Assessment of brachytherapy-induced prostate edema on postimplant dosimetric analysis in patients treated with magnetic resonance imaging–assisted radiosurgery (MARS)

Purpose: Brachytherapy-induced prostate edema (PE) can result in altered target dose coverage in prostate cancer patients treated with low-dose-rate (LDR) brachytherapy. While visualization of PE on CT is limited, PE is evident on magnetic resonance imaging (MRI) day 0 (D0) postimplant assessment, a critical step in the MRI-assisted radiosurgery (MARS) framework that improves LDR brachytherapy quality assurance. This study investigated PE in patients treated with MARS to analyze the effect on postimplant dosimetry and further optimize the treatment-planning process. Materials and Methods: We identified 317 patients with low-risk to intermediate-risk prostate cancer treated with cesium-131, iodine-125, or palladium-103 MARS definitive monotherapy from 2016 to 2021. Postimplant dosimetry was performed using MRI on D0. Simple linear regression with Pearson correlation analysis and ordinary ANOVA were used for analysis. Results: The median D0 prostate volume was 29% higher (IQR, 16%–42%) compared with preimplant measurement, with no significant differences in PE magnitude by isotope (p = 0.33) or number of needles implanted (p = 0.70). PE magnitude decreased with increasing preplan prostate size (p < 0.001). Greater PE was associated with decreased D0 prostate V₁₀₀ (p < 0.001), V₁₅₀ (p < 0.001), and D₉₀ (p < 0.001), but was not associated with V₂₀₀ (p = 0.06), and >98% implants achieved D₉₀/prescription dose >90%. Greater PE was associated with decreased rectum V₁₀₀ (p = 0.02). Conclusions: MRI on D0 confirmed PE after LDR brachytherapy. PE was not significantly different between isotopes, supporting the use of the same preplan target-volume margins among isotopes. Greater PE was associated with decreased prostate V₁₀₀, V₁₅₀, and D₉₀, without significant impact on overall implant quality.