The use of positron emission tomography/computed tomography imaging in radiation therapy: a phantom study for setting internal target volume of biological target volume

BACKGROUND: Fluorodeoxyglucose ((18)F-FDG) positron emission tomography/computed tomography (PET/CT) is an important method for detecting tumours, planning radiotherapy treatment, and evaluating treatment responses. However, using the standardized uptake value (SUV) threshold with PET imaging may be suitable not to determine gross tumour volume but to determine biological target volume (BTV). The aim of this study was to extract internal target volume of BTV from PET images. METHODS: Three spherical densities of (18)F-FDG were employed in a phantom with an air or water background with repetitive motion amplitudes of 0–30 mm. The PET data were reconstructed with attenuation correction (AC) based on CT images obtained by slow CT scanning (SCS) or helical CT scanning (HCS). The errors in measured SUV(max) and volumes calculated using SUV threshold values based on SUV(max) (TH(max)) in experiments performed with varying extents of respiratory motion and AC were analysed. RESULTS: A partial volume effect (PVE) was not observed in spheres with diameters of ≥ 28 mm. When calculating SUV(max) and TH(max), using SCS for AC yielded smaller variance than using HCS (p < 0.05). For spheres of 37- and 28-mm diameters in the phantom with either an air or water background, significant differences were observed when mean TH(max) of 30-, 20-, or 10-mm amplitude were compared with the stationary conditions (p < 0.05). The average TH(max) values for 37-mm and 28-mm spheres with an air background were 0.362 and 0.352 in non-motion, respectively, and the mean TH(max) values for 37-mm and 28-mm spheres with a water background were 0.404 and 0.387 in non-motion and 0.244 and 0.263 in motion, respectively. When the phantom background was air, regardless of sphere concentration or size, TH(max) was dependent only on motion amplitude. CONCLUSIONS: We found that there was no PVE for spheres with ≥ 28-mm diameters, and differences between SUV(max) and TH(max) were reduced by using SCS for AC. In the head-and-neck and the abdomen, the standard values of TH(max) were 0.25 and 0.40 with and without respiratory movement, respectively. In the lungs, the value of TH(max) became the approximate expression depending on motion amplitude.