8/16/2011

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 crossfiringof many beams.

* Target size determines the dose falloff characteristics beyond the target boundary

Small target, narrow beamsHigh dose is focused to wherebeams intersect over the target

Large target, broad beamsIncreased beam overlap beyond target boundary Figures: Jürgen Arndt

8/16/2011

2

First Gamma Unit Design for SRS (1967)

Early Gamma Unit collimator with elliptical beam collimation designed for functional SRSThe 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

GammaKnife System Evolution

Model U Gamma Unit (1986)201 sources4, 8, 14, 18 mm helmetsManual positioning

Model 4C: computer control and APS (2004)

GK Perfexion Unit (2006)192 sourcesNo collimator helmetsOnly 4, 8, 16 mm collimation

Original Linac Radiosurgery System at the Joint Center (~1986)

Photo courtesy of Wendell Lutz, PhD

Radiation is delivered via small cones in multiple arc geometry with gantry motionPatient immobilization and setup is achieved with a floor stand

MLC Based Linac Radiosurgery by Varian/BrainLAB: Technology Evolution

NOVALIS: 6 MV treatment beam + M3 MLC + ExacTrac x-rayimagingNOVALIS TX: Dual Mode Machine + HDMLC + OBI + ExacTrac x-ray imagingTrueBeam STX: Refinement of Imaging and treatment delivery (FFF mode for SRS)Linacs for dedicated radiosurgery are also available from other vendors

8/16/2011

3

Kilby et al, “The CyberKnife® Robotic Radiosurgery System in 2010,” Technology in Cancer Research and Treatment (2010)

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

Why need a tertiary collimation system?

Smith et al Radiation Oncology Investigations (1993)

Tertiary Collimation minimizes the geometric penumbra andpositioning error

Upper Jaw

Lower Jaw

Tray Collimator

Tertiary Collimator

Distance from isocenter (mm) 72 62 35 23

Geometrical penumbra for 2mm focal spot (mm)

5.1 3.3 1.1 0.6

Positional error due to 0.5mm displacement of X-ray target (mm)

1.3 0.8 0.3 0.15

Positional error due to 0.5 mm displacement of collimator (mm)

1.8 1.3 0.8 0.65

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

8/16/2011

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

?

Images Courtesy of Y Cao, Univ. of Michigan

CT is primary imaging modality (except for GK), structural discrimination is based on relative atomic composition (electron density info), has high spatial fidelityMR provides improved soft tissue contrast based on nuclear spin properties of Hydrogen atoms in tissues, imaging is subject to many sources of errors (distortions)

Inter-observer variability for GTV delineation using CT alone and impact of MRI

C. Weltens et al. / Radiotherapy and Oncology (2001)

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)

Yamazaki et al. Radiation Oncology 2011

8/16/2011

5

CT and MR Registration

Registration uncertainties are ~1mm (Wang et al JACMP 2009)

AVM Localization on DSA

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.

Rectangular Fiducial Markers

AVM Target Delineation Process

Angiography helps identification of the nidus position and differentiation from feeding arteries and draining veins, not easily identifiable on CT or MR images

DSA images are registered to CT/MR through stereotactic localization

Embolized AVM volumeNidus

GammaKnife Planning

• Cobalt-201 sources uniformly distributed over an angular 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

8/16/2011

6

Linac Based Circular Arc Techniques

Conventional arcs with circular cones

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

Standard University of Florida five-arc set

200

3400

550

27003050

Arcs are achieved through couch and gantry rotations

• 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

Target volume Sphere packing arrangement

Rx isodose (64%) surface superimposed over target volume.

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

3D wireframe representation of target volume and target volume with sphere packing arrangement (5, 10, 12, 20mm).

Wagner et al IJROBP (2000)

70%

35%14%

67%

38%

13%

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

8/16/2011

7

Linac Conformal Radiosurgery with micro MLC

Conformal ArcsConventional arc Geometry with Conformal beamshaping

Conformal StaticBeamsUtilization of BEV fieldShaping and simplification ofplanning 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-iso4 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)

8/16/2011

8

Prescription Isodose Line : 80 % or 90%?

Dose fall-off along axial plane

0

10

20

30

40

50

60

70

80

90

100

-50 -45 -40 -35 -30 -25 -20 -15 -10

lateral (mm)

rela

tiv

e d

os

e

d80-40=4.3 mm

d90-45=5.8 mm

d80-40=2.7 mm

d90-45=3.3 mm

Single isocenter (arcs or static fields): 80% is near the steepest point of dose falloff . Multi-isocenter : steepest dose falloff region moves near 70% IDLGammaKnife: 50% is near the steepest dose falloff

A Trigeminal Neuralgia Case

8/16/2011

9

Does MLC Leaf Size Matter for SRS?

3 mm MLC 5 mm MLC

Representative Acoustic Neuroma

5 mm MLC

3 mm MLC

Tumor volume of 1.7 cc13 non-coplanar beamsat 4 couch rotations1 mm uniform plan margin

0

20

40

60

80

100

0 5 10 15 20 25

Dose (Gy)

Vo

lum

e (

%) PTV 5mm

PTV 3mm

Brainstem 5mm

Brainstem 3mm

R Cochlea 5mm

R Cochlea 3mm

Normal Brain Dose (5mm vs 3mm MLC)

V10Gy (5 mm)

V10Gy (3 mm)

V10Gy Ratio=

1.00

1.05

1.10

1.15

1.20

1.25

1.30

1.35

1.40

V10Gy V5Gy V2Gy

V**

Rat

io (

5mm

/3m

m) <2 cc

2-4 cc4-6 cc6-8 cc>8 cc

Single Lesion (47 cases)

• 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 isodoselines and lower for lower dose levels

1.00

1.05

1.10

1.15

1.20

1.25

1.30

1.35

1.40

1.45

V10Gy V5Gy V2Gy

V**

Ratio

(5m

m/3

mm

)

<2 cc

2-4 cc

4-6 cc

6-8 cc

>8 cc

Multiple Lesions (19 cases)

• Normal tissue dose difference is significant only for lesions less than 2cc

Surucu and Yenice, AAPM 2010

8/16/2011

10

Stereotactic Radiotherapy (SRT)

• Fractionation –Conventional or hypo-fractionation

• Radiobiology

• Immobilization – GTC frame, mask or frameless approach with IGRT

• More labor intensive!

• Tumors > 4cm

• Tumors involved with a critical structure (<4mm), or benign tumors (acoustic neuromas, meningiomas, pituitary adenomas)

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 txBrainstem: Dmax= 60 Gy

• Organ at risk: brainstem

• Beam arrangement:

14 non-coplanar fields at 5 planes:

8/16/2011

11

Plan Comparison

• Static • Intensity Modulated

PTV

0

20

40

60

80

100

0 20 40 60 80 100 120

Relative Dose (%)

Rela

tive V

olu

me (

%)

Static

IM

Brainstem

0

20

40

60

80

100

0 10 20 30

Relative Dose (%)

Rela

tive V

olu

me (

%)

Static

IM

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 significantly for the last 25 years

So you think you can hit the target?

Target = 1.9 cm diameter nylon sphere

Parameters Linac (509) GammaKnife(125)

Dose to Target

93% 91%

Treated Volume

90% 98%

Meas. TxVol/Tx Vol

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.

PASS RATES FOR PARTICIPATING INSTITUTIONS

8/16/2011

12

Team work around the clock (1968)

Dr E-O Backlund,Prof. L Leksell,Dr Åström and engineer Bengt Jernberg

Slide Courtesy of Kristiina Hautanen

We have come a long way!

Photo Courtesy of Kristiina Hautanen

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

“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