For interim scans a substantial but incomplete reduction in metabolic activity may be sufficient to indicate an adequate response to treatment. The optimum threshold for interpreting interim PET scans depends on expected patient prognosis and proposed escalation or de-escalation of treatment. In particular, where standard treatment is omitted on the basis of a negative interim PET (e.g. fewer cycles of chemotherapy and or no radiotherapy), a lower threshold of positivity, using a score of 3, is recommended to prevent possibility of under treatment.

Both positive and negative results at end of treatment need to be considered in the light of the inherent limitations of FDG PET imaging. Infection and inflammatory uptake can lead to more intense false-positive scans of 4 or more. Positive scans should therefore, wherever possible, be confirmed with histology prior to subjecting patients to treatment escalation. If it is not possible or practical to obtain histology, follow-up imaging following adequate treatment for any reversible benign causes of uptake, e.g. infection, may be helpful in supporting detection of residual disease. Finally, a negative PET scan cannot exclude the presence of residual microscopic disease which is especially relevant in considering whether patients should undergo radiotherapy following chemotherapy.