MOVING TOWARD FOCAL THERAPY IN PROSTATE CANCER: DUAL-ISOTOPE PERMANENT SEED IMPLANTS AS A POSSIBLE SOLUTION

Purpose To compare the ability of single- and dual-isotope prostate seed implants to escalate biologically effective dose (BED) to foci of disease while reducing prescription dose to the prostate. Methods and Materials Nine plans, using 125 I, 103 Pd, and 131 Cs alone and in combination were created retrospectively for 2 patients. Ultrasound and MRI/MRS datasets were used for treatment planning. Voxel-by-voxel BED was calculated for single- and dual-isotope plans. Equivalent uniform BED (EUBED) was used to compare plans. The MRS-positive planning target volumes (PTV i ) were delineated along with PTV (prostate + 5 mm), rectum, and urethra. Single-isotope implants, prescribed to conventional doses, were generated to achieve good PTV coverage. The PTV i were prospectively used to generate implants using mixtures of isotopes. For mixed-radioisotope implants, we also explored the impact on EUBED of lowering prescription doses by 15%. Results The EUBED of PTV i in the setting of primary 125 I implant increased 20–66% when 103 Pd and 131 Cs were used compared with 125 I boost. Decreasing prescription dose by 15% in mixed-isotope implants results in a potential 10% reduction in urethral EUBED with preservation of PTV coverage while still boosting PTV i (up to 80%). When radiobiologic parameters corresponding to more-aggressive disease are assigned to foci, faster-decaying isotopes used in mixed implants have the potential to preserve the equivalent biological effect of mono-isotope implants considering less-aggressive disease distributed in the entire prostate. Conclusions This is a hypothesis-generating study proposing a treatment paradigm that could be the middle ground between whole-gland irradiation and focal-only treatment. The use of two isotopes concurrent with decreasing the minimal peripheral dose is shown to increase EUBED of selected subvolumes while preserving the therapeutic effect at the level of the gland.