PUB111 Delineating Mediastinal Lymph Nodal Clinical Target Volume: Comparison of Two Atlases and Practical Implications

Abstract

Variations in target volume delineation represent a significant hurdle in conformal radiotherapy. In Non-Small Cell Lung Cancer, the delineation of nodal clinical target volume (CTV) has been studied. We compared Chapet recommendations (IJROBP, 2005) with our institutional atlas (“A Guide for Delineation of Lymph Nodal Clinical Target Volume in Radiation Therapy”, Ausili Cefaro G. et al. 2008) about the definition of the radiologic boundaries of Mountain and Dresler's thoracic lymph node stations (LNS) on CT. Chapet’s and our contouring atlas were reviewed. Regional lymph nodes were delineated as separate CTVs using Mountain and Dresler classification (1997). Two different modalities to define LNS localization were compared. Chapet combined stations 1 and 2 on the right (R) and lest (L) side, assuming that they often cover a short vertical distance and that station 2R is “virtual” in some patients. We distinguished these stations and proposed a separate description of the boundaries of stations 1R- 1L and 2R-2L. While Chapet indicated left subclavian artery and left common carotid artery as vessels that describe the anterior boundary of the combined 1L-2L, we considered them as posterior part of 1L. This discrepancy may be explained by the different estimation of the upper limit of 1L that in our atlas is more cranial (thoracic inlet) because we considered a longer segment of subclavian and common carotid arteries thus including their posterior course. In our atlas, a plane touching the top of sternal manubrium and a horizontal plane passing through the superior margin of aortic arch are the cranial and caudal boundaries of 3A, respectively. Conversely, Chapet proposed the carina as the 3A inferior limit. Chapet located the origin of the right middle lobe bronchus in the inferior limit of 7, whereas we chose the right pulmonary artery. As station 6 caudal limit, we proposed a horizontal plane passing the auricle of right atrium while for Chapet was the lowest image where the right pulmonary artery is viewed. In our atlas the diaphragm is the caudal limit of station 8, as the gastroesophageal junction was for Chapet. Chapet described better limits for hilar LNS (10-11 R and L) than us. Standardized delineation of nodal areas is important. In our daily experience, our atlas is easy to use and reproducible with clearly recognizable limits. However, the atlas should be adapted to the needs and to the planning techniques (3DCRT, IMRT-VMAT, SBRT) employed in each single institution.