How does prostate radiation cause urge incontinence?

Mechanism of Urge Incontinence Following Prostate Radiation Therapy

Prostate radiation therapy causes urge incontinence primarily through damage to bladder vasculature and smooth muscle fibers, resulting in edema, cell death, and fibrosis that reduces bladder capacity and damages the bladder trigone area.1, 2

Pathophysiological Mechanisms

• Radiation-induced damage to bladder vasculature and vascular endothelial cells is the primary mechanism, developing with a latency period of 1-25 years after treatment 1, 2
• Reduced bladder capacity occurs due to damage to smooth muscle fibers, resulting in edema, cell death and fibrosis 1
• High dose radiation to the bladder neck and trigone area is particularly associated with urgency, frequency and incontinence symptoms 1
• Bladder dysfunction manifests as urgency, frequency and incontinence, significantly affecting quality of life 1
• Detrusor overactivity in post-radiation patients has been attributed to partial denervation of the bladder during treatment 3

Incidence and Timing

• Acute urinary symptoms occur in approximately 50% of patients receiving pelvic radiation therapy, typically after a dose of 20 Gy to the bladder 1, 2
• Acute symptoms (urgency, dysuria, nocturia) generally subside 2-3 weeks after treatment completion 1
• Long-term effects develop gradually, with urge incontinence becoming more prevalent over time 1
• Comparing surgical vs. radiation treatment, surgery patients experience more incontinence initially, but radiation effects increase over time 1
• Urinary urge incontinence has been shown to increase between 2 and 3 years post-radiation therapy (42% vs. 50%) 4

Risk Factors

• History of abdominal surgery, pelvic inflammatory disease, hypertension, diabetes mellitus, and smoking increase risk of late genitourinary toxicity 1, 2
• Older age significantly impacts incontinence risk due to potentially higher bladder neck doses during treatment 1, 2
• Obesity and overweight are established risk factors for urinary incontinence and frequency 1
• Prior transurethral resection of the prostate (TURP) significantly increases risk of incontinence following radiation therapy 1, 5
• Adjuvant radiation after prostatectomy is particularly harmful to urinary health, increasing risk of both stress and urge incontinence 1

Anatomical Considerations

• The ICRU bladder point dose >75 Gy is a strong predictor of incontinence, as it is located near the trigone, bladder neck and urethra 1
• Dose to the bladder trigone is particularly predictive of severe late urinary toxicity 1
• To reduce severe urinary complications to at least 15%, a D2cm³ ≤80 Gy EQD2 should be used 1
• Different IMRT (intensity-modulated radiation therapy) modalities may reduce rates of acute and late high-grade urinary toxicity 1
• Ureteral dose of D0.1 cc of 23.1 Gy EQD2 is connected to a 10% chance of grade 3 or greater urinary toxicity 1

Clinical Implications

• Urinary symptoms after radiation are often multifactorial, involving both irritative/obstructive components and incontinence 1, 3
• Urge incontinence is characterized by involuntary urine leakage preceded by urgency 1
• Management options include anticholinergic medications (e.g., oxybutynin) and urodynamic testing 1
• Botulinum toxin A injection into the detrusor muscle can be used when drug therapy is ineffective 2
• Radiation-induced cystitis and associated urinary symptoms can significantly impact quality of life, with fecal incontinence, fecal urge, and tenesmus being associated with lower global quality of life levels 6

Understanding these mechanisms helps clinicians better counsel patients about the risk of urinary complications following prostate radiation therapy and guides appropriate management strategies.