Small Field Measurements Research Articles

Determination of the penumbra width of Elekta SRS cone collimator for 6 MV FF and 6 MV FFF energies using gradient-based edge detection

The dosimetric measurements of small fields of radiation are undertaken with detectors such as: GafchromicTM films, diodes, diamond detectors and ionization chambers of small volumes. The penumbra width, e.g. the spatial distance between 80% and 20% dose, is smaller in small fields. This fact increases the curvature and hence the volume effect in the penumbra region. The accuracy in the penumbra calculation is important at the QA test for implementation of the cranial Stereotactic Radiosurgery. The objective of this work was to apply a Gradient-Based Edge Detection plugin to calculate the penumbra of a radiation beam defined with the Elekta SRS Cone Collimators for beams of 6 MV (Flattening Filter – FF) and 6 MV (Flattening Filter Free – FFF) energies using the beam profile obtained with GafchromicTM film. The results corroborate those from the literature, and they allow a quantitative evaluation of the width of dosimetric penumbra by Gradient-Based Edge Detection.

Commissioning and testing of treatment planning system model of an Edge accelerator with a high-definition multi-leaf collimator

Objective To investigate the commissioning and testing of the Eclipse model of an Edge accelerator with high-definition multi-leaf collimator (HD-MLC). Methods The percentage depth dose (PDD), profile, output factor measured by Razor and CC13 were statistically compared with the standard data. Penumbra, transmission factor (TF), leakage, concave-convex groove, accuracy of movement and dosimetry leaf gap (DLG) were measured with EBT3, electronic portal image device (EPID) and PTW SRS1000&SRS1500. The optimal DLG/TF was acquired when the γ pass rate of test cases was the highest. The point dose of regular fields, intensity-modulated radiation therapy (IMRT) and volume-modulated radiation therapy (VMAT) was verified with FC65-G. The planar dose of these case was verified with Octavius 4D and EBT3. Results The measured PDD data were consistent with the standard data. The measured penumbra of 3 cm and 4 cm square fields was smaller, whereas that of 6 cm square field was larger than the standard values. The left and right edge, field size, center of the field were distributed within the range of -1.0-0.4 mm、0.2-1.7 mm, -0.3-1.9 mm and -0.1-0.8 mm, respectively. The mean penumbra of the left and right MLC in different positions were (2.5±0.042) mm and (2.7±0.005) mm. The leakage of MLC was 0.009-0.016. The measured DLG/TF was 0.1861 cm/0.0116 and the optimal DLG/TF was 0.015 cm/0.014. The differences of point dose of all the test cases except the one which was in the low-dose area were within ±3%. Local and global γ pass rates of all IMRT were 79.81%-100% and 96.3%-100% (3%/3 mm), 71.3%-98.9% and 94.3%-99.8% for VMAT cases. Conclusions This method can accurately test and commission the Eclipse treatment planning model of Edge Linac equipped with HD-MLC. Key words: Commission of treatment planning model; Small-field measurement; HD-MLC feature parameter testing; Edge Linac

Optimizing beam models for dosimetric accuracy over a wide range of treatments

This work presents a systematic approach for testing a dose calculation algorithm over a variety of conditions designed to span the possible range of clinical treatment plans. Using this method, a TrueBeam STx machine with high definition multi-leaf collimators (MLCs) was commissioned in the RayStation treatment planning system (TPS). The initial model parameters values were determined by comparing TPS calculations with standard measured depth dose and profile curves. The MLC leaf offset calibration was determined by comparing measured and calculated field edges utilizing a wide range of MLC retracted and over-travel positions. The radial fluence was adjusted using profiles through both the center and corners of the largest field size, and through measurements of small fields that were located at highly off-axis positions. The flattening filter source was adjusted to improve the TPS agreement for the output of MLC-defined fields with much larger jaw openings. The MLC leaf transmission and leaf end parameters were adjusted to optimize the TPS agreement for highly modulated intensity-modulated radiotherapy (IMRT) plans. The final model was validated for simple open fields, multiple field configurations, the TG 119 C-shape target test, and a battery of clinical IMRT and volumetric-modulated arc therapy (VMAT) plans. The commissioning process detected potential dosimetric errors of over 10% and resulted in a final model that provided in general 3% dosimetric accuracy. This study demonstrates the importance of using a variety of conditions to adjust a beam model and provides an effective framework for achieving high dosimetric accuracy.

Community approach for reducing small field measurement errors: Experience over 24 centres

PurposeThe complexity of the modern Stereotactic Body Radiation Therapy (SBRT) techniques requires comprehensive quality assurance programs, to ensure the right treatment to the patient. Dosimetry of small radiation fields is a challenge especially for radiotherapy centres starting to work on this issue. The matter to be discussed here concerns the need of detailed measurement procedures and cross checks to be paired to the usual recommendations on detectors and correction factors. Materials and MethodsThe presented work involved 24 Italian radiotherapy centres, with the specific purpose to minimize systematic errors in output factor measurements over different radiotherapy centres. Using the unshielded silicon diode IBA Razor, reference curves for the relative signal ratio (RSR) as a function of beam size were created for each Linac family. ResultsWith this study we have demonstrated consistency of small field dosimetry on all the centres involved, moreover all radiotherapy centres using Razor are allowed to compare measurements amongst each other and centres with values deviating more than 5% from the reference curve are advised to repeat their measurements. With this procedure, some critical issues were detected from two centres in RSR measurements, that, if implemented into the own treatment planning system, would induce an unwanted overdosage larger than 5%. ConclusionsThe proposed approach could allow one to envision high-skilled therapy centres providing support to those featuring minor experience and could represent an important strategy for the clinical implementation of emerging technologies at high quality levels. The methodology adopted exploits crowd knowledge methods which could be applied in others areas of radiation dosimetry.

Characterization of a CMOS sensor array for small field fluence measurement of a low energy proton beam

Small fields and sharp dose gradients typically encountered in proton irradiation for preclinical research require high spatial resolution dosimetric measurements, especially below 10 mm beam diameter. This study aims to characterize a complementary metal oxide semiconductor (CMOS) sensor for fluence measurement of small radiation fields (2 mm, 5 mm and 10 mm diameter) delivered by a 25 MeV proton beam dedicated to preclinical irradiations. The beam fluence was characterized in air with a MIMOSA28 CMOS sensor and EBT3 films and compared to Geant4 simulations. The measured flatness for the 10 mm field was found within ±2.5 % with both the CMOS sensor and EBT3 films. Lateral profiles were in agreement within a distance of 0.1 mm between the two detectors and Geant4 simulations. Also, with a fixed particle rate, the fluence measured by the CMOS sensor was constant at the plateau of the Bragg curve, regardless the field size, while EBT3 showed a 10% increased fluence for the 2 mm collimated beam. The CMOS sensor demonstrated a high spatial resolution and its fluence-response is independent of the radiation field sizes that seems promising for future developments in dosimetry of small field proton beams for precise irradiation platforms dedicated to preclinical research, as well as in nuclear research and laser accelerated particle beam characterization

OA237] The output factor measurements of cyberknife system using various detectors

Purpose In Stereotactic Radiosurgery, the extremely high radiation dose is delivered to the target in a single or few fractions using narrow radiation fields. The radiation fields are collimated with 12 fixed circular collimators ranging from 5 to 60 mm in diameter in CyberKnife which is a robotic radiosurgery system. The accuracy of the output factors directly affects the accuracy of treatment delivery to the patients. The output factor (OF) measurement of small fields is very problematic and the small field measurements are more sensitive to physical properties of detectors. The aim of the present study is to perform the relative OF measurements of 12 fixed collimators in CyberKnife system using different detectors. Methods In this study, the OF measurements of 12 fixed collimators were made with CyberKnife system at maximum dose depth in water equivalent RW3 slab phantom using PTW 60019 microDiamond, PTW 31014 0.015 cc PinPoint, PTW 31010 0.125 cc Semiflex, EBT3 Gafchromic film and GR-200A TLD. All detectors were positioned at the center of the radiation field with the point laser from treatment head. The measured data were compared with Monte Carlo (MC) factors. Results The OF values measured at 30 mm and greater collimator sizes were found similar for all detectors. The OF values decrease rapidly as the collimator size becomes smaller. The measured data with TLD and microDiamond detector at the smallest collimator sizes such as 7.5, 10, 12.5 and 15 mm were found very similar with MC factors. Conclusions 0.125 cc Semiflex ion chamber may lead to large uncertainties in the OF measurements for the smallest collimator sizes. TLD and microDiamond detector show similar results for all collimator sizes except 5 mm. TLD is an utilizable dosimetric tool in the small field OF measurements by means of its small active volume.

P265] Small field dosimetry comparison of 6 MV beam profiles aquired with diodesrs vs 0.125cc ionization chamber

Purpose Compare TPS calculated profiles based on 0.125 cc ionization chamber measurements for linac commissioning with beam profiles acquired using IAEA small beam recommendations with DiodeSRS. Methods At Eclipse™ 11.0 we calculated dose distribution for 2 × 2 cm, 3 × 3 cm and 4 × 4 cm MLC collimated fields at 100 cm SSD with AAA algorithm based on data acquired with 0.125cc PTW ionization chamber. Using TPS dose profile tool, we got the dose values at 10 cm depth. 2 × 2 cm, 3 × 3 cm and 4 × 4 cm MLC collimated field profiles with SSD = 100 cm at 10 cm depth, were acquired following IAEA Technical Reports Series N° 483, in water phantom using DiodeSRS PTW and Mephysto™ mc2 PTW. Transverse beam profiles were compared measuring the full with at half maximum (FWHM). Results For each field, we found a decrease in the FWHM between 2% and 10% for the DiodeSRS measurements. Conclusions Using the IAEA protocol for small field measurements we found that DiodeSRS detectors are more accurate measuring small field relative dosimetry. We intend to recommission our linac small fields according to IAEA report and evaluate the impact on our IMRT/VMAT plans quality assurance.

A high resolution 2D array detector system for small-field MRI-linac applications

A monolithic silicon small-field array detector is proposed for relative dosimetry applications in hybrid MRI-linac systems. The detector has high sampling resolution, with 512 active elements arranged with 2 mm pitch over a 46 mm × 46 mm detection area. Experimental measurements were performed in a custom-designed permanent magnet device that is compatible with a standard clinical linear accelerator. It can be configured in both inline and perpendicular magnetic-field-to-photon-beam orientations and produces magnetic field strengths 0.95 T and 1.20 T, respectively. Monte Carlo simulation data, obtained using the GEANT4 toolkit, are presented to supplement experimental data. Beam profiles show agreement to EBT3 film within 0.5 mm for FWHM and penumbral width measurement of small square fields (width ranging from 0.75 cm to 2.25 cm), in both inline and perpendicular magnetic field orientations. The detector can be used to accurately resolve normalised beam profiles in magnetic fields. The impact of electron return effects (ERE) in a small air gap surrounding the detector was also quantified. For the perpendicular orientation, a reduced profile intensity was observed for an increasing air gap width above the detector (10% at 2 mm) due to ERE. In the inline orientation, a very small increase in response relative to the zero field case was observed with an air gap above the detector (2% at 2 mm). Calibration of the device in a magnetic field will therefore be necessary; the zero field calibration is non-transferable. The MagicPlate-512 provides a high-resolution real time alternative to accurately measure normalised beam profiles in magnetic fields, and is expected to be a suitable array detector for use in magnetic field environments typical of MRI-linacs.

EP-1946: Effect of small field (SF) output factor (OF) measurements on FSBRT VMAT plans

EP-1946: Effect of small field (SF) output factor (OF) measurements on FSBRT VMAT plans

Avaliação de modelos de footprint para análise de fluxos obtidos por Eddy-Covariance em pequenas-áreas

Two models for footprint calculations are compared employing flux measurements in the planetary boundary layer. The calculationsare based on the analytical models by Kormann e Meixner (2001) [An analytical footprint model for non-neutral stratification.Boundary-Layer Meteorology 99, 207–224] and by Schuepp et al. (1990) [Footprint prediction of scalar fluxes from analytical solutions of the difussion equation. Boundary-Layer Meteorology 50, 355-373]. The footprint density functions of a flux sensor are determined using eddy-covariance data. Those functions are integrated over surfaces given by quadrangular rectangles, in this case an agricultural field. This work ilustrates the features of each footprint model employing flux measurements with an eddy-covariance system of the SULFLUX network, installed on a agricultural field. Finally, it is presented the model that describes in a better way the flux measurements in small fields.

The long-term variation of the effective magnetic field of the active star ε Eridani

AbstractLong-term periodicities of magnetic fields in cool stars are usually studied from activity indicators, which are only indirectly related to the presence of the field. Direct detections are complicated issues since even a complex magnetic structure, as the solar one, has a very low disk integrated magnetic signal, which is usually hidden in the noise level. We introduce a method for the direct measurement of small integrated longitudinal stellar magnetic fields (effective magnetic fields), called multi-line slope method, based on the regression of the Stokes V signal with respect to the first derivative of Stokes I. We present the results of the application of this technique to a dataset of 9 yr of observations of the active star epsilon Eridani, obtained with the spectropolarimeters Narval, HARPSpol and CAOS, showing that the long-term variation of the effective magnetic field corresponds to the period of the cycle retrieved by the activity indicators.

Dosimetric characterization of Elekta stereotactic cones

PurposeDosimetry of small fields defined by stereotactic cones remains a challenging task. In this work, we report the results of commissioning measurements for the new Elekta stereotactic conical collimator system attached to the Elekta VersaHD linac and present the comparison between the measured and Monte Carlo (MC) calculated data for the 6 MV FFF beam. In addition, relative output factor (ROF) dependence on the stereotactic cone aperture variation was studied and penumbra comparison for small MLC‐based and cone‐based fields was performed.MethodsCones with nominal diameters of 15 mm, 12.5 mm, 10 mm, 7.5 mm, and 5 mm were employed in our study. Percentage depth dose (PDD), off‐axis ratios (OAR), and ROF were measured using a stereotactic field diode (SFD). BEAMnrc code was used for MC simulations.ResultsMC calculated and measured PDDs for all cones agreed within 1%/0.5 mm, and OAR profiles agreed within 1%/0.5 mm. ROF obtained from the measurements and MC calculations agreed within 2% for all cone sizes. Small‐field correction factors for the SFD detector Kfield,3 × 3(SFD) were derived using MC calculations as a baseline and were found to be 0.982, 0.992, 0.997, 1.015, and 1.017 for the 5, 7.5, 10, 12.5, and 15‐mm cones respectively. The difference in ROF was about 10%, 6%, 3.5%, 3%, 2.5%, and 2% for ±0.3 mm variations in 5, 7.5, 10, 12.5, and 15‐mm cone aperture respectively. In case of single static field, cone‐based collimation produced a sharper penumbra compared to the MLC‐based.ConclusionsAccurate MC simulation can be an effective tool for verification of dosimetric measurements of small fields. Due to the very high sensitivity of output factors on the cone diameter, manufacture‐related variations in cone size may lead to considerable variations in dosimetric characteristics of stereotactic cones.

Penumbra and Output Factors Measurements in Small and Non-Small Fields

Penumbra and Output Factors Measurements in Small and Non-Small Fields

Comparison of Measurements in Small Fields from an MR-LINAC

To investigate beam profiles for small square fields from a 1.5 T MRI – 7 MV linear accelerator system (MR-Linac) measured with Gafchromic film, Fricke gel films, and a microDiamond detector. EBT-3 radiochromic films were irradiated to 4 Gy at isocenter distance at 5 cm depth inside a water phantom. 2x2, 3x3, 5x5, and 10x10 cm2 fields were delivered by an MR-Linac while the magnetic field (B-field) was turned off. Fricke-type gels were manufactured in-house, poured into 5-mm thick sheets and stored at 4⁰C for 24 hours prior to irradiation. The gel sheets were placed at isocenter at 5 cm depth in a solid water phantom and irradiated to 4Gy for each field size. All gels and films were scanned with an Epson 10000XL flatbed scanner pre- and post-irradiation. In-plane and cross-plane beam profiles were measured for all four field sizes with a PTW60019 microDiamond detector at 5 cm depth inside a water phantom under the same conditions. In-plane and cross-plane profiles measured with the planar detectors were analyzed and compared to profiles obtained with the microDiamond detector. All measurements were repeated with the B-field turned on. With the B-field off, in-plane and cross-plane profiles measured with film agreed to approximately 1% with microDiamond measurements. Compared to the microdiamond data, the 10x10 cm2 cross-plane profile from film measurements indicated slightly more than 1% higher signal more than 3 cm away from the central axis. The 80/20 penumbra regions overlapped for both detectors. The shape of the cross-plane profiles measured with gel dosimetry conformed with both film and microDiamond. The gel measurements exhibited more signal variations inside the radiation field and reported lower doses in the penumbra. With the B-field on, the effect of the magnetic field was clearly seen. In-plane profiles appeared symmetric while cross-plane profiles were noticably asymmetric. Film and microDiamond measurements agreed within 1-2% for all four fields. The shape of the profiles measured with gels agreed with both film and microDiamond. In regions of non-uniform thickness the gels demonstrated variations in signal. Both microDiamond and EBT-3 film offer a valuable means to measure small fields with steep dose gradients from the MR-Linac. Maintaining good quality control during the fabrication process of gel sheets is crucial to obtain uniform sheet thickness. Further investigation of gel sheets will be pursued as an alternative to film measurements.

Small field characterization of a Nanochamber prototype under flattening filter free photon beams

IntroductionNanochambers present some advantages in terms of energy independence and absolute dose measurement for small field dosimetry in the SBRT scenario. Characterization of a micro-chamber prototype was carried out both under flattened and flattening-filter-free (FFF) beams with particular focus on stem effect. MethodsThe study included characterization of leakage and stem effects, dose rate and dose per pulse dependence, measurement of profiles, and percentage depth doses (PDDs). Ion collection efficiency and polarity effects were measured and evaluated against field size and dose per pulse. The 6_MV, 6_MV_FFF and 10_MV FFF beams of a Varian EDGE were used. Output factors were measured for field sizes ranging from 0.8×0.8cm2 to 20×20cm2 and were compared with other detectors. ResultsThe 2mm diameter of this chamber guarantees a high spatial resolution with low penumbra values. In orthogonal configuration a strong stem (and cable) effect was observed for small fields. Dose rate and dose per pulse dependence were <0.3% and 0.6% respectively for the whole range of considered values. The Nanochamber exhibits a field size (FS) dependence of the polarity correction >2%. The OF values were compared with other small field detectors showing a good agreement for field sizes >2×2cm2. The large field over-response was corrected applying kpol(FS). ConclusionsNanochamber is an interesting option for small field measurements. The spherical shape of the active volume is an advantage in terms of reduced angular dependence. An interesting feature of the Nanochamber is its beam quality independence and, as a future development, the possibility to use it for small field absolute dosimetry.

Monte Carlo and experimental determination of correction factors for gamma knife perfexion small field dosimetry measurements

Detector-, field size- and machine-specific correction factors are required for precise dosimetry measurements in small and non-standard photon fields. In this work, Monte Carlo (MC) simulation techniques were used to calculate the and correction factors for a series of ionization chambers, a synthetic microDiamond and diode dosimeters, used for reference and/or output factor (OF) measurements in the Gamma Knife Perfexion photon fields. Calculations were performed for the solid water (SW) and ABS plastic phantoms, as well as for a water phantom of the same geometry. MC calculations for the correction factors in SW were compared against corresponding experimental results for a subset of ionization chambers and diode detectors. Reference experimental OF data were obtained through the weighted average of corresponding measurements using TLDs, EBT-2 films and alanine pellets. values close to unity (within 1%) were calculated for most of ionization chambers in water. Greater corrections of up to 6.0% were observed for chambers with relatively large air-cavity dimensions and steel central electrode. A phantom correction of 1.006 and 1.024 (breaking down to 1.014 from the ABS sphere and 1.010 from the accompanying ABS phantom adapter) were calculated for the SW and ABS phantoms, respectively, adding up to corrections in water. Both measurements and MC calculations for the diode and microDiamond detectors resulted in lower than unit correction factors, due to their denser sensitive volume and encapsulation materials. In comparison, higher than unit results for the ionization chambers suggested field size depended dose underestimations (being significant for the 4 mm field), with magnitude depending on the combination of contradicting phenomena associated with volume averaging and electron fluence perturbations. Finally, the presence of 0.5 mm air-gap between the diodes’ frontal surface and their phantom-inserts may considerably influence OF measurements, reaching 4.6% for the Razor diode.

Initial testing of a pixelated silicon detector prototype in proton therapy.

As technology continues to develop, external beam radiation therapy is being employed, with increased conformity, to treat smaller targets. As this occurs, the dosimetry methods and tools employed to quantify these fields for treatment also have to evolve to provide increased spatial resolution. The team at the University of Wollongong has developed a pixelated silicon detector prototype known as the dose magnifying glass (DMG) for real‐time small‐field metrology. This device has been tested in photon fields and IMRT. The purpose of this work was to conduct the initial performance tests with proton radiation, using beam energies and modulations typically associated with proton radiosurgery. Depth dose and lateral beam profiles were measured and compared with those collected using a PTW parallel‐plate ionization chamber, a PTW proton‐specific dosimetry diode, EBT3 Gafchromic film, and Monte Carlo simulations. Measurements of the depth dose profile yielded good agreement when compared with Monte Carlo, diode and ionization chamber. Bragg peak location was measured accurately by the DMG by scanning along the depth dose profile, and the relative response of the DMG at the center of modulation was within 2.5% of that for the PTW dosimetry diode for all energy and modulation combinations tested. Real‐time beam profile measurements of a 5 mm 127 MeV proton beam also yielded FWHM and FW90 within ±1 channel (0.1 mm) of the Monte Carlo and EBT3 film data across all depths tested. The DMG tested here proved to be a useful device at measuring depth dose profiles in proton therapy with a stable response across the entire proton spread‐out Bragg peak. In addition, the linear array of small sensitive volumes allowed for accurate point and high spatial resolution one‐dimensional profile measurements of small radiation fields in real time to be completed with minimal impact from partial volume averaging.

EP-1434: Comparison of small-field output factor measurements

EP-1434: Comparison of small-field output factor measurements

Dosimetric characterization of small fields using a plastic scintillator detector: A large multicenter study

PurposeIn modern radiation therapy accurate small fields dosimetry is a challenge and its standardization is fundamental to harmonize delivered dose in different institutions. This study presents a multicenter characterization of MLC-defined small field for Elekta and Varian linear accelerators. Measurements were performed using the Exradin W1 plastic scintillator detector. Materials and methodsThe project enrolled 24 Italian centers. Each center performed Tissue Phantom Ratio (TPR), in-plane and cross-plane dose profiles of 0.8×0.8cm2 field, and Output Factor (OF) measurements for square field sizes ranging from 0.8 to 10cm. Set-up conditions were 10cm depth in water phantom at SSD 90cm. Measurements were performed using two twin Exradin W1 plastic scintillator detectors (PSD) correcting for the Cerenkov effect as proposed by the manufacturer. ResultsData analysis from 12 Varian and 12 Elekta centers was performed. Measurements of 7 centers were not included due to cable problems. TPR measurements showed standard deviations (SD)<1%; SD<0.4mm for the profile penumbra was obtained, while FWHM measurements showed SD<0.5mm. OF measurements showed SD<1.5% for field size greater than 2×2cm2. Median OFs values were in agreement with the recent bibliography. ConclusionsHigh degree of consistency was registered for all the considered parameters. This work confirmed the importance of multicenter dosimetric intercomparison. W1 PSD could be considered as a good candidate for small field measurements.

The practical application of scintillation dosimetry in small-field photon-beam radiotherapy

Plastic scintillation detectors are a new instrument of stereotactic photon-beam dosimetry. The clinical application of the plastic scintillation detector Exradin W1 at the Siemens Artiste and Elekta Synergy accelerators is a matter of current interest. In order to reduce the measurement uncertainty, precautions have to be taken with regard to the geometrical arrangement of the scintillator, the light-guide fiber and the photodiode in the radiation field. To determine the “Cerenkov light ratio” CLR with a type A uncertainty below 1%, the Cerenkov calibration procedure for small-field measurements based on the two-channel spectral method was used. Output factors were correctly measured with the W1 for field sizes down to 0.5×0.5cm2 with a type A uncertainty of 1.8%. Measurements of small field dose profiles and percentage depth dose curves were carried out with the W1 using automated water phantom profile scans, and a type A uncertainty for dose maxima of 1.4% was achieved. The agreement with a synthetic diamond detector (microDiamond, PTW Freiburg) and a plane parallel ionization chamber (Roos chamber, PTW Freiburg) in relative dose measurements was excellent. In oversight of all results, the suitability of the plastic scintillation detector Exradin W1 for clinical dosimetry under stereotactic conditions, in particular the tried and tested procedures for CLR determination, output factor measurement and automated dose profile scans in water phantoms, have been confirmed.