Dosimetric differences between cesium-131 and iodine-125 brachytherapy for the treatment of resected brain metastases

PURPOSE: To compare treatment plans and evaluate dosimetric characteristics of permanent cesium-131 ((131)Cs) vs. iodine-125 ((125)I) implants used in brain brachytherapy. MATERIAL AND METHODS: Twenty-four patients with (131)Cs implants from a prospective phase I/II trial were re-planned with (125)I implants. In order to evaluate the volume of brain tissue exposed to radiation therapy (RT), the dose volume histogram was generated for both radioisotopes. To evaluate the dosimetric differences of the two radioisotopes we compared homogeneity (HI) and conformity indices (CI), and dose covering 100% (D(100)), 90% (D(90)), 80% (D(80)), and 50% (D(50)) of the clinical target volume (CTV). RESULTS: At the 100%, 90%, 80%, and 50% isodose lines, the (131)Cs plans exposed less mean volume of brain tissue than the (125)I plans (p < 0.001). The D(100), D(90), D(80), and D(50) were smaller for (131)Cs (p < 0.001). The HI and CI for (131)Cs vs. (125)I were 19.71 vs. 29.04 and 1.31 vs. 1.92, respectively (p < 0.001). CONCLUSIONS: Compared to (125)I, (131)Cs exposed smaller volumes of brain tissue to equivalent doses of radiation and delivered lower radiation doses to equivalent volumes of the CTV. (131)Cs exhibited a higher HI, indicating increased uniformity of doses within the CTV. Lastly, (131)Cs presented a CI closer to 1, indicating that the total volume receiving the prescription dose was closer to the desired CTV volume. These results suggest that (131)Cs is dosimetrically superior to (125)I and may explain the reason for the 0% incidence of radiation necrosis (RN) in our previously published prospective study using (131)Cs.