Radiological presentation of skeletal involvement
Radiographic abnormalities are detected in nearly 40% of patients at the time of diagnosis. Bone tissue can be affected both focally and diffusely:
- Permeative destruction: A pattern resembling Ewing sarcoma. The cortical layer is eroded from the endosteal side and can be completely destroyed. This permeative (permeative) pattern of changes occurs in 18% of children with leukaemia.
- Osteolytic lesions: Appear as a "moth-eaten" (moth-eaten) pattern. Most commonly located in the metaphyses of long tubular bones (in 10–40% of patients), accompanied by the formation of metaphyseal radiolucent bands and predisposing to pathological fractures.
- Periosteal reaction: Detected in 2–50% of cases. Can be either isolated or accompany cortical destruction. It occurs due to subperiosteal haemorrhages or direct proliferation of leukaemic deposits.
- Osteosclerosis: Isolated sclerotic changes in the metaphyses are rare (about 6% of cases) and can occur spontaneously or during therapy. Mixed lytic-sclerotic lesions are identified in approximately 18% of children.
- Nonspecific cortical destruction: Often affects the medial surface of the proximal humerus, tibia, or femur.
Note: In adult patients, skeletal lesions in leukaemia are significantly less common, so careful differential diagnosis with bone metastases or primary sarcomas is always required.
Myeloid sarcoma (chloroma)
A specific focal process in leukaemia is myeloid sarcoma (myeloid sarcoma). This is a tumour mass consisting of a dense accumulation of leukaemic cells.
- Most commonly occurs in the setting of acute myeloid leukaemia (AML).
- In children, favoured locations include the skull, spine, sternum, and ribs.
- Radiographs visualise this as bone destruction with the formation of a pronounced soft tissue component (e.g., destruction of the ulna with involvement of the forearm muscles).
- On MRI, this extraosseous mass is isointense on T1-WI, hyperintense relative to muscles on PDW/T2-WI, and shows avid contrast enhancement.
MRI in bone marrow evaluation
Magnetic resonance imaging is a key and most sensitive tool for visualising the bone marrow.
- Basic semiotics: Due to bone marrow infiltration by leukaemic cells, there is a diffuse decrease in signal intensity (SI) on T1-weighted images (T1-WI). On fat-suppressed sequences (STIR) or T2-WI, a corresponding signal increase is detected. Patchy (patchy) infiltration is less common.
- Contrast enhancement: Following administration of a paramagnetic contrast agent, uniform and mild enhancement of the affected bone marrow is noted.
- Dynamics and treatment response: As the disease progresses, the signal changes from normal to nodular, and then to diffusely low against the background of increasing lesion size. In remission, signal intensity normalises. Relapse may manifest as well-defined nodules in the bone marrow or as extramedullary lesions in the muscles.
- Therapy complications: MRI excellently visualises osteonecrosis (occurs in 6.5–15% of cases) and bone marrow necrosis (the result of microvascular occlusion). The latter appears as a "geographic" zone with a non-enhancing centre and a paramagnetic-enhancing periphery.
MRI is extremely useful for focal lesions—it can be used to guide biopsies. The method can detect pathology even when a blind iliac crest biopsy yields a false-negative result.
Role of PET/CT
FDG (FDG) PET/CT demonstrates areas of focal or diffuse hypermetabolism. However, the method is considered nonspecific for leukaemia. Abnormally high radiopharmaceutical uptake is often associated with recent chemotherapy or bone marrow stimulation by granulocyte colony-stimulating factor (GCSF), which significantly complicates scan interpretation.
Briefly: answers to topical questions
What diseases should be considered in the differential diagnosis of bone changes in leukaemia?
Differential diagnosis of bone changes in leukaemia should include Ewing sarcoma, metastases, and primary malignant bone tumours.
- Ewing sarcoma (Ewing sarcoma) — extensive involvement in leukaemia can lead to diffuse permeative bone destruction, similar to the picture seen in Ewing sarcoma.
- Metastases and primary malignant bone tumours — especially important in the differential diagnosis of skeletal lesions in adult patients.
- In cases of pathological vertebral fractures in children, hypercorticism must also be excluded.
What types of bone necrosis occur in leukaemia and what are their specific MRI features?
Osteonecrosis and bone marrow necrosis occur in leukaemia, which are detected on MRI as complications of treatment.
- Osteonecrosis (osteonecrosis) — on MRI (in STIR mode), involvement of articular heads (e.g., humeral head) with subchondral fracture can be visualised.
- Bone marrow necrosis (bone marrow necrosis) — occurs due to microvascular occlusion and manifests as a "geographic" zone with peripheral contrast enhancement and a non-enhancing centre. Bone marrow infarction may also be visualised.
What bone complications, besides necrosis, can be visualised in children undergoing prolonged leukaemia chemotherapy?
In addition to necrosis, skeletal sclerotic changes and mixed lytic-sclerotic lesions may be visualised in leukaemia.
- Osteosclerosis — sclerotic changes in the metaphyses of long bones; they can occur spontaneously or as a result of therapy and are seen in approximately 6% of cases.
- Mixed lytic-sclerotic lesions are identified in approximately 18% of children.
- Following chemotherapy, PET/CT may show persistent nonspecific abnormal activity in the form of focal or diffuse increased radiopharmaceutical uptake.
To remember the radiological pattern of destruction in leukaemia, the rule "POP" can be used: Permeative destruction (as in Ewing sarcoma), Osteolytic lesions ("moth-eaten"), Periosteal reaction.
Class and exam questions
What does bone marrow look like on MRI in leukemia?
There is a diffuse decrease in signal intensity on T1-WI and its increase on STIR or fat-suppressed T2-WI. Contrast administration reveals mild, uniform enhancement.
What is myeloid sarcoma?
This is a growing tumour composed of an accumulation of leukaemic cells, which is most commonly found in acute myeloid leukaemia and typically affects the skull, spine, ribs, or sternum.
Does PET/CT help establish the diagnosis of leukemia?
PET/CT shows focal or diffuse radiopharmaceutical uptake, but the method is nonspecific, as a similar picture can be produced by reactive bone marrow after chemotherapy or GCSF administration.
How to differentiate leukemia remission from relapse on MRI?
During remission, the signal intensity of the bone marrow returns to normal. In relapse, pathological lesions reappear in the bone marrow, often as well-defined nodules.
What else to study on this topic
- Lymphomas: general classification principles and radiological differential diagnosis
- Hodgkin's disease: signs of primary and secondary bone involvement
- "Ivory vertebra" sign in haematological malignancies
- Comparison of diagnostic value of whole-body MRI (Whole body MRI with DWI) and PET/CT
- Primary bone lymphoma (PBL) and bone sequestrum formation
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