Changes in positron emission tomography (PET) uptake of 18F-fluorodeoxyglucose (18FDG), 18F-fluoro-3'-deoxy-3'-L-fluorothymidine (18FLT) and Copper (II) diacetyl-bis(N4-methylthiosemicarbazone) (64Cu-ATSM) before, during and after radiotherapy of a canine fibrosarcoma