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 (¹³¹Cs) vs. iodine-125 (¹²⁵I) implants used in brain brachytherapy.

Material and methods
Twenty-four patients with ¹³¹Cs implants from a prospective phase I/II trial were re-planned with ¹²⁵I 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% (D₁₀₀), 90% (D₉₀), 80% (D₈₀), and 50% (D₅₀) of the clinical target volume (CTV).

Results
At the 100%, 90%, 80%, and 50% isodose lines, the ¹³¹Cs plans exposed less mean volume of brain tissue than the ¹²⁵I plans (p < 0.001). The D₁₀₀, D₉₀, D₈₀, and D₅₀ were smaller for ¹³¹Cs (p < 0.001). The HI and CI for ¹³¹Cs vs. ¹²⁵I were 19.71 vs. 29.04 and 1.31 vs. 1.92, respectively (p < 0.001).

Conclusions
Compared to ¹²⁵I, ¹³¹Cs exposed smaller volumes of brain tissue to equivalent doses of radiation and delivered lower radiation doses to equivalent volumes of the CTV. ¹³¹Cs exhibited a higher HI, indicating increased uniformity of doses within the CTV. Lastly, ¹³¹Cs 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 ¹³¹Cs is dosimetrically superior to ¹²⁵I and may explain the reason for the 0% incidence of radiation necrosis (RN) in our previously published prospective study using ¹³¹Cs.