Daily image-guidance in prostate cancer radiotherapy - implications for treatment margins

Our purpose was to evaluate interfractional organ and patient movement during radiotherapy of prostate cancer and to calculate the necessary CTV to PTV margins. Fifty patients irradiated between 2009 and 2011were included into the prospective study. The 2D-2D KV system combined with the intraprostatic fiducial marker were used for daily position verification. Based on the 8629 measurements of isocentre displacement an interfractional motion of pelvis and prostate was evaluated. The CTV to PTV margins were calculated. Margins of 0.7 cm (AP), 1 cm (SI) and 0.35 cm (LR) are necessary when only bony based IGRT is performed. Margins of 1.0 cm, 1.8 cm and 0.5 cm in AP, SI and LR directions respectively are necessary in case of no IGRT.There was no clinically relevant changes in patient/target mobility throughout the whole treatment. The IGRT is essential for state of art radiotherapy of prostate cancer. Necessary CTV to PTV margins are much bigger in case of no IGRT performed. Changing of margins size throughout regular treatment is not necessary.