The impact of rectal and bladder variability on target coverage during post-prostatectomy intensity modulated radiotherapy.

BACKGROUND AND PURPOSE: Accuracy when delivering post-prostatectomy intensity modulated radiotherapy (IMRT) is crucial. The aims of this study were to quantify prostate bed movement and determine what amount of bladder or rectum size variation creates the potential for geographic miss.

METHODS AND MATERIALS: The Cone Beam CT (CBCT) images (n=377) of forty patients who received post-prostatectomy IMRT with daily on-line alignment to bony anatomy were reviewed. Prostate bed movement was estimated using the location of surgical clips in the upper and lower sections of the PTV and correlated with rectal and bladder filling (defined as changes in the cross sectional diameter at defined levels). The number of potential geographic misses caused by bladder and rectum variation was calculated assuming a uniform CTV to PTV expansion of 1cm except 0.5 cm posteriorly.

RESULTS: Variations in bladder filling of >2 cm larger, ±1 cm, or >2 cm smaller occurred in 3.4%, 56.2%, and 15.1% of images respectively with potential geographic misses in the upper prostate bed of 61.5%, 9.9% and 26.3% respectively. Variations in rectal filling in the upper prostate bed of >1.5 cm larger, 1.5 cm larger to 1cm smaller, and >1cm smaller occurred in 17.2%, 75.6%, and 7.2% of images respectively. These variations resulted in geographic misses in the upper prostate bed in 29.2%, 12.3%, and 63.0% of images respectively. Variations in bladder and rectal filling in the lower prostate bed region had minimal impact on geographic misses.

CONCLUSIONS: Bladder and rectal size changes at treatment affect prostate bed coverage, especially in the upper aspect of the prostate bed. The greatest potential for geographic miss occurred when either the bladder increased in size or when the rectum became smaller. Ensuring a full bladder and empty rectum at simulation will minimise this risk. Our data also support anisotropic PTV margins with larger margins superiorly than inferiorly.