CT-Based Study on Potential Mediastinal Lymph Node Spread of Patients with Lung CancerContribution to 3-D Treatment Planning for Adjuvant Radiotherapy of the Mediastinum

AIMS: To provide a schematized and graduated CT reference series of the mediastinum showing CT-detectable lymph node areas of lung cancer patients to support 3-D treatment planning and documentation for adjuvant irradiation of the mediastinum. PATIENTS AND METHODS: Cross sections of mediastinal structures from one male individual were schematized and overlaid with a 1 cm2 reference grid. Mediastinal CT scans of 100 consecutive patients with histologically proven lung cancer and CT-detectable lymph nodes were evaluated. For each patient the outlines of all identifiable nodes were transferred into the schematized CT series according to their localization in relation to anatomical landmarks. The outlines were centered in a predefined way into relevant squares of the reference grid. The number of patients with nodes projecting to individual CT cross sections was determined. Further, the number and percentage of patients locating to positive grid squares were analyzed. RESULTS: A differentiated pattern of CT-detectable lymph nodes was confirmed. Along the longitudinal axis the CT-detectable nodes followed a Gaussian distribution with its maximum (89% of all patients) at the level of the tracheal bifurcation. At different transverse levels closely circumscribed central areas with node positive grid squares were identified harboring up to 72% of the patients (locating to the front of the trachea, the bifurcation and below the carina). Peripheral areas showed only sporadically node positive grid squares, usually representing less than 5% of the patients. CONCLUSIONS: These schematized and graduated CT cross sections show areas of CT-detectable lymph node spread from the view of 3-D planning. They are of additional help to individualize mediastinal target volumes and/or dose distributions, particularly when administering adjuvant irradiation to patients without mediastinal lymph node involvement. Classifying mediastinal structures by using a grid square easily allows to describe and document target volumes and dose distributions per cross section.