That's gigantic! Did you ultrasound it? Might help figure out the make up of the mass and whether it's normal tissue.
That's a fair assessment. Cancers can do what they want of course, but general enlarged organs typically have discrete size limit, while tumors just continue to grow as they ignore all the signals for the cells to stop dividing. Still, it could in fact be gigantic hyperplasia, and I wouldn't rule it out just yet.
You're looking at four possible causes:
Benign Prostatic Hyperplasia
By far the most common issue with the canine prostate is benign prostatic hyperplasia (BPH), which is when the cells of the prostate continue to grow out of control, and is a natural part of aging in around 80% of intact dogs. The prostate requires testosterone to function and uses an enzyme, 5-alpha-reductase, to transform this testosterone into a hormone that causes its growth. Each individual dog's prostate will grow to different sizes, and if it stays small enough, you may not see any clinical signs or effects. In cases where the prostate is large enough to be causing issues, the two ways of treating this condition are via drugs that inhibit that enzyme or removing the source of testosterone via neutering.
These last three are much less common than BPH:
Bacterial Prostatitis
Bacterial prostatitis is caused by infection of the prostate by primarily E coli, Staphylococcus and Streptococcus, all of which are found in normal amounts on and in a healthy dog. These bacteria can land there via the bloodstream in an acute case or by slowly climbing up the urethra in a chronic case. Acute infections can be life threatening – having bacteria in the bloodstream is the same emergency conditions as pyometra in female dogs, multiple organ infection and shock, and will require hospitalization and treatment. Chronic infection usually occurs secondary to BPH, and will have almost no signs except a recurrent urinary tract infection–and in order to find it you have to run microbial cultures on the prostatic ejaculation fraction. Most cases will resolve simply by treating the BPH, and antibiotics alone won’t do much without also addressing it.
Prostatic Cysts
Occasionally, we find large cysts on the prostate. These are pockets of cells that are abnormalities of the structure of the prostate, and can become filled with fluid as well as get pretty large. The only treatment for these is surgical removal.
Prostatic adenocarcinoma
Based on the current literature, the incidence of prostatic cancer in male dogs is 0.35%. The clinical signs are very similar to BPH and other prostatic diseases, and the most common sites that this cancer metastasizes to the regional lymph nodes, the lumbar (back) vertebrae, and the bones of the pelvis – all areas around the prostate. Sadly, we don’t have any way of curing or combating this cancer–radiation therapy destroys the bladder as well, and when trying to remove the cancerous prostate, you cause incurable urinary incontinence.
I'm just making a theoretical guess because I've got no data or research to back this up, but the thing I'd be concerned about regarding sex is prostatic contraction potentially damaging the structure of a cyst and causing it to burst or knocking off cells to metastasize a cancer faster. If it's BHP, there's not much to worry about other than getting the testosterone under control, since the production of it spikes during ejaculation. I'd recommend that it's better to figure out the cause and then go from there.
