Cyberknife robotic radiosurgery system

For treatment planning purposes, four gold fiducial markers were inserted into the prostate gland. One week later, a computed tomography (CT) scan was performed (slice thickness, 2 mm) followed by T2-weighted magnetic resonance imaging (MRI). The CT and MRI images were then fused for treatment planning. The clinical target volume (CTV) was defined by the prostate capsule and the proximal seminal vesicles. The planning target volume (PTV) was created by expanding the CTV margins by 3 mm posteriorly and 5 mm in all other directions. The following structures were contoured as organs at risk (OAR): rectum, bladder, penile bulb, and both femurs. The prostatic urethra was not included as an OAR because the prescription isodose line was limited to ≥75% to restrict the maximum dose to 125% of the prescription dose. SBRT was delivered to patients using the CyberKnife robotic radiosurgery system (Accuray Inc., Sunnyvale, CA, USA). Fiducial-based tracking was used to account for inter- and intra-fraction prostate motion.
A total dose of 35 or 36.25 Gy to the PTV was delivered in five fractions of 7.0 or 7.25 Gy. Before simulation (planning CT and MRI) and prior to each SBRT fraction, patients were instructed to drink approximately 500 mL of water 20–30 min before the session to fill their bladder to a comfortable level. They were also instructed to present with an empty rectum.

All patients were treated using the CyberKnife® Robotic Radiosurgery System (Accuray, Inc., Sunnyvale, CA) with frameless skull base real time image-guided tracking. Pretreatment 1.25 mm thin-slice, high-resolution computer tomographic scans and matching gadolinium-enhanced magnetic resonance imaging (MRI) scans (MPRAGE sequence) were acquired and fused for treatment planning. Treatment plans were generated using an inverse planning method by the Accuray MultiPlan treatment software. An example of a treatment plan from patient #2 is shown in Fig. 1. Doses were prescribed to the isodose surface that encompassed the margin of the tumor. Patients received 4 mg of dexamethasone immediately after each treatment. For multisession treatments, the typical time interval between fractions was 24 h. The quality of treatment plans was assessed by evaluating target coverage, dose heterogeneity, and conformity. Digitally reconstructed radiograms were computationally synthesized to allow near real-time patient tracking throughout radiosurgery. Treatment safety and efficacy were determined by: 1) Quantifying changes in tumor volume on subsequent MRI scans at the end of each patient’s follow-up period, and 2) Assessment of preservation of optic and olfactory nerve functions.

This retrospective study includes 20 patients with unresectable pancreatic cancer (11 women, 9 men) who were treated between December 2011 and August 2012. We used the CyberKnife® Robotic Radiosurgery System and Synchrony respiratory tracking system (Accuray Inc., Sunnyvale, CA). Plans were designed to cover 95% of PTV (CTV based on CT/MRI registration during relaxed exhale + 3-mm safety margin) with the prescribed dose. A total of 20 tumors were treated with three fractions of 10 Gy every other day. In total, 60 fractions were analyzed. Four gold markers (fiducials) were implanted percutaneously under the CT control (each fiducial within 30 mm from tumor center and fiducial constellation centroid within 10 mm of the tumor center). It is assumed that motion of the fiducial’s center of mass (COM) closely approximates to the motion of the tumor’s COM. Constrains for OARs were set: 10 ml < 21Gy, 5 ml < 15Gy, 700 ml < 17Gy and 0,25 ml < 18Gy for stomach, duodenum, liver and spinal cord respectively. Patients were asked to breathe normally during the irradiation.