Quality assurance and procedures for implementing a new SRS program

Quality assurance procedures and equipment for implementing a new Linac-based SRS programZeynep Karakas

OverviewRemember SRSDelivery optionsPhases of implementing a new SRS programPicking the right dosimeterPicking the right phantom Relevant QA tests

What is SRS?SRS = stereotactic radiosurgeryIts a single-fraction radiation therapy procedure for treating brain lesionsConcentrates dose to the target and spares as much brain tissue as possibleIt uses stereotactic frame + 3D imaging (CT/MRI) to localize the lesion and plan/deliver treatment Uses small field, circular beams delivered by rotating gantry around targets isocenter SRS delivery options

Phases of implementing a new SRS programresearch relevant protocols and practice guidelinescommission the treatment planning system for radiotherapy device and SRS techniques **verify commissioning datavendor training of TPS-specific hardware and softwarephantom studies **documentation development mock procedures

** WE CAN MEASURE Commissioning phasepicking the right dosimeterCommissioning for SRS techniquesInvolves accurately collecting photon beam data of small fields (as small as 0.4 cm x 0.4 cm)The three SRS dosimetric quantities of interest: central axis depth dosecross-beam profilesoutput factorsNote: getting accurate measurements of these quantities can be difficult for small fields and can be a major source of treatment error lack of charged particle equilibrium dosimeters can only be so small

Picking the right dosimeterLimitations of small field SRS measurements can be overcome by: using small sized dosimeters (smallest: TLDs/diodes)dosimeters with high spatial resolution (highest: film)AAPM TG-42 recommends: depth dose and beam profiles be measured with film SRS output factors be measured with an ionization chamber with a diameter less than or equal to 3 mm (most precise) Phases of implementing a new SRS programresearch relevant protocols and practice guidelinescommission the treatment planning system for radiotherapy device and SRS techniques **verify commissioning datavendor training of TPS-specific hardware and softwarephantom studies **documentation development mock procedures

** WE CAN MEASURE Phantom studiespicking the right phantomSRS phantom studies SRS phantom studies encompass end-to-end quality assurance (QA) testing of: CT/MRI scanscontouring image fusion treatment planningtarget localization dose delivery AAPM TG-42 states possible phantoms include: polysterene, anthromorphic phantoms, water and water-equivalent phantomsPicking the right phantomBecause SRS uses rotating fields, spherical phantoms can be used for QA testsThere are many 3D phantoms that, while not necessarily spherical in spherical in shape, can be used for SRS QAOnly three possible SRS phantoms will be discussed hereLucy 3D QA Phantom by Standard Imaging

Lucy 3D QA PhantomSpherically shaped acrylic with a central square cavity that accommodates dosimetry and imaging QA insertsDosimetry inserts: used to obtain absolute dose measurements at the isocenter and for exact positions off the isocenterionization chamber at geometric center, 3 in x 3 in film, TLD detectors (spaced 1 mm or 3 mm apart)Imaging inserts: contain volumes of known geometries and densities specific to MRI or CTcheck imaging integrity when moving from one imaging system to another

Lucy 3D QA PhantomCapable of achieving unrivaled 0.1 mm accuracy of end-to-end QA testsSo, can be used to measure cumulative uncertainties of: imaging planning and dose delivery mechanical and geometric alignment

QUASAR Verification Phantom by ModusQA

QUASAR Verification PhantomCylindrically shaped acrylic with a central rectangular cavity that accommodates 25 cm x 15 cm film Can also accommodate an ionization chamber that can be placed every 2.5 cm along the diameter of the cylinderDoes not provide a means of checking imaging accuracy; limited to dose delivery checks ArcCHECK by Sun Nuclear

ArcCHECKCylindrically shaped diode array with 1386 silicon diodes embedded in acrylic (spaced 1 cm apart)Checks fields as small as 1.5 cm x 1.5 cm Has a central cylindrical cavity that can accommodate an ionization chamber that can measure isocentric dose Without the ionization chamber, the diode array measures the entrance and exit dose of the beamCan be used for treatment alignment and patient-specific dosimetric QA checks (only truly 3D detector we can use to do this) Which phantom is best?Lucy 3D QA Phantom pros: high spatial accuracy (0.1 mm)cons: maybe cumbersome to change inserts QUASAR Verification Phantompros: simultaneous film and ionization chamber dose measurementscons: cannot be used to conduct all SRS QA testsArcCHECK pros: only truly 3D dose/position verification phantom cons: cannot verify fields smaller than 1.5 cm x 1.5 cm

ArcCHECK by Sun Nuclear

Relevant SRS QA testsTwo categories of QA tests: Periodic testsMechanical tests Periodic tests: Check for whether the targets isocenter and Linacs isocenter agree to 1 mm Mechanical tests: Needed for radiation therapy devices not used solely for SRS treatment, i.e. Linacs at UTMCCheck for the alignment of the radiation beam with patients coordinate system or lasers in the treatment room

SummaryRemember SRS = stereotactic radiosurgery Delivery options: charged particles, Co-60, x-raysPhases of SRS implementation that we can measure: commissioning and phantom studiesPicking the right dosimeter: small size, high resolution Picking the right phantom: end-to-end testing and ease of use Relevant tests: periodic and mechanical