my kidney cancer/heart issue?

My clear cell kidney cancer has unfortunately spread to my bones<right humerus now has 4.5cm tumor>and I've just started radiation tx. on it. Also, there's been a sack/growth with a fluid texture that has shown up on my scans/mri on/around my heart. Reportedly it's a bit of a mystery and I'm told they are not quite sure what it is - but my oncologist will discuss it and everything with me in about one week. My blood pressure is okay, and i'm not having shortness of breath or irregular heart beats - but I'm of course concerned and wondering what this growth could be??? Most of the info. I've read just confuses me and leaves me with more questions. I've read that the fluid sac/growth could be cancer related but how so? Is it like a tumor or just a sac full of cancer cells? Or something else? I'm trying to better understand the process....of what's going on there.

I know I'm in a nasty stage and the cancer has been an aggressive g4 one ie. I had a 10.5 cm tumor/15cm mass removed with my right kidney about 6mos ago. I was hoping I'd be lucky - but it's back/metastatic...and my right arm is now weakened and very fragile. I'm very active and look well on the surface - but of course I'm a bit scared and expecting the worst hoping for the best or at least some some light...

As yet I'm not on any meds. yet other than pain meds, and now the radiation which has only a limited effect on rcc... The 'heart issue/growth' puzzles me...