In this study, we prospectively compared the sensitivity of PET and planar SPET (collimated gamma camera) 18F-FDG imaging in patients with lung and gastrointestinal tract cancer and analysed their respective impact on patient management. Twenty-eight patients with lung cancer and 14 with gastro-intestinal tract tumours were scanned on the same day with a PET and a collimated planar SPET gamma camera. The planar SPET procedure consisted of whole-body planar views and a tomographic acquisition centred over the torso or the abdomen, with the total imaging time within the same range as the whole-body PET procedure. The staging of lung cancer patients was accurate in 86% with PET and 64% with planar SPET. Planar SPET would have led to inappropriate therapeutic decisions in 8 of 28 patients, mainly due to undetected distant metastases. In patients with suspected gastrointestinal tract cancer, planar SPET identified 7 of 15 (47%) proven tumour sites, whereas PET identified 14 of 15 (93%). Our results suggest that collimated planar SPET cameras are not a substitute for dedicated PET scanners. The sensitivity for the detection of tumours is unacceptably low and can impair patient management. The use of multiple tomographic acquisitions could improve the sensitivity but would require a longer scanning time.