Figure 1. Theoretical image of radiotherapy planning volumes incorporating a PET-defined biological target volume (BTV) for dose escalation. GTV, gross tumour volume defined using high spatial resolution CT or MRI; CTV, clinical target volume defined by a clinician to account for microscopic tumour spread; PTV, planning target volume defined by a physicist to incorporate margins allowing for tumour motion and set-up errors; NTI, normal tissue; NTII, normal tissue at increased risk of radiation damage defined by molecular imaging. Careful planning of the 40% isodose (ISO) line to exclude NTII enables the delivery of a reduced dose to the high-risk area.