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.

BackgroundThis study investigates the variation in segmentation of several pelvic anatomical structures on computed tomography (CT) between multiple observers and a commercial automatic segmentation method, in the context of quality assurance and evaluation during a multicentre clinical trial.MethodsCT scans of two prostate cancer patients (‘benchmarking cases’), one high risk (HR) and one intermediate risk (IR), were sent to multiple radiotherapy centres for segmentation of prostate, rectum and bladder structures according to the TROG 03.04 “RADAR” trial protocol definitions. The same structures were automatically segmented using iPlan software for the same two patients, allowing structures defined by automatic segmentation to be quantitatively compared with those defined by multiple observers. A sample of twenty trial patient datasets were also used to automatically generate anatomical structures for quantitative comparison with structures defined by individual observers for the same datasets.ResultsThere was considerable agreement amongst all observers and automatic segmentation of the benchmarking cases for bladder (mean spatial variations < 0.4 cm across the majority of image slices). Although there was some variation in interpretation of the superior-inferior (cranio-caudal) extent of rectum, human-observer contours were typically within a mean 0.6 cm of automatically-defined contours. Prostate structures were more consistent for the HR case than the IR case with all human observers segmenting a prostate with considerably more volume (mean +113.3%) than that automatically segmented. Similar results were seen across the twenty sample datasets, with disagreement between iPlan and observers dominant at the prostatic apex and superior part of the rectum, which is consistent with observations made during quality assurance reviews during the trial.ConclusionsThis study has demonstrated quantitative analysis for comparison of multi-observer segmentation studies. For automatic segmentation algorithms based on image-registration as in iPlan, it is apparent that agreement between observer and automatic segmentation will be a function of patient-specific image characteristics, particularly for anatomy with poor contrast definition. For this reason, it is suggested that automatic registration based on transformation of a single reference dataset adds a significant systematic bias to the resulting volumes and their use in the context of a multicentre trial should be carefully considered.

PurposeThe aim of this work was to evaluate the use of an angularly independent silicon detector (edgeless diodes) developed for dosimetry in megavoltage radiotherapy for Cyberknife in a phantom and for patient quality assurance (QA).MethodThe characterization of the edgeless diodes has been performed on Cyberknife with fixed and IRIS collimators. The edgeless diode probes were tested in terms of basic QA parameters such as measurements of tissue‐phantom ratio (TPR), output factor and off‐axis ratio. The measurements were performed in both water and water‐equivalent phantoms. In addition, three patient‐specific plans have been delivered to a lung phantom with and without motion and dose measurements have been performed to verify the ability of the diodes to work as patient‐specific QA devices. The data obtained by the edgeless diodes have been compared to PTW 60016, SN edge, PinPoint ionization chamber, Gafchromic EBT3 film, and treatment planning system (TPS).ResultsThe TPR measurement performed by the edgeless diodes show agreement within 2.2% with data obtained with PTW 60016 diode for all the field sizes. Output factor agrees within 2.6% with that measured by SN EDGE diodes corrected for their field size dependence. The beam profiles’ measurements of edgeless diodes match SN EDGE diodes with a measured full width half maximum (FWHM) within 2.3% and penumbra widths within 0.148 mm. Patient‐specific QA measurements demonstrate an agreement within 4.72% in comparison with TPS.ConclusionThe edgeless diodes have been proved to be an excellent candidate for machine and patient QA for Cyberknife reproducing commercial dosimetry device measurements without need of angular dependence corrections. However, further investigation is required to evaluate the effect of their dose rate dependence on complex brain cancer dose verification.

Purpose Specialized treatment machines such as the CyberKnife, TomoTherapy, or the GammaKnife, utilize flattening filter free (FFF) photon beams and may not be able to generate a 10 cm x 10 cm reference field. A new Code of Practice has recently been published (IAEA TRS483) to give recommendations for these machines. Additionally, some standard laboratories provide measured beam quality correction factors for the user’s reference chamber, which can be used instead of the published tabulated beam quality correction factors. The purpose of this study was first to assess how different recommendations, as outlined below, affect the reference dosimetry at the CyberKnife, and second, to assess the impact of using measured rather than tabulated beam quality correction factors on measured dose. Methods Following recommendations in TRS398, three field chambers (IBA CC04, Exradin A19, and Exradin A12S) were cross‐calibrated with a user’s reference chamber (IBA FC65‐G), which was calibrated in a Cobalt‐60 (Co‐60) beam by a primary standards laboratory. The chamber response was corrected for influence quantities such as temperature, pressure, ion recombination, polarity, and beam quality. Additionally, correction factors for volume averaging and differences due the FFF beam spectrum were determined for the CyberKnife beam. Three different methods were utilized ‐ TRS398; Intermediate (i.e. TRS398 with additional published recommendations); and TRS483. The measurements were undertaken in a 10 cm × 10 cm field defined by jaws for a uniform flattened (WFF) 6 MV photon beam of a Varian TrueBeam linear accelerator (linac) with a source to detector distance (SDD) of 100 cm, and in a 60 mm diameter circular field for a 6 MV flattening filter free (FFF) Accuray CyberKnife beam with SDD of 80 cm. All measurement was performed at 10 cm deep in a full scatter phantom as defined in TRS398. Results Differences between the three methods in volume averaging correction factors ranged from 0.01% to 0.45% depending on the chamber assessed. As expected, an increased chamber length leads to a larger correction factor. The differences in beam spectrum correction factors range from 0.09% to 0.3%. Negligible differences in beam quality correction factors were observed; however, differences up to 1% were found between measured and tabulated values. Differences in cross‐calibrated chamber calibration coefficients range from 0.05% to 0.51% depending on the chamber assessed. Differences in measured dose are up to 0.87% between Method TRS398 and Intermediate, again chamber dependent, and 0.28% between Method Intermediate and TRS483. Conclusion Using chambers cross‐calibrated in the linac beam can lead to differences in measured dose per Monitor Unit (MU) in the CyberKnife beam of approximately 0.5% between chambers. Using Method Intermediate vs using recommendations given in TRS483 led to a difference of 0.28% in measured dose per MU, which is due to differences in volume averaging and beam spectrum correction factors. Using TRS483 is recommended as the...

Purpose Mechanical sag in the radiotherapy linear accelerator gantry and multi‐leaf collimator (MLC) carriage effectively causes systematic deviations in the isocenter with respect to gantry angle. To minimize the impact of this error on treatment, a tolerance value of a 1‐mm mechanical isocenter shift is commonly accepted for intensity‐modulated radiation therapy quality assurance (QA). However, this tolerance value has not been firmly established for volumetric modulated arc therapy (VMAT) treatments. The purpose of this study is therefore to evaluate the impact of gantry and MLC carriage sag on VMAT clinical performance. Methods A published dataset of Elekta and Varian sag measurements served as a starting point for the investigation. Typical sag profiles were chosen and modeled as continuous isocenter deviations in three dimensions. The data were then incorporated into existing Digital Imaging and Communications in Medicine protocol, extended for radiotherapy plans via a “beam‐splitting” algorithm. Three treatment sites were investigated in parallel: head and neck, prostate, and prostate with surrounding lymph nodes. Monte Carlo‐simulated dose distributions were obtained for varying magnifications of the modeled sag. The resulting dose distributions, including that for no error, were compared qualitatively and quantitatively, against multiple metrics. Results The dose‐volume histograms (DVHs) for all plans exhibited a decrease in planning target volume (PTV) dose uniformity with increasing sag magnification, whereas dose to organs at risk exhibited no coherent trend. The prostate plan was shown to be the most vulnerable to mechanical sag across all considered metrics. However, all plans with peak isocenter deviation less than 1 mm were well within typical cutoff points for each metric. Conclusions All avenues of investigation presented substantiate the commonly accepted tolerance value of a 1‐mm peak isocenter shift in annual linac QA.