Figure 1

Planning CT (A-C) and CBCT images of a patient immediately before (D-F) and after (G-I) an IMRT fraction. Prostate position differs due to differences in rectal filling during therapy caused by rectal gas not present on treatment planning (in this case -7.4 mm in AP, 1.1 mm in left-right and -1.0 mm in cranio-caudal directions). Planning CT was acquired due to the intitutional policy using enema. This patient marks the maximum unidirectional displacement observed. In general patients with larger differences in rectal filling between treatment planning CT and daily online cone beam CT are asked to empty their rectum and drink water before another treatment attempt.