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Short read · Oncology

A Residual Mass After Lymphoma Treatment: What Does PET Add?

Oncology · 2 min read

CT measures the mass. PET asks whether anything in it is still alive.

After treatment, a mass often remains on CT. It may be scar, necrosis or live lymphoma, and size alone cannot tell them apart. 18F-FDG PET asks a different question: is there still metabolically active tumour inside?

Why CT falls short

  • Tumour dies faster than the mass shrinks. Fibrosis and necrosis persist, typically in bulky mediastinal Hodgkin lymphoma and primary mediastinal B-cell lymphoma.
  • So a mass that has stopped shrinking is not proof of live disease, and a smaller one is not proof of cure.
  • PET separates viable tumour from inactive tissue. It reads glucose use, not size.
Pearl

A negative PET ends the argument; a positive PET starts one.

How to read it

  • Score the residual mass with Deauville. A score of 1 to 3 is complete metabolic response, with or without a residual mass. A score of 4 or 5 is residual metabolic disease.
  • Time it right. At least 3 weeks after chemotherapy, preferably 6 to 8 weeks; 8 to 12 weeks after radiotherapy; and at least 2 weeks after G-CSF. Earlier scans catch inflammation.
  • Compare with baseline. Deauville 4 or 5 with less uptake than baseline is a partial metabolic response. At the end of treatment, that still means residual disease.

What the evidence shows

  • A negative PET is reliable. In non-Hodgkin lymphoma, 56 of 67 PET-negative patients stayed in remission, while all 26 PET-positive patients relapsed.
  • In a systematic review, pooled specificity was 90% in Hodgkin and 100% in non-Hodgkin lymphoma. Sensitivity was lower, 84% and 72%, so a negative scan cannot exclude microscopic disease.
  • PET can decide who needs radiotherapy. In advanced Hodgkin lymphoma (HD15, 2126 patients analysed), a residual mass of 2.5 cm or more was irradiated only if PET-positive. The negative predictive value at 12 months was 94.1%, and only 11% of patients needed radiotherapy.
  • Radiotherapy can be omitted after complete metabolic response. In primary mediastinal B-cell lymphoma (IELSG37), 30-month progression-free survival was 96.2% without radiotherapy and 98.5% with it.
  • Deauville 4 is not Deauville 5. In the same trial, patients with Deauville 4, 86% of whom received radiotherapy alone, had 5-year progression-free survival of 95.8%. With Deauville 5 it was only 60.3%.

A positive residual mass: be careful

  • Positive predictive value is modest. After chemotherapy in Hodgkin lymphoma it is only about 25% to 40%, against a negative predictive value of 86% to 94%.
  • False positives come from treatment-related inflammation, rebound thymic hyperplasia, post-radiotherapy change and infection.
  • A negative scan reassures. Deauville 4 is read in context. With Deauville 5, biopsy before changing treatment when the answer would change management, or at least repeat the scan if suspicion is low.
Take home
  • CT shows the mass; PET shows whether it is alive. Deauville 1 to 3 is complete metabolic response even with a residual mass.
  • A negative end-of-treatment PET has been reliable enough to omit consolidation radiotherapy in trials (HD15, IELSG37).
  • A positive PET is far less certain: biopsy before changing treatment.
Sources
  1. Cheson BD, Fisher RI, Barrington SF, et al. Recommendations for initial evaluation, staging, and response assessment of Hodgkin and non-Hodgkin lymphoma: the Lugano classification. J Clin Oncol. 2014;32(27):3059-68.
  2. Juweid ME, Stroobants S, Hoekstra OS, et al. Use of positron emission tomography for response assessment of lymphoma: consensus of the Imaging Subcommittee of International Harmonization Project in Lymphoma. J Clin Oncol. 2007;25(5):571-8.
  3. Spaepen K, Stroobants S, Dupont P, et al. Prognostic value of positron emission tomography (PET) with fluorine-18 fluorodeoxyglucose ([18F]FDG) after first-line chemotherapy in non-Hodgkin's lymphoma: is [18F]FDG-PET a valid alternative to conventional diagnostic methods? J Clin Oncol. 2001;19(2):414-9.
  4. Zijlstra JM, Lindauer-van der Werf G, Hoekstra OS, et al. 18F-fluoro-deoxyglucose positron emission tomography for post-treatment evaluation of malignant lymphoma: a systematic review. Haematologica. 2006;91(4):522-9.
  5. Engert A, Haverkamp H, Kobe C, et al. Reduced-intensity chemotherapy and PET-guided radiotherapy in patients with advanced stage Hodgkin's lymphoma (HD15 trial): a randomised, open-label, phase 3 non-inferiority trial. Lancet. 2012;379(9828):1791-9.
  6. Martelli M, Ceriani L, Ciccone G, et al. Omission of radiotherapy in primary mediastinal B-cell lymphoma: IELSG37 trial results. J Clin Oncol. 2024;42(34):4071-83.
  7. Gallamini A, Kurlapski M, Zaucha JM. FDG-PET/CT for the management of post-chemotherapy residual mass in Hodgkin lymphoma. Cancers (Basel). 2021;13(16):3952.
  8. Barrington SF, Kluge R. FDG PET for therapy monitoring in Hodgkin and non-Hodgkin lymphomas. Eur J Nucl Med Mol Imaging. 2017;44(Suppl 1):97-110.
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