Radiographic Imaging in Monitoring Cancer Therapy Response
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Monitoring therapy response and disease progression is an essential part of a lung cancer patient’s treatment journey. To date, radiographic imaging remains the gold standard for this surveillance. The imaging techniques used in lung cancer surveillance include:
- Chest X-ray
- Computed tomography (CT)
- Positron emission tomography / computed tomography (PET/CT)
- Magnetic Resonance Imaging (MRI)
The current evaluation of tumor shrinkage is Response Evaluation Criteria In Solid Tumors (RECIST), a standard scoring method based on whether tumors
- Shrink
- Remain the same
- Increase in size
RECIST and World Health Organizations (WHO) categorized the response to cancer therapy into four main groups:
Complete response: Complete response refers to the disappearance of all target tumors. In other words, there should be no evidence of disease progression, and all signs and symptoms of cancer should have resolved. This indicates that the treatment has effectively eradicated the cancer cells.
Partial response: Partial response is characterized by a significant decrease in the size of target tumors. Although not all tumors may disappear, there should be at least a 30% reduction in the total tumor size. This indicates that the treatment has led to substantial tumor shrinkage.
Stable disease: Stable disease occurs when the tumor remains relatively unchanged in size and does not meet the criteria for either partial response or progressive disease. This suggests that the treatment has stabilized the cancer and prevented further growth.
Progressive disease: Progressive disease refers to an increase in the size of target tumors or the appearance of new tumors. This indicates that the cancer has continued to grow despite treatment and may require a change in therapy or additional interventions.
There are limitations and errors in this radiographic methodology:
- Changes in tumor growth or size measured by CT scan are only informative at an interval of months
- Errors in tumor size measurement
- Errors in the selection of tumor targets
- Measurement variation between pathologists
- Does not differentiate scarring tissue and the influx of immune cells (if inflammation occurs) from viable tumor
- It does not assess the molecular response or a reduction in driver mutations in the case of targeted therapies
Researchers have come up with strategies to overcome the limitations and challenges in the existing radiographic methodologies. They include molecular tests like liquid biopsies and new radiographic imaging. Some experts have suggested the use of liquid biopsy alongside imaging techniques as a validation tool to improve the assessment of therapeutic response. It is hopeful that researchers will continue to explore and improve these methods in order to provide better treatment methods for patients. By doing so, patients can receive the most accurate and reliable information about their treatment response and disease progression, leading to more successful outcomes.
Related: Minimal Residual Disease Assays – Detecting Circulating Tumor DNA
18F-FDG PET/CT in monitoring the therapeutic response of immune checkpoint inhibitors
The use of PET/CT with radiolabeled glucose molecule, 2-Deoxy-2-[fluorine-18]-fluoro-D-glucose (18F-FDG) is useful in distinguishing benign from malignant tumors. This technique works on the basis that tumor cells consume more energy, thus more sugar uptake, than normal cells. The 18F-FDG accumulates in the tumor and emits signals that can be detected by PET/CT. The radioactive fluorine tag 18F is not strong enough to cause significant health risks.
Refer to the video below for a more detailed explanation of 18F-FDG PET/CT.
18F-FDG PET/CT can address the limitation of CT scans in assessing tumor growth-based therapy response:
- 18F-FDG PET can help determine whether a small lesion detected on a CT scan is malignant (cancerous) or benign (non-cancerous). A high FDG uptake on PET suggests a higher likelihood of malignancy
- Combining information from both modalities is crucial for accurate staging of lung cancer. CT scans provide anatomical information about tumor size and location, while PET scans help identify any metastatic spread of cancer to lymph nodes or distant organs
After treatment, follow-up PET scans can assess treatment response by evaluating changes in metabolic activity within the tumor. A decrease in FDG uptake may indicate a positive response to therapy. A new study has confirmed that 18F-FDG PET/CT is effective in monitoring NSCLC patients’ response to immunotherapy at 4-9 weeks after taking immune checkpoint inhibitors.
Related: Monitoring Therapy Response and Progression of Lung Cancer
