- Purpose To evaluate dose reduction caused by the implantation of an interstitial inflatable and biodegradable balloon device aiming to achieve lower rectal doses with virtual 3D conformal external beam radiation treatment. Materials and methods An inflatable balloon device was placed, interstitially and under transrectal ultrasound guidance, into the rectal–prostate interspace prior treatment initiation of 26 patients with localized prostate cancer, who elected to be treated with radiotherapy (3D CRT or IMRT). The pre- and post-implant CT imaging data of twenty two patients were collected (44 images) for the purpose of the 3D conformal virtual planning presented herein. Results The dorsal prostate–ventral rectal wall separation resulted in an average reduction of the rectal V70% by 55.3% (±16.8%), V80% by 64.0% (±17.7%), V90% by 72.0% (±17.1%), and V100% by 82.3% (±24.1%). In parallel, rectal D2 ml and D0.1 ml were reduced by 15.8% (±11.4%) and 3.9% (±6.4%), respectively. Conclusions Insertion of the biodegradable balloon into the prostate–rectum interspace is similar to other published invasive procedures. In this virtual dose distribution analysis, the balloon insertion resulted in a remarkable reduction of rectal volume exposed to high radiation doses. This effect has the potential to keep the rectal dose lower especially when higher than usual prostate dose escalation protocols or hypo-fractionated regimes are used. Further prospective clinical investigations on larger cohorts and more conformal radiation techniques will be necessary to define the clinical advantage of the biodegradable interstitial tissue separation device.