Motion-specific internal target volumes for FDG-avid mediastinal and hilar lymph nodes

Background and purpose To quantify the benefit of motion-specific internal target volumes for FDG-avid mediastinal and hilar lymph nodes generated using 4D-PET, vs. conventional internal target volumes generated using non-respiratory gated PET and 4D-CT scans. Materials and methods Five patients with FDG-avid tumors metastatic to 11 hilar or mediastinal lymph nodes were imaged with respiratory-correlated FDG-PET (4D-PET) and 4D-CT. FDG-avid nodes were contoured by a radiation oncologist in two ways. Standard-of-care volumes were contoured using conventional un-gated PET, 4D-CT, and breath-hold CT. A second, motion-specific, set of volumes were contoured using 4D-PET.Contours based on 4D-PET corresponded directly to an internal target volume (ITV_4D), whereas contours based on un-gated PET were expanded by a series of exploratory isotropic margins (from 5 to 13 mm) based on literature recommendations on lymph node motion to form internal target volumes (ITV_3D). Results A 13 mm expansion of the un-gated PET nodal volume was needed to cover the ITV _4D for 10 of 11 nodes studied. The ITV_3D based on a 13 mm expansion included on average 45 cm³ of tissue that was not included in the ITV_4D. Conclusions Motion-specific lymph-node internal target volumes generated from 4D-PET imaging could be used to improve accuracy and/or reduce normal-tissue irradiation compared to the standard-of-care un-gated PET based internal target volumes.