What kidney constraints would you use for a patient with horseshoe kidney?  

It depends on the clinical scenario (treating pa region prophylactic or definitive). If prophylactic, then be more conservative and plan for a mean dose of less than 10 Gy (16 Gy volume to less than 5 percent). If gross disease around, then the mean dose can be higher but try to focus on the highest...

I completely agree with Dr. @Sushil Beriwal's comments. A great comprehensive paper on kidney dose constraints was completed by @Laura Dawson and colleagues a few years ago and might be helpful for people facing this clinical situation. One might constrain the single horseshoe kidney volume similarl...