Current status of PET/CT for tumour volume definition in radiotherapy treatment planning for non-small cell lung cancer (NSCLC).

Target volume delineation of lung cancer is well known to be prone to large inter-observer variability. The advent of PET/CT devices, with co-registered functional and anatomical data, has opened new exciting possibilities for target volume definition in radiation oncology. PET/CT imaging is rapidly being embraced by the radiation oncology community as a tool to improve the accuracy of target volume delineation for treatment optimization in NSCLC. Several studies have dealt with the feasibility of incorporating FDG-PET information into contour delineation with the aim to improve overall accuracy and to reduce inter-observer variation. A significant impact of PET-derived contours on treatment planning has been shown in 30-60% of the plans with respect to the CT-only target volume. The most prominent changes in the gross tumour volume (GTV) have been reported in cases with atelectasis and following the incorporation of PET-positive nodes in otherwise CT-insignificant nodal areas. Although inter-observer variability is still present following target volume delineation with PET/CT, it is greatly reduced compared to conventional CT-only contouring. PET/CT may also provide improved therapeutic ratio compared to conventional CT planning. Increased target coverage and often reduced target volumes may potentially result in PET/CT-based planning to yield better tumour control probability through dose escalation, while still complying with dose/volume constrains for normal tissues. Despite these exciting results, more clinical studies need to be performed to better define the role of combined PET/CT in treatment planning for NSCLC.