Normal Tissue Complication Probability (NTCP) Modeling of Late Rectal Bleeding Following Iodine-125 Prostate Brachytherapy Combined With External Beam Radiation Therapy

Abstract

prostate size in patients with localized prostate cancer undergoing intensity modulated RT (IMRT) alone by magnetic resonance imaging (MRI). We also analyzed the relationship between the prostate size and prostatespecific antigen (PSA) levels. Materials/Methods: Among consecutive 525 patients with localized prostate cancer (T1-T2N0M0 according to the TNM classification 2009) undergoing IMRT between February 2006 and August 2010, 72 patients (median age, 70 years) treated without androgen deprivation therapy were retrospectively analyzed. The prescribed dose of tomotherapy-based IMRT was 70 to 78 Gy (2 Gy/fraction) to the prostate and proximal seminal vesicles. All patients underwent MRI examinations of the pelvis before and once or more after IMRT using a 1.5-Tesla imager. T2-weighted axial images (retention time: 2400 ms; echo time: 99 ms) were evaluated to measure the diameters of the entire prostate and transitional zone in the left-right (L-R) and anterior-posterior (A-P) directions. Results: Median follow-up was 74 months. After IMRT, 212 follow-up MRI scans were performed (mean, 3 times/patient). The average relative diameter of the entire prostate at 3,6, 12, 24, and 36 months after IMRT compared to the pre-IMRT diameter was 0.94, 0.90, 0.89, 0.89, and 0.90 in the L-R direction and 0.93, 0.92, 0.91, 0.91, and 0.90 in the A-P direction. The average relative diameter of the transitional zone at 3, 6, 12, 24, and 36 months after IMRT compared to the pre-IMRT diameter was 0.93, 0.88, 0.91, 0.87, and 0.89 in the L-R direction and 0.96, 0.90, 0.91, 0.87, and 0.88 in the A-P direction. There was no significant difference in the PSA level according to the change in the relative diameter of the entire prostate and transitional zone at 3, 6, and 12 months after IMRT. At 12 months after IMRT, the average relative diameter of the transitional zone in the A-P direction was significantly lower in 8 recurrent cases (0.85) than in 64 nonrecurrent cases (0.93). Conclusion: The entire prostate and transitional zone diameters in both LR and A-P directions were reduced by about 10% at 12 months after IMRT alone for localized prostate cancer. The size of the entire prostate and transitional zone diameter may help predict the relapse of prostate cancer as well as obstructive symptoms after IMRT. Author Disclosure: M. Matsuo: None. T. Murai: None. C. Sugie: None. T. Yanagi: None. S. Ishihara: None. H. Nishibori: None. Y. Shibamoto: None.