For patients with solitary plasmacytoma of the ureter undergoing definitive XRT (40-50 Gy), what dose constraint do you use for the ureter?  

Well, since this is a solitary plasmocytoma of the ureter, I presume that parts of the GTV encompass the OAR here. I do not think that you can set any meaningful constraint for the ureter, bearing in mind that this is a serial OAR. You can try to avoid hotspots in the ureter, but that's about it.

Ureteral stenosis is a relatively rare (or, potentially, under-reported) side effect of radiation and is seen most frequently at dose/fractionation schemes much higher than the doses mentioned in the OP.The below paper shows that the risk of stenosis to a point regarding procedural intervention (G3)...

Rectal fields, prostate fields, endometrial fields, and bladder fields, all can deliver 45-50 Gy to the ureter without ureteral stenosis/stricture being a common side effect. There shouldn't be any reason then that there would be such a side effect treating to the same dose for plasmacytoma, which i...