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摘要
Purpose: To assess the target localization error (TLE) in terms of the distance between the target and the localization point estimated from the surrogates (vertical bar TMD vertical bar), the average of respiratory motion for the surrogates and the target (vertical bar aRM vertical bar), and the number of fiducial markers used for estimating the target (n). Methods: This study enrolled 17 lung cancer patients who subsequently underwent four fractions of real-time tumor tracking irradiation. Four or five fiducial markers were implanted around the lung tumor. The three-dimensional (3D) distance between the tumor and markers was at maximum 58.7 mm. One of the markers was used as the target (P-t), and those markers with a 3D vertical bar TMDn vertical bar <= 58.7 mm at end-exhalation were then selected. The estimated target position (P-e) was calculated from a localization point consisting of one to three markers except P-t. Respiratory motion for P-t and P-e was defined as the root mean square of each displacement, and vertical bar aRM vertical bar was calculated from the mean value. TLE was defined as the root mean square of each difference between P-t and P-e during the monitoring of each fraction. These procedures were performed repeatedly using the remaining markers. To provide the best guidance on the answer with n and vertical bar TMD vertical bar, fiducial markers with a 3D vertical bar aRM >= 10 mm were selected. Finally, a total of 205, 282, and 76 TLEs that fulfilled the 3D vertical bar TMD vertical bar and 3D vertical bar aRM vertical bar criteria were obtained for n = 1, 2, and 3, respectively. Multiple regression analysis (MRA) was used to evaluate TLE as a function of vertical bar TMD vertical bar and vertical bar aRM vertical bar in each n. Results: vertical bar TMD vertical bar for n = 1 was larger than that for n = 3. Moreover, vertical bar aRM vertical bar was almost constant for all n, indicating a similar scale for the marker's motion near the lung tumor. MRA showed that vertical bar aRM vertical bar in the left-right direction was the major cause of TLE; however, the contribution made little difference to the 3D TLE because of the small amount of motion in the left-right direction. The TLE calculated from the MRA ((TLE)-T-MRA) increased as vertical bar TMD vertical bar and vertical bar aRM vertical bar increased and adversely decreased with each increment of n. The median 3D (TLE)-T-MRA was 2.0 mm (range, 0.6-4.3 mm) for n = 1, 1.8 mm (range, 0.4-4.0 mm) for n = 2, and 1.6 mm (range, 0.3-3.7 mm) for n = 3. Although statistical significance between n = 1 and n = 3 was observed in all directions, the absolute average difference and the standard deviation of the (TLE)-T-MRA between n = 1 and n = 3 were 0.5 and 0.2 mm, respectively. Conclusions: A large vertical bar TMD vertical bar and vertical bar aRM vertical bar increased the differences in TLE between each n; however, the difference in 3D (TLEs)-T-MRA was, at most, 0.6 mm. Thus, the authors conclude that it is acceptable to continue fiducial marker-based radiotherapy as long as vertical bar TMD vertical bar is maintained at <= 58.7 mm for a 3D vertical bar aRM vertical bar >= 10 mm.
- 出版日期2016-4
