8/16/2011

Intracranial Stereotactic Radiosurgery (SRS) and Stereotactic Radiotherapy (SRT)

Kamil M. Yenice, PhD University of Chicago

Radiosurgery * Focused high intensity radiation dose requires crossfiring of many beams. * Target size determines the dose falloff characteristics • The use of radiation as a “surgical” beyond the target boundary 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

Small target, narrow beams Large target, broad beams High dose is focused to where Increased beam overlap beyond target beams intersect over the target boundary Figures: Jürgen Arndt

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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

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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)

SRS Treatment Process: Linac and GammaKnife Why need a tertiary collimation system? • Frame placement: rigid immobilization for imaging and treatment • Imaging Tertiary Collimation minimizes the – CT, MR, and Angiography/DSA geometric penumbra and • Treatment Planning positioning error – Stereotactic localization and image registration – Target and structure delineation Upper Lower Tray Tertiary – Beam (shot) selection and placement Jaw Jaw Collimator Collimator – Iterative optimization Distance from isocenter (mm) 72 62 35 23 – Dose selection and normalization Geometrical penumbra for 2mm 5.1 3.3 1.1 0.6 • Treatment Plan Evaluation focal spot (mm) – Dose distribution Positional error due to 0.5mm 1.3 0.8 0.3 0.15 – DVH analysis for target and critical structures displacement of X-ray target (mm) – Various Conformity measures Positional error due to 0.5 mm 1.8 1.3 0.8 0.65 displacement of collimator (mm) • Treatment Plan QA • Treatment Machine and Patient QA Smith et al Radiation Oncology Investigations (1993) • Setup and treatment

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Stereotactic Frames Radiosurgery Target Delineation – 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 CT T1 Flair • patient setup must guarantee accurate placement of target CT is primary imaging modality (except for GK), structural discrimination is based coordinates to the nominal isocenter on relative atomic composition (electron density info), has high spatial fidelity of the linac 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 Inter-observer variability in delineating target volume and organs at risk in benign and impact of MRI 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 2011

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CT and MR Registration AVM Localization on DSA

Rectangular Fiducial Markers

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 Registration uncertainties are ~1mm (Wang et al JACMP 2009) stereotactic coordinate system.

AVM Target Delineation Process GammaKnife Planning

• Cobalt-201 sources uniformly distributed over an angular Nidus Embolized AVM volume segment of160°×60°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 2009 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

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Linac Based Circular Arc Techniques Standard University of Florida five-arc set

Conventional arcs with circular cones

0 3050 270 550

3400

200 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 67% multiple spherical high-dose regions that will be used to fill up 70% 35% 38% the target volume 14%

13%

Clinical Plan (20 isocenters, 68 arcs, PITV=1.05) Test Plan (20 isocenters, 100 arcs, PITV=1.27)

Target volume Sphere packing arrangement 3D wireframe representation of target volume and target volume with sphere packing Rx isodose (64%) surface superimposed over target volume. arrangement (5, 10, 12, 20mm).

Wagner et al, “A geometrically based automated radiosurgery planning” IJROBP 2000 Wagner et al IJROBP (2000)

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Linac Conformal Radiosurgery with micro MLC Static Conformal Beam Stereotactic Radiosurgery • Beam Geometry Conformal Arcs – Maximize the solid angle Conventional arc irradiated: 2p or 4p Geometry with Conformal beam – Use a reasonable shaping number of beams • How many beams are reasonable? • The higher the number of fields the lower the peripheral dose Conformal Static Beams – Use unopposed fields Utilization of BEV field – Diminishing gains Shaping and beyond 11 static beams simplification of compared to a single-iso planning process 4 arc plan

Bourland and McCollough IJROBP 1993

Linac Radiosurgery: Isocenter placement Dose Selection/Prescription

• Dose selection depends on – Usually at the – lesion volume geometrical center of – lesion location the PTV – pre-existing neurologic deficit – collimator size is set to – proximity to radiosensitive structures encompass most of the – lesion pathology target volume – previous treatments – Multiple non-coplanar beams (8-12) or 4-5 arcs used • Dose prescribed to an isodose line (shell) that conforms to the periphery of the target – Limit no of beams/arcs – typically 80% line (sharper dose fall-off outside in ANT/POST directions the target)

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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

100

90

80

70

60 d80-40=2.7 mm d80-40=4.3 mm d90-45=3.3 mm d90-45=5.8 mm 50

relative dose relative 40

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20

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0 -50 -45 -40 -35 -30 -25 -20 -15 -10 lateral (mm)

A Trigeminal Neuralgia Case

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Does MLC Leaf Size Matter for SRS?

3 mm MLC 5 mm MLC

Representative Acoustic Neuroma Normal Brain Dose (5mm vs 3mm MLC)

1.40 Tumor volume of 1.7 cc <2 cc 1.35 2-4 cc 1.30 4-6 cc 13 non-coplanar beams 1.25 6-8 cc V10Gy Ratio= V10Gy (5 mm) 1.20 >8 cc at 4 couch rotations 1.15 V10Gy (3 mm) 1.10

1 mm uniform plan margin 1.05 V** Ratio (5mm/3mm) Ratio V** 1.00 100 • V Ratio of 5mm to 3mm V10Gy V5Gy V2Gy

5 mm MLC decreases with increasing Single Lesion (47 cases) 80 volume for 10, 5 and 2 Gy 1.45 <2 cc PTV 5mm 1.40 PTV 3mm • V Ratio of 5mm to 3mm 2-4 cc 60 1.35 Brainstem 5mm 4-6 cc was higher for high isodose 1.30 Brainstem 3mm 6-8 cc 1.25 R Cochlea 5mm lines and lower for lower Volume(%) 40 >8 cc R Cochlea 3mm 1.20 dose levels 1.15 1.10

• Normal tissue dose (5mm/3mm) Ratio V** 20 1.05 difference is significant only 1.00 V10Gy V5Gy V2Gy 0 for lesions less than 2cc 0 5 10 15 20 25 Multiple Lesions (19 cases) 3 mm MLC Dose (Gy) Surucu and Yenice, AAPM 2010

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Stereotactic Radiotherapy (SRT)

• Tumors > 4cm • Fractionation – • Tumors involved with a Conventional or hypo- critical structure fractionation (<4mm), or benign • Radiobiology tumors (acoustic • Immobilization – GTC neuromas, frame, mask or meningiomas, pituitary frameless approach adenomas) with IGRT • More labor intensive!

Example: Glioblastoma Multiform Static Conformal vs Intensity Modulated Stereotactic Fractionated SRT Radiosurgery • PTV =55.35 cc • Previous radiation tx • Static Conformal • Intensity Modulated Brainstem: Dmax= 60 Gy Field shape conforms Field intensity varies • Organ at risk: brainstem • Beam arrangement: to the outline of PTV, across the field to 14 non-coplanar fields at 5 uniform intensity achieve optimum dose planes: across the field distribution

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100 100 Brainstem PTV Plan Comparison 80 80 60 • Static • Intensity Modulated 60 40 Static 40 IM

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Relative Volume (%) Relative Relative Volume (%) Relative 20 Static 0 IM 0 0 10 20 30 0 20 40 60 80 100 120 Relative Dose (%) Relative Dose (%) PTV B-stem Normal Tissue

Plan CI V80 V90 UI D02 V24Gy V12Gy

Static 1.45 100.0% 96.3% 1.22 3.0 Gy 25.0 cc 106.8 cc

IM 1.38 100.0% 99.7% 1.05 3.0 Gy 21.1 cc 105.1 cc

IM90 1.21 - - - 2.7 Gy 11.58 cc 83.9 cc

Things that have not changed So you think you can hit the target? significantly for the last 25 years

PASS RATES FOR PARTICIPATING INSTITUTIONS Parameters Linac (509) GammaKnife (125) Dose to 93% 91% Target Treated 90% 98% Volume Meas. Tx 92% 88% Vol/Tx Vol Min Dose 80% 49%

Target = 1.9 cm diameter nylon sphere 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.

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Team work around the clock (1968) We have come a long way!

Gamma Knife Dose planning on the light table (~1968)

Photo Courtesy of Kristiina Hautanen “Water tank” used for early radiosurgery dosimetry at Dr E-O Backlund,Prof. L Leksell,Dr Åström and engineer Bengt Jernberg the Joint Center Slide Courtesy of Kristiina Hautanen Courtesy of Wendell Lutz, PhD

You only get one chance with radiosurgery! and never forget

FOOLS WITH TOOLS ARE STILL FOOLS

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