Purpose/Objective(s): Intensity modulated radiation therapy (IMRT) for prostate cancer has a favorable toxicity profile but can result in urinary symptoms. Urethra length changes have been implicated in post-prostatectomy lower urinary tract symptoms. Moreover, radiation therapy to the prostate causes changes to urethra length, prostate volume, and bladder neck enhancement as measured on MRI. We hypothesized that greater changes in bladder enhancement, prostate volume, and urethra length are associated with urinary morbidity.

Materials/Methods: Patients treated with IMRT for prostate cancer who underwent MRI before treatment and six months after treatment were retrospectively reviewed. American Urological Association (AUA) symptom scores and medications for urinary symptoms were recorded at baseline, 1-month, and 6-month post-radiation therapy (post-RT). Irritative (frequency, urgency, and nocturia) and obstructive (incomplete emptying, intermittency, weak stream, and straining) AUA sub-scores were analyzed. Worsening urinary tract symptoms were defined as an increase in AUA score > 5 or an increase in medications for urinary symptoms. Urethra length (membranous plus prostatic urethra) was measured on sagittal T2W images. Bladder neck enhancement was evaluated on coronal T2W images as signal intensity (SI) normalized to bladder dome SI (outside of the irradiated volume). MRI measurements were performed by readers blinded to clinical outcomes. Changes in bladder neck enhancement, prostate volume and urethra length were evaluated by Wilcoxon signed-rank tests. Differences in MRI findings between worsened versus stable/improved urinary symptom groups were determined by Wilcoxon rank-sum tests.

Results: Thirty-nine patients that received radiation treatment to the pelvic lymph nodes (45 Gy) and prostate (mean 78.1 Gy) were included. From pre to 6 month post-treatment MRIs, median normalized bladder neck SI increased (0.89 to 1.07, p = 0.01); median urethra length decreased (60.65 mm to 57.73 mm, p = 0.03); and prostate volume decreased (39 mL to 28 mL, p <0.0001). Twenty-five of 38 patients at 1 month post-RT and 11 of 33 patients at 6 months post-RT had worsening urinary symptoms from baseline. Greater decrease in prostate volume at post-RT MRI was associated with early (1 month AUA) worsening of urinary symptoms (p = 0.05). At 6 months post-RT, patients with longer urethra length on MRI had stable or decreasing obstructive urinary symptoms (p = 0.01). Increased bladder neck enhancement was not associated with worsening overall or irritative urinary symptoms at either time point (p >0.05).

Conclusion: Radiation therapy is associated with decrease in prostate volume, shortening of urethra length, and increase in bladder neck enhancement. Post-RT MRI at 6 months revealed an association between decrease in prostate volume and worsening urinary symptoms at early follow-up. An association between post-RT urethra length and stable or improving obstructive urinary function was observed.