Tracey
I would not be happy in your position. Just because your Dad's PSA is low does not mean the disease could not have spread. There are men on here with bone mets who started out with a PSA in single digits. PSA is not the be all and end all of every situation it is just a normally reliable guideline.
Lyn was correct when she said mets are not usually found in the long bones initially like arms and legs the most common places are pelvic area. hips, ribs and spine. However I would never presume anything.
If your Dad had a successful prostatectomy which kept things at bay for a while only to have his PSA start to rise then something is creating that. If HT was given and that has brought it back down again that is all well and good but the MRI and then after that if necessary a bone scan should really be arranged to check exactly where that PSA is being created from. The MRI will show up any potential area for concern either in the organs, lymph nodes or bones to some degree.
The one thing I would say is that some of the HT drugs do cause breathlessness and weakness for some Men much worse than others. several men here cannot take Bicultamide (casodex) for that reason. Another cause of muscular aches and pains can be higher than normal calcium levels. This can happen if the bones are not processing in the way that they should, there are two main functions in bone maintenance cells die and disperse causing a calcium output and new cells are created, these normally go at a matched rate so we notice nothing but it one gets out of synch with the other it can cause problems that are not PCa related.
That is a very basic explanation but hopefully it gives you an insight.
When you see the DR on Friday make sure you tell him your concerns and let him know that you feel very anxious that more is not being done to invesigate the underlying problems. If it turns out to be non cancer related the relief in itself may help improve the situation.
I will be thinking of you on Friday
best wishes
xx
Mo