International Journal of Radiation Oncology*Biology*Physics
Clinical investigation: lungAnalysis of intrathoracic tumor mobility during whole breathing cycle by dynamic MRI
Introduction
Major improvements in the optimization of three-dimensional conformal radiotherapy (RT) of lung tumors has largely been a result of improvement in target volume localization. Still, RT beams are commonly defined on static CT scans, taking into account the patient's predictable motions from one session to another and tumor motion during inspiration and expiration (1).
Imaging of respiratory mechanics and volumetry has received little attention in therapy planning. Information on tumor mobility for RT planning is commonly generated using fluoroscopy. Fluoroscopy visualizes tumors poorly, especially small tumors, and only provides crude data on craniocaudal and mediolateral mobility (2). Single markers implanted in tumors have also been used for this purpose (3), but this represents an invasive and technically difficult procedure, and, for optimal detection of tumor rotation and distortion, up to four markers are required (4). Thus, it is common practice to add standard safety margins to clinical target volumes, which are derived from a spiral CT scan (5). These safety margins are estimated arbitrarily, potentially resulting in either excessive exposure of normal tissues or insufficient target volume coverage 6, 7.
Recently developed, fast MRI acquisition techniques permit direct dynamic visualization of respiratory motion, including assessment of the parenchyma, chest wall, and diaphragm, with high spatial and temporal resolution 8, 9, 10, 11, 12. Continuous documentation and measurement of the motion of intrathoracic organs and the mobility of tumors during the breathing cycle by MRI has not yet been published. However, MRI might provide a noninvasive method to measure and adapt target volumes on an individual basis, especially for high-precision RT planning (13).
The aim of the current study was to characterize the mobility of tumors and tumor-involved lung regions in a cohort of patients with Stage I non–small-cell lung carcinoma (NSCLC) or solitary intrathoracic metastases. Tumor mobility was compared in different regions and different breathing cycles. These population-based data on mobility were correlated with the anatomic location. On the basis of tumor mobility in quiet respiration on MRI, we evaluated the role of a generalized vs. an individual safety margin concept in RT planning.
Section snippets
Methods and materials
Twenty patients with a histologically proven intrathoracic solitary tumor (13 men, 7 women, mean age 59 years, range 43–74) were included in this study (Table 1). Fifteen patients had Stage I NSCLC. All these patients were referred for potential curative therapy; 5 patients had solitary lung metastases. After the nature of the procedure had been fully explained, all patients provided written informed consent under an institutionally approved subjects research protocol. The study was performed
Results
In all patients, dynamic MRI showed regular synchronous diaphragm and chest wall motions with good mobility and a good contrast to the surrounding tissue. The diaphragm showed upward deflections during expiration and downward deflections during inspiration, and the chest wall showed inspiratory expansion and expiratory contraction (Fig. 1). The acquisition of three images per second allowed for continuous recording during the breathing cycle even in forced respiration.
Discussion
Our results indicate that MRI using trueFISP sequences is a highly precise technique for direct and noninvasive visualization of dynamic respiratory motions of the diaphragm and chest wall and the mobility of intrathoracic tumors. The motion of different lung regions, intrathoracic structures, and tumors can be measured continuously in quiet respiration, as well as in deep inspiration and expiration.
Mobility of the tumor during the breathing cycle is a potential cause of RT failure in lung
Conclusion
The motions of thoracic structures during extreme phases of breathing and quiet respiration are highly variable and significantly dependent on the localization. Dynamic MRI using trueFISP sequences is a feasible noninvasive technique to document continuous diaphragm and chest wall motion, as well as tumor mobility, with high precision. Identification and quantification of these motions are essential to define a sufficient safety margin around the clinical target volume. Because of the
Acknowledgements
The authors thank Dr. Frank Lohr and Prof. Ivan Zuna for their excellent help with the preparation of this report.
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