Impact of rectal balloon-filling materials on the dosimetry of prostate and organs at risk in photon beam therapy

The use of rectal balloon in radiotherapy of prostate cancer is shown to be effective in reducing prostate motion and minimizing rectal volume, thus reducing rectal toxicity. Air-filled rectal balloon has been used most commonly, but creates dose perturbation at the air-tissue interface. In this study, we evaluate the effects of rectal balloon-filling materials on the dose distribution to the target and organs at risk. The dosimetric impact of rectal balloon filling was studied in detail for a typical prostate patient, and the general effect of the balloon filling was investigated from a study of ten prostate patients covering a wide range of anterior-posterior and left-right separations, as well as rectal and bladder volumes. Hounsfield units (HU) of the rectal balloon filling was changed from -1000 HU to 1000 HU at an interval of 250 HU, and the corresponding changes in the relative electron density (RED) was calculated. For each of the HU of the rectal balloon filling, a seven-field IMRT plan was generated with 6 MV and 15 MV photon beams, respectively. Dosimetric evaluation was performed with the AAA algorithm for inhomogeneity corrections. A detailed study of the rectal balloon filling shows that the GTV, PTV, rectal, and bladder mean dose decreased with increasing values of RED in the rectal balloon. There is significant underdosage in the target volume at the rectum-prostate interface with an air-filled balloon as compared to that with a water-filled balloon for both 6 MV and 15 MV beams. While the dosimetric effect of the rectal balloon filling is reduced when averaged over ten patients, generally an air-filled balloon results in lower minimum dose and lower mean dose in the overlap region (and possibly the PTV) compared to those produced by water-filled or contrast-filled balloons. Dose inhomogeneity in the target volume is increased with an air-filled rectal balloon. Thus a water-filled or contrast-filled rectal balloon is preferred to an air-filled rectal balloon in EBRT of prostate treatment.