The purpose of our study was to compare acute and late toxicities of high dose rate (HDR) brachytherapy and low dose rate (LDR) permanent iodine or palladium seeds as monotherapy in patients with low and favorable intermediate risk prostate cancer. Materials/Methods: We retrospectively examined patients with biopsy proven, low to favorable intermediate risk prostate cancer treated with interstitial brachytherapy monotherapy from 1998-2016 at a single institution. Patients selected were restricted to T stage T2c, Gleason score 7, and prostate specific antigen (PSA) less than 20 ng/ml. For each case of prostate brachytherapy, needles/seeds were implanted transperineally via trans-rectal ultrasound guidance. Dose for LDR was 145 Gy and HDR was 27 Gy in 2 fractions. The Common Toxicity Criteria scale, version 2.0 and 3.0 were used to score the acute (<12 months after brachytherapy) and late (! 12 months after brachytherapy) toxicities. Chi square analysis tested for associations between toxicity and treatment technique. Logistic regression was used in multivariable analyses (MVA). Results: A total of 572 patients were included, including 97 treated with HDR using 192 iridium ( 192 Ir), and 475 treated with LDR using 125 iodine ( 125 I) or 103 palladium ( 103 Pd). The median follow up time was 9.2months (range 3.1-145.3) for HDR patients and 53.1 months (range 0.2-198.6) for LDR patients. The two treatment groups were well-balanced with respect to age, T stage, ethnicity, and implanted prostate gland volume. A majority of patients had T1c disease and pretreatment PSA less than 10 ng/ml. 69% of HDR patients had Gleason score of 7 while 2.5% of LDR patients had Gleason score of 7. On univariate analysis, HDR brachytherapy was associated with lower rate of grade 2+ acute GU toxicity (14% vs 38%, p <0.0001) and grade 2+ late GU toxicity (10% vs 31%, p< 0.0001). Acute GI toxicity was low at 3% for both HDR and LDR; late GI toxicity were 6% and 7% (pZ 0.63) for HDR and LDR, respectively. In MVA, only LDR was independently associated with increased risk of grade 2+ acute GU toxicity (OR 2.4, 95% CI 1.1-5.3), but it was no longer associated with increased risk of grade 2+ late GU toxicity (OR 1.0, 95% CI 0.5-2.3). MVA demonstrated no difference between the two treatment methods with regard to acute or late GI toxicity. Conclusion: Acute and late GI toxicity was rare in patients treated with brachytherapy. The use of HDR brachytherapy as monotherapy was associated with a significantly lower risk of grade 2+ acute GU toxicity but similar grade 2+ late GU toxicity compared to LDR. Our results suggest HDR may be the preferred brachytherapy option in patients with clinically localized prostate cancer due to the better acute safety profile.

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