Radiosurgery 8/16/2011. Intracranial Stereotactic Radiosurgery (SRS) and Stereotactic Radiotherapy (SRT) Kamil M. Yenice, PhD University of Chicago

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1 Intracranial Stereotactic Radiosurgery (SRS) and Stereotactic Radiotherapy (SRT) Kamil M. Yenice, PhD University of Chicago Radiosurgery The use of radiation as a surgical tool Small volumes of tissues within the brain are treated with large doses delivered in a single fraction Normal tissues are protected by the rapid dose falloff and by delivering the treatment with high precision * Focused high intensity radiation dose requires crossfiring of many beams. * Target size determines the dose falloff characteristics beyond the target boundary Small target, narrow beams High dose is focused to where beams intersect over the target Large target, broad beams Increased beam overlap beyond target boundary Figures: Jürgen Arndt 1

2 First Gamma Unit Design for SRS (1967) GammaKnife System Evolution Model 4C: computer control and APS (2004) Model U Gamma Unit (1986) 201 sources 4, 8, 14, 18 mm helmets Manual positioning GK Perfexion Unit (2006) 192 sources No collimator helmets Only 4, 8, 16 mm collimation Early Gamma Unit collimator with elliptical beam collimation designed for functional SRS The Co-60 sources are evenly distributed over the surface of the hemispherical source core so that each beam is directed at a common focal spot at the center Original Linac Radiosurgery System at the Joint Center (~1986) MLC Based Linac Radiosurgery by Varian/BrainLAB: Technology Evolution NOVALIS: 6 MV treatment beam + M3 MLC + ExacTrac x-rayimaging NOVALIS TX: Dual Mode Machine + HDMLC + OBI + ExacTrac x-ray imaging TrueBeam STX: Refinement of Imaging and treatment delivery (FFF mode for SRS) Linacs for dedicated radiosurgery are also available from other vendors Radiation is delivered via small cones in multiple arc geometry with gantry motion Patient immobilization and setup is achieved with a floor stand Photo courtesy of Wendell Lutz, PhD 2

3 CyberKnife: dedicated robotic linac radiosurgery system Beam Shaping Small fields shaped by tertiary collimating system precisely machined closer to patient - smaller geometric penumbra diverging beam shaping further minimizes penumbra Kilby et al, The CyberKnife Robotic Radiosurgery System in 2010, Technology in Cancer Research and Treatment (2010) Why need a tertiary collimation system? Tertiary Collimation minimizes the geometric penumbra and positioning error Upper Jaw Lower Jaw Tray Collimator Distance from isocenter (mm) Geometrical penumbra for 2mm focal spot (mm) Positional error due to 0.5mm displacement of X-ray target (mm) Positional error due to 0.5 mm displacement of collimator (mm) Tertiary Collimator Smith et al Radiation Oncology Investigations (1993) SRS Treatment Process: Linac and GammaKnife Frame placement: rigid immobilization for imaging and treatment Imaging CT, MR, and Angiography/DSA Treatment Planning Stereotactic localization and image registration Target and structure delineation Beam (shot) selection and placement Iterative optimization Dose selection and normalization Treatment Plan Evaluation Dose distribution DVH analysis for target and critical structures Various Conformity measures Treatment Plan QA Treatment Machine and Patient QA Setup and treatment 3

4 Stereotactic Frames patient immobilization rigid fixation of cranial anatomy Is it really rigid? 0.36±0.2 mm (Li et al Med Phys 2011) target localization precise identification of target coordinates in a stereotactic coordinate frame treatment setup patient setup must guarantee accurate placement of target coordinates to the nominal isocenter of the linac Radiosurgery Target Delineation? CT T1 Flair CT is primary imaging modality (except for GK), structural discrimination is based on relative atomic composition (electron density info), has high spatial fidelity MR provides improved soft tissue contrast based on nuclear spin properties of Hydrogen atoms in tissues, imaging is subject to many sources of errors (distortions) Images Courtesy of Y Cao, Univ. of Michigan Inter-observer variability for GTV delineation using CT alone and impact of MRI Inter-observer variability in delineating target volume and organs at risk in benign tumor for SRS (analyzed 21 plans made by 11 clinicians in seven CyberKnife centers) C. Weltens et al. / Radiotherapy and Oncology (2001) Yamazaki et al. Radiation Oncology

5 CT and MR Registration AVM Localization on DSA Rectangular Fiducial Markers Registration uncertainties are ~1mm (Wang et al JACMP 2009) AP view Lateral view Conventional Angiography: as contrast material is injected through cerebral vasculature, Orthogonal x-ray transmission images capture cerebral architecture with respect to a stereotactic coordinate system. AVM Target Delineation Process Nidus Embolized AVM volume DSA images are registered to CT/MR through stereotactic localization Angiography helps identification of the nidus position and differentiation from feeding arteries and draining veins, not easily identifiable on CT or MR images GammaKnife Planning Cobalt-201 sources uniformly distributed over an angular segment of uses the idea of the 2p geometry Single iso plan The shot location and size Plug pattern Multi-iso plan: sphere packing-manual or algorithm the number of shots The shot sizes The shot locations The shot weights Iterative optimization of above See the talk by D. Shepard AAPM

6 Linac Based Circular Arc Techniques Conventional arcs with circular cones Standard University of Florida five-arc set Arcs are achieved through couch and gantry rotations Most techniques were developed to mimic GK delivery by early investigators First attempt at conformal planning with circular cones and jaws were explored by the JCRT group Multiple isocenter linear accelerator radiosurgery treatment planning optimization based on optimal sphere packing arrangement with circular cones. Planning reduces to determining positions and sizes of the multiple spherical high-dose regions that will be used to fill up the target volume 70% 35% 14% 67% 38% 13% Clinical Plan (20 isocenters, 68 arcs, PITV=1.05) Test Plan (20 isocenters, 100 arcs, PITV=1.27) Target volume Sphere packing arrangement Rx isodose (64%) surface superimposed over target volume. Wagner et al, A geometrically based automated radiosurgery planning IJROBP D wireframe representation of target volume and target volume with sphere packing arrangement (5, 10, 12, 20mm). Wagner et al IJROBP (2000) 6

7 Linac Conformal Radiosurgery with micro MLC Conformal Arcs Conventional arc Geometry with Conformal beam shaping Conformal Static Beams Utilization of BEV field Shaping and simplification of planning process Static Conformal Beam Stereotactic Radiosurgery Beam Geometry Maximize the solid angle irradiated: 2p or 4p Use a reasonable number of beams How many beams are reasonable? The higher the number of fields the lower the peripheral dose Use unopposed fields Diminishing gains beyond 11 static beams compared to a single-iso 4 arc plan Bourland and McCollough IJROBP 1993 Linac Radiosurgery: Isocenter placement Usually at the geometrical center of the PTV collimator size is set to encompass most of the target volume Multiple non-coplanar beams (8-12) or 4-5 arcs used Limit no of beams/arcs in ANT/POST directions Dose Selection/Prescription Dose selection depends on lesion volume lesion location pre-existing neurologic deficit proximity to radiosensitive structures lesion pathology previous treatments Dose prescribed to an isodose line (shell) that conforms to the periphery of the target typically 80% line (sharper dose fall-off outside the target) 7

8 relative dose 8/16/2011 Prescription Isodose Line : 80 % or 90%? Single isocenter (arcs or static fields): 80% is near the steepest point of dose falloff. Multi-isocenter : steepest dose falloff region moves near 70% IDL GammaKnife: 50% is near the steepest dose falloff Dose fall-off along axial plane d80-40=2.7 mm d90-45=3.3 mm d80-40=4.3 mm d90-45=5.8 mm lateral (mm) A Trigeminal Neuralgia Case 8

9 Volume (%) V** Ratio (5mm/3mm) V** Ratio (5mm/3mm) 8/16/2011 Does MLC Leaf Size Matter for SRS? 3 mm MLC 5 mm MLC Representative Acoustic Neuroma Normal Brain Dose (5mm vs 3mm MLC) Tumor volume of 1.7 cc 13 non-coplanar beams at 4 couch rotations 1 mm uniform plan margin V10Gy Ratio= V10Gy (5 mm) V10Gy (3 mm) <2 cc 2-4 cc 4-6 cc 6-8 cc >8 cc 5 mm MLC 3 mm MLC Dose (Gy) PTV 5mm PTV 3mm Brainstem 5mm Brainstem 3mm R Cochlea 5mm R Cochlea 3mm V Ratio of 5mm to 3mm decreases with increasing volume for 10, 5 and 2 Gy V Ratio of 5mm to 3mm was higher for high isodose lines and lower for lower dose levels Normal tissue dose difference is significant only for lesions less than 2cc Surucu and Yenice, AAPM V10Gy V5Gy V2Gy Single Lesion (47 cases) V10Gy V5Gy V2Gy Multiple Lesions (19 cases) <2 cc 2-4 cc 4-6 cc 6-8 cc >8 cc 9

10 Stereotactic Radiotherapy (SRT) Tumors > 4cm Tumors involved with a critical structure (<4mm), or benign tumors (acoustic neuromas, meningiomas, pituitary adenomas) Fractionation Conventional or hypofractionation Radiobiology Immobilization GTC frame, mask or frameless approach with IGRT More labor intensive! Static Conformal vs Intensity Modulated Stereotactic Radiosurgery Static Conformal Field shape conforms to the outline of PTV, uniform intensity across the field Intensity Modulated Field intensity varies across the field to achieve optimum dose distribution Example: Glioblastoma Multiform Fractionated SRT PTV =55.35 cc Previous radiation tx Brainstem: Dmax= 60 Gy Organ at risk: brainstem Beam arrangement: 14 non-coplanar fields at 5 planes: 10

11 Relative Volume (%) Relative Volume (%) 8/16/2011 Plan Comparison PTV Brainstem Static Intensity Modulated Static IM Relative Dose (%) Relative Dose (%) Static IM PTV B-stem Normal Tissue Plan CI V80 V90 UI D02 V24Gy V12Gy Static % 96.3% Gy 25.0 cc cc IM % 99.7% Gy 21.1 cc cc IM Gy cc 83.9 cc Things that have not changed significantly for the last 25 years So you think you can hit the target? Target = 1.9 cm diameter nylon sphere PASS RATES FOR PARTICIPATING INSTITUTIONS Parameters Linac (509) GammaKnife (125) Dose to Target Treated Volume Meas. Tx Vol/Tx Vol 93% 91% 90% 98% 92% 88% Min Dose 80% 49% All four 54% 39% Failure to plan adequate target coverage and/or deliver adequate target coverage contributed to the low percentages for minimum dose to target compliance. 11

12 Team work around the clock (1968) We have come a long way! Gamma Knife Dose planning on the light table (~1968) Dr E-O Backlund,Prof. L Leksell,Dr Åström and engineer Bengt Jernberg Slide Courtesy of Kristiina Hautanen Photo Courtesy of Kristiina Hautanen Water tank used for early radiosurgery dosimetry at the Joint Center Courtesy of Wendell Lutz, PhD You only get one chance with radiosurgery! and never forget FOOLS WITH TOOLS ARE STILL FOOLS 12