Bladder, urethral and prostatic tumors are primarily a local problem in dogs and cats. These tumors can spread to the regional lymph nodes and eventually to other organs. However, the majority of patients die or are euthanized because of progression of their local tumor, causing progression of clinical signs and eventual obstruction of either the urethra or ureters. Chemotherapy and non-steroidal anti-inflammatory drugs, (NSAIDS) have all been shown to result in tumor responses and may help control the disease for a period of time. However, long term control of bladder tumors is rare. (Henry, 2003)
Given the local or loco-regional nature of these cancers, it would make sense to consider radiation therapy as an additional treatment option, since it can allow for the treatment to be directed to where the problem is.
Background/The Clinical Problem
The main issue with radiation for bladder tumors is that treating this area usually requires giving high doses of radiation to the colon and normal bladder tissue. More recent studies have demonstrated that by using a low dose per fraction (less than 3 Gy) relatively high doses of radiation (around 54 Gy) can be given safely to the pelvic area. (Arthur, 2008; Anderson, 2002) However, it is not clear whether this is enough radiation to result in a significant effect on tumor control.
In early studies one method attempted to try to get around this issue was to perform intraoperative RT. This allowed large single doses of radiation to be given directly to the tumor, with most of the critical normal structures pushed out of the way. This was very effective in controlling the bladder tumors. However, almost half of the dogs treated this way developed significant problems caused by the radiation, including incontinence, stranguria, pollakiuria and cystitis. (Walker 1987) Another concern is that giving a single high dose to the urinary tract can significantly increase the risk of radiation induced tumors. (Johnstone, 1996)
One way that radiation has been used in an effective manner recently for bladder tumors has been palliative radiation, in combination with chemotherapy and NSAIDS. In the only study published to date evaluating this combination, 9 out of 10 dogs had amelioration of their clinical signs and average survival was approximately 11 months. (Poirier, 2004)
Probably the most exciting radiation option for urogenital tumors is intensity modulated radiation (IMRT). This technique allows the dose of radiation to be targeted more accurately to the tumor, while avoiding the critical normal structures such as the colon. With this technique it is possible to give 54 to 58 Gy to the tumor over 20 treatments. Survival time in these dogs was almost 22 months with minimal significant side effects. (Nolan, 2012). This new and exciting treatment modality may help us to prolong survival and quality of life for dogs with these tumors.
Submittted by Dr. John Farrelly DVM, MS,
ACVIM (Oncology), ACVR (Radiation Oncology)
Radiation Oncologist/Medical Oncologist at The Veterinary Cancer Center