In the majority of classical Hodgkin lymphoma treatment protocols and trials (RAPID, RATHL, HD17), a Deauville score of 3 at interim PET is treated as 'negative' (equivalent to CMR) and supports treatment de-escalation — specifically, omission of bleomycin in the RATHL trial paradigm, and consideration of radiotherapy omission in the RAPID trial. However, some protocols (particularly those enrolling high-risk patients) may treat score 3 as positive. Always check the specific protocol or institutional guideline your patient is being managed under. For DLBCL, Lugano classification leaves score 3 at end-of-treatment as ambiguous — correlate with CT morphology (if residual mass >2 cm on CT with score 3, many centers classify as PR).

This is almost certainly a false-positive from G-CSF-stimulated marrow hyperplasia, not true marrow disease. G-CSF (filgrastim, pegfilgrastim) given after chemotherapy stimulates bone marrow progenitor cells, increasing FDG avidity in the axial skeleton and spleen. The pattern is typically diffuse and symmetric (entire spine, pelvis, ribs), whereas lymphoma marrow involvement tends to be focal and asymmetric. To minimize this issue, PET should ideally be performed ≥3 weeks after the last G-CSF dose. If interim PET is positive in a marrow distribution suggesting G-CSF effect, document this in the report, do not assign Deauville 5 based on marrow uptake alone, and consider repeat imaging or biopsy of a suspicious site if truly uncertain.

Residual mediastinal masses are common in Hodgkin lymphoma and nodular sclerosing subtype often leaves fibrotic scar tissue. The key is whether this residual mass is metabolically active. If the mass shows Deauville ≤3 uptake (no uptake above liver), it represents scar tissue — this is still CMR per PET criteria even if the mass remains large on CT. This is a critical point: Lugano Classification CMR is defined by PET negativity, not CT size normalization. A Deauville 4–5 residual mass indicates viable lymphoma and warrants treatment modification. When in doubt, biopsy the hottest area of the mass (guided by PET/CT fusion) to confirm histology.

Deauville scoring was developed and validated for FDG-avid lymphomas — primarily classical Hodgkin lymphoma and DLBCL. For indolent lymphomas with variable FDG avidity (follicular lymphoma grade 1–2, marginal zone lymphoma, small lymphocytic lymphoma/CLL), PET-CT is less reliable for response assessment, and Deauville criteria have limited applicability. For these lymphomas, CT-based response criteria (Lugano CT response: CR, PR, SD, PD) are more appropriate. If PET is performed (baseline and follow-up), Deauville can be used, but interpretation requires caution — low-grade follicular lymphoma often has lower baseline FDG uptake, making score 3 vs 4 particularly difficult to interpret.

While most guidelines support empiric treatment escalation for Deauville 4–5 at interim PET in Hodgkin lymphoma (switch to BEACOPP) or end-of-treatment PET in DLBCL (salvage therapy), biopsy is preferred when: (1) the positive finding is atypical (isolated extranodal site, non-lymphoma distribution), (2) the patient has received prior radiation or chemotherapy that could cause false-positive uptake at a treatment field site, (3) the positive result would lead to very high-risk treatment (conditioning chemotherapy for stem cell transplant), (4) the clinical picture is discordant (patient clinically improving despite Deauville 5). A false-positive PET leading to unnecessary escalation can expose patients to life-threatening toxicity — when in doubt, biopsy before escalating.