When contouring presacral space for gynecological tumors for IMRT plans, how far inferiorly do you take your contour?   

For example, RTOG1203 says to include "1-2 cm of tissue anterior to the S1, S2 and S3 sacral segments" but have also heard of stopped at S2 to limit rectal dose, as well as stopping at top of piriformis muscle which can be higher than S3.