Abstract
PurposeThe challenges of accurate dosimetry for stereotactic radiotherapy (SRT) with small unflattened radiation fields have been widely reported in the literature. In this case, suitable dosimeters would have to offer a submillimeter spatial resolution. The CyberKnife® (Accuray Inc., Sunnyvale, CA, USA) is an SRT‐dedicated linear accelerator (linac), which can deliver treatments with submillimeter positional accuracy using circular fields. Beams are delivered with the desired field size using fixed cones, the InCise™ multileaf collimator or a dynamic variable‐aperture Iris™ collimator. The latter, allowing for field sizes to be varied during treatment delivery, has the potential to decrease treatment time, but its reproducibility in terms of output factors (OFs) and dose profiles (DPs) needs to be verified.MethodsA 2D monolithic silicon array detector, the “Octa”, was evaluated for dosimetric quality assurance (QA) for a CyberKnife system. OFs, DPs, percentage depth‐dose (PDD) and tissue maximum ratio (TMR) were investigated, and results were benchmarked against the PTW SRS diode. Cross‐plane, in‐plane and 2 diagonal dose profiles were measured simultaneously with high spatial resolution (0.3 mm). Monte Carlo (MC) simulations with a GEANT4 (GEometry ANd Tracking 4) tool‐kit were added to the study to support the experimental characterization of the detector response.ResultsFor fixed cones and the Iris, for all field sizes investigated in the range between 5 and 60 mm diameter, OFs, PDDs, TMRs, and DPs in terms of FWHM measured by the Octa were accurate within 3% when benchmarked against the SRS diode and MC calculations.ConclusionsThe Octa was shown to be an accurate dosimeter for measurements with a 6 MV FFF beam delivered with a CyberKnife system. The detector enabled real‐time dosimetric verification for the variable aperture Iris collimator, yielding OFs and DPs consistent with those obtained with alternative methods.
Highlights
The CyberKnife® system can deliver stereotactic radiotherapy (SRT) treatments with high doses in a few fractions using small radiation fields, with submillimeter positional accuracy.[1,2] The linear accelerator, mounted on a robotic arm, is operated without a flattening filter and the treatment beam is shaped using fixed circular cones, the InCiseTM multileaf collimator or the variable aperture IrisTM collimator (Fig. 1).[1,3] The latter, allowing for the radiation field size to be varied during treatment delivery, has the potential to decrease the peripheral dose compared to fixed collimators[4] and to reduce treatment time.[3]
For fixed cones and the Iris, for all field sizes investigated in the range between 5 and 60 mm diameter, output factors (OFs), percentage depth‐dose (PDD), tissue maximum ratio (TMR), and dose profiles (DPs) in terms of FWHM measured by the Octa were accurate within 3% when benchmarked against the SRS diode and Monte Carlo (MC) calculations
The detector enabled real‐time dosimetric verification for the variable aperture Iris collimator, yielding OFs and DPs consistent with those obtained with alternative methods
Summary
The CyberKnife® system can deliver stereotactic radiotherapy (SRT) treatments with high doses in a few fractions using small radiation fields, with submillimeter positional accuracy.[1,2] The linear accelerator (linac), mounted on a robotic arm, is operated without a flattening filter and the treatment beam is shaped using fixed circular cones, the InCiseTM multileaf collimator or the variable aperture IrisTM collimator (Fig. 1).[1,3] The latter, allowing for the radiation field size to be varied during treatment delivery, has the potential to decrease the peripheral dose compared to fixed collimators[4] and to reduce treatment time.[3]. Small‐field dosimetry, known to be challenging due to volume averaging effects and a lack of charged particle equilibrium (CPE), has been extensively discussed in the literature.[6,7] The problems associated with small‐field dosimetry for flattened beams are likely to be compounded in flattening filter free (FFF) beams, given their inherently higher dose gradients, not just the penumbral region and in the central beam, and higher doses per pulse.[8,9]
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