Cone-beam computed tomography in hypofractionated stereotactic radiotherapy for brain metastases

BACKGROUND: To assess interfraction translational and rotational setup errors, in patients treated with image-guded hypofractionated stereotactic radiotherapy, immobilized by a thermoplastic mask and a bite-block and positioned using stereotactic coordinates. METHODS: 37 patients with 47 brain metastases were treated with hypofractionated stererotactic radiotherapy. All patients were immobilized with a combination of a thermoplastic mask and a bite-block fixed to a stereotactic frame support. Daily cone-beam CT scans were acquired for every patient before the treatment session and were matched online with planning CT images, for 3D image registration. The mean value and standard deviation of all translational (X, Y, Z) and rotational errors (θ(x), θy, θ(z)) were calculated for the matching results of bone matching algorithm. RESULTS: A total of 194 CBCT scans were analyzed. Mean +/- standard deviation of translational errors (X, Y, Z) were respectively 0.5 +/- 1.6 mm (range -5.7 and 5.9 mm) in X; 0.4 +/- 2.7 mm (range -8.2 and 12.1 mm) in Y; 0.4 +/- 1.9 mm (range -7.0 and 14 mm) in Z; median and 90th percentile were respectively within 0.5 mm and 2.4 mm in X, 0.3 mm and 3.2 mm in Y, 0.3 mm and 2.2 mm in Z. Mean +/- standard deviation of rotational errors (θ(x), θy, θ(z)) were respectively 0.0 degrees+/- 1.3 degrees (θ(x)) (range -6.0 degrees and 3.1 degrees); -0.1 degrees +/- 1.1 degrees (θy) (range -3.0 degrees and 2.4 degrees); -0.6 degrees +/- 1.4 degrees (θ(z)) (range -5.0 degrees and 3.3 degrees). Median and 90th percentile of rotational errors were respectively within 0.1 degrees and 1.4 degrees (θ(x)), 0.0 degrees and 1.2 degrees (θy), 0.0 degrees and 0.9 degrees (θ(z)). Mean +/- SD of 3D vector was 3.1 +/- 2.1 mm (range 0.3 and 14.9 mm); median and 90th percentile of 3D vector was within 2.7 mm and 5.1 mm. CONCLUSIONS: Hypofractionated stereotactic radiotherapy have the significant limitation of uncertainty in interfraction repeatability of the patient setup; image-guided radiotherapy using cone-beam computed tomography improves the accuracy of the treatment delivery reducing set-up uncertainty, giving the possibility of 3-dimensional anatomic informations in the treatment position.