FDG PET Scan in Lymphoma Management

Lymphoma, a type of cancer that originates in the lymphatic system, can affect lymph nodes and organs throughout the body. Accurate assessment of disease extent, response to therapy, and detection of recurrence are critical components of effective management. This is where the FDG PET scan plays a pivotal role.

An FDG PET scan (Fluorodeoxyglucose Positron Emission Tomography) is a nuclear imaging technique that highlights areas of increased metabolic activity in the body—often corresponding to cancerous tissues. In lymphoma, FDG PET is widely used for staging, evaluating treatment response, detecting residual disease, and guiding biopsy decisions. It is especially valuable in both Hodgkin lymphoma (HL) and high-grade non-Hodgkin lymphomas (NHL).

What is an FDG PET Scan?

An FDG PET scan uses a small amount of radioactive glucose (FDG) injected into the bloodstream. Cancer cells, due to their high metabolic rate, absorb more glucose than normal cells. The PET scanner detects the radiation emitted by FDG and generates 3D images that reveal the location and activity of lymphoma in the body.

The scan is often combined with a CT (Computed Tomography) scan for anatomical correlation, forming a PET/CT scan, which provides both functional and structural information in one session.

Purpose of FDG PET Scans in Lymphoma

FDG PET scans serve multiple roles across the lymphoma treatment journey, including:

• Initial staging – Identifying the extent of disease at diagnosis.
• Treatment planning – Helping choose appropriate chemotherapy or radiation strategies.
• Interim assessment – Evaluating response after a few cycles of chemotherapy.
• End-of-treatment evaluation – Confirming remission or detecting residual disease.
• Surveillance – Monitoring for disease recurrence in some cases.
• Biopsy guidance – Identifying the most metabolically active lymph nodes for sampling.

Procedure of an FDG PET Scan

The scan typically takes 2–3 hours, including preparation and imaging. It is a non-invasive, painless test.

1. Preparation:

• Fasting for 4–6 hours before the scan is usually required to ensure accurate glucose uptake.
• Blood sugar levels are checked prior to injection, especially in diabetic patients.
• You should inform the healthcare team about:
  • Medications
  • Pregnancy or breastfeeding
  • Prior treatments like chemotherapy or radiotherapy
• Avoid strenuous activity 24 hours prior to the scan, as it may affect FDG uptake.

2. During the Scan:

• A small dose of FDG is injected into a vein, and you will rest quietly for about 60 minutes while the tracer distributes through the body.
• You will then lie on a scanning table that moves slowly through the PET/CT machine.
• You must remain still during imaging, which usually lasts 20–30 minutes.
• The scanner detects areas with high FDG uptake, producing detailed images of metabolic activity.

3. After the Scan:

• You can resume normal activities immediately.
• Drink plenty of water to help flush the tracer from your body.
• A nuclear medicine specialist or radiologist interprets the images and sends a report to your oncologist.

Benefits of FDG PET in Lymphoma Management

FDG PET is considered a cornerstone in lymphoma imaging due to its numerous advantages:

• Accurate Staging: PET/CT reveals both nodal and extranodal disease with high precision.
• Early Response Assessment: Interim PET scans can identify non-responders early, allowing treatment modification.
• Improved Prognostication: PET results correlate with outcomes and guide risk-adapted therapies.
• Detection of Residual Disease: Helps differentiate between active cancer and scar tissue after therapy.
• Non-invasive: Offers a whole-body view without surgical procedures.
• High Sensitivity and Specificity: Especially in FDG-avid lymphomas like HL and aggressive NHLs.

Limitations and Considerations

While highly effective, FDG PET scans have certain limitations:

• False Positives: Inflammation or infection can also show high FDG uptake.
• False Negatives: Some slow-growing lymphomas (e.g., small lymphocytic lymphoma) may not be FDG-avid.
• Radiation Exposure: PET/CT involves exposure to radiation, though the benefit often outweighs the risk.
• Cost and Availability: PET scans may not be available in all healthcare settings and can be expensive.
• Timing Issues: Scans done too soon after treatment may show inflammation rather than residual disease, leading to inaccurate interpretation.

Lymphoma Types Best Suited for FDG PET

FDG PET is particularly valuable in:

• Hodgkin Lymphoma (HL) – Highly FDG-avid and PET-responsive; PET is used throughout management.
• Diffuse Large B-Cell Lymphoma (DLBCL) – Aggressive NHL with excellent PET correlation.
• Primary Mediastinal B-cell Lymphoma
• Burkitt Lymphoma

In contrast, indolent lymphomas like follicular lymphoma, marginal zone lymphoma, and chronic lymphocytic leukemia/small lymphocytic lymphoma may not show high FDG uptake and are less commonly evaluated with PET.

Interpreting FDG PET Results

PET results are interpreted using standardized criteria such as:

• Deauville Score (a 5-point scale):
  • 1–3: Complete metabolic response (good)
  • 4–5: Partial or no response (may require treatment adjustment)
• SUV (Standard Uptake Value): A semi-quantitative measure of FDG uptake, useful for comparison during follow-up.

When Is FDG PET Recommended in Lymphoma?

PET/CT scans are recommended at different stages of lymphoma treatment:

1. At Diagnosis: To determine staging (Ann Arbor stage), which influences treatment.
2. After 2–4 Chemotherapy Cycles: Interim scan to assess early response.
3. At End of Treatment: To confirm remission or residual disease.
4. During Surveillance (Selective Cases): Particularly in high-risk patients or when relapse is suspected.

Conclusion

FDG PET scans have revolutionized the way lymphoma is diagnosed, staged, and monitored. Their ability to visualize metabolic activity offers a dynamic view of the disease that goes far beyond structural imaging alone. As a result, oncologists can make better-informed decisions, adapt treatment strategies, and improve overall patient outcomes. While not without limitations, FDG PET remains a cornerstone tool in modern lymphoma management.

Frequently Asked Questions (FAQs)

What is an FDG PET scan?

It’s an imaging test that uses radioactive glucose (FDG) to detect areas of high metabolic activity in the body, often indicating cancer.

Is FDG PET scan safe?

Yes, it’s generally safe. It involves low levels of radiation and is non-invasive. Allergic reactions are rare.

How long does the scan take?

The entire process takes about 2–3 hours, including preparation, injection, and imaging.

Can FDG PET detect all types of lymphoma?

It is highly effective for aggressive and FDG-avid types like Hodgkin lymphoma and DLBCL but may be less useful in indolent lymphomas.

Will I need sedation?

No, sedation is not typically required unless the patient is claustrophobic or very young.

Why do I need to fast before the scan?

Fasting ensures low blood glucose levels, which improves the accuracy of FDG uptake in cancer cells.

How many FDG PET scans can I have safely?

There is no strict limit, but PET scans are spaced out to balance medical benefit and radiation exposure.

What does a “hot spot” on a PET scan mean?

It indicates an area of increased FDG uptake, which could be due to cancer, inflammation, or infection.

Can PET scan differentiate between scar tissue and active disease?

Yes, one of its major strengths is distinguishing metabolically active tissue from inert scar tissue after treatment.