Quantitative assessment of image-guided radiotherapy for paraspinal tumors.

ABSTRACT

PURPOSE: To evaluate stereotactic positioning uncertainties of patients with paraspinal tumors treated with fractionated intensity-modulated radiotherapy; and to determine whether target-point correction via rigid registration is sufficient for daily patient positioning. PATIENTS AND METHODS: Forty-five patients with tumors at the cervical, thoracic, and lumbar spine received regular control computed-tomography (CT) scans using an in-room CT scanner. All patients were immobilized with the combination of Scotch cast torso and head masks. The positioning was evaluated regarding translational and rotational errors by applying a rigid registration algorithm based on mutual information. The registration box was fitted to the target volume for optimal registration in the high-dose area. To evaluate the suitability of the rigid registration result for correcting the target volume position we subsequently registered three small subsections of the upper, middle, and lower target volume. The resulting residual deviations reflect the extent of the elastic deformations, which cannot be covered by the rigid-body registration procedure. RESULTS: A total of 321 control CT scans were evaluated. The rotational errors were negligible. Translational errors were smallest for cervical tumors (-0.1 +/- 1.1, 0.3 +/- 0.8, and 0.1 +/- 0.9 mm along left-right, anterior-posterior, and superior-inferior axes), followed by thoracic (0.8 +/- 1.1, 0.3 +/- 0.8, and 1.1 +/- 1.3 mm) and lumbar tumors (-0.7 +/- 1.3, 0.0 +/- 0.9, and 0.5 +/- 1.6 mm). The residual deviations of the three subsections were <1 mm. CONCLUSIONS: The applied stereotactic patient setup resulted in small rotational errors. However, considerable translational positioning errors may occur; thus, on the basis of these data daily control CT scans are recommended. Rigid transformation is adequate for correcting the target volume position.