Purpose: To assess diaphragm, lung region, and tumor mobility during the whole breathing cycle using dynamic MRI. A generalized safety margin concept for radiotherapy planning was calculated and compared with an individualized concept.
Methods and materials: The breathing cycles of 20 patients with solitary lung tumors (15 Stage I non-small-cell lung carcinoma, 5 small solitary metastases) were examined with dynamic MRI (true Fast imaging with steady precision, three images per second). The deep inspiratory and expiratory positions of the diaphragm, upper, middle, and lower lung regions, and the tumor were measured in three dimensions. The mobility of tumor-bearing and corresponding tumor-free regions was compared. Tumor mobility in quiet respiration served as an MRI-based safety margin concept.
Results: The motion of the lung regions was significantly greater in the lower regions than in the upper regions (5 +/- 2 cm vs. 0.9 +/- 0.4 cm, p < 0.05). Tumor-bearing lung regions showed a significantly lower mobility than the corresponding noninvolved regions (p < 0.05). In quiet respiration, tumor mobility showed a high variability; a safety margin of 3.4 mm in the upper, 4.5 mm in the middle, and 7.2 mm in the lower region was calculated.
Conclusion: Dynamic MRI is a simple, noninvasive method to evaluate intrathoracic tumor mobility for therapy planning. Because of the high variability of tumor mobility, an individual safety margin is recommended.