INTRODUCTION To report the impact on target volume delineation and dose to normal tissue using anatomic versus biological imaging ((18) F-FDG-PET) for bone metastases. METHODS Patients with uncomplicated painful bone metastases were randomized (1:1:1) and blinded to receive either 8 Gy in a single fraction with conventionally planned radiotherapy (ConvRT-8 Gy) or 8 Gy in a single fraction with dose-painting-by-numbers (DPBN) dose range between 6 and 10 Gy) (DPBN-8 Gy) or 16 Gy in a single fraction with DPBN (dose range between 14 and 18 Gy) (DPBN-16 Gy). The primary endpoint was overall pain response at 1 month. Volumes of the gross tumour volume (GTV) - both biological (GTVPET ) and anatomical (GTVCT ) -, planning target volume (PTV), dose to the normal tissue and maximum standardized-uptake values (SUVMAX ) were analysed (secondary endpoint). RESULTS Sixty-three percent of the GTVCT volume did not show (18) F-FDG-uptake. On average, 20% of the GTVPET volume was outside GTVCT . The volume of normal tissue receiving 4 Gy, 6 Gy and 8 Gy was at least 3×, 6× and 13× smaller in DPBN-8 Gy compared to ConvRT-8 Gy and DPBN-16 Gy (P < 0.05). CONCLUSION Positron emitting tomography-information potentially changes the target volume for bone metastases. DPBN between 6 and 10 Gy significantly decreases dose to the normal tissue compared to conventional radiotherapy.