Three things cause this.
Actually having regular erections is necessary for the erectile tissue in the penis to continue working properly. Even when not having sex or masturbating, healthy men have many nocturnal erections each night and occasional spontaneous erections, and this is good enough to maintain the erectile tissue. However, hormone therapy (HT) tends to stop the nocturnal erections and occasional spontaneous erections, and removes libido so you don't have any desire for sex or to masturbate.
When you are not having erections, fibrous tissue starts to grow in the erectile tissue, and this reduces the extent that it can increase in size, reducing length and girth of erections. Ironically, it also causes the flaccid penis to be a bit larger, although mostly that's not noticed.
So to get around this, you must regularly generate erections even when you have no libido and no enjoyment in doing so. If you are physically unable to do so but hope to be able to in the future, span this gap with the use of a pump.
The other thing that can help is a regular low dose PDE5 inhibitor, such as 5mg tadalafil, although this is expensive and not all areas will provide it.
You can ask for a referral to an Erectile Dysfunction (ED) clinic, and they should go through this in detail. Your hospital team may be able to get you an appointment faster than you GP can. In my case, the Macmillan nurse organised it before I had any issues, and the ED nurse went through what to do to protect myself.
This is known as Penile Rehabilitation. Look it up for more details and get working on it now, even before you get an ED appointment. I don't think it can recover lost length/girth, but it should stop it getting any worse.
The second cause of this is that the RT can damage the erectile tissue in the base of the penis, and then you lose the length that would previously have extended by. It can also cause veinous leakage and/or damage the nerves that generate erections, reducing strength of erections, and this can show up a long time later. I don't think there's anything you can do to recover this, but there are some workarounds.
The third cause is a radical prostatectomy (although not relevant in your case). Some of the urethra is lost with the prostate, and penis length is lost where more urethra needs to be drawn in to reconnect with the bladder. In the case of nerve sparing where erections controlled by the spared nerves take some time to recover, it is important to continue having erections during this recovery period using Penile Rehabilitation or further shortening will occur for same reason as with HT. This could involve using pumps and possibly a PDE5 inhibitor - either a daily low dose (which is unlikely to generate erections but improves blood flow which provides some protection from damage), or a periodic normal dose to generate erections.
Edited by member 15 May 2019 at 18:19
| Reason: Add the third (radical prostatectomy) reason, for completeness.