Professor Judith Trotman

PET is more accurate than CT alone as an imaging modality in follicular lymphoma – but the jury is still waiting more evidence on its application in some situations.

According to a Perspective article in Blood, FDG-PET is the gold standard imaging modality for staging and response assessment of follicular lymphoma.

“The sensitivity of PET supports a PET-guided approach to initial therapy with supplementary bone marrow biopsy in selected cases,” it says.

However PET as yet has no role in remission surveillance, and use of the quantitative measure of total metabolic tumour volume (TMTV) is also premature without standardisation.

The article, co-authored by Professor Judith Trotman from Concord Hospital and the University of Sydney, and Professor Andrew Pettitt from the University of Liverpool (UK), included a review of the PET literature between 2000 and 2020.

It noted additional sites are detected by PET in approximately two-thirds of cases, with upstaging estimated in 10-60%, especially those with apparently limited stage disease on CT.

“Directly demonstrating the clinical benefit of new imaging modalities presents major challenges due to many confounding factors that influence, or are influenced by, their application,” they said.

“Consequently, they are often adopted into routine practice based on improved sensitivity alone.”

Too early to guide therapy

Professors Trotman and Pettitt said end-of-induction (EOI) PET is strongly predictive of outcomes but it is yet to be shown how it can be used to guide subsequent therapy.

“This is particularly important in the current pandemic when the balance between the beneficial effects (improved PFS but not OS) and unwanted effects (increased susceptibility to infection) of continued therapy with anti-CD20 maintenance is being re-evaluated,” they said.

“Likewise, further studies are necessary to confirm if EOI PET status in the modern therapeutic era overrides the prognostic value of pretreatment risk scores.”

“There are currently no data to support pre-emptive intervention in patients who remain PET-positive and particularly for this poor risk population, the results of current trials involving EOI PET-adapted approaches are awaited with interest,” they concluded.

Professor Trotman told the limbic PET provides a platform for studies of further response to therapy.

“At the moment end of induction PET is prognostic but you generally don’t change your management based on your end of treatment PET. It helps you and the patient feel much more comfortable about a prolonged remission whereas if the patient is still PET-positive, their progression free and overall survival is inferior.”

She said, regarding maintenance treatment, the FOLL12 study had identified a PFS advantage to patients receiving rituximab even if they were PET negative and MRD negative.

“What PETReA is wanting to do is validate that data but also measure the trade-off in terms of the toxicity of antibody maintenance which is so relevant in the current COVID era.”