Is there a strong rationale for contouring small and large bowel structures for GI cases of IMRT/VMAT/3D-CRT separately?

It depends on the clinical situation. However, keep in mind that these structures are not all mobile. An example is that the C-loop of the duodenum always retains its relationship to the pancreas. For large bowel, there is relatively little mobility of the ascending and descending colon (although th...