Whole-body MRI and PET-CT in multiple myeloma and bone metastases: staging and treatment response

Radiology

Multiple myeloma (MM) is a haematological malignancy characterised by the clonal proliferation of plasma cells in the bone marrow and the production of monoclonal immunoglobulins or their components. Bone disease is a major feature of MM, while other clinical manifestations include anaemia, hypercalcaemia, and renal impairment. Imaging plays a central role in disease detection, initial staging, assessment of disease extent, and evaluation of treatment response. Whole-body magnetic resonance imaging (MRI) and positron emission tomography–computed tomography (PET-CT) provide complementary information for assessing MM and bone metastases from other malignancies. Whole-body MRI is highly sensitive to bone marrow infiltration, including focal and diffuse disease that may precede visible cortical destruction. PET-CT combines metabolic and anatomical information to identify metabolically active lesions, detect extramedullary involvement, and monitor metabolic response to treatment. The choice of imaging modality depends on the suspected or established malignancy, the clinical question, baseline findings, and the treatment setting. Understanding the strengths and limitations of these techniques is essential for accurate staging and consistent assessment of treatment response.[1]Whole-body MRI for staging and disease assessmentDetect marrow infiltration: Whole-body MRI, particularly diffusion-weighted imaging, can identify focal and diffuse marrow abnormalities before structural bone destruction becomes apparentAssess myeloma burden: MRI helps identify the distribution and number of focal lesions, supporting the evaluation of suspected myeloma and selected patients with smoldering diseaseEvaluate complications: Targeted MRI is important when spinal cord compression, nerve involvement, or other soft-tissue complications are suspectedInterpret response cautiously: Changes in marrow signal and diffusion characteristics may persist during treatment or recovery, so imaging findings should be interpreted alongside clinical and laboratory evidenceUse standardized reporting: The Myeloma Response Assessment and Diagnosis System (MY-RADS) supports consistent whole-body MRI acquisition, interpretation, and response assessmentFig: MRI illustration of a diffuse medullary infiltration pattern in myeloma using T1w (a) and STIR (b) sequences. It is characterized by decreased signal intensity of the bone marrow on T1w sequences (a) and a correspondingly increased signal on STIR (b) sequences. On T1w images (a), the signal intensity of the bone marrow is equal to the intervertebral disc and the muscle, which is known as the “bright disc” sign[2]PET-CT for staging and treatment responseIdentify metabolically active disease: Fluorine-18 fluorodeoxyglucose (FDG) PET-CT detects glucose-avid lesions and provides information about disease activity alongside CT findingsEvaluate extramedullary involvement: PET-CT can identify metabolically active soft-tissue disease outside the bone marrow and help establish disease distributionAssess treatment response: In multiple myeloma, PET-CT can demonstrate reduced or absent metabolic activity following treatment, including when residual structural abnormalities remainRecognize limitations: False-negative findings can occur in FDG-nonavid disease, while inflammation, infection, and treatment-related changes can produce false-positive uptakeMaintain comparability: When serial imaging is indicated, comparison with baseline studies using a consistent modality and standardized interpretation criteria improves assessment of response or progressionImaging of bone metastasesMRI is particularly useful for suspected spinal or epidural involvement, marrow disease, and assessment of possible metastatic spinal cord compressionPET-CT may help identify metabolically active skeletal lesions and additional sites of disease, depending on the primary tumor's FDG avidityCT remains important for evaluating cortical destruction, structural integrity, and fracture risk, which functional imaging alone cannot adequately establishImaging findings should be reviewed alongside symptoms, examination findings, primary tumor characteristics, and other investigations to guide multidisciplinary managementSelecting imaging for the clinical questionClinical considerationImplication for imaging selectionSuspected myeloma with negative or inconclusive initial imagingAdditional whole-body imaging may be required because different modalities detect different patterns of disease.Suspected impending pathological fractureDedicated CT or radiographs may be needed to assess cortical integrity and structural stability.Lesion with uncertain metabolic activityCorrelation with anatomic imaging, clinical findings, and interval assessment may help clarify significance.Discordant imaging and laboratory responseMultidisciplinary review is appropriate before changing treatment based on imaging alone.ConclusionWhole-body MRI is increasingly used to evaluate disease extent across a range of malignancies, although standardised imaging protocols and reporting approaches remain important for consistent interpretation. In multiple myeloma and bone metastases, whole-body-MRI and PET-CT provide complementary information and should be selected according to the malignancy and clinical question. MRI is particularly valuable for detecting bone marrow involvement and assessing spinal and other soft-tissue complications, whereas PET-CT provides metabolic information that supports disease staging and treatment-response assessment. Consistent imaging protocols, comparison with baseline studies, and integration of imaging findings with clinical and laboratory data are essential for accurate disease evaluation, treatment planning, and follow-up.[3]Reference(s):1. Mulé S, Reizine E, Blanc-Durand P, et al. Whole-Body Functional MRI and PET/MRI in Multiple Myeloma. Cancers (Basel). 2020;12(11):3155. Published 2020 Oct 27. doi:10.3390/cancers12113155. https://pmc.ncbi.nlm.nih.gov/articles/PMC7693006/2. Treitl KM, Ricke J, Baur-Melnyk A. Whole-body magnetic resonance imaging (WBMRI) versus whole-body computed tomography (WBCT) for myeloma imaging and staging. Skeletal Radiol. 2022;51(1):43-58. doi:10.1007/s00256-021-03799-4. https://pmc.ncbi.nlm.nih.gov/articles/PMC8626374/3. Parillo M, Mallio CA. The Whole-Body MRI Reporting and Data System Guidelines for Prostate Cancer (MET-RADS-P), Multiple Myeloma (MY-RADS), and Cancer Screening (ONCO-RADS). Cancers (Basel). 2025;17(2):275. Published 2025 Jan 16. doi:10.3390/cancers17020275. https://pmc.ncbi.nlm.nih.gov/articles/PMC11763526/