Went to see Dr Craig today - this is roughly 6weeks after last chemo appointment.
Explained that I'd been scanned for radiation and then my insurance had thrown a fit. So I had the scan and marker tattoos but hadn't DONE anything yet.
He in turn explained the 6 weeks instead of 3 weeks thing for radiation - which was very helpful. Basically, they have to map how you breathe in the hope of not terribly irradiating your organs too much by mistake.
If you're rather old, or rather large, they don't care so much because it's less likely to bite you or them in the arse. But if you are smaller, younger, or thinner... issues! and hence complex mapping.
This also explains why when I was getting 'mapped' one of the doctors kept telling the other - "SHE CAN'T HOLD HER BREATH THAT LONG WTF? WE NEED TO DO EXTRA SCAN STUFF?!!'
Or as Dr Craig put it, "They wouldn't care about me - I have plenty of padding! But you..." and he just sort of waved his hands to convey a lack. Which is especially funny/notfunny given my body issues and annoyance about 'swoll' steroid effects that I'm trying to get rid of.