Intrafractional Prostate Shifts During Stereotactic Radiation Therapy

Abstract

In this study we analyzed prostate intrafraction motion during the course of stereotactic radiotherapy (STRT). STRT (5 fraction of 7.25Gy) was performed in 31 primary patients with prostate cancer. At least 3 gold fiducial markers were implanted in prostate of every patient 3-5 days before simulation. Prostate position on treatment table was verified with cone beam CT just before and immediately after the end of each radiotherapy session. This data help to determine prostate displacement in cranio-caudal, anterior-posterior and lateral axes. Average values of intra-fractional prostate displacement were as follows: 0,8+/-1,2mm - in cranio-caudal, 0.9+/-1,0mm - in lateral and 1.3+/-1,3mm - in anterior-posterior directions. In 5%-8,3% cases prostate displacement in cranio-caudal and lateral directions exceeded 2mm with maximal value of 5mm. In accordance with obtained average shifts we recommend following PTV margins: 1mm - in anterior-posterior and 3mm - in all other directions. In this case average intrafractional prostate shifts would not compromise dose delivery to prostate (V100 - 98%, D90 - 101%) and in comparison with standard (3mm, 5mm, 5mm, 5mm) margins would permit 16% reduction of rectum volume incorporated in 80% isodose. We propose that small average intrafractional displacement of prostate permits the use of narrower PTV margins without compromising coverage of the target and significant reduction of rectum volume covered by 80% isodose.