Role of PET imaging in adaptive radiotherapy for lymphoma

Positron emission tomography/Computed tomography (PET/CT) is an essential part of modern radiotherapy for patients with lymphoma. PET/CT can be used to adapt treatment algorithms in Hodgkin lymphoma, reserving consolidation radiotherapy for patients with residual fluoro-D-glucose (FDG) avidity after treatment with intensive chemotherapy such as escalated BEACOPP and limiting the need for radiotherapy for some patients with complete metabolic response on PET if radiotherapy may be associated with increased toxicity. More importantly, PET/CT is now mandatory to define sites of initial disease for radiotherapy planning where smaller volumes are to be used rather than historical extended field treatments, such as mantle radiotherapy or even involved field radiotherapy. Involved node radiotherapy (INRT) treats only the initially involved nodes and is possible when the pretreatment PET/CT scan has been performed in the radiotherapy treatment position. Involved site radiotherapy (ISRT) builds in a margin for uncertainty when a pretreatment PET/CT is available, but has not been performed in the radiotherapy treatment position. Studies suggest that PET/CT changes radiotherapy volumes in approximately one third of patients by mapping the extent of initial disease better than using CT alone. PET/CT has also been used to adjust radiotherapy dose for patients who may be at increased risk of radioresistance, by virtue of residual FDG avidity post chemotherapy or patients with relapsed disease. This article will discuss the role of PET in selecting patients for radiotherapy, its influence on the choice of target volume and radiotherapy dose and the practicalities of how PET/CT scanning is incorporated into the radiotherapy planning process.