Lymphoma PET-CT

Understanding Lymphoma PET-CT Imaging

Lymphoma PET-CT has revolutionized the management of both Hodgkin and non-Hodgkin lymphoma. Unlike many other cancers where CT remains the primary staging tool, PET-CT is integral to every phase of lymphoma care—from initial staging to assessing treatment response and detecting recurrence.

Lymphomas are cancers of the lymphatic system, specifically affecting lymphocytes (a type of white blood cell). What makes lymphoma uniquely suited to PET imaging is that most lymphoma subtypes are highly metabolically active, readily absorbing the FDG (fluorodeoxyglucose) tracer used in PET scans. This characteristic allows for precise detection of disease throughout the body.

The Critical Role of PET-CT in Lymphoma Care

PET-CT serves multiple essential functions in lymphoma management that go beyond simple disease detection:

Accurate staging is fundamental to selecting appropriate therapy. Lymphoma staging uses the Ann Arbor system, which categorizes disease from stage I (single lymph node region) to stage IV (involvement of extralymphatic organs like liver or bone marrow). PET-CT often upstages patients compared to CT alone by detecting occult disease sites.

Treatment response assessment is perhaps the most valuable application. Unlike solid tumors where size reduction indicates response, lymphoma can leave behind a residual mass even after successful treatment. This mass may represent only scar tissue, but CT alone cannot distinguish scar from residual cancer. PET-CT solves this dilemma by showing metabolic activity—residual cancer remains FDG-avid while scar tissue does not.

Interim PET performed after 2-4 cycles of chemotherapy can predict final treatment outcome and potentially guide therapy escalation or de-escalation. Patients whose lymphoma shows complete metabolic response early have excellent long-term outcomes.

PET-CT Appearances in Lymphoma

Nodal Disease Patterns

Lymphoma typically involves lymph nodes in characteristic distributions that differ by subtype:

Hodgkin lymphoma typically spreads contiguously from one lymph node region to the next. It most commonly involves:

• Cervical/supraclavicular nodes: Often the initial site
• Mediastinal nodes: Present in ~60% of cases
• Axillary nodes: Less commonly involved
• Abdominal/pelvic nodes: In later stages

Non-Hodgkin lymphoma can involve lymph nodes more unpredictably:

• Nodal chains: Any nodal group may be involved
• Waldeyer's ring: Tonsils and adenoids
• Mesenteric nodes: Common in certain subtypes
• Retroperitoneal nodes: Frequent involvement

On PET-CT, involved lymph nodes appear as focally FDG-avid masses. SUVmax values typically range from 5 to 20 for aggressive lymphomas, though indolent subtypes may show lower uptake (3-8).

Extranodal Disease

Lymphoma can involve organs outside the lymphatic system, known as extranodal disease:

Spleen involvement appears as either focal FDG-avid lesions or diffusely increased uptake throughout the splenic parenchyma. The spleen normally shows mild physiologic FDG uptake, but lymphomatous infiltration demonstrates more intense activity.

Liver involvement typically shows multiple FDG-avid lesions, though diffuse infiltration can also occur. Differentiating lymphoma from other liver malignancies often requires correlation with clinical history and sometimes biopsy.

Bone marrow involvement may appear as focal FDG-avid lesions or diffusely increased marrow uptake throughout the skeleton. This finding is particularly important as it indicates stage IV disease.

Unusual extranodal sites can include:

• Gastrointestinal tract: Stomach or small bowel involvement
• Lung: Parenchymal lesions or pleural effusions
• Bone: Focal lytic lesions with associated FDG uptake
• Brain: Rare, but can occur in aggressive subtypes

The Deauville Score: Standardizing Response

Understanding the 5-Point Scale

The Deauville score is a standardized 5-point system used to assess treatment response in lymphoma based on the intensity of FDG uptake relative to reference sites:

Deauville 1: No uptake Deauville 2: Uptake less than or equal to mediastinum Deauville 3: Uptake greater than mediastinum but less than or equal to liver Deauville 4: Uptake moderately higher than liver Deauville 5: Uptake markedly higher than liver or any new sites of disease

Complete metabolic response (CMR) is defined as Deauville 1-3, meaning any residual uptake is no greater than normal liver. This correlates with excellent prognosis.

Partial metabolic response (Deauville 4-5) indicates residual active disease and may require additional treatment.

When PET-CT Changes Management

Interim PET: Early Treatment Guidance

Perhaps the most powerful application of PET-CT in lymphoma is the interim scan, performed after 2-4 cycles of chemotherapy. This early assessment can predict final treatment outcome:

• Patients achieving complete metabolic response at interim have excellent outcomes, with cure rates exceeding 90% in many subtypes
• Patients with persistent FDG-avid disease at interim have higher relapse rates, potentially indicating need for treatment escalation

In Hodgkin lymphoma, interim PET is increasingly used to guide treatment de-escalation. Patients with early complete response may receive fewer cycles of chemotherapy or lower radiation doses, reducing long-term treatment toxicity while maintaining cure rates.

Distinguishing Residual Mass from Active Disease

A common dilemma after lymphoma treatment is the presence of a residual mass on CT. This occurs in up to 60% of Hodgkin lymphoma patients and represents either:

• Fibrotic scar tissue: Benign, no further treatment needed
• Residual active lymphoma: Requires additional therapy

PET-CT resolves this dilemma by showing metabolic activity. FDG-avid residual masses indicate persistent disease requiring further treatment, while non-avid masses represent scar tissue and require only observation.

Transplant Assessment

For patients undergoing stem cell transplantation for relapsed or refractory lymphoma, PET-CT serves as a critical baseline before transplant and assessment tool afterward. Patients who achieve complete metabolic response before transplant have significantly better outcomes.

Evidence-Based Outcomes

Prognostic Significance

PET-CT results provide powerful prognostic information:

• Complete metabolic response after treatment correlates with progression-free survival of 80-90% in most lymphoma subtypes
• Interim PET response is the strongest predictor of outcome in Hodgkin lymphoma and diffuse large B-cell lymphoma
• Total metabolic tumor volume at baseline predicts outcomes even after accounting for other factors

Special Considerations

Indolent Lymphomas

Not all lymphoma subtypes are reliably FDG-avid. Indolent lymphomas like follicular lymphoma grade 1-2 and small lymphocytic lymphoma may show lower FDG uptake, potentially leading to false-negative results. However, most indolent lymphomas still demonstrate sufficient uptake for detection, especially when compared to background tissues.

Transformation Detection

Indolent lymphomas can transform to aggressive disease, a critical event requiring treatment change. PET-CT can detect transformation by identifying new foci of intensely FDG-avid disease that were not previously present, often correlating with clinical deterioration.

Post-Treatment Changes

After treatment, inflammatory changes can cause false-positive FDG uptake:

• Radiation changes: Typically appear 2-3 months after treatment and may persist for 6-12 months
• Chemotherapy effects: May cause temporary bone marrow activation
• Growth factor stimulation: G-CSF causes diffuse marrow uptake

Knowledge of treatment timing is crucial for accurate interpretation.

Preparing for Your Scan

Before the Appointment

Preparation for lymphoma PET-CT is similar to other PET indications:

• Fast for 6 hours before tracer injection (water is permitted)
• Avoid strenuous exercise for 24 hours prior
• Bring prior scans for comparison
• Inform of recent treatments especially chemotherapy, radiation, or growth factors
• Manage blood sugar if diabetic

Timing Considerations

For lymphoma patients, scan timing is particularly important:

• Staging scan: Before treatment initiation
• Interim scan: Usually after 2-4 cycles, at least 2 weeks after last chemotherapy
• End-of-treatment scan: 2-4 weeks after treatment completion
• Surveillance scans: Based on risk and clinical indication

Understanding Your Results

Frequently Asked Questions

How often will I need PET-CT scans?

Typical lymphoma monitoring includes: staging at diagnosis, interim after 2-4 cycles, end-of-treatment, and then as clinically indicated for surveillance. The exact schedule depends on your lymphoma subtype, treatment protocol, and response.

Can PET-CT replace bone marrow biopsy?

Not entirely. While PET-CT can detect focal bone marrow involvement and diffuse infiltration, it may miss patchy involvement. Bone marrow biopsy remains the gold standard for assessing marrow disease, particularly in subtypes where marrow involvement changes staging.

What if I have a residual mass after treatment?

This is common and not necessarily concerning. The key question is whether the mass shows FDG uptake. A non-avid residual mass typically represents scar tissue and requires only observation, while an FDG-avid mass indicates residual disease needing further treatment.

Does PET-CT work for all lymphoma types?

Most lymphoma subtypes are FDG-avid and well-evaluated with PET-CT. Exceptions include some very indolent subtypes and certain rare variants. Your hematologist will determine whether PET-CT is appropriate for your specific lymphoma type.

References

1. National Comprehensive Cancer Network. NCCN Clinical Practice Guidelines in Oncology: Hodgkin Lymphoma and Non-Hodgkin Lymphoma. Version 4.2024.
2. Barrington SF, et al. Role of imaging in the staging and response assessment of lymphoma. Blood. 2023.
3. International Conference on Malignant Lymphoma. Consensus recommendations on PET-CT in lymphoma. 2023.
4. Journal of Nuclear Medicine. Deauville scoring criteria validation studies. 2022.

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Medical Disclaimer: This information is educational only. Always discuss findings with your healthcare provider for personalized medical advice.