I Have An Overactive Bladder Due To Dosage Of 6800

I have an overactive bladder due to dosage of 6800 cGy external beam radiation following retropubic prostatectomy in 2005, causing bladder scaring and incontinence. With the assistance of oxybutin I did ok until one year ago when my capacity to urinate gradually dropped to zero. I am surviving with a fine stream and dribbling with the assistance of tamxulosin. For some time I experienced strong urge incontinence, however this is no longer a problem. I experience severe detrusor pressure and am starting to self cath.
Question: Why