How much margin do we need for pelvic lymph nodes irradiation in the era of IGRT?

Background and purpose: Image guided radiotherapy (IGRT) without 6 degree of freedom couch can only correct the translational setup errors of pelvic radiotherapy. But errors introduced by rotation and deformation of CTV can't be adjusted in most of IGRT systems. This article is to evaluate these errors and to provide recommendations on the margin needed in the era of IGRT. Material and methods: 218 patients who received pelvic radiotherapy in PUMC Hospital from 2012 to 2014 were included. A simulation CT and a CBCT were acquired for every patient. 3D and 6D registrations of CT and CBCT were applied. 9 bony landmarks were marked and distances of each landmark between CT and CBCT were measured in three directions. Results: Without image guidance, movements of landmarks in the directions of LR, AP and SI were 0.4 ± 2.5 mm, 1.3 ± 3.8 mm and 1.5 ± 5.0 mm respectively, with 3D-registration, movements were 0.0 ± 1.5 mm, 0.7± 2.8 mm and 0.6± 3.2 mm, and with 6D-registration, movements were 0.0 ± 0.5 mm, 0.2 ± 1.0 mm and 0.2 ± 1.1 mm in each direction. Conclusions: IGRT could reduce setup errors. IGRT with 6D treatment couches could further reduce setup errors compared to 3D couches. For centers without IGRT, we suggest CTV-PTV margins of 6 mm, 9 mm and 12 mm in LR, AP and SI directions respectively, margins of 3 mm, 6.5 mm and 7 mm for the use of daily IGRT with 3D couch and 2 mm, 3 mm and 3 mm for 6D couch.