Gosh @cranky_granny - what an ungrateful lot your bosses are! You've been busting a gut to work around your treatments & scans for them - and this is how they react!!
I know that you 'like' your job - but it really is essential that you put YOU first in this case! xx. You are SO CLOSE to your retirement 'date' just now .... I reckon (if you can afford to), you should bring it forward & start doing what YOU want to do from now on, and not worry about what THEY want you to do!
As @AllyJay said - they WILL replace you as soon as you DO leave - so there is no need for you feeling 'irresponsible' for leaving (BTW, that's why they give you holidays - so you already 'know' that they can 'do without you' ....)
And you've been keeping them up to date with your treatments & scans since your mets diagnosis - so it is hardly a surprise that you now need to look after No 1 - YOU xxx
take care & all the best for the scans xx