Interim PET Deauville 4 with Recent G-CSF
A 34-year-old man with stage II bulky diffuse large B-cell lymphoma has a baseline PET/CT showing a 12 cm mediastinal mass with SUVmax 24 and no other sites of disease. He receives two cycles of R-CHOP. Interim FDG PET/CT shows a residual 4.1 cm mediastinal mass with focal SUVmax 4.3; liver SUVmean is 3.1 and mediastinal blood pool SUVmean is 2.4. The uptake corresponds to the residual mass. He received pegfilgrastim 72 hours before the scan. He is asymptomatic, and his LDH is normal. The scan was acquired 60 minutes after 10.2 mCi of FDG, with a blood glucose of 110 mg/dL. The referring oncologist wants to switch to salvage chemotherapy because the Deauville score is 4. The patient asks whether the scan proves the lymphoma is still active.
1. Assign the Deauville score and explain whether it should drive an immediate change in therapy.
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The score is Deauville 4 because the focal mediastinal uptake is greater than the liver background. However, a Deauville 4 at interim does not by itself prove treatment failure in DLBCL, and it should not automatically trigger a switch to salvage chemotherapy. In the Lugano classification, a score of 4 or 5 at interim is equivocal and requires clinical correlation, especially when the uptake is focal and close to the liver reference. The high false-positive rate reflects inflammation, post-treatment changes, and the fact that the liver reference itself can vary. The patient is asymptomatic with a normal LDH, which further argues against acting on the scan alone. The appropriate response is to discuss the uncertainty with the oncologist, consider a biopsy, or repeat imaging after a short interval rather than escalate immediately.
2. What additional information would you use to resolve the equivocal interim PET?
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First, compare the residual mass with the baseline scan: a marked reduction in size and SUVmax from 24 to 4.3 is encouraging, even if the absolute value is above liver. Second, review the CT morphology, because a residual mass with low-density or fibrotic features may not be active tumor. Third, account for pegfilgrastim: marrow and spleen uptake can alter the reference background, although it does not usually cause focal mediastinal uptake. Fourth, consider a repeat PET/CT in 2 to 4 weeks after the marrow activation has resolved, or a biopsy if it can be done safely. Fifth, check whether the liver reference was measured in a representative region and whether the uptake is truly focal rather than thymic rebound, which is common in young patients after chemotherapy.
3. How does the recent pegfilgrastim affect interpretation, and what timing would you recommend for a repeat scan?
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Pegfilgrastim increases FDG uptake in bone marrow and the spleen, which can raise background activity and alter the liver reference; it can also produce diffuse marrow uptake that mimics disease. It does not typically cause focal mediastinal uptake, so it is unlikely to explain this patient's residual mass activity, but it can affect the Deauville score by changing the liver denominator. For a repeat scan, wait at least 5 to 7 days after pegfilgrastim, and preferably longer if the marrow uptake was intense, so that the reference tissues have returned to baseline. Also standardize the acquisition at 60 minutes and document the blood glucose. The repeat should be interpreted with the same 5-point scale and compared side by side with the baseline and interim studies.