OBJECTIVETo measure the intrafraction displacement of the mediastinal metastatic lymph nodes of non-small cell lung cancer (NSCLC) based on four-dimensional computed tomography (4D-CT), and to provide the basis for the internal margin of metastatic mediastinal lymph nodes.

METHODSTwenty-four NSCLC patients with mediastinal metastatic lymph nodes confirmed by contrast enhanced CT (short axis diameter ≥ 1 cm) were included in this study. 4D-CT simulation was carried out during free breathing and 10 image sets were acquired. The mediastinal metastatic lymph nodes and the dome of ipsilateral diaphragma were separately delineated on the CT images of 10 phases of breath cycle, and the lymph nodes were grouped as the upper, middle and lower mediastinal groups depending on the mediastinal station. Then the displacements of the lymph nodes in the left-right, anterior-posterior, superior-inferior directions and the 3-dimensional vector were measured. The differences of displacement in three directions for the same group of metastatic lymph nodes and in the same direction for different groups of metastatic lymph nodes were compared. The correlation between the displacement of ipsilateral diaphragma and mediastinal lymph nodes was analyzed in superior-inferior direction.

RESULTSThe displacements in left-right, anterior-posterior and superior-inferior directions were (2.24 ± 1.55) mm, (1.87 ± 0.92) mm and (3.28 ± 2.59) mm for the total (53) mediastinal lymph nodes, respectively. The vectors were (4.70 ± 2.66) mm, (3.87 ± 2.45) mm, (4.97 ± 2.75) mm and (5.23 ± 2.67) mm for the total, upper, middle and lower mediastinal lymph nodes, respectively. For the upper mediastinal lymph nodes, the displacements in left-right, anterior-posterior and superior-inferior directions showed no significant difference between each other (P > 0.05). For the middle mediastinal lymph nodes, the displacements merely in anterior-posterior and superior-inferior directions showed significant difference (P = 0.005), while the displacements were not significantly different in the left-right and anterior-posterior, left-right and superior-inferior directions (P > 0.05). The displacements of the total and the lower mediastinal lymph nodes in left-right and superior-inferior, or anterior-posterior and superior-inferior directions were significantly different (P < 0.05), but was not significantly different in left-right and anterior-posterior directions (P > 0.05). The displacements of different group of mediastinal lymph nodes in a single direction or vector showed no significant difference (P > 0.05). In the superior-inferior direction, the correlation between the displacements of ipsilateral diaphragma and mediastinal lymph nodes were not statistically significant (P > 0.05).

CONCLUSIONSDuring free breathing, the differences between the intrafractional displacement of mediastinal metastatic lymph nodes in the same direction and its station were not statistically significant. The displacements of the total mediastinal metastatic lymph nodes in the superior-inferior direction were greater than that in the left-right and anterior-posterior directions, especially for the middle and lower mediastinal metastatic lymph nodes. There was no significant correlation between the displacements of ipsilateral diaphragma and the mediastinal metastatic lymph nodes in the superior-inferior direction, so it was unreasonable to estimate and predict the displacement of mediastinal metastatic lymph nodes by the displacement of ipsilateral diaphragma.