Background: The challenge of prostate and rectal motion during radiotherapy has been firmly established, both these problems are exacerbated by rectal instability. A number of different mechanical methods have been explored to see if the rectum can be stabilised. This study aimed to establish whether there is a clinical advantage in the use of ERB’s for the treatment of prostate patients, assessing inter-fractional prostate stability and dose to the rectum. Methods: This planning study was based on 10 patients with 2 sets of CT and Cone beam CT (CBCT) data with and without an ERB in situ. The ERB was a 100cc air filled RectalPro balloon. CBCT scan with and without the ERB were taken on the same day. The scans were contoured and planned using pinnacle version 9.8 with rotational arc therapy to a dose of 60Gy in 20 fractions, for an Elekta Agility treatment unit. An assumption was made that the prostate stability was directly related to the rectal stability. Based on this assumption, comparisons of the rectal position and dose on the CBCT verses the respective CBCT were made. The rectal DVH on the plans for the ERB verses no ERB were compared and a two-tailed T-test was used for statistical analysis of all comparisons. Results: Positional analysis demonstrated 12% improvement in the dice coefficient similarity score of the rectum for the ERB (P=0.0003). This only correlated to a significant improvement in the AP movement at mid prostate for the ERBs (P=0.031). The ERB position was seen to be influenced by the angle of insertion. Removal of the CBCT data from the 4 ERBs with largest deviations in insertion angle significantly reduced the inter-fractional movement seen with the ERBs. The lower rectal doses showed an improvement of around 8% for the ERB group, although for the 57Gy dose was 2.5% better with no-ERB as shown in the Table. Conclusions: There is a clinical and statistical benefit in the dose reduction for the rectal wall in the use of ERB’s for prostate radiotherapy. Although the ERB position needs to be confirmed with IGRT to remove the potential for inter-fractional positional error.