Five-year changes in urinary function and prostate volume in patients with localized prostate cancer treated with carbon ion radiotherapy: A prospective study.

378 Background: The potential of carbon ion radiation therapy (CIRT) as a curative treatment for localized prostate cancer (PCa) has garnered attention due to its characteristic dose distribution. There is no data on long-term urinary function or changes in prostate volume in prostate patients after CIRT. We prospectively collected and analyzed over five years to investigate the outcomes related to the urinary function of localized PCa treated with CIRT. Methods: This study included patients with localized PCa classified as T1-T3N0M0 according to the TNM staging system. A total of 309 patients who underwent carbon-ion radiotherapy (CIRT) between 2010 and 2013 were enrolled. Patients with T1c-T2bN0M0, an initial PSA < 10 ng/ml, and a Gleason score (GS) ≤ 6 were classified as the low-risk group (none androgen deprivation therapy, ADT), while those with ≥T3, an initial PSA ≥ 20 ng/ml, or a GS ≥ 8 were categorized as the high-risk group (neo adjuvant ADT 6 months and adjuvant ADT 1.5 year). The remaining patients were classified into the intermediate-risk group (neo adjuvant ADT 6 months). CIRT was administered 57.6 Gy (RBE) in 16 fractions. Pre- and post-treatment evaluations, including UFM, transrectal ultrasound, and residual urine tests, were performed, with data collected over a 5-year period. This study was approved by the Clinical Ethics Committee of Gunma University Hospital (approval number 693). Results: The median age of the patients was 66.5 years. Analysis of the UFM results showed that the maximum flow rate (mean, ml/s) significantly worsened at 1 month post-treatment (Post) compared to pre-treatment (Pre) but improved to the pre-treatment level by 3 months post-treatment (Pre: 20.0 ± 10.6 ml/s, Post: 14.5 ± 7.3 ml/s, p < 0.05, 3 m: 18.7 ± 8.9 ml/s). The mean flow rate also significantly worsened at 1 month post-treatment compared to pre-treatment but returned to pre-treatment levels at 3 months post-treatment and showed a significant improvement at 3 years post-treatment, which was maintained for up to 5 years (Pre: 8.3 ± 4.0 ml/s, Post: 6.0 ± 3.2 ml/s, p < 0.05, 3 m: 7.7 ± 4.3 ml/s, 3 y: 10.5 ± 4.9 ml/s, p < 0.05). The prostate volume in ADT (-) cases significantly decreased one year after treatment, reaching approximately 80% of the initial volume by three years and remaining stable through five years (1 year: 85.6 ± 18.6%, p < 0.05; 3 years: 78.3 ± 21.9%, p < 0.05). The multivariate analysis identified T classification (T1 vs. ≥T2, p < 0.001) and prostate volume ( p = 0.014) as significant predictive factors for late genitourinary tract complications. Conclusions: The long-term prospective study conducted over five years has elucidated the changes in urinary function and prostate volume in patients with prostate cancer following CIRT. Based on these data, it was suggested that complications in CIRT patients might be predictable through pre-treatment evaluations.