Figure 2. ¹⁸FDG PET/CT case study. This patient presented with a right sided level II node containing metastatic squamous cell carcinoma. Conventional imaging with a diagnostic contrast enhanced CT scan showed the right neck node but the primary tumour was occult. (a) The non-contrast enhanced CT component of the PET/CT scan illustrates this (arrow points at node). (b) ¹⁸FDG PET/CT demonstrated the right neck node and but also identified an area of ¹⁸FDG uptake in the right tongue base which on biopsy was proven to be the primary site. (c) The planning target volume (PTV) defined in blue shows the target volume planned with conventional data only whereas the PTV defined in green shows the expansion of this when the PET/CT identifies the tongue base tumour and therefore includes the oropharynx and contralateral neck. The PTV increased by 467 cm³ with the addition of data from the PET/CT.