeloma requiring treatment 1, 5

Management Approaches

• Bisphosphonates are indicated for all patients with multiple myeloma and bone disease to reduce skeletal-related events 1, 5
• Radiation therapy is effective for localized painful bone lesions or those at risk of pathological fracture 5
• For vertebral compression fractures, balloon kyphoplasty or vertebroplasty may be considered to stabilize the spine and reduce pain 5
• Treating the underlying disease (e.g., anti-myeloma therapy) is essential for addressing bone disease 5

Monitoring

• WBLD-CT is recommended for initial evaluation and follow-up of lytic bone lesions 1, 5
• Radiographic monitoring of lytic lesions often shows little change even in patients experiencing disease remission, making it less valuable for assessing treatment response 1
• PET/CT may be more useful for monitoring treatment response as it can assess the metabolic activity of lesions 1

Understanding lytic bone lesions is crucial for proper diagnosis, staging, and management of multiple myeloma and other malignancies with bone involvement.