Tumour-to-background ratio (TBR)

Tumour-to-background ratio (TBR), also called tissue-to-background or lesion-to-background ratio, divides a measure of tracer uptake in a lesion (typically SUVmax or SUVpeak) by a measure of uptake in a reference region, which may be a fixed organ such as liver or blood pool, gluteal muscle, the contralateral normal tissue, or the tissue immediately surrounding the lesion ("local TBR"). Because PET and SPECT images are reconstructed and displayed with contrast-dependent scaling, TBR is a practical proxy for lesion conspicuity and is used both qualitatively (as a component of the Krenning score and PSMA-RADS) and quantitatively in response criteria.

In amino acid PET for brain tumours, the RANO/EANM/SNMMI/EANO consensus (PET RANO 1.0) defines PET-positive tumour as any voxel with uptake at least 1.6 times the mean background activity measured in a reference region of normal frontal lobe, and tracks change in TBRmax and TBRmean over time as response criteria (for example, an increase of 30% or more in TBRmax defines PET-based progression). The choice of reference tissue changes the number materially: a lesion's ratio to liver, muscle, and the immediately surrounding tissue can differ by an order of magnitude, so any reported TBR should state which reference region and which SUV metric were used.

TBR is sensitive to technical factors including reconstruction algorithm, attenuation correction, and time after injection, so serial comparisons for treatment response are only valid when acquisition protocol is held constant.