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KV and MV Imaging for SBRT

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KV and MV Imaging for SBRT Powered By Docstoc
					MV and kV Imaging for SBRT
Michael Lovelock PhD

Physics Department, Memorial Sloan-Kettering Cancer Center, New York City

Lovelock ACMP 2005

Acknowledgements
Josh Yamada MD, Michael Zelefsky MD Radiation Oncology Department Mark Bilsky MD Neurosurgery Chiaho Hua PhD, Kamil Yenice PhD, Hai Pham MS, Ping Wang MS, Sean Toner MS, Margie Hunt MS Medical Physics Department Memorial Sloan-Kettering Cancer Center
Lovelock ACMP 2005

Outline
• Several different approaches all incorporate an imaging system:
– Brainlab, Cyberknife, Tomotherapy, CT –Linac, …

• At MSKCC, started SBRT program in 2000. Originally had a conventional Linac equipped with an aSi imager.
– kV capability was added summer 2004

• This talk: QA and clinical procedures we had to institute in order to develop our Linac based image guided SBRT program
Lovelock ACMP 2005

Patient Treatment
• Primary Disease (osteo-sarcoma
– Osteo-sarcoma, fibroma, chondro-sarcoma, chordoma,… – Standard fractionation, 2Gy x 35 fr.

• Re-treatment
– Prior treatment has taken cord to 45 Gy – Hypo-fractionation, 4 Gy x 5 fr.

• Oligometastases
– Variety of histologies – single fraction, 22-24 Gy x 1 fr.

• • • •

Spine is the most common site All treatments planned using IMRT with dynamic delivery Approximately 1100 precise setups delivered to 110 patients Workload: ~500 image guided precise setups annually

Lovelock ACMP 2005

Requirements for SBRT using conventional Linac with MV / kV cone beam localization images
• • • • Imaging (Geometric) Accuracy Assessment QA program for imaging accuracy Patient immobilization Registration tools

Lovelock ACMP 2005

Sources of geometric uncertainty
• Reference Image (DRR)
– Slice spacing

• Location of radiation isocenter
– Field edge – Central axis location

• Patient rotation • Arm stability as a function of gantry rotation • Registration error tolerance
– 2 mm at MSKCC
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Routine QA – MV Imaging
• Beam axis / isocenter projection • Graticule projected on to all localization films • Registration done with respect to the graticule – dual registration technique* • Monthly QA for graticule alignment
*Varian Medical systems Portal Vision software

Lovelock ACMP 2005

kV Imaging system
• kV source, kV detector, and MV detector all mounted on robotic arms • Accuracy depends on beam axis intersecting imaging plate at the central pixels • Daily QA checks on location of mechanical isocenter as reported by kV imaging system • Monthly checks on alignment of kV beam axis with respect to axis of gantry rotation

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Test of arm stability vs rotation
If arms drop due to gravity, image of isocenter BB would be displaced anteriorly

kV source

kV imaging plate
Note pixel coordinates of isocenter BB in orthogonal images

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Shift is isocenter pixels will occur if any sag occurs in source or imaging arm

BB will appear too anterior in this image

Will observe shift in pixel coordinates of the isocenter BB if arms flex due to gravity

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Typical imaging done using oblique field because of poor quality of lateral images. Important to test system in this configuration

Lovelock ACMP 2005

Routine QA – kV Imaging
• Cube phantom setup using room lasers each morning • Orthogonal imaging, and registration to reference image • Error required to be at most 1 mm. • Performed daily

Lovelock ACMP 2005

Routine QA – Cone Beam Imaging
• Isocenter phantom setup using room lasers • Cone beam scan taken • Location mechanical isocenter with respect to imaging isocenter Transverse slice through given by BB mechanical isocenter • Discrepancy < 1 mm
Lovelock ACMP 2005

Test of overall accuracy
• Place ‘patient’ in body frame • CT scan patient • define isocenter tattoo patient, note frame coordinates • Move ‘patient’ and body frame to treatment machine • do approximate setup • image patient, determine corrections • apply correction • note displacements in frame coordinates

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Patient Immobilization
• Tracking patient motion impractical  Patients must be immobilized • Patient motion
– Gradual –relaxation – Sudden change patient discomfort Immobilization Cradle
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Immobilization Cradle

• Initial customization of cradle to patient takes about 30 minutes. • Four sturdy paddles and alpha cradle cushion are adjusted to comfortably constrain the patient • Remain fixed in position for duration of treatment course
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Immobilization cradle design goals
• • • • All setups within 2 mm of planned Intra-fractional motion <= 1 mm CT MR PET compatible Arms at sides for comfort

Lovelock ACMP 2005

Monitoring Intra-fractional Motion
Cradle with 3 -4 infra-red reflectors

Stereotactic infra-red camera detects 3-4 reflectors attached to the cradle

Infra-red camera with integral emitters

Position and orientation of cradle known to ~ 1 mm

Additional reflectors attached to patient
Monitor possible motions of patient with respect ot the cradle during treatment Lovelock ACMP 2005

Monitoring Intra-fractional motion - continued
Camera and IR emitters Markers attached to patient

Lovelock ACMP 2005

Monitoring Intra-fractional motion - continued
Observe motion of each IR reflector in coordinate frame of the cradle Track motion in 3D, through couch rotations

Require mean position of a stable marker (sternum to vary < 2mm throughout treatment
Lovelock ACMP 2005

Registration: Hardware / Fiducials
• Most patients have titanium hardware rigidly attached to the spine – use this as a target surrogate (fiducial marker) • All patients without hardware have 6 gold markers implanted. This is done for both kV and MV imaging • DCR constructed to get good images of the hardware or fiducials
Lovelock ACMP 2005

DRR / DCR
• DRR often lacks clean edges needed for accurate registration • DCR* (digitally composited radiograph) always used if patient has gold markers or titanium hardware

DRR

DCR

*Philips Medical systems AcQSim software

Lovelock ACMP 2005

Implanted Gold Fiducials
• All patients without hardware undergo implantation of 6 gold markers into the spine (2 per vertebra) • Neurosurgery procedure • DCR used for reference images • Image processing used to improve visualization of the markers in the aSi image
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Implanted Gold Fiducials
DRR
DCR

aSi portal image

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kV cone beam guided setup
• Volumetric imaging of patient in treatment position • Visibility of soft tissue enables image guided treatment • Time required:
– – – – – Data acquisition in one gantry rotation Reconstruction time: (Transfer of 2 GB of data) Registration time: Total 1 minute 2 minutes 2 minutes 5 minutes 10 minutes

Lovelock ACMP 2005

Cone beam guided treatment of lymph node
• 22 Gy delivered in 1 fraction to target while avoiding nearby bowel • Both node and bowel move in response to changes in filling, gas
 Must image at time of treatment

• Gold marker inserted into node • Cone beam scans also taken after correction, and post-treatment
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Cone beam guided setup – soft tissue

Planning
Implantation of marker into node Node (green), Bowel (red) contoured on planning scan IMRT Treatment plan developed

Lovelock ACMP 2005

Cone beam guided setup – soft tissue

Patient setup
Patient positioned in immobilization cradle Stereoscopic Infra-red camera used to monitor possible patient motion during treatment Cone beam scan taken

Lovelock ACMP 2005

Cone beam guided setup – soft tissue

Cone beam scan
Bowel has moved slightly from position seen in planning scan

Registration achieved by overlaying CT and CB isocenters

Lovelock ACMP 2005

Cone beam guided setup – soft tissue

Registration
Registration achieved by overlaying CT and CB isocenters

CT CB

Lovelock ACMP 2005

Cone beam guided setup – soft tissue

Setup Correction
Bowel has moved slightly from position seen in planning scan

Registration achieved by overlaying CT and CB isocenters

Axial Coronal

Lovelock ACMP 2005

Summary
• SBRT program can be successfully implemented using conventional Linac + aSi imager • Requires
– – – – Quantification of imaging uncertainties Daily / Monthly QA procedures Patient immobilization Monitoring of possible patient motion during treatment

• kV imaging
– QA / immobilization requirements similar – Enables entirely new ways of accurately targeting soft tissue structures
Lovelock ACMP 2005


				
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