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 risk volume (like reduce CTV in right paracaval region where the risk of disease is lowest in comparison to left pa and aortocaval).

I completely agree with Dr. @Dr. First Last's comments. A great comprehensive paper on kidney dose constraints was completed by @Dr. First Last and colleagues a few years ago and might be helpful for people facing this clinical situation. One might constrain the single horseshoe kidney volume similarly to how we constrain bilateral kidneys, with the level of vigilance depending on the clinical situation considering underlying kidney function, comorbidities, and goals of care.