Background and purpose: Detailed knowledge on mediastinal lymph nodes position variability is lacking. In this study we quantified the variability over the irradiation course in non-small cell lung cancer (NSCLC) patients.
Methods: A 0.35×5 mm gold fiducial marker was inserted in mediastinal lymph nodes of 14 stage III NSCLC patients. A respiration correlated 4D-planning-CT (pCT) and daily 4D-Cone Beam (CB)CT-scans were acquired. To calculate the systematic and random baseline variations, and respiratory motion variability of the lymph nodes, all CBCT scans were registered to both the bony anatomy and marker in the pCT. Patient population statistics of the peak-to-peak amplitude and time averaged mean position relative to the bony anatomy were calculated.
Results: The average peak-to-peak amplitude was 0.21 cm, 0.52 cm and 0.20 cm in the Left-Right, Cranial-Caudal and Anterior-Posterior direction respectively, while the amplitude variability was ≤0.1 cm in each direction. Inter-fraction lymph node baseline variation was 0.21/0.2 cm, 0.34/0.23 cm, and 0.17/0.15 cm systematic/random. PTV margins for these variations were 0.92 cm, 1.24 cm, 0.82 cm for an online bone match and could be reduced to 0.77 cm, 0.82 cm and 0.86 cm for an online carina match.
Conclusions: Substantial and anisotropic, systematic and random mediastinal lymph node baseline variations were found in NSCLC patients indicating that non-uniform margins could be beneficial.
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