CyberKnife Radiosurgery System Research Articles

Radical cyberknife radiosurgery with tumor tracking: an effective treatment for inoperable small peripheral stage I non-small cell lung cancer

ObjectiveCurative surgery is not an option for many patients with clinical stage I non-small-cell lung carcinoma (NSCLC), but radical radiosurgery may be effective.MethodsInoperable patients with small peripheral clinical stage I NSCLC were enrolled in this study. Three-to-five fiducial markers were implanted in or near tumors under CT guidance. Gross tumor volumes (GTVs) were contoured using lung windows. The GTV margin was expanded by 5 mm to establish the planning treatment volume (PTV). A dose of 42–60 Gy was delivered to the PTV in 3 equal fractions in less than 2 weeks using the CyberKnife radiosurgery system. The 30-Gy isodose contour extended at least 1 cm from the GTV. Physical examination, CT imaging and pulmonary function testing were completed at 6 months intervals for three years following treatment.ResultsTwenty patients with an average maximum tumor diameter of 2.2 cm (range, 1.1 – 3.5 cm) and a mean FEV1 of 1.08 liters (range, 0.53 – 1.71 L) were treated. Pneumothorax requiring tube thoracostomy occurred following CT-guided fiducial placement in 25% of the patients. All patients completed treatment with few acute side effects and no procedure-related mortality. Transient chest wall discomfort developed in 8 of the 12 patients with lesions within 5 mm of the pleura. The mean percentage of the total lung volume receiving a minimum of 15 Gy was 7.3% (range, 2.4% to 11.3%). One patient who received concurrent gefitinib developed short-lived, grade III radiation pneumonitis. The mean percent predicted DLCO decreased by 9% and 11% at 6 and 12 months, respectively. There were no local failures, regional lymph node recurrences or distant metastases. With a median follow-up of 25 months for the surviving patients, Kaplan-Meier overall survival estimate at 2 years was 87%, with deaths due to COPD progression.ConclusionRadical CyberKnife radiosurgery is a well-tolerated treatment option for inoperable patients with small, peripheral stage I NSCLC. Effective doses and adequate margins are likely to have contributed to the optimal early local control seen in this study.

Measurements of the Relative Output Factors for Radiosurgical CyberKnife Collimators Using a Glass Rod Dosimeter

The purpose of this study was to evaluate the accuracy of a glass rod dosimeter (GRD) for relative output factor measurements in the CyberKnife radiosurgery system. The output factors measured with the GRD were compared to those obtained with an ionization chamber, a diode and a Gafchromic film. The GRD was irradiated in a water phantom using an in-house custom designed holder stand. The relative output factors measured with the four dosimeters showed very similar results for collimator diameters larger than 20 mm. The mean value of the output factors of the GRD for the 5 mm collimator was 0.695. The output factor measured with the ionization chamber was approximately 13.7% lower than the corresponding GRD values for the 5 mm collimator. The diode output factors were 3.1% and 1.9% higher than the GRD for the 5 mm and 7.5 mm collimators, respectively. However, the GRD results were in agreement with the measurements obtained with a Gafchromic film for all of the collimator diameters.

SU‐GG‐T‐262: Total Scatter Factors of Small 6 MV Photon Beams Estimated by Simulating the Response of Small Detectors

Purpose: To estimate total scatter factors (sc,p) of small 6 MV photon beams from a Siemens Primus linear accelerator. Method and Materials: A method of estimation of sc,p of small beams previously applied to circular fields of the Cyberknife radiosurgery system has now been applied to square fields of 0.3 cm and 0.5 cm field size (reference field 10 cm). Experimental measurements were made with a PTW31014 PinPoint chamber and with 3 diodes (PTW60012, PTW60008 and SunNuclear EdgeDetector™). The method consists of Monte Carlo simulation (BEAMnrc) of the active volume of the detectors and of the treatment head in the hypothesis of 6 different values of FWHM of the electron beam incident on the target (1.0 to 2.5 mm). Possible outcomes are then checked for consistency with experimental results to determine correction factors to be applied to experimental sc,p. Results: Corrected sc,p values of the 2 fields were 0.085 and 0.243 with the PinPoint chamber compared to 0.049 and 0.234 resulting from uncorrected measurement. Corrected sc,p values of the 2 fields were on average 0.075 and 0.256 with the diodes compared to 0.082 and 0.272. The FWHM of the electron beam was consistently estimated between 1.8 and 2.0 mm for all the detectors. Conclusion: This strategy has the advantage of avoiding any “a priori” beam tuning of the electron beam width in the Monte Carlo model because it takes into account the whole range of possible values and uses the actual shape and chemical composition of the detectors in the simulation. Results are consistent with underestimation of sc,p from the ion chamber, partly due to volume effect, and overestimation from the diodes due to non‐water equivalence of the silicon substrate (all) and of the metal shield (PTW60008 and SunNuclear). The method will be extended to include more field sizes.

Total scatter factors of small beams: A multidetector and Monte Carlo study

The scope of this study was to estimate total scatter factors (S(c,p)) of the three smallest collimators of the Cyberknife radiosurgery system (5-10 mm in diameter), combining experimental measurements and Monte Carlo simulation. Two microchambers, a diode, and a diamond detector were used to collect experimental data. The treatment head and the detectors were simulated by means of a Monte Carlo code in order to calculate correction factors for the detectors and to estimate total scatter factors by means of a consistency check between measurement and simulation. Results for the three collimators were: S(c,p) (5 mm) = 0.677 +/- 0.004, S(c,p) (7.5 mm) = 0.820 +/- 0.008, S(c,p) (10 mm) = 0.871 +/- 0.008, all relative to the 60 mm collimator at 80 cm source-to-detector distance. The method also allows the full width at half maximum of the electron beam to be estimated; estimations made with different collimators and different detectors were in excellent agreement and gave a value of 2.1 mm. Correction factors to be applied to the detectors for the measurement of S(c,p) were consistent with a prevalence of volume effect for the microchambers and the diamond and a prevalence of scattering from high-Z material for the diode detector. The proposed method is more sensitive to small variations of the electron beam diameter with respect to the conventional method used to commission Monte Carlo codes, i.e., by comparison with measured percentage depth doses (PDD) and beam profiles. This is especially important for small fields (less than 10 mm diameter), for which measurements of PDD and profiles are strongly affected by the type of detector used. Moreover, this method should allow S(c,p) of Cyberknife systems different from the unit under investigation to be estimated without the need for further Monte Carlo calculation, provided that one of the microchambers or the diode detector of the type used in this study are employed. The results for the diamond are applicable only to the specific detector that was investigated due to excessive variability in manufacturing.

The Development of Radiosurgery

Radiosurgery commenced when the Gamma Knife was invented by Lars Leksell in 1951.Thirty years have passed since radiosurgery was applied clinically.Millions of patients have benefited from it and have avoided traditional surgery.Early radiosurgery was frame-based and was mainly used for intracranial lesions, providing excellent clinical results.The linear accelerator-based radiosurgical technique was established in the 1980s,and radiosurgery entered a new epoch of framelessness.However,accuracy was not satisfactory and this limitation restricted its clinical application.Since 1990,with the development of imaging techniques and computer science,more and more new methods such as image guidance,multileaf collimators,dynamic field shaping,image fusion,scanning beam techniques,and inverse treatment planning systems have been applied in clinical practice.The first revolutionary Cyberknife radiosurgery system was used in the clinic in 1994.A compact,lightweight linear accelerator is always aimed toward the target as it moves along different trajectories around the patient via a computer-driven robotic arm with six degrees of freedom.It continuously monitors and corrects for changes in patient position and motion by comparing pretreatment CT-based digitally reconstructed radiographs to digital radiographs acquired throughout the radiosurgical treatment.The Cyberknife received FDA approval to be used throughout the body in 2001,and now it is widely used all over the world where it provides fantastic clinical results.We have summarized the general history of radiosurgery and have introduced the working principle and clinical application of the Cyberknife.

MO‐D‐AUD‐06: Dependence of Total Scatter Factors of Small Beams On the Radial Distribution of the Electron Beam Incident On the Target: A Multi‐Detector and Monte Carlo Study

Purpose: To investigate dependence of total scatter factors (sc,p) of small beams used in radiosurgery on the radial distribution of the electron beam incident on the target. The study was performed in order to clarify discrepancies observed in sc,p measurements performed by means of different detectors at the Cyberknife radiosurgery system and was aimed at obtaining a means to infer the electron radial distribution of a specific Cyberknife unit. Method and Materials: PTW PinPoint31014 and Exradin #a16 microchambers, PTW30012 diode and TM60003 diamond were used to measure sc,p. The same detectors were simulated by means of the Monte Carlo code BEAMnrc to calculate correction factors for sc,p. BEAMnrc was also used to calculate theoretical values of sc,p. Accuracy of Monte Carlo simulation depends on the choice of energy, divergence and radial distribution of the electron beam incident on the target. The energy was determined by comparison to experimental TMRs. Radial distribution of the electron beam (expressed as FWHM, gaussian shape) was chosen to be optimal when correction factors of the 4 detectors were such that corrected sc,p converged to the same value within +/−1%. Results: measured sc,p of the 5mm collimator averaged 0.638 −4%+11% with the 4 detectors. E=7.2MeV best matched calculated to experimental TMRs. Optimal FWHM=2.3mm gave sc,p=0.673 −0.7%+0.3%. Correction factors decreased with increasing FWHM, while Monte Carlo‐calculated sc,p increased. Measured (corrected) and calculated sc,p matched at 0.673 only for FWHM=2.3mm. Conclusion: Variations of electron beam focussing can explain significant variations of sc,p. If one of the investigated detectors is used, it is possible to infer actual FWHM values and thus appropriate correction factors by comparison to pure Monte Carlo‐calculated sc,p values as a function of FWHM. This fact could be exploited by centres who do not have access to Monte Carlo codes to simulate their own system.

Technical description, phantom accuracy, and clinical feasibility for fiducial-free frameless real-time image-guided spinal radiosurgery

The authors describe the technical application of the Xsight Spine Tracking System, data pertaining to accuracy obtained during phantom testing, and the initial clinical feasibility of using this fiducial-free alignment system with the CyberKnife in spinal radiosurgery. The Xsight integrates with the CyberKnife radiosurgery system to eliminate the need for implantation of radiographic markers or fiducials prior to spinal radiosurgery. It locates and tracks spinal lesions relative to spinal osseous landmarks. The authors performed 10 end-to-end tests of accuracy using an anthropomorphic head and cervical spine phantom. Xsight was also used in the treatment of 50 spinal lesions in 42 patients. Dose planning was based on 1.5-mm-thick computed tomography slices in which an inverse treatment planning technique was used. All lesions could be treated using the fiducial-free tracking procedure. Phantom tests produced an overall mean targeting error of 0.52 +/- 0.22 mm. The setup time for patient alignment averaged 6 minutes (range 2-45 minutes). The treatment doses varied from 12 to 25 Gy to the median prescription isodose of 65% (40 to 70%). The tumor volume ranged between 1.3 and 152.8 cm3. The mean spinal cord volume receiving greater than 8 Gy was 0.69 +/- 0.35 cm3. No short-term adverse events were noted during the 1- to 7-month follow-up period. Axial and radicular pain was relieved in 14 of 15 patients treated for pain. Fiducial-free tracking is a feasible, accurate, and reliable tool for radiosurgery of the entire spine. By eliminating the need for fiducial implantation, the Xsight system offers patients noninvasive radiosurgical intervention for intra- and paraspinal tumors.

TU‐C‐224C‐07: A Comprehensive Dosimetric Protocol for the Cyberknife Radiosurgery System

Purpose: To propose dosimetric guidelines specifically designed for the Cyberknife radiosurgery system. Non‐availability of 10×10cm2 field and use of small circular collimators (5mm to 60mm) pose serious problems, that have been faced in this study by means of 8 different detectors and Monte Carlo simulation. This work is oriented to measurement of total scatter factors (Sc,p) and to reference dosimetry, though indications will also be given in view of a comprehensive guideline. Method and Materials: PTW PinPoint 31014, Exradin A16 and T14P microchambers, TN 502RDM micromosfet, PTW 30008 diode and TM60003 diamond, MD55 and EBT radiochromic films were used to measure Sc,p. Monte Carlo simulations (BEAMnrc) were used to produce phase space descriptions at the exit plane of each collimator, to calculate: 1) theoretical Sc,p values in water, and 2) correction factors to be applied to Sc,p as measured by 5 detectors (PinPoint, A16, T14P, diode, diamond), obtained by simulating shape and chemical composition of each detector. BEAMnrc was also used to calculate stopping power ratios and chamber correction factors for the Cyberknife linac, to decide whether values of kQ from the IAEA398 protocol could be applied without using a 10×10cm2 field. Results: Sc,p of the 5mm collimator as measured by simulated detectors averaged 0.653 − 9%+14%. Variation for larger collimators was smaller. After Monte Carlo correction, Sc,p of the 5mm collimator became 0.686 −2%+1%. Pure Monte Carlo calculation gave Sc,p=0.715 +/−1%. Calculation of correction factors showed that kQ values for the investigated chambers could be chosen when using IAEA398, introducing +/−0.2% uncertainty. Conclusion: Pure Monte Carlo calculation gave higher values of Sc,p compared to Monte Carlo‐corrected measurement. The latter is to be preferred because correction factors are less sensitive to beam parameters than pure calculation of Sc,p. For determination of Sc,p use of microchambers and Monte Carlo correction is recommended.

SU-FF-T-238: Dosimetry of Small Beams Used in Radiosurgery: A Comparison Between Different Detectors and Monte Carlo Simulation

Purpose: To compare experimental results from different detectors to Monte Carlo simulation in the dosimetry of small fields employed in the Cyberknife radiosurgery system. The Monte Carlo code has also been used to decide whether protocols that require a 10×10 cm2 field can be used for dosimetry under reference conditions. Method and Materials: Total scatter factors (sc,p) have been measured by means of the PTW PinPoint, Exradin A16 and T14P microchambers, Thomson and Nielsen micromosfet, PTW diamond and diode detectors, and MD 55-2 radiochromic films. A Monte Carlo calculation (BEAM code) was used to simulate the Cyberknife linac and to calculate sc,p and correction factors to be applied to raw data for various detectors. Two Farmer-type chambers were also simulated in order to calculate the ratio between kQ factors for the Cyberknife and of for a standard 6MV linac. Results: Monte Carlo simulations gave results in agreement within ±1% with experimental results for sc,p, PDD and profiles for collimators above 10 mm. Results for sc,p of smaller collimators (5 to 10 mm) agreed with radiochromic film measurements within ± 2%, while results with ionization chambers and diamond suffer from a systematic underestimation, resolved after application of the Monte Carlo correction factor. Micro-mosfet detectors showed good results as compared to Monte Carlo and radiochromic films, but suffered from poor reproducibility. The PTW diode detector seemed to systematically overestimate sc,p values; for the 5 mm collimator, a value of 0.746 has been obtained versus values in the range 0.67–0.71 obtained with Monte Carlo, radiochromic films, ionization chambers and diamond. The ratio between kQ factors at different beam qualities is 1.002 ± 0.5%. Conclusion: Monte Carlo can be used in the dosimetry of small beams to help solve experimental problems due to the finite dimension and to non-water equivalence of the detectors.

Robotic whole body stereotactic radiosurgery: clinical advantages of the Cyberknife® integrated system

Radiosurgery is defined as the delivery of high doses of ionising radiation, in mono- or hypo- fractionated treatments, to destroy tumours or focal areas of pathology. The clinical requirements of designing a radiosurgical treatment system include providing: a) a highly precise beam delivery to targets located throughout the body, b) a highly conformal dose distribution, c) the ability to irradiate both small and/or large complex-shaped lesions while minimising the dose to adjacent radiosensitive tissues and d) the ability to interactively track lesion motion due to normal patient motion. To accomplish this, the CyberKnife radiosurgery system has pioneered in this area by taking advantage of the inherent geometrical targeting precision of a commercial arm-based robotic system carrying a compact X-band linear accelerator and integrated with X-ray imaging and visualisation feedback systems. The arm-mounted linear accelerator, equipped with patient specific anatomical models, registered to the patient in real-time with image guidance, dynamically and safely delivers conformal and homogeneous radiation for therapeutic benefit. This paper details the components of the CyberKnife system and their integration in the clinical workflow of radiosurgery.

An analysis of the accuracy of the CyberKnife: a robotic frameless stereotactic radiosurgical system.

The use of stereotactic radiosurgical systems to treat intracranial and extracranial tumors and other lesions requires a high degree of accuracy in target identification and localization. The purpose of this study was to evaluate the total system accuracy of the CyberKnife (Accuray, Inc., Sunnyvale, CA), a frameless, image-guided, stereotactic radiosurgery system. Clinically relevant accuracy or application accuracy of the CyberKnife radiosurgery system is based on 1) the beam delivery accuracy, which combines the robot and the camera image tracking system, and 2) target localization accuracy, which combines computed tomographic (CT) imaging and treatment planning. Clinically relevant accuracy can be measured by delivering a radiation dose to phantoms, in which the target is defined on a set of CT images using all components of the CyberKnife system, including the treatment planning software, the robot, the camera tracking system, and the linear accelerator. Clinically relevant accuracy was measured in head phantoms loaded with packs of radiochromic film. The accuracy measured is the displacement of the dose contours from the treatment plan to that measured in the radiosurgically exposed phantom. Measurements of mean errors of the second-generation CyberKnife system at Stanford University Medical Center, installed in 2001, ranged from 0.7 mm for a CT slice thickness of 0.625 mm to 1.97 mm for a CT slice thickness of 3.75 mm. The frameless, image-guided, second-generation CyberKnife radiosurgery system has a clinically relevant accuracy of 1.1 +/- 0.3 mm when CT slice thicknesses of 1.25 mm are used. CyberKnife precision is comparable to published localization errors in current frame-based radiosurgical systems.