4 Essentials of CK Physics 8/2/2012. SRS using the CyberKnife. Disclaimer/Conflict of Interest

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1 SRS using the CyberKnife Sonja Dieterich, PhD, DABR Associate Professor University of California Davis Disclaimer/Conflict of Interest Consulting agreements with Broncus Medical and CyberHeart, Inc. Scientific Advisory Board, MGS Research 4 Essentials of CK Physics 1. Dose Delivery Accuracy 2. Small-field Dosimetry for SRS 3. Patient Localization 4. Treatment Planning 1

2 Image courtesy of Accuray Inc. The CyberKnife System 1-30 Fractions in min 1000 Mu/min Image-guidance during treatment Corrects: 10 mm in translation, 1.5 degree roll/pitch angle 3 degree yaw angle <0.95 mm accuracy as defined by E2E test 1. Dose Delivery Accuracy Winston-Lutz Test Reference: W. Lutz, K. R. Winston and N. Maleki, "A system for stereotactic radiosurgery with a linear accelerator," Int J Radiat Oncol Biol Phys 14, (1988). Aimed at frame-based SRS Image-based setup not included 2

3 Modified Winston-Lutz: AQA Modified Winston-Lutz: 2 beams Image-guided setup using fiducials Performed daily AQA is lacking: Dose distribution is overlay of many beams 4 localization modalities Real-time tracking Modified Winston-Lutz: End-to-End (E2E) Process & Results of E2E Test E2E includes complete process: Simulation Contouring Planning Setup Localization Treatment delivery Specification < 0.95 mm Actuals 0.3 mm 0.7 mm 3

4 Delivery Accuracy in Patient Near real-time image guidance Frequency based on acceptable PTV margin Small PTV margin vs. image dose and Tx time Murphy, M. J. (2009). "Intrafraction Geometric Uncertainties in Frameless Image-Guided Radiosurgery." International Journal of Radiation Oncology*Biology*Physics 73(5): Furweger et al 2010 SAMS Question 1 Which test defines the overall accuracy of a CyberKnife? 1. Winston-Lutz test 2. Daily beam pointing accuracy (AQA) 3. Robot calibration 4. Delivery QA of patient plan (DQA) 5. End-to-end test (E2E) 10 Countdown SAMS Answer 1 Which test defines the overall accuracy of a CyberKnife? End-to-end (E2E)test Simulates patient treatment from CT scan through delivery All localization algorithms in anthropomorphic phantom 0.3 mm 0.7 mm in clinical practice References: Chang, S. D., W. Main, et al. (2003). "An analysis of the accuracy of the CyberKnife: a robotic frameless stereotactic radiosurgical system." Neurosurgery 52(1): ; discussion Yu, C., W. Main, et al. (2004). "An Anthropomorphic Phantom Study of the Accuracy of CyberKnife Spinal Radiosurgery." Neurosurgery 55(5):

5 2. Small Field Dosimetry IAEA Formalism for Reference Dosimetry Alfonso, R., P. Andreo, et al. (2008). "A new formalism for reference dosimetry of small and nonstandard fields." Med Phys 35(11): TG-51 for Flattening-Filter Free Beam TG51 uses %dd(10) x Details in: Kalach & Rogers, Med Phys 30 (2003)

6 TG 51: How do we get k Q? MC simulation of k Q 0.3% difference in linac vs. CK Use 6MV linac k Q or measure with 60 mm collimator Araki, Med Phys (2006), 2955 TG-51: Measuring %dd(10) x CK: 60 mm cylindrical collimator at 80 cm SAD Measure at 10 cm depth, 100 cm SSD, 60 mm collimator Calculate equivalent square Interpolate to 80 cm SAD using the BJR data TG-51: Chamber selection Dose flatness insufficient for Farmer-type chamber Cavity length should be 1 cm or shorter Option: cross calibrate a short chamber with Farmer-type chamber Kawachi et al, Med Phys (2008)

7 SAMS Question 2 Why does the dosimeter reading of a Farmer-type chamber need to be corrected for reference dosimetry? 1. Difference in k Q for flattening filter vs. 6 MV flattening filter free beam 2. To adjust for detector alignment uncertainties 3. Off-axis beam profile for non-flattened beam changes over chamber length 4. High dose rate effect (1000 MU/min) on chamber 5. To adjust for energy-dependent detector response 10 Countdown SAMS Answer 2 Why does the dosimeter reading of a Farmer-type chamber need to be corrected for reference dosimetry? Off-axis beam profile for nonflattened beam changes over chamber length Reference: Kawachi, T., H. Saitoh, et al. (2008). "Reference dosimetry condition and beam quality correction factor for CyberKnife beam." Med Phys 35(10): Patient Localization 7

8 The Principle of 2D-3D registration Fu, D. and G. Kuduvalli (2008). "A fast, accurate, and automatic 2D-3D image registration for image-guided cranial radiosurgery." Med Phys 35(5): Cranial Localization XSight Spine Localization D. Fu, G. Kuduvalli, C. J. Maurer, J. Allision and J. Adler, "3D target localization using 2D local displacements of skeletal structures in orthogonal x-ray images for image-guided spinal radiosurgery," Int J CARS 1, (2006). 8

9 Spine Localization Accuracy Fürweger, C., C. Drexler, et al. (2010). "Patient Motion and Targeting Accuracy in Robotic Spinal Radiosurgery: 260 Single- Fraction Fiducial-Free Cases." International Journal of Radiation Oncology*Biology*Physics 78(3): SAMS Question 3 Which tracking algorithm would be used for a C3 spine target? 1. Cranial tracking 2. Spine Tracking 3. Fiducial tracking 4. LOT tracking 5. Spine segmentation 10 Countdown SAMS Answer 3 Which tracking algorithm would be used for a C3 spine target? Spine Tracking C3 barely visible in DRR Highly flexible cervical spine means cranial tracking not accurate Spine segmentation removes mandible interference, allows for stable tracking Reference: Fürweger, C., C. Drexler, et al. (2010). "Patient Motion and Targeting Accuracy in Robotic Spinal Radiosurgery: 260 Single-Fraction Fiducial-Free Cases." International Journal of Radiation Oncology*Biology*Physics 78(3):

10 4. Treatment Planning Sequential Optimization Example 1. Initial optimization step, optimize the minimum dose in the PTV 2. Set a constraint on the minimum dose, optimize the conformality 3. Set a constraint on conformality, optimize total beam weight: MU MU Sequential Optimization Schlaefer, A. and A. Schweikard (2008). "Stepwise multi-criteria optimization for robotic radiosurgery." Med Phys 35(5):

11 Example of Sequential Planning PDD Calculations for 4 Collimators with Raytrace and Monte Carlo Change in PDD most pronounced for small collimators SRS near air cavities, e.g. nasal sinuses MC calculation time: min for 2% uncertainty at d max SAMS Question 4 What is the treatment planning algorithm used to for inhomogeneity corrections? 1. Convolution-Superposition 2. Equivalent tissue-air-ratio 3. Monte Carlo 4. Linear Boltzman transport equation 5. Simulated annealing 10 Countdown 11

12 SAMS Question 4 What is the treatment planning algorithm used to for inhomogeneity corrections? Monte Carlo Use might be necessary near air cavities or large metal implants AVMs: CT artefact vs. accuracy of dose calculation? References: Wilcox, E. E. and G. M. Daskalov (2008). "Accuracy of dose measurements and calculations within and beyond heterogeneous tissues for 6 MV photon fields smaller than 4 cm produced by Cyberknife." Med Phys 35(6): Summary CK dose delivery accuracy in E2E meets SRS standards Well understood small field reference dosimetry Integrated image guidance system for frameless & fiducial-less localization and tracking MC dose calculation & Sequential optimization 12