PET-based treatment planning in radiotherapy: a new standard?

Molecular imaging, in particular, PET, has brought an additional dimension to management for patients with cancer. 18F-FDG, which is the most widely available tracer, has been shown to be of value for the selection of target volumes in radiation oncology. Depending on its sensitivity and specificity, 18F-FDG has been shown to influence the selection of target volumes for non-small cell lung cancers (NSCLC) or for esophageal tumors. On the other hand, for tumors such as head and neck squamous cell carcinomas (HNSCC) and rectal carcinomas, convincing data on the value of 18F-FDG for target volume selection are still lacking. For target volume delineation, given that an adequate method is used for volume segmentation, the added value of 18F-FDG has been demonstrated for HNSCC and NSCLC. For both types of tumors, modifications in target volume delineation translated into differences in dose distribution compared with the results of CT scan-based plans. Studies are in progress for rectal tumors. Novel markers of tumor hypoxia or proliferation have the potential to modify the delineation of target volumes, allowing for "dose painting" in selected subvolumes. Finally, variations in tumor volume and viability during radiotherapy also are under intense investigation, potentially paving the way for so-called "theragnostic" or adaptive dose distribution during treatment. This review discusses how PET/CT might modify the current state of the art of radiotherapy planning.