Detection of treatment setup errors between two CT scans for patients with head and neck cancer

Accuracy of treatment setup for head and neck patients undergoing intensity-modulated radiation therapy is of paramount importance. The conventional method using orthogonal portal images can only detect translational setup errors while the most frequent setup errors for head and neck patients could be rotational errors. With the rapid development of image-guided radiotherapy, three-dimensional images are readily acquired and can be used to detect both translational and rotational setup errors. The purpose of this study is to determine the significance of rotational variations between two planning CT scans acquired for each of eight head and neck patients, who experienced substantial weight loss or tumor shrinkage. To this end, using a rigid body assumption, we developed an in-house computer program that utilizes matrix transformations to align point bony landmarks with an incremental best-fit routine. The program returns the quantified translational and rotational shifts needed to align the scans of each patient. The program was tested using a phantom for a set of known translational and rotational shifts. For comparison, a commercial treatment planning system was used to register the two CT scans and estimate the translational errors for these patients. For the eight patients, we found that the average magnitudes and standard deviations of the rotational shifts about the transverse, anterior-posterior, and longitudinal axes were 1.7 +/- 2.3 degrees, 0.8 +/- 0.7 degrees, and 1.8 +/- 1.1 degrees, respectively. The average magnitudes and standard deviations of the translational shifts were 2.5 +/- 2.6 mm, 2.9 +/- 2.8 mm, 2.7 +/- 1.7 mm while the differences detected between our program and the CT-CT fusion method were 1.8 +/- 1.3 mm, 3.3 +/- 5.4 mm, and 3.0 +/- 3.4 mm in the left-right, anterior-posterior, and superior-inferior directions, respectively. A trend of larger rotational errors resulting in larger translational differences between the two methods was observed. In conclusion, conventional methods used for verifying patient positioning may misinterpret rotational shifts as translational shifts, and our study demonstrated that rotational errors may be significant in the treatment of head and neck cancer.