Inter-fraction prostate motion during intensity-modulated radiotherapy for prostate cancer

Introduction: This study aimed to analyse the inter-fraction prostate motion and setup error during intensity-modulated radiotherapy for prostate cancer at the National Cancer Centre Singapore. Methods: Gold seeds were implanted as fiducial markers. Daily portal films were taken and displacements of the gold seeds from the isocentre in each axis were recorded. Random and systematic errors were used to derive a margin recipe for each axis based on the van Herk formula. Results: 1,077 fractions from 36 patients were analysed. 89.8 percent, 85.2 percent and 83.6 percent of the setup errors were within +/- 2 mm for the right-left (RL), superior-inferior (SI) and anterior-posterior (AP) axes, respectively. The population systematic errors were 0.71 mm, 0.84 mm and 0.87 mm; the population random errors were 1.32 mm, 1.59 mm and 1.70 mm; the overall population mean setup errors were -0.14 (range -2.27 to 1.15) mm, 0.11 (range -2.32 to 1.69) mm and 0.08 (range -1.33 to 1.46) mm; and the van Herk margin recipes were 2.69 mm, 3.22 mm and 3.37 mm for the RL, SI and AP axes, respectively. Conclusion: The setup errors and inter-fraction prostate movements were small. Gold seed implantation is a feasible and easy method of verifying the prostate position.