What is the accuracy of a Positron Emission Tomography (PET) scan in detecting lymphoma?

Accuracy of PET Scan for Lymphoma Detection

PET scans demonstrate high accuracy for lymphoma detection with pooled sensitivity of 84% (95% CI, 71-92%) and specificity of 90% (95% CI, 84-94%) for Hodgkin lymphoma, and sensitivity of 72% (95% CI, 61-82%) and specificity of 100% (95% CI, 97-100%) for aggressive non-Hodgkin lymphoma. 1

Diagnostic Performance by Lymphoma Type

The accuracy of PET scans varies significantly depending on the lymphoma subtype:

Highly FDG-Avid Lymphomas (Excellent Detection)

• Hodgkin lymphoma (HL): 84% sensitivity, 90% specificity 1
• Diffuse large B-cell lymphoma (DLBCL): 72% sensitivity, 100% specificity 1
• Follicular lymphoma: Routinely FDG avid 1
• Mantle cell lymphoma: Routinely FDG avid 1

Variably FDG-Avid Lymphomas (Lower Detection Rates)

• T-cell lymphomas: Variable FDG avidity 1
• Extranodal marginal zone MALT lymphoma: 54.5% overall detection rate 2
  • Gastric MALT: 38.9% sensitivity
  • Non-gastric MALT: 75% sensitivity
• Small lymphocytic lymphoma: Modest FDG avidity 1

Factors Affecting PET Scan Accuracy

Disease Stage

• Advanced disease (Stage III-IV): 100% detection rate 2
• Early stage disease (Stage I-II): 42.3% detection rate 2

Anatomical Location

• Gastric involvement: Lower sensitivity (38.9%) 2
• Non-gastric sites: Higher sensitivity (75%) 2
• Cervical, supraclavicular, and extranodal regions: Better detected with PET than CT alone 3
• Para-aortic to iliac regions: Better detected with combined PET/CT approaches 3

Timing of Scan

• Post-chemotherapy: Wait at least 3 weeks after chemotherapy 1, 4
• Post-radiotherapy: Wait 8-12 weeks after completion 1, 4
• Early scans: High risk of false positives due to inflammatory changes 4

PET/CT vs. Other Imaging Modalities

PET/CT significantly outperforms individual modalities:

• PET/CT: 97% sensitivity, 96-97% specificity 5
• PET alone: 82% sensitivity, 81% specificity 5
• CT alone: 91% sensitivity, lower negative predictive value 5

For restaging or follow-up:

• PET alone: 86.1% sensitivity, 99.4% specificity 3
• Side-by-side reading: 96.0% sensitivity, 99.4% specificity 3
• Fused images: 98.0% sensitivity, 99.4% specificity 3
• CT alone: 59.4% sensitivity, 96.1% specificity 3

Special Considerations

Bone Marrow Assessment

• PET has limited sensitivity (29%) for bone marrow involvement 5
• Bone marrow biopsy remains the standard procedure 1
• Diffusely increased bone marrow uptake post-therapy is usually due to marrow hyperplasia, not lymphoma 1

False Positives

• Inflammatory changes post-therapy can mimic residual disease 1, 4
• Mild and diffuse FDG uptake at residual mass sites is common after therapy 4
• Uptake lower than or equal to mediastinal blood pool structures should be considered negative 4

Optimal Protocol for Lymphoma Assessment

1. Pre-therapy baseline PET:

   • Strongly recommended for routinely FDG-avid lymphomas (HL, DLBCL, follicular, mantle cell) 1
   • Mandatory for variably FDG-avid lymphomas 1

2. Timing of post-therapy PET:

   • At least 3 weeks after chemotherapy 1, 4
   • 8-12 weeks after radiotherapy or chemoradiotherapy 1, 4

3. Interpretation criteria:

   • Visual assessment is adequate for post-therapy evaluation 1
   • Residual masses >1.5 cm with uptake ≥ liver/spleen should be considered positive 1
   • Residual masses <1.5 cm with uptake > liver/spleen should be considered positive 1

PET/CT has emerged as the modality of choice for staging, restaging, and treatment monitoring in lymphoma patients, offering high sensitivity and specificity for distinguishing between viable tumor and necrosis or fibrosis in residual masses 6.