Iris Collimator Research Articles

Clinical application of a GPU-accelerated monte carlo dose verification for cyberknife M6 with Iris collimator

PurposeTo apply an independent GPU-accelerated Monte Carlo (MC) dose verification for CyberKnife M6 with Iris collimator and evaluate the dose calculation accuracy of RayTracing (TPS-RT) algorithm and Monte Carlo (TPS-MC) algorithm in the Precision treatment planning system (TPS).MethodsGPU-accelerated MC algorithm (ArcherQA-CK) was integrated into a commercial dose verification system, ArcherQA, to implement the patient-specific quality assurance in the CyberKnife M6 system. 30 clinical cases (10 cases in head, and 10 cases in chest, and 10 cases in abdomen) were collected in this study. For each case, three different dose calculation methods (TPS-MC, TPS-RT and ArcherQA-CK) were implemented based on the same treatment plan and compared with each other. For evaluation, the 3D global gamma analysis and dose parameters of the target volume and organs at risk (OARs) were analyzed comparatively.ResultsFor gamma pass rates at the criterion of 2%/2 mm, the results were over 98.0% for TPS-MC vs.TPS-RT, TPS-MC vs. ArcherQA-CK and TPS-RT vs. ArcherQA-CK in head cases, 84.9% for TPS-MC vs.TPS-RT, 98.0% for TPS-MC vs. ArcherQA-CK and 83.3% for TPS-RT vs. ArcherQA-CK in chest cases, 98.2% for TPS-MC vs.TPS-RT, 99.4% for TPS-MC vs. ArcherQA-CK and 94.5% for TPS-RT vs. ArcherQA-CK in abdomen cases. For dose parameters of planning target volume (PTV) in chest cases, the deviations of TPS-RT vs. TPS-MC and ArcherQA-CK vs. TPS-MC had significant difference (P < 0.01), and the deviations of TPS-RT vs. TPS-MC and TPS-RT vs. ArcherQA-CK were similar (P > 0.05). ArcherQA-CK had less calculation time compared with TPS-MC (1.66 min vs. 65.11 min).ConclusionsOur proposed MC dose engine (ArcherQA-CK) has a high degree of consistency with the Precision TPS-MC algorithm, which can quickly identify the calculation errors of TPS-RT algorithm for some chest cases. ArcherQA-CK can provide accurate patient-specific quality assurance in clinical practice.

Non-coplanar lung SABR treatments delivered with a gantry-mounted x-ray tube

Objective. To create two non-coplanar, stereotactic ablative radiotherapy (SABR) lung patient treatment plans compliant with the radiation therapy oncology group (RTOG) 0813 dosimetric criteria using a simple, isocentric, therapy with kilovoltage arcs (SITKA) system designed to provide low cost external radiotherapy treatments for low- and middle-income countries (LMICs). Approach. A treatment machine design has been proposed featuring a 320 kVp x-ray tube mounted on a gantry. A deep learning cone-beam CT (CBCT) to synthetic CT (sCT) method was employed to remove the additional cost of planning CTs. A novel inverse treatment planning approach using GPU backprojection was used to create a highly non-coplanar treatment plan with circular beam shapes generated by an iris collimator. Treatments were planned and simulated using the TOPAS Monte Carlo (MC) code for two lung patients. Dose distributions were compared to 6 MV volumetric modulated arc therapy (VMAT) planned in Eclipse on the same cases for a Truebeam linac as well as obeying the RTOG 0813 protocols for lung SABR treatments with a prescribed dose of 50 Gy. Main results. The low-cost SITKA treatments were compliant with all RTOG 0813 dosimetric criteria. SITKA treatments showed, on average, a 6.7 and 4.9 Gy reduction of the maximum dose in soft tissue organs at risk (OARs) as compared to VMAT, for the two patients respectively. This was accompanied by a small increase in the mean dose of 0.17 and 0.30 Gy in soft tissue OARs. Significance. The proposed SITKA system offers a maximally low-cost, effective alternative to conventional radiotherapy systems for lung cancer patients, particularly in low-income countries. The system’s non-coplanar, isocentric approach, coupled with the deep learning CBCT to sCT and GPU backprojection-based inverse treatment planning, offers lower maximum doses in OARs and comparable conformity to VMAT plans at a fraction of the cost of conventional radiotherapy.

Dosimetric characterization of CyberKnife® robotic SRS using MOSFET-based "MOskin" detector

AimThis study assesses the implementation of MOSkin detectors for the SRS dosimetry of the CyberKnife® system. MethodThe performance of the MOSkin detectors was evaluated by using Iris collimators on water-equivalent phantoms in the CyberKnife® system. The dosimetric evaluation included measurements of the tissue–phantom ratio (TPR), output factor, and off-axis ratio. The results of the MOSkin sensors were compared with those of PTW 60016 and EDGE diodes. ResultsThe TPR measurements obtained by the MOSkin devices exhibited conformity lower than 2.9% compared with the results of the PTW60016 diodes across the field sizes considered. The output factor was consistent to within 2.8% of that of the EDGE detectors, following adjustment based on the appropriate field correction factors. The off-axis ratio of the MOSkin detectors corresponded to that of the EDGE diodes, with an FWHM of 4.3% and a penumbra narrower than 0.22 mm. ConclusionMOSkin sensors are strong contenders for Cyberknife® quality assurance of both machines and patients, as they are capable of reproducing measurements obtained by commercial sensors with an acceptable degree of precision.

Evaluation of a planar diode matrix for SRS patient-specific QA in comparison with GAFchromic films.

We validate the routine use of a two-dimensional (2D) diode matrix for patient specific pre-treatment verification for Cyberknife (CK) stereotactic radiosurgery and to compare it with film dosimetry. A total of 46 patients were selected according to the most frequent diseases treated at our institution with the CK system, that is, brain metastases, meningiomas, spine metastases, and prostate tumors. All cases were evaluated with GAFChromic EBT-3 films and SRS MapCHECK for Fixed cone, IRIS, and MLC collimators of the CK. The highest mean passing rate was observed for the SRS MapCHECK system compared to films. In order to assess if the two techniques provide statistically different results, a Wilcoxon Signed-Rank non-parametric test was performed (p<0.05) and we found gamma values significantly lower for EBT-3 films with respect to the SRS MapCHECK. We noticed a moderately significant association between the two techniques using Spearman's rank correlation coefficient (rs>0.4). We also performed the Bland-Altman statistical method: less than 5% of the differences resulted outside the range (mean±1.96×SD), so the two methods can be considered interchangeable within the combined inaccuracy. The use of SRS MapCHECK for CK patient specific quality assurance (QA) is feasible for a variety of clinical districts and could be reliably used as a replacement for radiochromic films.

Evaluation of a high resolution diode array for CyberKnife quality assurance.

The CyberKnife quality assurance (QA) program relies mainly on the use of radiochromic film (RCF). We aimed at evaluating high-resolution arrays of detectors as an alternative to films for CyberKnife machine QA. This study will test the SRS Mapcheck (Sun Nuclear, Melbourne, Florida, USA) diode array and its own software, which allows three tests of the CyberKnife QA program to be performed. The first one is a geometrical accuracy test based on the delivery of two orthogonal beams (Automated Quality Assurance, AQA). Besides comparing the constancy and repeatability of both methods, known errors will be introduced to check their sensitivity. The second checks the constancy of the iris collimator field sizes (Iris QA). Changes in the field sizes will be introduced to study the array sensitivity. The last test checks the correct positioning of the multileaf collimator (MLC). It will be tested introducing known systematic displacements to whole banks and to single leaves. The results of the RCF and diode array were equivalent (maximum differences of 0.18±0.14mm) for the AQA test, showing the array a higher reproducibility. When known errors were introduced, both methods behaved linearly with similar slopes. Regarding Iris QA, the array measurements are highly linear when changes in the field sizes are introduced. Linear regressions show slopes of 0.96-1.17 with r2 above 0.99 in all field sizes. Diode array seems to detect changes of 0.1mm. In MLC QA, systematic errors of the whole bank of leaves were not detected by the array, while single leaf errors were detected. The diode array is sensitive and accurate in the AQA and Iris QA tests, which give us the possibility of substituting RCF with a diode array. QA would be performed faster than using the film procedure, obtaining reliable results. Regarding the MLC QA, the inability to detect systematic displacements make it difficult to confidently use the detector.

Radioluminescence imaging feasibility for robotic radiosurgery field size quality assurance.

To investigate the feasibility of radioluminescence imaging (RLI) as a novel 2D quality assurance (QA) dosimetry system for CyberKnife®. We developed a field size measurement system based on a commercial complementary metal oxide semiconductor (CMOS) camera facing a radioluminescence screen located at the isocenter normal to the beam axis. The radioluminescence light collected by a lens was used to measure 2D dose distributions. An image transformation procedure, based on two reference phantoms, was developed to correct for projective distortion due to the angle (15°) between the optical and beam axis. Dose profiles were measured for field sizes ranging from 5 mm to 60 mm using fixed circular and iris collimators and compared against gafchromic (GC) film. The corresponding full width at half maximum (FWHM) was measured using RLI and benchmarked against GC film. A small shift in the source-to-surface distance (SSD) of the measurement plane was intentionally introduced to test the sensitivity of the RLI system to field size variations. To assess reproducibility, the entire RLI procedure was tested by acquiring the 60mm circle field three times on two consecutive days. The implemented procedure for perspective image distortion correction showed improvements of up to 1 mm using the star phantom against the square phantom. The FWHM measurements using the RLI system indicated a strong agreement with GC film with maximum absolute difference equal to 0.131 mm for fixed collimators and 0.056 mm for the iris. A 2D analysis of RLI with respect to GC film showed that the differences in the central region are negligible, while small discrepancies are in the penumbra region. Changes in field sizes of 0.2 mm were detectable by RLI. Repeatability measurements of the beam FWHM have shown a standard deviation equal to 0.11mm. The first application of a RLI approach for CyberKnife® field size measurement was presented and tested. Results are in agreement with GC film measurements. Spatial resolution and immediate availability of the data indicate that RLI is a feasible technique for robotic radiosurgeryQA.

Implementing and evaluating a high-resolution diode array for patient-specific quality assurance of robotic brain stereotactic radiosurgery/radiotherapy.

The purpose of the study was to introduce and evaluate a high‐resolution diode array for patient‐specific quality assurance (PSQA) of CyberKnife brain stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT). Thirty‐three intracranial plans were retrospectively delivered on the SRS MapCHECK using fixed cone, Iris, and multileaf collimator (MLC). The plans were selected to cover a range of sites from large tumor bed, single/multiple small brain metastases (METs) to trigeminal neuralgia. Fiducial tracking using the four fiducials embedded around the detector plane was used as image guidance. Results were analyzed before and after registration based on absolute dose gamma criterion of 1 mm distance‐to‐agreement and 0.5%–3% dose‐difference. Overall, the gamma passing rates (1 mm and 3% criterion) before registration for all the patients were above 90% for all three treatment modalities (96.8 ± 3.5%, the lowest passing rate of 90.4%), and were improved after registration (99.3 ± 1.5%). When tighter criteria (1 mm and 2%) were applied, the gamma passing rates after registration for all the cases dropped to 97.3 ± 3.2%. For trigeminal neuralgia cases, we applied 1 mm and 0.5% criterion and the passing rates dropped from 100 ± 0.0% to 98.5 ± 2.0%. The mean delivery time was 33.4 ± 11.7 min, 24.0 ± 4.9 min, and 17.1 ± 2.6 min for the fixed cone, Iris, and MLC, respectively. With superior gamma passing rates and reasonable quality assurance (QA) time, we believe the SRS MapCHECK could be a good option for routine PSQA for CyberKnife SRS/SRT.

Assessment of HDR brachytherapy-replicating prostate radiotherapy planning for tomotherapy, cyberknife and VMAT.

A dosimetric study was undertaken to assess the ability of Cyberknife (CK), Volumetric Modulated Arc Therapy (VMAT), and TomoTherapy (Tomo) to generate treatment plans that mimic the dosimetry of high dose-rate brachytherapy (HDR BT) for prostate cancer. The project aimed to assess the potential of using stereotactic body radiotherapy (SBRT) for boost treatment of high-risk prostate cancer patients where HDR BT in combination with conformal external beam radiotherapy (EBRT) is the standard of care. The datasets of 6 prostate patients previously treated with HDR BT were collated. VMAT, CK, and TomoTherapy treatment plans were generated for each dataset using the target and organ-at-risk structures as defined by the Radiation Oncologist during the HDR BT treatment process. The HDR BT plan isodoses were also converted into planning structures to assist the other modalities to achieve a HDR BT-like dose distribution. CK plans were created using both the iris collimator (IC) and a multileaf collimator (MLC). Comparison of the techniques was made based on dose-volume indices. Each plan was created at centres experienced using the respective treatment planning systems (TPS). Planning target volume (PTV V100%), i.e., the volume of the planning target volume (PTV) receiving 100% of the relative dose, in VMAT and TomoTherapy SBRT plans was higher than HDR BT plans. PTV V150% and V200%, i.e., volume of the PTV receiving 150% and 200% of the relative dose, were approached on all the CK MLC and TomoTherapy SBRT plans. However, it is not presently achievable for "virtual brachytherapy" SBRT to replicate the same high intraprostatic doses as HDR BT while meeting the constraints on the organs-at-risk (OARs). Half of the CK IC plans achieved PTV V150% but this was at the expense of high rectal dose. TomoTherapy and CK MLC plans achieved PTV V150% and V200% but the bladder dose was higher compared to CK IC plans. VMAT exhibited excellent PTV coverage based on V100 and OAR sparing, but without any ability to achieve the high intra-prostatic doses of HDR (V150% and V200%). SBRT techniques can be used to deliver hypofractionated radiotherapy to the PTV V100%. Based on the comparison of "physical" dose distributions, SBRT cannot presently achieve the same high intraprostatic doses as HDR BT while respecting the OAR constraints. SBRT still remains an attractive treatment option for delivering hypofractionated treatments for prostate cancer compared to HDR BT, in particular as it is less invasive and less resource intensive. Long-term outcomes of clinical trials comparing HDR BT and SBRT "prostate boosts" may show whether the high intraprostatic doses are clinically significant and correlate with outcomes.

Output factor measurements with multiple detectors in CyberKnife® Robotic Radiosurgery System.

The aim of this study was to measure and compare the output factor (OF) of a CyberKnife Robotic Radiosurgery System with eight different small field detectors and validate with Technical Report Series (TRS) report 483. Accurate dosimetry of CyberKnife system is limited due to the challenges in small field dosimetry. OF is a vital dosimetric parameter used in the photon beam modeling and any error would affect the dose calculation accuracy. In this study, the OF was measured with eight different small-field detectors for the 12 IRIS collimators at 800 mm SAD setup at 15 mm depth. The detectors used were PTW 31016 PinPoint 3D, IBA PFD shielded diode, IBA EFD unshielded diode, IBA SFD unshielded diode (stereotactic), PTW 60008 shielded diode, PTW 60012 unshielded diode, PTW 60018 unshielded diode (stereotactic), and PTW 60019 CVD diamond detector. OF was obtained after correcting for field output correction factors from IAEA TRS No. 483. The field OFs in CyberKnife are derived from the measured data by applying the correction factors from Table 23 in TRS 483 for the eight small field detectors. These field OFs matched within 2% of peer-reviewed published values. The range and standard deviation showed a decreasing trend with collimator diameter. The field OF obtained after applying the appropriate correction factor from TRS 483 matched well with the peer-reviewed published OFs. The inter-detector variation showed a decreasing trend with increasing collimator field size. This study gives physicists confidence in measuring field OFs while using small field detectors mentioned in this work.

Dosimetric Comparison of Robotic and Linear Accelerator Multi-Leaf Collimator-Based Stereotactic Radiosurgery for Arteriovenous Malformation.

Purpose:To investigate the dosimetric comparison of different collimators which are used in robotic radiosurgery (cyberknife-CK) and linear accelerator (LINAC) for stereotactic radiosurgery (SRS) in arteriovenous malformation (AVM).Materials and Methods:Twenty-five AVM patients were planned in CK using FIXED cone, IRIS collimator, and multi-leaf collimator (MLC) based in LINAC. Dosimetric comparison was performed using Paddick conformity index (CIPaddick) and International Commission on Radiation Units and measurements (ICRU) homogeneity index (HIICRU), gradient score (GS), normal brain dose received by 10cc (D10cc) and critical structure (brain stem, optic chiasma, optic nerves) doses. Paired sample t-test was used for statistical analysis.Results:Mean treatment volume was 3.16cc (standard deviation ± 4.91cc). No significant deviation (P =0.45, 0.237 for FIXED vs. IRIS and FIXED vs. MLC, respectively) was found in target coverage. For CIPaddick, the mean difference (MD) between FIXED- and MLC-based plans was 0.16(P = 0.001); For HIICRU, difference between FIXED and IRIS was insignificant (0.5, P = 0.823); but, when FIXED versus MLC, the deviation was 7.99% (P = 0.002). In FIXED- and MLC-based plans, significant difference was found in GS70 and GS40 (P < 0.041 and 0.005, respectively). MD between FIXED- and MLC-based plans for normal brain for 5Gy, 10Gy, 12Gy, and 20Gy were 36.08cc (P = 0.009), 7.12cc (P = 0.000), 5.84cc (P = 0.000) and 1.56cc (P = 0.000), respectively. AVM volume <0.7cc should be treated with CK FIXED and >0.7cc were treated by using FIXED or IRIS collimators. AVM volume > 1.4cc can be treated by either LINAC MLC-based SRS or CK.Conclusion:Our study shows CK collimator (IRIS and FIXED) could be able to treat brain AVMs in any size. Linac MLC-based SRS has some limitations in terms of conformity and low-dose spillage, and advantages like reduced treatment time and MU.

Integrating CVH and LVH metrics into an optimization strategy for the selection of Iris collimator for Cyberknife Xsight lung tracking treatment.

PurposeWe conducted this study to construct a target coverage‐volume histogram (CVH) and leakage‐volume histogram (LVH) metrics and optimization strategy for the selection of the Iris collimator in Cyberknife Xsight lung tracking treatment through a retrospective analysis of target structures and clinical data.Methods and MaterialsCVH and LVH metrics were retrospectively analyzed for 37 lung cancer patients. CVH and LVH were the same as dose‐volume histogram (DVH), but with a coverage and leakage replacing dose. For each patient, Iris collimator was optimized and selected based on CVH and LVH metrics. The CVH and LVH metrics were then compared to ascertain differences in 95% (C95) or 90% (C90) of the target coverage thresholds. The planning target volume (PTV) C95 and C90 coverage, absolute mean leakage value, leakage/coverage ratio, selected collimator diameter (Φ), Φ/length of the long axis of PTV (Amax), and Φ/length of the short axis (Amin) of PTV were compared. The correlation of the absolute mean leakage value, leakage/coverage ratio, Φ/Amin and Φ/Amax were evaluated.ResultsFor each patient, the PTV C95 coverage (70.45 vs 63.19) and C90 coverage (77.25 vs 69.96) were higher in the C95 coverage threshold group compared to the C90 coverage threshold group. The leakage/coverage ratio (0.56 vs 0.69) and absolute mean leakage value (0.56 vs 0.61) were lower in C90 coverage threshold group than in C95 coverage threshold group. The Spearmen correlation test showed the Φ/Amin were significantly correlated with leakage/coverage ratio and absolute mean leakage value. Upon analysis of the selected collimator diameters, the mean value of Φ/Amin of the optimized collimator diameters was found to be 1.10.ConclusionThe CVH and LVH analysis is able to quantitatively evaluate the tradeoff between target coverage and normal tissue sparing.

Commissioning of two different algorithms for stereotactic Radiosurgery M6 FI+ CyberKnife system

Introduction: Cyber Knife M6 FI+ a very precise robotic stereotactic radiosurgery system which is capable of delivering very high radiation dose to the tumour while minimizing radiation exposure to normal organs and tissues. MultiPlan treatment planning system is used with the CyberKnife unit and the algorithms used for optimization are Ray-Tracing and Monte Carlo. Our CyberKnife M6 FI+ system commissioning is done as per the vendor recommendation and meeting the local regulatory guidelines. Materials and Methods: Clinical beam data measurement is carried out using Radiation Field Analyzer, Diode E, Pinpoint chamber, Semiflex ionization chamber and Unidose E Electrometer. FC-65 ionization chamber used in absolute dose calibration. The mechanical accuracy of the robot and image stability was verified using Radiochromic film (EBT3), E2E and Iris QA toolkits along with software. StereoPHAN is used for the patient-specific QA point dose measurement. Results: Tissue phantom ratio, Off-centre ratio, Output factor, Percentage depth dose, open beam profile and absolute dose calibration are done as per the protocols. E2E performed for two different modes- static and motion. Iris aperture size measured for all the field sizes. The patient-specific QA delivered for both algorithms. Discussion: Clinical beam data measurements are within ±1% of composite data set, overall standard deviation for Output Factors of the fixed and Iris collimators are 0.0026 and 0.0063. The absolute dose was calibrated to 1cGy per MU. E2E, Iris QA and Laser and radiation coincidence values are within the tolerance. Patient-specific QA point dose measurement variation for Ray-Tracing and Monte Carlo is 3% and 2%. Conclusion: These exercises are mandatory to achieve the accurate, precise and high quality of treatment which also includes patient safety.

15 OF evaluation for the Cyberknife M6: Comparison between two measurement correction methods for different detectors

Introduction For small beams used in stereotactic radiotherapy, extra precautions need to be taken into account when determining dosimetric data used in the modelling of dose calculation algorithms [1] . In this work, we focused on the output factors determination of the Cyberknife M6 (Accuray Inc., Sunnyvale, USA). Measurements were performed with active and passive detectors and with fix collimation. We compared two new sets of corrective factors published by the IAEA (TRS 483) and by Francescon et al., recommended by Accuray [2] and we compared theses corrected results with radiochromic films. Methods OF were measured with three punctual PTW detectors: 60018 (SRS) and 60008 diodes, 60019 microdiamond and EBT3 Gafchromic films (Ashland Inc., USA) for all the fixed collimators in the reference conditions (SAD = 80 cm, depth = 1.5 cm), 200 MU for each beam. Five measurements were performed for each punctual detector and three for the films. Films were analyzed with the Epson V750 Pro (Epson, Japan) and the ImageJ software (NIH, USA). Results Application of these two sets of corrective factors enable the reduction of discrepancies (related to small beams measurements challenges) between detectors for collimators greater than or equal to 7.5 mm diameter (see Fig. 1). For the 5 mm fixed collimator, the maximal discrepancy between two detectors is 9.6% without corrections and is reduced to 4.4% and 8% with the Francescon’s corrections and with the IAEA respectively. In average, corrected measurements are closer to films measurements with a maximal discrepancy varying from 3.8% to 2% and 2.1% for Francescon and for the IAEA respectively. Download : Download high-res image (502KB) Download : Download full-size image Conclusions The two sets of corrective factors give similar OF results with a good agreement between detectors, allowing a better precision for OF modelling in the TPS. Those factors could be extended for the Iris collimator [3] .

A new plan quality objective function for determining optimal collimator combinations in prostate cancer treatment with stereotactic body radiation therapy using CyberKnife.

Stereotactic body radiation therapy with CyberKnife for prostate cancer has long treatment times compared with conventional radiotherapy. This arises the need for designing treatment plans with short execution times. We propose an objective function for plan quality evaluation, which was used to determine an optimal combination between small and large collimators based on short treatment times and clinically acceptable dose distributions. Data from 11 prostate cancer patients were used. For each patient, 20 plans were created based on all combinations between one small (⌀ 10–25 mm) and one large (⌀ 35–60 mm) Iris collimator size. The objective function was assigned to each combination as a penalty, such that plans with low penalties were considered superior. This function considered the achievement of dosimetric planning goals, tumor control probability, normal tissue complication probability, relative seriality parameter, and treatment time. Two methods were used to determine the optimal combination. First, we constructed heat maps representing the mean penalty values and standard deviations of the plans created for each collimator combination. The combination giving a plan with the smallest mean penalty and standard deviation was considered optimal. Second, we created two groups of superior plans: group A plans were selected by histogram analysis and group B plans were selected by choosing the plan with the lowest penalty from each patient. In both groups, the most used small and large collimators were assumed to represent the optimal combination. The optimal combinations obtained from the heat maps included the 25 mm as a small collimator, giving small/large collimator sizes of 25/35, 25/40, 25/50, and 25/60 mm. The superior-group analysis indicated that 25/50 mm was the optimal combination. The optimal Iris combination for prostate cancer treatment using CyberKnife was determined to be a collimator size between 25 mm (small) and 50 mm (large).

Quality assurance of Cyberknife robotic stereotactic radiosurgery using an angularly independent silicon detector.

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.

CyberKnife MLC-based treatment planning for abdominal and pelvic SBRT: Analysis of multiple dosimetric parameters, overall scoring index and clinical scoring

PurposeThis study evaluated the plan quality of CyberKnife MLC-based treatment planning in comparison to the Iris collimator for abdominal and pelvic SBRT. Multiple dosimetric parameters were considered together with a global scoring index validated by clinical scoring. Methods and materialsIris and MLC plans were created for 28 liver, 15 pancreas and 13 prostate cases including a wide range of PTV sizes (24–643 cm3). Plans were compared in terms of coverage, conformity (nCI), dose gradient (R50%), homogeneity (HI), OAR doses, PTV gEUD, MU, treatment time both estimated by TPS (tTPS) and measured. A global plan quality score index was calculated for IRIS and MLC solutions and validated by a clinical score given independently by two observers. ResultsCompared to Iris, MLC achieved equivalent coverage and conformity without compromising OAR sparing and improving R50% (p < 0.001). MLC gEUD was slightly lower than Iris (p < 0.05) for abdominal cases. MLC reduced significantly MU (−15%) and tTPS (−22%). Time reduction was partially lost when measured. The global score index was significantly higher for MLC solutions which were selected in 73% and 64% of cases respectively by the first and second observer. ConclusionIris and MLC comparison was not straightforward when based on multiple dosimetric parameters. The use of a mathematical overall score index integrated with a clinical scoring was essential to confirm MLC plans advantages over Iris solutions.

CyberKnife® fixed cone and Iris™ defined small radiation fields: Assessment with a high-resolution solid-state detector array.

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.

Beam Commissioning Data Acquisition for the Iris Variable Aperture Collimator of CyberKnife VSI

Objective: To minimize the risk inherent to the use of medical radiation and robotic radiosurgery, this research introduced the basis and method of beam commissioning data acquisition for the Iris variable aperture collimator of CyberKnife VSI. Methods: In this study, clinical dosimetry measurements of the Iris collimator such as tissue-phantom ratios (TPRs), off-center ratios (OCRs), and secondary collimator output factors (OFs) were conducted, and the data was contrasted with that of the Fixed collimator. Results: The Iris data and the Fixed data were not quite different in TPRs in the same radiation field size. Since the difference of the hardware structure,the OCRs and OFs were quite different, especially in small aperture (5, 7.5 and 10 mm) under the same condition of SAD. For example, the maximum discrepancy of the OFs was 26.67% (5mm aperture, 650mm SAD). Conclusion: When Iris aperture makes some change, tungsten segments move fast and form an aperture smaller than that we need, and then recover to the right size, which may affect the results of the measurement, and also the dodecagonal design of Iris results in a problem of leakage radiation on the edge. Keywords: Beam commissioning, Iris Variable Aperture Collimator, tissue phantom ratio, off center ratio, output factor, CyberKnife VSI™.

EP-1550: Investigating the advantages of CyberKnife M6 MLC over Iris collimator for Liver SBRT plans

EP-1550: Investigating the advantages of CyberKnife M6 MLC over Iris collimator for Liver SBRT plans

40. Evaluation of the Incise2™ multileaf collimator for stereotactic radiotherapy on Cyberknife®

Introduction The CyberKnife® (Accuray) is a stereotactic robotic system using historically circular collimators to define the beam apertures. A new multileaf collimator (MLC) is now available on the Cyberknife® M6 version. The purpose of this study was to evaluate the quality of treatment plans using MLC compared to treatment plans using a variable circular aperture collimator IRIS™ for radiosurgery (SRS) and stereotactic body radiotherapy (SBRT). Methods The study was performed on a Cyberknife® M6™ v10.6. A total of 17 cases including cranial, liver, spinal and prostatic cases were investigated. For each case, two treatment plans were generated with Multiplan® v5.3 treatment planning system: one plan using the Incise2™ MLC associated with the Finite Size Pencil Beam (FSPB) dose calculation algorithm and one using the IRIS™ collimator associated with Ray Tracing or Monte Carlo dose calculation algorithm. The dose was prescribed near the 80% isodose level relative to max dose. Plans were normalized to have the same target coverage at ±0.5%. The quality of the MLC and IRIS™ treatment plans was assessed using two criteria: - Dose distribution by reporting the conformal index (COIN), Homogeneity Index (HI) and dose gradient index (GI). - Efficiency by reporting the number of delivered monitor units (MUs) and the treatment time. To evaluate the accuracy of dose calculation algorithm within heterogeneity, a typical lung treatment plan for each collimator was created on a QUASAR™ Respiratory phantom (Modus Medical Inc) with a lung target insert. Calculated planar dose was compared with measurements by radiochromic EBT3 films (Ashland). The analysis was done using gamma analysis 3% (local) / 2 mm (threshold: 30% of the max dose) Results Results are summarized in Table 1. Dose conformality and homogeneity were equivalent for both collimator systems (mean differences less than 3%) except for targets with volumes smaller than 2 cm3 for which dose conformality was better of 14% on average with the IRIS collimator. The dose gradient index was improved by 10.5% on average with the MLC and up to 26% on average for volume larger than 100 cm3. The number of MUs decreased by 25% on average (up to 70%) and the treatment times were systematically reduced with a mean reduction of 28% (max 54%) with the MLC. The comparison between calculated and measured dose in lung phantom showed a gamma passing rate of 51.6 %, 45.5 % and 98.7% for FSPB, Ray Tracing and Monte Carlo respectively. Conclusion The MLC Incise2™ on Cyberknife® allows a significant reduction of MUs and treatment time compared to circular collimators while maintaining a high degree of conformality and a steep dose gradient. For treatment of target volumes smaller than 2 cm3, circular collimators should be still used. The use of MLC for SBRT of lung targets should not be recommended currently due to the absence of type B dose calculation algorithm.