Rectal volume variations and estimated rectal dose during 8 weeks of image-guided radical 3D conformal external beam radiotherapy for prostate cancer

•Modern IGRT has given new insight regarding organ motion in radiotherapy.•Rectal volume variation may increase the risk of biochemical and local failure.•Rectal volume decreased significantly during eight weeks of radiotherapy.•The percentage of irradiated rectal volume did not change statistically significant.•Our study shows that IGRT ensures a close to stable dose to the rectum.