Assessment of PTV Margins Accounting for Prostate Intrafraction Motion in SBRT with Online IGRT

Purpose: The aim of this work was to assess PTV margins suitable for SBRT of prostate cancer uncertainties after daily online correction. Moreover, intra-fraction prostate motion is analyzed with the aim to identify its main causes Material and Methods: Between 2013 and 2014, 43 patients with low or intermediate risk prostate cancer were treated with 7-fraction SBRT in supine position, with implanted Fiducial Markers (FM), empty rectum and full bladder. The van Herk’s formula was applied to calculate the PTV margins of prostate/seminal vescicles. To investigate the causes of organ motion, the bladder volume and the rectum wall distension and the treatment time. Results: systematic increase of bladder and the rectal distension not influence the prostate displacement (p = 0.55 and p = 0.32 respectively). Significant correlation was observed between the intrafraction composite shift of the prostate volume and the elapsed treatment time (p = 0.036). Conclusion: our data suggest a good control of intrafraction motion. The prostate intrafraction motion is shown to be dependent on elapsed treatment time.