Multiple myeloma (MM) is a cancer originating in the plasma cells, a type of white blood cell housed within the bone marrow. These malignant cells can crowd out healthy blood cells and produce abnormal proteins that damage organs. A computed tomography (CT) scan is a non-invasive imaging technique that uses specialized X-ray equipment to create detailed cross-sectional images of the body. CT scans are used to evaluate the extent of the disease and guide therapeutic decisions.
The Role of CT in Multiple Myeloma Diagnosis and Staging
The primary clinical utility of a CT scan in the workup for multiple myeloma is to accurately assess the presence and extent of bone destruction. Multiple myeloma cells stimulate specialized cells called osteoclasts, which break down bone tissue, leading to structural damage. Whole-body low-dose CT (WBLDCT) is now the standard of care for establishing a baseline of bone involvement at the time of initial diagnosis.
WBLDCT is significantly more sensitive than conventional X-ray images, known as a skeletal survey, particularly in areas like the ribs, scapulae, and sternum. The ability to detect subtle erosions and small lesions helps determine if the patient meets the criteria for active disease. The presence of one or more lytic bone lesions measuring 5 millimeters or larger is defined as a myeloma-defining event, necessitating treatment initiation.
This precise mapping of bone lesions is fundamental to the staging process, influencing the International Staging System (ISS) and the Revised International Staging System (R-ISS). CT scans also guide biopsies by identifying the most representative or suspicious site for tissue sampling, ensuring an accurate diagnosis.
Interpreting the Images: What the CT Scan Reveals
The radiologist examines the CT images specifically for visual markers that indicate the presence and activity of multiple myeloma. The most common finding is the lytic lesion, which appears as a well-defined, “punched-out” dark area within the bone. These dark regions represent areas where the bone matrix has been destroyed by myeloma cells, registering as a void on the scan.
In addition to these focal lesions, the scan can reveal diffuse osteopenia, which is a generalized thinning of the bone density across the skeleton. This widespread bone loss indicates systemic bone weakening due to the disease process. The CT scan is effective at demonstrating cortical disruption, where the hard outer layer of the bone has been broken down, significantly increasing the risk of fracture.
The radiologist also looks for soft tissue masses of myeloma cells outside of the bone, known as extramedullary plasmacytomas. These masses may be identified near the spine or in other organs, appearing as dense, abnormal tissue collections that often enhance with intravenous contrast dye. The CT scan is also used to identify pathological fractures, which are breaks that occur in bones already weakened by the cancer, often without significant trauma.
Comparing Imaging Techniques for Multiple Myeloma
While the CT scan is excellent for visualizing bone destruction, it is one tool within a broader diagnostic strategy that includes other imaging modalities. Whole-body low-dose CT has largely replaced the traditional skeletal survey because of its superior sensitivity in detecting small lytic lesions. Conventional X-rays often require 30 to 50 percent of the bone mineral to be lost before a lesion becomes visible, a limitation CT overcomes with its cross-sectional detail.
Magnetic Resonance Imaging (MRI) specializes in the visualization of soft tissues, including the bone marrow itself. MRI is considered the gold standard for detecting bone marrow infiltration, showing diffuse disease or precursor lesions that have not yet caused bone destruction. Physicians often choose MRI when there is suspicion of spinal cord compression or to assess the marrow before treatment, while CT is preferred for a rapid assessment of bone stability.
Positron Emission Tomography (PET) combined with a CT scan (PET/CT) merges anatomical information with metabolic activity. The PET component uses a radioactive tracer that accumulates in highly active cancer cells, providing a measure of the tumor’s biological activity. PET/CT is valuable for assessing the metabolic response to therapy and detecting extramedullary disease, which can be an important prognostic indicator.
Preparing for and Undergoing the CT Procedure
The CT scan procedure is generally straightforward and non-invasive, though some preparation may be required depending on the type of scan. For a whole-body low-dose CT focused primarily on bone structure, little to no preparation, such as fasting, is necessary. Patients must remove any metal objects, such as jewelry or belts, as these can interfere with the X-ray beams and distort the images.
During the scan, the patient lies on a motorized table that slides into the large, donut-shaped CT scanner. The X-ray tube rotates around the patient, capturing images from multiple angles, which can be accompanied by whirring or buzzing sounds. The entire procedure is relatively fast, often lasting only a few minutes, though the overall appointment time may be longer.
In some cases, an intravenous (IV) contrast dye, which is iodine-based, may be injected to enhance the visibility of blood vessels or soft tissue masses. The contrast material can cause a temporary feeling of warmth or a metallic taste in the mouth. Since multiple myeloma can affect kidney function, blood tests (such as creatinine levels) are performed before administering the contrast agent, as the kidneys filter the dye. The exposure to radiation from a CT scan is minimal, and the diagnostic benefit typically outweighs this small risk.