This study concerns patients with inoperable (respiratory failure), stage I or II, lung cancers. It aims to establish a reliable procedure for ventilation–perfusion (V-Q) SPECT-CT study and to estimate its impact on treatment planning. Fifteen patients underwent V-Q SPECT and CT studies in the prone position in an expendable foam immobilization device (EFID). This position was well tolerated even for patients requiring part-time oxygen supply. SPECT data induced a modification of radiotherapy planning in all patients. The V-Q SPECT-CT study in the prone position is safe, accurate and useful for irradiation treatment planning.