I don't have any literature on that issue but I have to say I think your colleagues are being a bit silly. If you follow the precautions - lid down when flushing, wipe the seat afterwards if necessary - I would imagine there is next to no risk for them.
It may be too that you won't be well enough to go to work on some of the days following chemo anyway, I know I wouldn't have been. And some of the days will be weekend days.
But if they persist in "freaking out" about it I would try asking your breast care nurse or doctor for a pamphlet or if they don't have that, ask them to write you a little note to re-assure them. Take care, I hope it you don't have too many icky side-effects. Janet.