Okay, to summarise to date:
My step-dad is in hospital. He went to see his GP two weeks ago exactly, can't believe how much has happened since then. He's been feeling breathless for a while and was retaining a lot of fluid on his ankles. GP sent him for a chest x-ray. Radiographer said "go back to your GP. Today." because there was a shadow on his heart that looked like a mass of some kind.
He was due to be seen at the chest clinic today but had a CT scan done last Wednesday that confirmed it was a mass and that there was a big build-up of fluid round his heart. On Thursday the hospital phoned to say "why aren't you here?" Errr, why would I be? Anyway, he should never have been allowed to leave after the CT, so they admitted him and on Friday they drained the fluid.
Unfortunately during the drain my step-dad went into shock and they thought he'd had a heart attack. (Crash team called, my mum in the waiting room watching them go by ... )Fortunately this wasn't the case and with some saline and oxygen he was fine. He feels a lot better now the fluid's been drained and we're waiting to hear whether they can make a diagnosis from the fluid itself, or if a needle biopsy will have to be done. If the latter, hopefully he'll be out today and await the results in a week's time.
The two possibilities which have been mentioned are lymphoma (not good - but there are 'grades' of lymphoma) and thymoma. Thymoma would be the preferred option as the majority of them (it's a tumour that grows on the thymus, which is some weirdo entity near the heart) are benign and 'just' require surgery to remove.
So a worrying time, but my mum is coping okay for now, my step-dad's quite chipper. Just got to take it as it comes, I guess.