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A Comparison Between Two Leading Stereotactic Platforms

in the Treatment of Multiple Metastases

Sandra Vermeulen MD, James Raisis MD, Francisco Li MS, Per Kjäll PhD

Northwest Hospital Gamma Knife, Seattle Seattle Cyberknife, Seattle

R & D Physics, Eletka Instruments AB, Stockholm, Sweden


Radiosurgical Stereotactic Platform Objectives

• Precision

• Reproducibility

• Operator Friendly

• Patient comfort

Treatment time constraints

Frame, No frame


Cyberknife Credentials

• Approved by the FDA since 2001

• More than 120 hospitals and medical centers provide CyberKnife technology

• Greater than 40,000 patients treated worldwide

• More than 400 peer review papers have been published

• Patient mix: 50% cranial, 50% extra-cranial


Cyberknife


Cyberknife Characteristic that Differ from Gamma Knife

• No immbolization frame– Real-time imaging required– Motion tracking required

• Capable of Fractionation – important if tumors engulf optic apparatus, spinal cord or

other sensitive structures– Large tumors

• Extra-cranial targeting below the clavicle possible• More staffing requirements • More complex QA requirements


Brain Metastases

• Most patients have multiple lesions– MRI screening evaluations show 80% of patients

have more than 1 metastases and 50% have 3 or more

• Many patients require more than one SRS session during the course of their disease

• Are CK and GK beam profiles the same for 1, 4 or 10 metastases?


Index Conformity (PIV/TV ratio) of Radiosurgical Treatment Plans

Defined in the RTOG radiosurgical guidelines 90-05 as a measure of the conformity of the Prescribed Dose (PIV) to the Target Volume (TV).


CK Cone Size Selection

CK cone diameters (mm) available:– 5, 7.5, 10, 12.5, 15, 20, 25, 30, 35, 40, 50, 60

An output factor is a ratio of dose for a given field size to that for a reference field size

The output factor of the 5 mm cone has not been

verified by the manufacturer


Extreme Brain Metastases Case

• MH is a 35y/o mother of 2 with a 4 year history of metastatic breast cancer involving brain, lung and bone.

• Systemic treatment includes chemotherapy, monoclonal antibodies and hormones

• SRS by CK has been given twice for BM (6 lesions have been treated over 2 years)

• Patient has been followed for 6 months with 10 intracranial lesions, all under 1 cm

• With a continued KPS of 80%, patient has refused WBRT because of the risk of dementia and the GK because of the frame placement


CyberKnife: Single Matrix and Multiple Matrix Plans

Multiple Matrix Single Matrix


Cyberknife: Conformity Index Multiple and Single Matrix

Multiple Matrix,

2.08 CI/2.17 nCI

Single Matrix,

1.68 CI/1.70 nCI


Cyberknife: “Dose Stacking”


Gamma Knife: Single Matrix and Multiple Matrix Plans

Multiple Matrix, nCI 1.04 Single Matrix, nCI 1.14


Cyberknife Limitations in the Treatment of Multiple Metastases

• When treating more than 4 lesions, target conformity is lost and normal brain inclusion within lower non-prescription isodose volumes is increased.

• Output measurement of 5 mm cone not verified by manufacturer (most center do not treat tumors smaller than 4 mm’s)


Conclusion

• The dosimetry of either GK or CK in the treatment of < 4 metastases which are greater than 5 mm’s is comparable by radiosurgical standards

• When treating > 4 metastases, a larger number of lesions can be treated with GK than CK with better dosimetry and less normal brain inclusion within lower isodose volumes.

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