Reference dose rates for single- and double-plane 192Ir implants.

The proper selection of the reference dose rate is critical in the irradiation of a tumor or tumor bed using interstitial implant. The selected reference dose rate should result in the delivery of highly homogeneous dose throughout the target volume, adequately cover the target volume, and minimize irradiation of the surrounding normal tissues. In this paper, the influence of the reference dose rate on the adequacy of the irradiation of idealized target volumes using single- and double-plane 192Ir implants was studied in terms of three volumetric irradiation indices. The results show that maximum relative dose homogeneity can be attained if the proper reference dose rate is chosen. This isodose rate contour exhibits a width larger than the thickness of the target volume in the central plane. The lowest dose rate within the idealized target volume is not recommended as the reference dose rate to avoid a large volume of the surrounding tissues from receiving dose rates equal to or greater than this dose rate, and also the inhomogeneous irradiation of the target volume.