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©2017 MFMER | slide-1
Leksell Gamma Knife Icon A New User’s Perspective
Steve Herchko, DMP
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Disclosures • None
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Outline • Icon Overview
• Mayo Clinic Experience
• Frame-Based System
• Mask-Based System
• Linac Single Isocenter Technique
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Icon Overview
• Essentially the same as the Perfexion with the addition of:
• Mask immobilization system • CBCT • High Definition Motion
Management (HDMM) system
• Framed, frameless, and fractionated treatments
• Allows for pre-treatment and during treatment verification of patient positioning
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Icon Overview • Commercially available in 2016
• 192 Co-60 sources
• ~6600 Ci at installation • ~35 Ci per source
• 4, 8, and 16 mm collimators • Sources divided into 8 sectors that can be collimated/blocked
independently (24 sources/sector) • Sources also divided into 5 rings
• Initial output ~3.5 Gy/min for the 16 mm collimator
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Mayo Clinic Florida Experience • First treatment in January 2018
• ~100 patients total • ~50% mask based
• ~1/3 of mask patients are fractionated • Mostly brain mets
• Treatment durations range from ~15 minutes to hours
• Number of mets range from 1 to 30+ • Vast majority of cases have 1-10 mets
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SRS vs Whole Brain • Recent trend towards treating more mets and less
whole brain
• Randomized data only exists for a small number of brain mets (1-3)
• Studies looking into more mets are underway (5-15)
• SRS is believed to be associated with less cognitive side effects than whole brain
• Other factors such as tumor volume, performance status, histology, etc. are also important
• Physician dependent
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Frame-Based Workflow @ MCF • Day before treatment
• MRI (without frame) • Preplan
• Morning of treatment • Frame placement • CT sim with fiducial box to establish stereotactic space • Co-register CT sim to preplan MRI
• Treatment Delivery • CBCT • Co-register CBCT to CT sim to verify position/dose distribution
• Can’t apply corrections
• Deliver treatment
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Framed Treatment Delivery
Dashed = Planned, Solid = Delivered
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Framed Treatment Delivery
Dashed = Planned, Solid = Delivered
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Framed Treatment Delivery
Planned:
Actual:
Dashed = Planned, Solid = Delivered
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Framed Treatment Delivery
IMAGE OSTEEN PIN DISPLACEMENT
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Do Frames Move?
• For properly mounted frames, 2% of patients had frame shifts > 1 mm [1]
• CBCT on the Icon allows these shifts and shifts due to improper frame placement to be detected and corrected prior to treatment
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Mask-Based System • CBCT must be used for stereotactic space
• CBCT field of view can be limiting for extreme target locations
• CBCT used to correct shot locations based on treatment position
• High Definition Motion Management (HDMM) system used to verify patient position throughout treatment
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CBCT Correction • Delivery of plan is automatically corrected according to
actual patient position (from CBCT)
• Correction is done so the delivery preserves the planned position of each individual isocenter (shot) in the patient anatomy [2]
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CBCT Correction Verification
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HDMM System • Looks at patient position (via nose marker) relative to
reference markers in the mask adapter
• If movement exceeds a user defined threshold, collimators move to blocked position
• Treatment is paused if above threshold for 30 seconds or exceeds threshold more than 5 times within a single shot
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HDMM System • System can accurately measure movements of 0.1 mm [3]
• Motion of nose marker using HDMM system has been found to be greater than or equal to target motion measured via CBCT [4]
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Mask-Based Workflow @ MCF • Day Before Treatment
• MRI • Create mask • CBCT (stereotactic reference) • CT Sim • Planning
• Treatment Delivery • CBCT • Co-register to previous CBCT • Verify corrected dose distribution • Deliver treatment
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Mask Treatment Delivery
4 Gy
12 Gy
Dotted Line: Planned Solid Line: Current (CBCT)
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Mask Treatment Delivery
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Gamma Knife Icon Summary • System allows efficient and accurate treatment delivery
• Addition of mask provides a lot of flexibility
• Addition of CBCT is necessary for mask-based treatments
• Provides a lot of value for frame patients as well
• HDMM system provides an acceptable way to track intra-fraction motion
• May cause excessive treatment stoppages due to the nose marker being used as a surrogate
• Gamma Knife vs. Linac?
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Linac Single Isocenter Technique • Treat multiple mets off-axis with a single isocenter
• Isocenter often placed in the geometric center of the targets • Intensity-modulated or dynamic conformal arc • User-defined or template-based planning
• Plan quality is comparable to multiple isocenter technique and Gamma Knife
• Margins? Prescription Isodose Line?
• Treatment time can be significantly shorter than other techniques
• QA time can be significantly longer than other techniques
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Mayo Clinic Florida Experience • 2-6 targets treated per isocenter
• Keep all targets within the HDMLCs
• Treatment time ~25 minutes
• Intra-fraction imaging used when possible
• Primarily used for patients who cannot tolerate longer treatment times
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Single Isocenter SRS QA
• Radiochromic film most commonly used
• Often requires multiple measurements due to off-axis target locations
• Choose measurement planes to minimize number of film measurements needed
• Passing Criteria: >95% @ 2%/2mm
• Recently began doing a secondary TPS calculation as well for comparison
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Questions?
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References 1. Sunil W. Dutta, Roman O. Kowalchuk, et al. Stereotactic Shifts During Frame-Based Image-Guided Stereotactic
Radiosurgery: Clinical Measurements, International Journal of Radiation Oncology*Biology*Physics, 2018, ISSN 0360-3016, https://doi.org/10.1016/j.ijrobp.2018.05.042.
2. Automatic positional delivery correction using a stereotactic CBCT in Leksell Gamma Knife Icon, White Paper.
3. High Definition Motion Management – enabling stereotactic Gamma Knife radiosurgery with non-rigid patient fixations. White Paper.
4. Li W, Bootsma G, Von Schultz O, et al. Preliminary Evaluation of a Novel Thermoplastic Mask System with Intra-fraction Motion Monitoring for Future Use with Image-Guided Gamma Knife. Muacevic A, Adler JR, eds. Cureus. 2016;8(3):e531. doi:10.7759/cureus.531.