Intra-fraction motion of the prostate during treatment with helical tomotherapy

Abstract Background and purpose To measure the geometric uncertainty resulting from intra-fraction motion and intra-observer image matching, for patients having image-guided prostate radiotherapy on TomoTherapy. Material and methods All patients had already been selected for prostate radiotherapy on TomoTherapy, with daily MV-CT imaging. The study involved performing an additional MV-CT image at the end of treatment, on 5 occasions during the course of 37 treatments. 54 patients were recruited to the study. A new formula was derived to calculate the PTV margin for intra-fraction motion. Results The mean values of the intra-fraction differences were 0.0 mm, 0.5 mm, 0.5 mm and 0.0° for LR, SI, AP and roll, respectively. The corresponding standard deviations were 1.1 mm, 0.8 mm, 0.8 mm and 0.6° for systematic uncertainties ( Σ ), 1.3 mm, 2.0 mm, 2.2 mm and 0.3° for random uncertainties ( σ ). This intra-fraction motion requires margins of 2.2 mm in LR, 2.1 mm in SI and 2.1 mm in AP directions. Inclusion of estimates of the effect of rotations and matching errors increases these margins to approximately 4 mm in LR and 5 mm in SI and AP directions. Conclusions A new margin recipe has been developed to calculate margins for intra-fraction motion. This recipe is applicable to any measurement technique that is based on the difference between images taken before and after treatment.