COMP Report: CPQR technical quality control guidelines for CyberKnife® Technology.

The Canadian Organization of Medical Physicists (COMP), in close partnership with the Canadian Partnership for Quality Radiotherapy (CPQR) has developed a series of Technical Quality Control (TQC) guidelines for radiation treatment equipment. These guidelines outline the performance objectives that equipment should meet in order to ensure an acceptable level of radiation treatment quality. This particular TQC contains detailed performance objectives and safety criteria for CyberKnife® Technology. The quality control recommendations in this document are based upon previously published guidelines and the collective experience of all Canadian sites using this technology. This TQC guideline has been field tested at the newest Canadian CyberKnife installation site and includes recommendations for quality control of the Iris™ and InCise™ MLC collimation systems.

INTRODUCTION

The Canadian Partnership for Quality Radiotherapy (CPQR) is an alliance among the three key national professional organizations involved in the delivery of radiation treatment in Canada: the Canadian Association of Radiation Oncology (CARO), the Canadian Organization of Medical Physicists (COMP), and the Canadian Association of Medical Radiation Technologists (CAMRT). Financial and strategic backing is provided by the federal government through the Canadian Partnership Against Cancer (CPAC), a national resource for advancing cancer prevention and treatment. The mandate of the CPQR is to support the universal availability of high quality and safe radiotherapy for all Canadians through system performance improvement and the development of consensus‐based guidelines and indicators to aid in radiation treatment program development and evaluation.

This document contains detailed performance objectives and safety criteria for CyberKnife® Technology. Please refer to the overarching document Technical Quality Control Guidelines for Canadian Radiation Treatment Centres1 for a programmatic overview of technical quality control, and a description of how the performance objectives and criteria listed in this document should be interpreted. The development of the individual TQC guidelines is spearheaded by expert reviewers and involves broad stakeholder input from the medical physics and radiation oncology community.2

All information contained in this document is intended to be used at the discretion of each individual center to help guide quality and safety program improvement. There are no legal standards supporting this document; specific federal or provincial regulations and licence conditions take precedence over the content of this document.

SYSTEM DESCRIPTION

In recent years, stereotactic ablative radiosurgery (SABR) has moved from using rigid frames fixed to a patient's skull to the use of noninvasive frameless techniques requiring in room image guidance which are capable of treating extracranial targets. One such system is the CyberKnife® from Accuray Inc. (Sunnyvale, CA, USA) which consists of a compact linear accelerator mounted to an industrial robotic arm. The CyberKnife® system delivers highly conformal radiation doses by delivering multiple radiation fields from many different noncoplanar directions. This is allowed for by the flexibility of the robotic arm and small size of the linac.

The central axes of these beams may share a common point of intersection (isocentric). This type of delivery provides highly conformal spherically shaped radiation dose distributions similar to those delivered using arc therapy with cones on a conventional linac. However, the vast bulk of CyberKnife® treatments use many nonisocentric beams with nonintersecting central axes to treat arbitrarily shaped tumors. For complex targets being treated with circular collimators, this can result in plans with 80–200 beams and tens of thousands of total monitor units per plan.

The most recent generation of the CyberKnife® system has three different secondary collimator systems. The first are the fixed collimators, consisting of 12 circular collimators with nominal diameters from 5 to 60 mm projected at 800 mm from the source. The second is the Iris™, a 12‐sided (two banks of six) regular polygonal variable sized collimators, which in its clinical implementation is restricted to the same equivalent field sizes as the fixed collimators. Use of this collimator decreases treatment time by allowing for changing field sizes and beam directions at each position the robot places the MV photon source (refered to as node positions). The final collimation system is the InCise™ multileaf collimator (MLC) consisting of 41 pairs of 2.5 mm wide leaves as projected at 800 mm from the source, each leaf capable of full interdigitation and over‐travel. The maximum field size of this collimator is 120 mm × 102.5 mm.

The CyberKnife® radiosurgery system uses two orthogonal kilovoltage x‐ray generators and two amorphous silicon flat panel digital detectors for image guidance. CyberKnife® employs several different algorithms to identify the target position in the x‐ray data including skull and spine tracking based on x‐ray contrast of bony anatomy; internally implanted fiducial tracking and tracking based on x‐ray contrast differences between solid tumors and surrounding lung tissue. The system can also compensate for respiratory motion in real time using a predictive algorithm for extracranial treatments. A predictive correlation model is created relating the internal motion of the target to external breathing motion. The external breathing motion is based on the positions of external markers (LED‐based, fiber optic tracking markers) located on the patient's exterior as measured using a stereoscopic camera system. The internal motion is based on the positions of fiducials (referred to as Synchrony® motion tracking), or on the position of a lung tumor itself (referred to as Xsight® Lung Tracking) or on the location of vertebral bodies (for respiratory compensated prone spine treatments). The robotic arm dynamically changes the direction of the linac central axis pointing it to the predicted location of the tumor throughout treatment while the beam is on. All treatments and quality control tests employing respiratory compensation should be observed carefully, listening for unusual noises or vibrations which may indicate problems with robot mastering, robot motion braking, or high levels of noise for the optical marker tracking system.

Comprehensive quality assurance guidelines for robotic radiosurgery were published by the American Association of Physicists in Medicine (AAPM)3 in 2011. These guidelines provided QA recommendations for all CyberKnife® tracking algorithms presently available but did not address the use of the Iris or InCise collimation systems. Most of the quality control recommendations in that report have been included in this document with minor modifications based on a consensus between Canadian cancer centers which presently use the technology. This document also includes quality control for the Iris™ and InCise™ MLC collimation systems but, like the AAPM task group report, acknowledges that many issues remain that require further research and development. Some of the quality control tests in both documents are part of the vendor recommended preventative maintenance program. In most centers, these tasks are performed by field service engineers from Accuray. Some tests are performed routinely while others only following hardware or software upgrades. These tests and procedures also evolve as the technology changes. The vendor has a responsibility to clearly communicate changes to its users and provide them with a means of accessing data from individual system components as necessary for quality control testing. It is the responsibility of the medical physicist to provide informed support for this work and adequate return to service testing for all service events. A comprehensive but practical routine quality assurance program for all aspects of this system is required to ensure the accurate and safe delivery of radiation for this unique system Tables 1, 2, 3, 4.

Table 1

Daily quality control tests

Designator	Test	Performance
		Tolerance	Action
Daily
DL1	Emergency robotic arm motion stop circuit (if present)	Functional
DL2	Robotic arm collision detection interlocks	Functional
DL3	Visual check of beam laser and a standard floor mark	n/a	1 mm
DL4	Accelerator output	2%	3%
DL5	Automated quality assurance test (alternate daily between fixed and Iris™ collimators and the InCise™ MLC)	0.75 mm in any direction	1 mm radial
DL6	Modified picket fence field tests for defocused MLC	Visual inspection of junctions

Table 2

Monthly quality control tests

Designator	Test	Performance
		Tolerance	Action
Monthly
ML1	Energy constancy (change in TPR or PDD ratio)	1%	2%
ML2	Accelerator output	2%	3%
ML3	Intracranial and extracranial isocentric end‐to‐end test; scheduled to cycle through each clinically used tracking method, path, and collimation system (fixed, Iris™, and InCise™ MLC)	Error in any direction: 0.75 mm (static); 1 mm (Synchrony®)	Radial error: 1 mm (static); 1.5 mm (Synchrony®)
ML4	Nonisocentric patient specific quality assurance; scheduled monthly to cycle through each clinically used tracking method, path, and collimation system at least quarterly (fixed, Iris™, and InCise™ MLC)	n/a	5% / 2 mm (static); 5% / 3 mm (Synchrony®)
ML5	Iris™ field size verification	±0.3 mm	±0.5 mm
ML6	Garden fence MLC test	n/a	±0.5 mm for 95% of leaf positions; <2 failures/leaf
ML7	Low contrast details visibility and spatial resolution of amorphous silicon detectors	n/a	Reproducible
ML8	Records	Complete

Table 3

Quarterly quality control tests

Designator	Test	Performance
		Tolerance	Action
Quarterly
QL1	Beam symmetry	2%	3%
QL2	Beam profile shape compared to beam data	2% / 2 mm	3% / 2 mm
QL3	Imager alignment center	0.5 mm	1 mm

Table 4

Annual quality control tests

Designator	Test	Performance
		Tolerance	Action
Annual
AL1	Reference dosimetry	1%	2%
AL2	TPR or PDD and output factors for each clinically used collimation system	1%	2%
AL3	Radial profile constancy	1%/1 mm	2%/2 mm
AL4	Dose output linearity to lowest MU/beam used	1%/1 MU (0.5 MU end monitor effect)	2%/2 MU (1 MU end monitor effect)
AL5	Verify relative location of the central axis beam laser to the radiation central axis to ensure it has not changed from the baseline and is coincident	Change from baseline: 0.5 mm	Coincidence of laser and central axes: 1 mm
AL6	Verification of the second order path calibration	n/a	Each node < 0.5 mm RMS < 0.3 mm
AL7	Run Synchrony® end‐to‐end test with at least 20° phase shift; analyze penumbra spread compared to static delivery	Radial Error: 1.0 mm 2 mm change in penumbra	Radial Error: 1.5 mm 3 mm change in penumbra
AL8	InCise™ MLC Leaf transmission	0.5%	1%
AL9	InCise™ MLC Leaf leakage between leaves	0.5%	1%
AL10	InCise™ MLC Transmission between abutting leaves	0.5%	1%
AL11	InCise™ MLC leaf alignment with jaws	0.5o	1.0o
AL12	Imager kVp, mA and timer accuracy, exposure linearity, exposure reproducibility	n/a	Reproducible
AL13	Quantitative assessment of contrast, noise, and spatial resolution of amorphous silicon detector	n/a	Reproducible
AL14	Independent review and update of quality assurance references	Complete

RELATED TECHNICAL QUALITY CONTROL GUIDELINES

In order to comprehensively assess CyberKnife® Technology performance, additional guideline tests, as outlined in related CPQR Technical Quality Control (TQC) guidelines must also be completed and documented, as applicable. Related TQC guidelines, available at cpqr.ca, include:

Safety Systems

Major Dosimetry Equipment

CONFLICT OF INTEREST

The Ottawa Hospital Cancer Centre holds research agreements with Accuray Incorporated. Eric Vandervoort is the principal investigator for a research grant funded by Accuray Incorporated.