All animal experiments were performed in accordance with the United Kingdom Home Office Animal (scientific procedures) Act 1986. 5 × 10 6 A549 human lung cancer cells in 100 µL Dulbecco's PBS were injected subcutaneously into the upper flank of female Balb/c nu/nu mice aged 6-9 weeks (Charles River Laboratories). Tumor dimensions were measured using an electronic caliper and the volume calculated using the following equation: volume = ((π/6) × h × w × l), where h, w and l represent, height, width and length, respectively. PET imaging studies with [ 18 F]FSPG took place when tumor volume reached approximately 100 mm 3 .