To study prostate bed deformation, and compare coverage by 5 mm and 10mm posterior expansion PTV margins.Fifty patients who completed post-prostatectomy radiotherapy had two expansion margins applied to the planning CT CTV: PTV10 (10 mm isometrically) and PTV5 (5 mm posteriorly, 10 mm all other directions). The CTV was then contoured on 477 pre-treatment CBCTs, and PTV5 and PTV10 coverage of each CBCT-CTVs was assessed. The maximum distance from the planning CT CTV to the combined CTV of all CBCTs including the planning CT CTV was measured for the superior part of the prostate bed, and the inferior part of the prostate bed, for every patient.The mean difference between largest and smallest CBCT-CTVs per patient was 18.7 cm(3) (range 6.3-34.2 cm(3)). Out of 477 CBCTs, there were 43 anterior geometric geographical misses for either PTV with a mean volume of 2.25 cm(3) (range 0.01-18.88 cm(3)). For PTV10, there were 26 posterior geometric geographical misses with a mean volume of 1.37 cm(3) (0.01-11.02 cm(3)). For PTV5, there were 46 posterior geometric geographical misses with a mean volume of 3.22 cm(3) (0.01-19.82 cm(3)). The maximum edge-to-edge distance for the superior prostate bed was anterior 19 mm, posterior 16 mm, left and right 7 mm. The maximum edge-to-edge distance for the inferior prostate bed was anterior 4mm, posterior 12 mm, left and right 7 mm.This study supports differential margins for the superior and inferior portions of the prostate bed. Because of the large deformation of CTV volume seen, adaptive radiotherapy solutions should be investigated further.