Field Output Factors Research Articles

A novel extrapolation method using OSL detectors for very small field output factor measurement for stereotactic radiosurgery.

Appropriate methods for the determination of very small X-ray beam output factors are essential to ensure correct clinical outcomes for stereotactic radiosurgery. To date, substantial work has been performed in identifying and quantifying suitable dosimeters for relative output factor (ROF) measurements including recent IAEA published recommendations. In this work, we provide a novel method using optically stimulated luminescent dosimeters (OSLDs) with different effective sizes of the readout area to determine ROFs. This involves applying an extrapolation technique to assess ROFs for 6MV SRS X-ray beams with field diameters ranging from 4 to 30mm as defined by the Brainlab SRS cones. By combining the use of multiple sized OSLDs and water droplets to remove air gaps located around the OSLD detectors, both volume averaging and density variation effects were minimised to estimate ROFs for an extrapolated zero volume detector. The measured results showed that for a 4mm diameter cone, the ROF was 0.660 ± 0.032 (2SD) as compared to 0.661 ± 0.01 and 0.651 ± 0.018 for the PTW 600019 microDiamond detector and Gafchromic EBT3 film respectively. Whilst the uncertainties were larger than conventional detectors, the technique shows promise and improvements in accuracy may be obtained by higher quality manufacturing techniques. Based on these results, using OSLDs with different effective sizes of readout area and an extrapolation technique shows promise for use as an independent verification tool for very small X-ray field ROFs in the clinical department.

The impact of corrected field output factors based on IAEA/AAPM code of practice on small-field dosimetry to the calculated monitor unit in eclipse™ treatment planning system.

The objective of this study was to investigate the effect of field output factors (FOFs) according to the current protocol for small‐field dosimetry in conjunction to treatment planning system (TPS) commissioning. The calculated monitor unit (MU) for intensity‐modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) plans in Eclipse™ TPS were observed. Micro ion chamber (0.01 CC) (CC01), photon field diode (shielded diode) (PFD), and electron field diode (unshielded diode) (EFD) were used to measure percentage depth doses, beam profiles, and FOFs from 1 × 1 cm2 to 10 × 10 cm2 field sizes of 6 MV photon beams. CC01 illustrated the highest percentage depth doses at 10 cm depth while EFD exhibited the lowest with the difference of 1.6% at 1 × 1 cm2. CC01 also produced slightly broader penumbra, the difference with other detectors was within 1 mm. For uncorrected FOF of three detectors, the maximum percent standard deviation (%SD) was 5.4% at 1 × 1 cm2 field size. When the correction factors were applied, this value dropped to 2.7%. For the calculated MU in symmetric field sizes, beam commissioning group from uncorrected FOF demonstrated maximum %SD of 6.0% at 1 × 1 cm2 field size. This value decreased to 2.2% when the corrected FOF was integrated. For the calculated MU in IMRT‐SRS plans, the impact of corrected FOF reduced the maximum %SD from 6.0% to 2.5% in planning target volume (PTV) less than 0.5 cm3. Beam commissioning using corrected FOF also decreased %SD for VMAT‐SRS plans, although it was less pronounced in comparison to other treatment planning techniques, since the %SD remained less than 2%. The use of FOFs based on IAEA/AAPM TRS 483 has been proven in this research to reduce the discrepancy of calculated MU among three beam commissioning datasets in Eclipse™ TPS. The dose measurement of both symmetric field and clinical cases comparing to the calculation illustrated the dependence of the types of detector commissioning and the algorithm of the treatment planning for small field size.

Characterization of a microSilicon diode detector for small-field photon beam dosimetry.

This study characterized a new unshielded diode detector, the microSilicon (model 60023), for small-field photon beam dosimetry by evaluating the photon beams generated by a TrueBeam STx and a CyberKnife. Temperature dependence was evaluated by irradiating photons and increasing the water temperature from 11.5 to 31.3°C. For Diode E, microSilicon, microDiamond and EDGE detectors, dose linearity, dose rate dependence, energy dependence, percent-depth-dose (PDD), beam profiles and detector output factor (OFdet) were evaluated. The OFdet of the microSilicon detector was compared to the field output factors of the other detectors. The microSilicon exhibited small temperature dependence within 0.4%, although the Diode E showed a linear variation with a ratio of 0.26%/°C. The Diode E and EDGE detectors showed positive correlations between the detector reading and dose rate, whereas the microSilicon showed a stable response within 0.11%. The Diode E and microSilicon demonstrated negative correlations with the beam energy. The OFdet of microSilicon was the smallest among all the detectors. The maximum differences between the OFdet of microSilicon and the field output factors of microDiamond were 2.3 and 1.6% for 5 × 5 mm2 TrueBeam and 5 mm φ CyberKnife beams, respectively. The PDD data exhibited small variations in the dose fall-off region. The microSilicon and microDiamond detectors yielded similar penumbra widths, whereas the other detectors showed steeper penumbra profiles. The microSilicon demonstrated favorable characteristics including small temperature and dose rate dependence as well as the small spatial resolution and output factors suitable for small field dosimetry.

SciFi detector and associated method for real-time determination of profile and output factor for small fields in stereotactic radiotherapy.

PurposeFor determining small‐field profile and output factor during stereotactic radiotherapy quality assurance (QA) procedures, we propose a novel system based on the scintillating fiber (SciFi) detector with output image acquisition and processing to allow real‐time monitoring of profile and output factor.Materials and methodsThe employed detector is a SciFi detector made of tissue‐equivalent scintillating plastic fibers arranged in 6‐layer fiber ribbons with a fiber pitch of 275 μm in each layer. The scintillating signal at the detector output is acquired by a sCMOS (scientific complementary metal–oxide–semiconductor) camera and represents the projected field profile along the fibers axis. An iterative reconstruction method of the field from its projected profile based on a priori knowledge of some features of the radiation field defined by the stereotactic cones is suggested. The detector with implemented data processing has been tested in clinical conditions, for determining beam profiles and output factors, using cone collimators of different sizes from 4 to 15 mm diameter. The detector under test was placed at 1.4 cm depth and 98.6 cm source to surface distance (SSD) in a water‐equivalent phantom and irradiated by a 6 MV photon beam.ResultsThe reconstructed field profiles obtained from the detector are coherent with data from EBT3 radiochromic films, with differences within ±0.32 mm for both the FWHM and the penumbra region. For real‐time determination of the field output factor, the measured data are also in good agreement with data independently determined by the French Institute for Radiological Protection and Nuclear Safety (IRSN) based on radiochromic films and thermoluminescent 1 × 1 mm2 micro‐cubes dosimeters (TLD). The differences are within ±1.6% for all the tested cone sizes.ConclusionsWe propose and have tested a SciFi plastic scintillating detector with an optimized signal processing method to characterize small fields defined by cone collimators. It allows the determination of key field parameters such as full width at half maximum (FWHM) and field output factors. The results are consistent with those independently measured using TLD and radiochromic films. As the SciFi detector does not require a correction factor, it is in line with the International Atomic Energy Agency (IAEA) and the American Association of Physicists in Medicine (AAPM) TRS‐483 recommendations, and can be suitable for online QA of small radiation fields used in photon beam radiotherapy, and is compatible with MRI‐LINAC.

An analytical formalism for the assessment of dose uncertainties due to positioning uncertainties

PurposeTo present an analytical formalism for the in depth assessment of uncertainties of field output factors in small fields related to detector positioning based on dose profile measurements. Additionally, a procedure for the propagation of these uncertainties was developed.MethodsBased on the assumption that one dimensional and two dimensional second‐order polynomial functions can be fitted to dose profiles of small photon beams, equations for the calculation of the expectation value, the variance, and the standard deviation were developed. The following fitting procedures of the dose profiles were considered: A one‐dimensional case (1D), a quasi two‐dimensional case (2Dq) based on independently measured line profiles and a full 2D case (2Df) which also considers cross‐correlations in a two‐dimensional dose distribution. A rectangular and a Gaussian probability density function (PDF) characterizing the probability of possible positions of the detector relative to the maximum dose were used. Uncertainty components such as the finite resolution of the scanning water phantom, the reproducibility of the determination of the position of the maximum dose, and the reproducibility of the collimator system were investigated. This formalism was tested in a 0.5 x 0.5 cm2 photon field where dose profiles were measured using a radiochromic film, a synthetic diamond detector, and an unshielded diode detector. Additionally, the dose distribution measured with the radiochromic film was convoluted with a convolution kernel mimicking the active volume of the unshielded diode.ResultsAnalytic expressions for the calculation of uncertainties on field output factors were found for the 1D, the 2Dq, and the 2Df case. The uncertainty of the field output factor related to the relative position of the detector to the maximum dose increased quadratically with increasing limits of possible detector positions. Analysis of the radiochromic film showed that the 2Dq case gave a more conservative assessment of the uncertainty compared to the 2Df case with a difference of < 0.1%. The 2Dq case applied to the film measurements agreed well with the same approach as was applied to the unshielded diode. The investigated uncertainty components propagated to an uncertainty of the field output factors of 0.5% and 0.4% for the synthetic diamond and the unshielded diode, respectively. Additionally, the expectation value was lower than the maximum dose. The difference was 0.4% and 0.3% for the synthetic diamond and the unshielded diode, respectively.ConclusionsThe assessment of uncertainties of field output factors related to detector positioning is feasible using the proposed formalism. The 2Dq case is applicable when using online detectors. Accurate positioning in small fields is essential for accurate dosimetry as its related uncertainty increases quadratically. The observed drop of the expectation value needs to be considered in small field dosimetry.

Small field dosimetry correction factors for circular and MLC shaped fields with the CyberKnife M6 System: evaluation of the PTW 60023 microSilicon detector

The PTW 60023 microSilicon is a new unshielded diode detector for small-field photon dosimetry. It provides improved water equivalence and a slightly larger sensitive region diameter in comparison to previous diode detectors in this range. In this study we evaluated the correction factors relevant to commissioning a CyberKnife System with this detector by Monte Carlo simulation and verified this data by multi-detector measurement comparison. The correction factors required for output factor determination were substantially closer to unity at small field sizes than for previous diode versions (e.g. = 0.981 at 5 mm field size which compares with corrections of 5%–6% with other stereotactic diodes). Because of these differences we recommend that corrections to small field output factor measurements generated specifically for the microSilicon detector rather than generic data taken from other diode types should be used with this new detector. For depth-dose measurements the microSilicon is consistent with a microDiamond detector to <1% (global), except at depths <10 mm where the diode gives a significantly lower measurement, by 6%–8% at the surface. For profile measurements, the microSilicon requires negligible corrections except in the low dose region outside the beam, where it underestimates off-axis-ratio (OAR) for small fields and overestimates for large fields. Where this effect is most noticeable at the largest field size and depth (115 mm × 100 mm and 300 mm depth) the microSilicon overestimates OAR by 2.3% (global) in the profile tail. This is consistent with other unshielded diodes.

Evaluation of the IAEA-TRS483 protocol for the dosimetry of small fields (square and stereotactic cones) using multiple detectors.

The IAEA TRS 483 protocol1 for the dosimetry of small static fields in radiotherapy was used to calculate output factors for the Elekta Synergy linac at the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA). Small field output factors for both square and circular fields were measured using nine different detectors. The “corrected” output factors (ratio of detector readings multiplied by the appropriate correction factor from the protocol) showed better consistency compared to the “uncorrected” output factors (ratio of detector readings only), with the relative standard deviation decreasing by approximately 1% after the application of the relevant correction factors. Comparisons relative to an arbitrarily chosen PTW 60019 microDiamond detector showed a reduction of maximal variation for the corrected values of approximately 3%. A full uncertainty budget was prepared to analyze the consistency of the output factors. Agreement within uncertainties between all detectors and field sizes was found, except for the 15 mm circular field. The results of this study show that the application of IAEA TRS 4831 when measuring small fields will improve the consistency of small field measurements when using multiple detectors contained within the protocol.

Small-field dosimetry of TrueBeamTM flattened and flattening filter-free beams: A multi-institutional analysis.

PurposeDetector‐dependent interinstitutional variations of the beam data may lead to uncertainties of the delivered dose to patients. Here we evaluated the inter‐unit variability of the flattened and flattening filter‐free (FFF) beam data of multiple TrueBeam (Varian Medical Systems) linear accelerators focusing on the small‐field dosimetry.MethodsThe beam data of 6‐ and 10‐MV photon beams with and without flattening filter measured for modeling of an iPLAN treatment planning system (BrainLAB) were collected from 12 institutions — ten HD120 Multileaf Collimator (MLC) and two Millennium120 MLC. Percent‐depth dose (PDD), off‐center ratio (OCR), and detector output factors (OF det) measured with different detectors were evaluated. To investigate the detector‐associated effects, we evaluated the inter‐unit variations of the OF det before and after having applied the output correction factors provided by the International Atomic Energy Agency (IAEA) Technical Reports Series no. 483.ResultsPDD measured with a field size of 5 × 5 mm2 showed that the data measured using an ionization chamber had variations exceeding 1% from the median values. The maximum difference from median value was 2.87% for 10 MV photon beam. The maximum variations of the penumbra width for OCR with 10 × 10 mm2 field size were 0.97 mm. The OF det showed large variations exceeding 15% for a field size of 5 × 5 mm2. When the output correction factors were applied to the OF det, the variations were greatly reduced. The relative difference of almost all field output factors were within ± 5% from the median field output factors.ConclusionIn this study, the inter‐unit variability of small‐field dosimetry was evaluated for TrueBeam linear accelerators. The variations were large at a field size of 5 × 5 mm2, and most occurred in a detector‐dependent manner.

Output correction factors for small static fields in megavoltage photon beams for seven ionization chambers in two orientations - perpendicular and parallel.

PurposeThe goal of the present work was to provide a large set of detector‐specific output correction factors for seven small volume ionization chambers on two linear accelerators in four megavoltage photon beams utilizing perpendicular and parallel orientation of ionization chambers in the beam for nominal field sizes ranging from 0.5 cm2 × 0.5 cm2 to 10 cm2 × 10 cm2. The present study is the second part of an extensive research conducted by our group.MethodsOutput correction factors kQclin,Qreffclin,fref were experimentally determined on two linacs, Elekta Versa HD and Varian TrueBeam for 6 and 10 MV beams with and without flattening filter for nine square fields ranging from 0.5 cm2 × 0.5 cm2 to 10 cm2 × 10 cm2, for seven mini and micro ionization chambers, IBA CC04, IBA Razor, PTW 31016 3D PinPoint, PTW 31021 3D Semiflex, PTW 31022 3D PinPoint, PTW 31023 PinPoint, and SI Exradin A16. An Exradin W1 plastic scintillator and EBT3 radiochromic films were used as the reference detectors.ResultsFor all ionization chambers, values of output correction factors kQclin,Qreffclin,fref were lower for parallel orientation compared to those obtained in the perpendicular orientation. Five ionization chambers from our study set, IBA Razor, PTW 31016 3D PinPoint, PTW 31022 3D PinPoint, PTW 31023 PinPoint, and SI Exradin A16, fulfill the requirement recommended in the TRS‐483 Code of Practice, that is, 0.95<kQclin,Qreffclin,fref<1.05, down to the field size 0.8 cm2 × 0.8 cm2, when they are positioned in parallel orientation; two of the ionization chambers, IBA Razor and PTW 31023 PinPoint, satisfy this condition down to the field size of 0.5 cm2 × 0.5 cm2.ConclusionsThe present paper provides experimental results of detector‐specific output correction factors for seven small volume ionization chambers. Output correction factors were determined in 6 and 10 MV photon beams with and without flattening filter down to the square field size of 0.5 cm2 × 0.5 cm2 for two orientations of ionization chambers — perpendicular and parallel. Our main finding is that output correction factors are smaller if they are determined in a parallel orientation compared to those obtained in a perpendicular orientation for all ionization chambers regardless of the photon beam energy, filtration, or linear accelerator being used. Based on our findings, we recommend using ionization chambers in parallel orientation, to minimize corrections in the experimental determination of field output factors. Latter holds even for field sizes below 1.0 cm2 × 1.0 cm2, whenever necessary corrections remain within 5%, which was the case for several ionization chambers from our set.TRS‐483 recommended perpendicular orientation of ionization chambers for the determination of field output factors. The present study presents results for both perpendicular and parallel orientation of ionization chambers. When validated by other researchers, the present results for parallel orientation can be considered as a complementary dataset to those given in TRS‐483.

Verification of representative data for output factors of SRS cones utilizing IAEA TRS 483 recommendations

Measurements of small fields continue to be a clinical challenge despite the recent work done to identify their characteristics. Due to this challenge, many physicists use representative data supplied by their vendors to verify their own measurements for small field output factors. However, with recent guidelines being released in IAEA TRS 483, the question remains if this representative data provides an accurate representation for small field dosimetry. A Sun Nuclear EDGE detector, PTW 60012 stereotactic diode, and GafChromic EBT3 films were used to measure the output factor for a set of Varian SRS cones (4 mm–17.5 mm diameters) on a TrueBeam linear accelerator. The measured output factors were then compared to the Varian provided SRS representative data. IAEA TRS 483 recommendations for measuring small field output factors were applied and the impact of those recommendations were examined. The EDGE detector showed good agreement with the representative data when correction factors were not applied (0.01%–1.64% difference) but the PTW 60012 diode showed larger deviation (0.61%–3.35% difference). The EBT3 film showed the largest difference with the representative data (0.66%–9.19%). After application of IAEA TRS 483 detector specific correction factors the output factors measured by the diodes showed good agreement with the EBT3 film for 6MV (<1.8% difference) but showed a large deviation with the representative data (up to 9% difference). The 6FFF energy output factors agreed between the EDGE, the PTW 60012, and EBT3 Film. This work shows that the use of uncorrected representative data on the Truebeam can lead to a significant over estimation of the SRS cone output factors.

Reconstruction of the electron source intensity distribution of a clinical linear accelerator using in-air measurements and a genetic algorithm.

Background and purposeThe electron source intensity distribution of a clinical linear accelerator has a great influence on the calculation of output factors for small radiation fields where source occlusion by the collimating devices takes place. The purpose of this study was to present a new method for the electron source reconstruction problem.Materials and methodsThe measurements were performed in-air using diode and 6 MV 1 × 1 cm2 photon field in flattening filter-free mode. In Monte Carlo simulation, an electron target area was divided into a number of square subsources. Then, the in-air doses in 2D silicon chip array were calculated individually from each subsource. A genetic algorithm search was applied in order to determine the optimal weight factors for all subsources that provide the best agreement between simulated and measured doses.ResultsIt was found that the reconstructed electron source intensity from a clinical linear accelerator has the two-dimensional elliptical double Gaussian distribution. The source intensity distribution consisted of two intensity components along the in-plane (x) and cross-plane (y) directions characterized by full width half-maximum (FWHM): FWHMx1 = 0.27 cm, FWHMx2 = 0.08 cm, FWHMy1 = 0.24 cm, FWHMy2 = 0.06 cm, where broader components are 81% and 53% of the total intensity along × and y axis respectively.ConclusionsThe obtained results demonstrated an elliptical double Gaussian intensity distribution of the incident electron source. We anticipate that the proposed method has universal applications independent of the type of linear accelerator, modality or energy.

Quantifying the ionization quenching effect in organic plastic scintillators used in MV photon dosimetry

Miniature organic plastic scintillators attached to optical fiber cables have found important uses in medical dosimetry, in particular for small-field dosimetry and field output factor measurements in megavoltage linear accelerator beams. Plastic scintillators are well suited for these applications due to their high degree of water equivalence and their small size. Plastic scintillators, however, are known to suffer from ionization quenching, meaning that the light production per absorbed dose decreases if the dose deposition is caused by low energy electrons. As MV beams also lead to dose deposition by low energy electrons, ionization quenching will also affect, for example, field output factor measurements in MV beams if the fraction of dose delivered by low energy electrons changes with field size or beam quality. While the influence of ionization quenching on scintillator dosimetry in beams of heavy charge particles or kV x-ray has been subject to much research, the effect in MV beams is less well studied, and it is often implicitly assumed that there is no effect. The purpose of this work therefore was to quantify the influence of ionization quenching on plastic scintillators, specifically BCF-60, for the two application in MV photon dosimetry: (i) field output factor measurements for field sizes from 0.5×0.5 cm2 to 10×10 cm2 for a 6 MV beam and (ii) ionization chambers beam quality correction factor (kQ-factors) measurements for beams between 4 MV and 15 MV at a fixed 10×10 cm2 field size. The quenching was quantified using Monte Carlo modelling of the MV beams, a variation of Birks formalism that accounted for the detailed dose deposition by secondary electrons, and quenching parameters established previously using experiments in kV x-ray beams. This enabled the computation of the light yield in a “quenching free” scintillator and therefore a computation of a correction factor due to ionization quenching. Ionization quenching was found to have a small but statistically significant influence on both of the studied applications: (0.6±0.2)% for the field output factor measurements between 0.5×0.5 cm2 and 10×10 cm2 and about (2±0.4)% for the kQ-factor application for beams between 4 MV and 15 MV. The modelling results were in agreement with experimental measurements. The results support that the ionization quenching effect has a small effect on field-output factor measurements and it can probably be neglected during clinical measurements. For direct scintillator-based measurements of beam quality correction factors for ionization chambers, ionization quenching is larger and this effect needs to be accounted for if anyone wants to measure or verify ionization chamber kQ-factors using organic plastic scintillators.

Effect of beam configuration with inaccurate or incomplete small field output factors on the accuracy of treatment planning dose calculation.

To evaluate the effect of beam configuration with inaccurate or incomplete small field output factors on the accuracy of dose calculations in treatment planning systems. Output factors were measured using various detectors and for a range of field sizes. Three types of treatment machines were configured in two treatment planning systems. In the first (corrected) machine, the Exradin W1 scintillator was used to determine output factors. In the second (uncorrected) machine, the measured output factors by the A1SL ion chamber without considering output correction factors for small field sizes were utilized. In the third (clinical) machine, measured output factors by the Exradin W1 were used but not for field sizes smaller than 2×2cm2 . The dose computed by the anisotropic analytical algorithm (AAA), Acuros XB (AXB) and collapsed cone convolution/superposition (CCC) algorithms in the three machines were delivered using static (jaw-, MLC-, and jaw/MLC-defined), and composite [intensity modulated radiation therapy (IMRT), volumetric modulated arc therapy (VMAT)] fields. The differences between measured and calculated dose values were analyzed. For static fields, the percentage differences between measured and calculated doses by the three algorithms in three configured machines were <2% for field sizes larger than 2×2cm2 . In jaw- and jaw/MLC-defined fields smaller than 2×2cm2 , the corrected machine presented better agreement with measurement. Considering output correction factors in MLC-defined fields, among the three configured machines, the accuracy of calculation improved to within ±0.5%. For MLC-defined field size of 1×1cm2 , AXB showed the smallest percentage difference (1%). In IMRT and VMAT plans, the percentage differences between measured and calculated doses at the isocenter, as well as the gamma analysis of different plans, which include field sizes larger than 3×3cm2 , did not vary noticeably. For smaller field sizes, using the corrected machine influences dose calculation accuracy. Configuration with corrected output factors improves accuracy of dose calculation for static field sizes smaller than 2×2cm2 . For very small fields, the robustness of the dose calculation algorithm affects the accuracy of dose as well. In IMRT and VMAT plans, which include small subfields, the size of the jaw-defined field is an important factor and using corrected output factors increases dose calculation accuracy.

Multi-site evaluation of the Razor stereotactic diode for CyberKnife small field relative dosimetry

PurposeThe aims of this study were: (i) to validate in a multi-site context the suitability of the IBA Razor silicon diode detector for CyberKnife relative dosimetry. (ii) to fit the multi-center experimental data into a function relating the field output factors to the effective field size (EFS). Methods and materialsRatio of detector readings in clinical and reference field (OFdet) and beam profiles were acquired on five CyberKnife units for fixed collimator diameters (range 5–60 mm), using both Razor and PTW 60017 diodes. Measured OFdet were corrected using published MonteCarlo correction factors to get field output factors ΩQclin,Qmsrfclin,fmsr. Profiles were analyzed in terms of penumbra and EFS. ΩQclin,Qmsrfclin,fmsr obtained in four centers were fitted as a function of EFS, while the data of the 5th center were used to validate the fitting curve. ResultsDifferences between Razor and PTW60017 ΩQclin,Qmsrfclin,fmsr were within 1.5% over all centers down to 7.5 mm aperture and within 3.5% for the 5 mm diameter. The fit showed a coefficient of determination R2 = 0.997. The mean deviation of measured points from the predictive curve was within 0.5%. Data of the 5th center showed a mean deviation of 0.4% from the curve, with maximum differences within 2.5% for the 7.5 mm aperture. ConclusionsThe results confirmed the suitability of Razor detector for CyberKnife dosimetry by comparison to the PTW 60017 diode which has been well characterized and is in widespread use. The proposed mathematical relation between ΩQclin,Qmsrfclin,fmsr and EFS is a robust predictive model applicable to different CyberKnife systems and detectors.

The characteristics of small field beam quality and output factor of 6 MV FFF

Flattening filter free linear accelerator (FFF LINAC) has been installed in Indonesia. To ensure the accuracy of FFF irradiation, we evaluate the characteristic of FFF in regular and small fields. We employed the Monte Carlo (MC) simulation of FFF LINAC 6 MV as a standard reference in this study. Then, we compared the calculation of FFF beam at central and lateral axis to the measurement and TPS at 10 × 10, 1 × 1, 2 × 2, 3 × 3, 4 × 4 cm2 field sizes. Output factor (OF) and beam quality such that TPR20,10 and penumbra of FFF LINAC were evaluated. TPR20,10(S) for 10 × 10 cm2 was 0.01 differ between MC and measurement whereas it varied from 0.588 to 0.703 for small fields. Dose difference of lateral axis was agreed within 3% except for the penumbra region. Output factor of small fields measurement indicated that field size of 1 × 1 cm2 had a large discrepancy to MC according to this works. The TPR20,10(10) of MC, TPS, and measurement was not a significant difference, while the OF was tending to a large deviation in 1 × 1 cm2. The results showed that our FFF LINAC could be used for small field until 2 × 2 cm2.

Evaluating small field dosimetry with the Acuros XB (AXB) and analytical anisotropic algorithm (AAA) dose calculation algorithms in the eclipse treatment planning system

AbstractBackground:An increasing number of external beam treatment modalities including intensity modulated radiation therapy, volumetric modulated arc therapy (VMAT) and stereotactic radiosurgery uses very small fields for treatment planning and delivery. However, there are major challenges in small photon field dosimetry, due to the partial occlusion of the direct photon beam source’s view from the measurement point, lack of lateral charged particle equilibrium, steep dose-rate gradient and volume averaging effect of the detector response and variation of the energy fluence in the lateral direction of the beam. Therefore, experimental measurements of dosimetric parameters such as percent depth doses (PDDs), beam profiles and relative output factors (ROFs) for small fields continue to be a challenge.Materials and Methods:In this study, we used a homogeneous water phantom and the heterogeneous anthropomorphic stereotactic end-to-end verification (STEEV) head phantom for all dose measurements and calculations. PDDs, lateral dose profiles and ROFs were calculated in the Eclipse Treatment Planning System version 13·6 using the Acuros XB (AXB) and the analytical anisotropic algorithms (AAAs) in a homogenous water phantom. Monte Carlo (MC) simulations and measurements using the Exradin W1 Scintillator were also accomplished for four photon energies: 6 MV, 6FFF, 10 MV and 10FFF. Two VMAT treatment plans were generated for two different targets: one located in the brain and the other in the neck (close to the trachea) in the head phantom (CIRS, Norfolk, VA, USA). A Varian Truebeam linear accelerator (Varian, Palo Alto, CA, USA) was used for all treatment deliveries. Calculated results with AXB and AAA were compared with MC simulations and measurements.Results:The average difference of PDDs between W1 Exradin Scintillator measurements and MC simulations, AAA and AXB algorithm calculations were 1·2, 2·4 and 3·2%, respectively, for all field sizes and energies. AXB and AAA showed differences in ROF of about 0·3 and 2·9%, respectively, compared with W1 Exradin Scintillator measured values. For the target located in the brain in the head phantom, the average dose difference between W1 Exradin Scintillator and the MC simulations, AAA and AXB were 0·2, 3·2 and 2·7%, respectively, for all field sizes. Similarly, for the target located in the neck, the respective dose differences were 3·8, 5·7 and 3·5%.Conclusion:In this study, we compared dosimetric parameters such as PDD, beam profile and ROFs in water phantom and isocenter point dose measurements in an anthropomorphic head phantom representing a patient. We observed that measurements using the W1 Exradin scintillator agreed well with MC simulations and can be used efficiently for dosimetric parameters such as PDDs and dose profiles and patient-specific quality assurance measurements for small fields. In both homogenous and heterogeneous media, the AXB algorithm dose prediction agrees well with MC and measurements and was found to be superior to the AAA algorithm.

Characterization of the plastic scintillation detector Exradin W2 for small field dosimetry.

Small field dosimetry has been an active area of research for over a decade. It is now known that large dosimetric errors can be introduced if proper detectors or correction factors are not used. The International Atomic Energy Agency (IAEA) through the technical report series No. 483 provides guidelines for small field dosimetry procedures as well as correction factors for most detectors available in the market. The plastic scintillator detector (PSD) Exradin W1 has been found to have a correction factor close to unity; however, it is not designed for beam scanning. To overcome this limitation, the new PSD Exradin W2 has been developed to be used as a scanning as well as a relative dosimeter. Characterization of this detector in small field dosimetry is presented in this study. A 6MV beam from a Varian-Edge linac was used to collect data for the characterization of a W2 detector. Cerenkov light ratio (CLR) is corrected through a separate new electrometer system that comes with the W2 detector. The parameters investigated include the dose and dose rate linearity, beam profiles, percent depth dose (PDD), field output factors, and temperature response. The results were compared with Gafchromic film (EBT-3 film) for beam profiles. The field output factor and temperature response were compared to the Exradin W1 detector. The dose linearity measured with 600MU/min dose rate showed minimal variations (<0.5%) even for small MU, and similar results were seen for dose rate linearity. The comparison of field output factors between the W2 and W1 showed small differences for various depth and field sizes. The temperature response showed small variation when the temperature was varied from to . The slope was and for the W2 and the W1 detector, respectively. The differences in profiles are 0.5% in umbra and penumbra region for field size when compared to the EBT-3 film profile. The W2 scintillator detector showed similar dosimetric and temperature properties to the W1 scintillator detector. The main advantage of the W2 detector among other plastic scintillators is the beam scanning capabilities that, combined with its correction factor of 1.0, make it an ideal detector for commissioning of SRS and SBRT techniques.

A novel method for the determination of field output factors and output correction factors for small static fields for six diodes and a microdiamond detector in megavoltage photon beams.

PurposeThe goal of this work is to provide a large and consistent set of data for detector‐specific output correction factors, kQclin,Qreffclin,fref, for small static fields for seven solid‐state detectors and to determine field output factors, ΩQclin,Qreffclin,fref, using EBT3 radiochromic films and W1 plastic scintillator as reference detectors on two different linear accelerators and four megavoltage photon beams. Consistent measurement conditions and recommendations given in the International Code of Practice TRS‐483 for small‐field dosimetry were followed throughout the study.Methods ΩQclin,Qreffclin,fref were determined on two linacs, Elekta Versa HD and Varian TrueBeam, for 6 and 10 MV beams with and without flattening filter and for nine fields ranging from 0.5 × 0.5 cm2 to 10 × 10 cm2. Signal readings obtained with EBT3 radiochromic films and W1 plastic scintillator were fitted by an analytical function. Volume averaging correction factors, determined from two‐dimensional (2D) dose matrices obtained with EBT3 films and fitted to bivariate Gaussian function, were used to correct measured signals. kQclin,Qreffclin,fref were determined empirically for six diodes, IBA SFD, IBA Razor, PTW 60008 P, PTW 60012 E, PTW 60018 SRS, and SN EDGE, and a PTW 60019 microDiamond detector.ResultsField output factors and detector‐specific kQclin,Qreffclin,fref are presented in the form of analytical functions as well as in the form of discrete values. It is found that in general, for a given linac, small‐field output factors need to be determined for every combination of beam energy and filtration (WFF or FFF) and field size as the differences between them can be statistically significant (P < 0.05). For different beam energies, the present data for kQclin,Qreffclin,fref are found to differ significantly (P < 0.05) from the corresponding data published in TRS‐483 mostly for the smallest fields (<1.5 cm). For the PTW microDiamond detector, statistically significant differences (P < 0.05) between kQclin,Qreffclin,fref values were found for all investigated beams on an Elekta Versa HD linac for field sizes 0.5 × 0.5 cm2 and 0.8 × 0.8 cm2. Significant differences in kQclin,Qreffclin,fref between beams of a given energy but with and without flattening filters are found for measurements made in small fields (<1.5 cm) at a given linac. Differences in kQclin,Qreffclin,fref are also found when measurements are made at different linacs using the same beam energy filtration combination; for the PTW microDiamond detector, these differences were found to be around 6% and were considered as significant.ConclusionsSelection of two reference detectors, EBT3 films and W1 plastic scintillator, and use of an analytical function, is a novel approach for the determination of ΩQclin,Qreffclin,fref for small static fields in megavoltage photon beams. Large set of kQclin,Qreffclin,fref data for seven solid‐state detectors and four beam energies determined on two linacs by a single group of researchers can be considered a valuable supplement to the literature and the TRS‐483 dataset.

A method of measuring the field output factor based on daisy–chaining

Objective improve the accuracy of the measurement results by using the field output factor measurement method based on daisy-chaining. Methods The Varian Edge Accelerator 6 MV X-ray data were measured using the IBA CC13 ionization chamber, IBA CC01 ionization chamber, IBA Razor semiconductor detector, IBA EFD semiconductor detector and Gafchromic EBT3 film, respectively. Results Compared with the daisy-chaining measurement method, the results obtained by the conventional measurement method using CC13 were smaller. The deviation value was 16.71% in the 1 cm × 1 cm field. The measurement results in a larger field via CC01 were bigger with a deviation of 8.39% in the 40 cm × 40 cm filed. The measurement results via Razor in a larger field were larger with a deviation of 9.40% in the 40 cm × 40 cm field. The measurement results were similar between EFD and Razor with a deviation of 9.14% in the 40 cm × 40 cm field. The results of the film measurement were equivalent to those obtained from the daisy-chaining method in a field of> 1 cm × 1 cm with a deviation within 1.60%, whereas the deviation was increased to 3.13% in the 1 cm × 1 cm field. The results were consistent with daisy-chaining measurement if the 3 cm × 3 cm or 4 cm × 4 cm fields were selected as the intermediate fields with the maximum deviation of 0.29%. Conclusions For the detectors with changing response along with the field size, daisy-chaining measurement method can be utilized to extend the measurement range and improve the accuracy of the measurement results. Key words: Daisy-Chaining; Output factor; Measurement method

Quantification the dosimetric parameters of asymmetric physical wedged-6MV photon beam

Introduction: Physical wedge as a useful tool has been utilized in radiotherapy to modify photon beam shape and intensity such that it distributes dose uniformly in tumor site and reduces hot points. Since during Linac commissioning dosimetric parameters like output factors and lateral dose profiles are measured only for symmetric open and wedged fields, so calculation the parameters for asymmetric wedged fields become necessary. The study aims to achieve output factors and dose profiles for symmetric and asymmetric wedged fields of 6 MV photon beams. Materials and Methods: The Siemens PRIMUS Linac head for 6 MV photon beam was simulated by BEAMnrc and all dose calculations were performed by DOSXYZnrc code. Percentage depth dose (PDD) and profiles for open and wedged (15° and 45°) fields were compared with corresponding measurements. Wedge factors for 10 x 10 cm2 field size as a function of lateral distance from central axis were obtained as well for half beam (negative or positive Y jaws were closed) wedged fields. All mesurments were carried out using 0.125 cm3 Farmer type ionization chamber with DOSE1 electrometer (FC65G, Scanditronix-Wellhofer, Germany) at the depth of 10 cm with source to surface distance (SSD) of 100 cm in 50 cm3 PTW-Blue water phantom and processed by dosimetry software RFAplus (Version 5.2, Scanditronix-Wellhofer, Germany). Results: The calculated doses were in agreement with measured data that confirmed the simulated MC model. The output factors on the central axis of symmetric wedged beams decreased to 0.693 and 0.307 for 15˚ and 45˚ wedges, respectively. The total photon fluence of 15˚ and 45˚ physical wedged fields reduced to 71.6% and 27.7% of fluence for open field, respectively. It was found that the output factor for asymmetric wedged fields is lower than ones from symmetric fields, particularly at field edges. Conclusion: The presences of physical wedge across the beamline decrease the photon fluence and cases beam hardening both for symmetric and asymmetric wedged fields. Lack of scattering photons near the half beam edges in asymmetric wedged field causes a dose fall off in these regions where have potential to overestimate the calculated dose by treatment planning system and consequent cold spots at target volume. Our calculated correction coefficients are recommended to calculate doses by TPS in case of asymmetric wedged fields.