COMP report: CPQR technical quality control guidelines for Gamma Knife radiosurgery

Abstract The Canadian Organization of Medical Physicists (COMP), in close partnership with the Canadian Partnership for Quality Radiotherapy (CPQR), has developed a series of Technical Quality Control (TQC) guidelines for radiation treatment equipment. These guidelines outline the performance objectives that equipment should meet in order to ensure an acceptable level of radiation treatment quality. The TQC guidelines have been rigorously reviewed and field tested in a variety of Canadian radiation treatment facilities. The development process enables rapid review and update to keep the guidelines current with changes in technology (the most updated version of this guideline can be found on the CPQR website). This particular TQC details recommended quality control testing for Gamma Knife radiosurgery.


| INTRODUCTION
The CPQR is an alliance among the three key national professional organizations involved in the delivery of radiation treatment in

| SYSTEM DE SCRIPTION
The Gamma Knife (GK) Perfexion™ (Elekta AB, Stockholm, Sweden) is used to treat intracranial lesions using stereotactic radiosurgery (SRS) procedures. Radiation is delivered by means of 192 60 Co sources arranged in rings with a common focus point. 3,4 By distributing the incident radiation over nearly the entire brain, a very large dose can be delivered to a well localized target with minimal harm to healthy brain tissue. These single fraction treatments are a less invasive alternative to cranial surgery. Before imaging, a frame is fixed to the head of the patient.
This serves two purposes: to define a coordinate system common to the imaging, planning, and treatment system, and to ensure that the patient cannot move during treatment. The patient positioning system (PPS; treatment couch) is rigidly affixed to the treatment unit, and the head frame is in turn locked into place on the PPS. Drive motors within the couch automatically position the patient to the prescribed isocenters during treatment. Head frame immobilization and high mechanical reproducibility allow for the accuracy required to deliver large doses to targets near relevant structures within the brain.

CONTROL GUIDELINES
In order to comprehensively assess GK performance, additional guideline tests, as outlined in related CPQR TQC guidelines must also be completed and documented, as applicable. Related TQC guidelines, available at cpqr.ca, include: • Safety Systems • Major Dosimetry Equipment Tables 1-3 list the daily/weekly, monthly/quarterly and annual recommended quality control tests.

D1
The GK inhibits beam on if the patient is not locked in place, at the correct gamma angle with the side protection panels engaged

D2
The GK timer agrees with an independent measurement (e.g., stopwatch). Linearity can be tested by cycling through shots of different durations over multiple days

M4
The position of the patient positioning system must be verified against physical reference positions over an appropriate clinical range in the directions of the three axes (x,y,z)

Q1
The sectors move to correct alignment with the 4, 8, or 16 mm collimators

A1
The positions of the radiation and mechanical isocenters agree with each other

A2
The GK timer is linear. Test over a larger range than daily testing

A3
The transit error is consistent with that measured during commissioning

A4
The measured profiles agree with those in the treatment planning system. Stated tolerance applies to the 50% isodose line for each collimator size

A5
The backup timer on the GK sector computer agrees with the console computer A6 The absolute dose rate in the treatment planning system matches the measured dose rate. Measurements must be made with a calibrated chamber using an accepted protocol (e.g., TG-21 5 ) The absolute dose is independently verified by an external service (e.g., IROC Houston OSLD/TLD [optically-stimulated/thermoluminescent dosimeter] Monitoring Program)

A8
An end-to-end phantom test is performed including frame placement, imaging, treatment planning, treatment, and verification that the intended treatment was delivered with the stated dose and positioning accuracy. The dosimetric accuracy depends on the dosimeter being used. For example, 1% accuracy would apply when using an ion chamber, whereas 5% would be appropriate for film A9-10 The configuration of these tests will depend on the design of the facility and equipment. As a minimum, Canadian Nuclear Safety Commission (CNSC) license conditions and applicable regulations must be followed

A11
To ensure redundancy and adequate monitoring, a second qualified medical physicist must independently verify the implementation, analysis, and interpretation of the quality control tests at least annually