What GTV to CTV expansion do you use for limited stage small cell lung cancer with IMRT?

It seems to me that in stage III NSCLC cases, the CTV of the primary tumor in the lung and the CTV of the nodal volumes would be drawn differently for maximum efficiency:

1. Primary: GTVp --> iGTVp --> + 5-7 mm = CTVp --> + 5 mm = PTVp
2. Nodal: GTVn --> + 5-7 mm + entire nodal station (if desired) + elective nodal (if any) = CTVn --> ITVn --> +5 mm = PTVn
3. Final PTV = PTVp + PTVn

This is logical to me because the CTV of the primary tumor is accurately represented by a circumferential expansion aro...

If giving a formal answer, I would expand by 5mm from GTV to CTV. Step-by-step would start with drawing GTV, add a 5 mm CTV expansion, then correct for motion to an ITV, and expand that by a further 5 mm to PTV. I do edit my CTV by expanding to include the entire nodal station of any levels that are partially covered by the expansion, and also shave out of normal tissue that would be a barrier for spread. Protocols do seem to place the ITV expansion at different steps (i.e., after CTV as above, ...