How often do you order follow up mammograms after breast radiotherapy?

We do it once a year, as doing them more frequently than that has not been shown to improve outcomes and this is endorsed by national societies. There is a recent paper which showed higher yields with 6 month mammograms but this has not shown to improve outcomes and our approach at present is a year...

The ASTRO Choose Wisely campaign for this year includes recommendation for yearly mammogram after breast conserving surgery and RT:

https://www.astro.org/Clinical-Practice/Choosing-Wisely/2014-Choosing-Wisely.aspx