Six-Dimensional Correction of Intra-Fractional Prostate Motion with CyberKnife Stereotactic Body Radiation Therapy

Abstract

Large fraction radiation therapy offers a shorter course of treatment and radiobiological advantages for prostate cancer treatment. The CyberKnife is an attractive technology for delivering large fraction doses based on the ability to deliver highly conformal radiation therapy to moving targets. In addition to intra-fractional translational motion (left-right, superior-inferior, and anterior-posterior), prostate rotation (pitch, roll, and yaw) can increase geographical miss risk. We describe our experience with six-dimensional (6D) intra-fraction prostate motion correction using CyberKnife stereotactic body radiation therapy (SBRT). Eighty-eight patients were treated by SBRT alone or with supplemental external radiation therapy. Trans-perineal placement of four gold fiducials within the prostate accommodated X-ray guided prostate localization and beam adjustment. Fiducial separation and non-overlapping positioning permitted the orthogonal imaging required for 6D tracking. Fiducial placement accuracy was assessed using the CyberKnife fiducial extraction algorithm. Acute toxicities were assessed using Common Toxicity Criteria v3. There were no Grade 3, or higher, complications and acute morbidity was minimal. Ninety-eight percent of patients completed treatment employing 6D prostate motion tracking with intra-fractional beam correction. Suboptimal fiducial placement limited treatment to 3D tracking in two patients. Our experience may guide others in performing 6D correction of prostate motion with CyberKnife SBRT.

Keywords: CyberKnife; hypo-fractionated radiation therapy and fiducial placement; intra-factional; prostate motion; six-dimensional.

Figure 1

(A) The 6D of possible prostate motion. The X-, Y-, and Z-axes represent translational motion, while pitch, roll, and yaw represent rotation around each of these axes. (B) Fiducial placement criteria for 6D tracking. (1) A minimum of three fiducials are implanted within the prostate. (2) The distance between any two fiducials must be greater than 20 mm (Spacing threshold). (3) All angles formed by the fiducial triplets must be greater than 15° (Co-linearity threshold).

Figure 2

Fiducial placement. (A) A fixed-frame system with needles is used to place fiducials under trans-rectal ultrasound guidance. (B) Axial and (C) sagittal views of ultrasound image guidance.

Figure 3

Axial images through the mid-prostate. (A) Thin cut CT scan visualizes fiducials well, but glandular tissue is indistinguishable from adjacent soft tissues. (B) Long echo time axial 2D T1-weighted MR images were obtained to optimally visualize fiducials. (C) Axial high-resolution turbo T2-weighted spin-echo MR images to assess prostatic soft tissues.

Figure 4

Six-dimensional fiducial tracking for stereotactic body radiation therapy with the CyberKnife system. Shown are digitally reconstructed radiographs (left column), real time orthogonal X-ray images (middle column), and co-registered overlay images (right column).