Group items tagged

Far more important than the SUVmax is the pattern rather than intensity of metabolic abnormality and the correlative CT findings

Descriptively, we define SUV < 5 as “low intensity”, 5–10 as “moderate”, 10–15 as “intense” and >15 as “very intense”

Evolving literature suggests that intensity of uptake is an independent prognostic factor and in some tumour subtypes superior to histopathologic characterisation.

aerobic glycolysis

Our practice of thresholding the grey and colour scale to liver as detailed above results in similar image intensity to a fixed upper SUV threshold of 8 to 10

The advantage of using the liver as a reference tissue is also aided by this organ having rather low variability in metabolic activity

When the liver is abnormal and cannot be used as a reference organ, we use the default SUV setting of an upper SUV threshold of 8

One of the most challenging aspects of oncologic FDG PET/CT review, however, is to recognise all the patterns of metabolic activity that are not malignant and which consequently confound interpretation

Many benign and inflammatory processes are also associated with high glycolytic activity

Future articles in the “How I Read” series will address the specific details of reading PET/CT in various cancers

The intensity of uptake in metastases usually parallels that in the primary site of disease

For example, discordant low-grade activity in an enlarged lymph node in the setting of intense uptake in the primary tumour suggests it is unlikely malignant and more likely inflammatory or reactive

By CT criteria the enlarged node is ‘pathologic’ but the discordantly low metabolic signature further characterises this is as non-malignant since such a node is not subject to partial volume effects and therefore the intensity of uptake should be similar to the primary site

The exception is when the lymph node is centrally necrotic as a small rim of viable tumour is subject to partial volume effects with expectant lower intensity of uptake; integrating the CT morphology is therefore critical to reaching an accurate interpretation

Small nodes that are visualised on PET are conversely much more likely to be metastatic as such nodes are subject to partial volume effects.

The exception to this rule is tumours with a propensity for tumour heterogeneity at different sites

The combination of FDG and a more specific tracer, which visualises the well-differentiated disease can be very useful to characterise this phenomenon

“metabolic signature”

For the majority of malignant processes, the intensity of metabolic abnormality correlates with degree of aggressiveness or proliferative rate.

a negative PET/CT study in a patient with biopsy proven malignancy would be considered false-negative

Warburg effect

There, however, are a significant minority of tumours that utilise substrates other glucose such as glutamine or fatty acids as a source of the carbon atoms required for growth and proliferation

This includes a subset of diffuse gastric adenocarcinomas, signet cell colonic adenocarcinomas and some sarcomas, particularly liposarcoma

There may be a role for other radiotracers such as fluorothymidine (FLT) or amino acid substrates in this setting.

Some tumours harbour mutations that result in defective aerobic mitochondrial energy metabolism, effectively simulating the Warburg effect

patients with hereditary paraganglioma and pheochromocytoma highlight this phenomenon

These have intense uptake on FDG PET/CT despite often having low proliferative rate.

Uterine fibroids, hepatic adenomas, fibroadenomas of the breast and desmoid tumours are benign or relatively benign lesions that can have quite high FDG-avidity.

Metabolic activity switches off rapidly following initiation of therapy

Common examples where patients have commenced active therapy but the referrer is requesting “staging” includes hormonal therapy (eg. tamoxifen) in breast cancer, oral capecitabine in colorectal cancer or high dose steroids in Hodgkin’s lymphoma

It is therefore critical to perform PET staging before commencement of anti-tumour therapy

The potential advantage of routine diagnostic CT is improved anatomic localisation and definition

Without intravenous contrast, additional identification of typical oncologic complications such as pulmonary embolism or venous thrombosis cannot be identified

If the study is performed as an “interim” restaging study after commencement of therapy but before completion, in order to reach a valid or clinically useful conclusion findings must be interpreted in the context of known changes that occur at a specific timing and type of therapy

The most well studied use of interim PET is in Hodgkin’s lymphoma where repeat PET after two cycles of ABVD-chemotherapy provides powerful prognostic information and may improve outcomes by enabling early change of management