Small Field Measurements Research Articles

Of the use of new EBT3 for dose profile and dose-area product measurements in small fields

)coupled through optical fibers to a two-channel photodetection module.The module is basically composed of two silicon photomultipliers syn-chronized together by a FPGA card. The two probes are positioned in aPMMA cylinder, at 18 mm away from its axis and spaced apart by 29 mm.A 2 mm diameter source transfer catheter is inserted into the cylinderaxis.Testing results have shown that, the seed activity can be estimated with aprecision better than 5 % when it is positioned in front of one of the twodosimetric probes. This positioning is chosen to estimate the dose with thehighest measurement SNR and the lowest sensitivity on seed positioningerrors (There is a low dose rate gradient when the seed is facing the probe).With the acquisition of a treatment plan used as a reference, the dwell posi-tion can be estimated with sub-milimetric accuracy over a range of 20 mm,and the dwell time with a mean error of 24 ms. It also integrates measure-ments of thetransit timeduringtheintroduction andremovalof the source.Thus, it is possible to define the dose at any point in space from the activityreferencesourcebyusingthedosecalculationformalismTG-43AAPM-ESTROconsensus [2]. The difference between the dose calculated by the systemsoftware and the TPS is less than 4% for the consideredpoints of interest.Theproposedsystemallowsanentirequalitycheckoftheafterloaderbeforetreatment.Bycomparingmeasuredvalueswithexpectedones,italsoallowsreal-time detection of different types of errors such as bad source activity,treatment protocol error, in-catheter curvature-induced position errors [3].Keywords: Brachytherapy, DoRGaN, Quality assuranceReferences[1]R.Nath,L.L.Anderson,J.A.Meli,A.J.Olch,J.A.Stitt,andJ.F.Williamson,“Code of practice for brachytherapy physics: Reportof the AAPM RadiationTherapy Committee Task Group No. 56,”Med. Phys. Vol 24 (1997),p.1557 e1598.[2] J. Perez-Calatayud, F. Ballester, R.K. Das, L.A. Dewerd, G.S. Ibbott, A.S.Meigooni, Z. Ouhib Z.M.J. Rivard, R.S. Sloboda, J.F. Williamson, “Dosecalculation for photon-emitting brachytherapy sources with average en-ergy higher than 50 keV: full report of the AAPM and ESTRO report of thehigh energy brachytherapy source dosimetry (HEBD) working group,”august 2012, ISBN: 978-1-936366-17-0, ISSN: 0271-7344.[3] A. Palmer, B. Mzenda, “Performance assessment of the BEBIG Multi-Source high dose rate brachytherapy treatment unit, “Phys Med Biol. Vol 54(2009) p. 7417-34.http://dx.doi.org/10.1016/j.ejmp.2014.10.048ESTABLISHMENT OF DOSIMETRIC REFERENCES IN SMALL FIELDSTHANKS TO A NEW QUANTITY OF INTERESTS. Dufreneix, J.M. Bordy, F. Delaunay, J. Daures, J. Gouriou, M. Le Roy, A.Ostrowsky, B. Rapp, L. Sommier CEA, LIST, Laboratoire National HenriBecquerel, 91191 Gif-sur-Yvette, FranceIntroduction: The output factor ratio measurement assumes that thecalibration factor of the ionization chamber in terms of absorbed dose towater is invariant with field size. A new approach was suggested in orderto verify that this hypothesis is true for a parallel-plate chamber whichsensitive volume is larger than the beam’s section. The quantity of interestis then no longer a dose at a point but a dose integrated over a surface.Methods: A graphite calorimeter and a parallel-plate ionization chamberwith the same sensitive volume (3 cm diameter) were built. They werefully characterized in a large 60Co beam and then used in small circularfields of 2, 1 and 0.75 cm diameter. The ratio of calorimetric againstionometric measurements was considered as a good approximation of thecalibration factor and thus studied.Results: The dose rate established in a large 60Co field with the newcalorimeter is in agreement within 0.4% with previous calorimeters. Theionization chamber shows good characteristics except for a 0.06% drift perhourinwater.Insmallfields,statisticaluncertaintyontheratioconsideredincreases with decreasing field size but stays under 0.5%. A difference of1.3% of this ratio was found for the three field sizes of interest.Conclusion: Results presented here highlight the possibility of measuringcalibration factors in small fields using a dose-area product. Monte Carlosimulations are underway in order to precisely determine the calibrationfactor of the parallel plate chamber in the three small fields studied.Keywords: Radiotherapy; Dosimetric reference; Small fields; Dose-areaproducthttp://dx.doi.org/10.1016/j.ejmp.2014.10.049

PRESAGE 3D dosimetry accurately measures Gamma Knife output factors

Small-field output factor measurements are traditionally very difficult because of steep dose gradients, loss of lateral electronic equilibrium, and dose volume averaging in finitely sized detectors. Three-dimensional (3D) dosimetry is ideal for measuring small output factors and avoids many of these potential challenges of point and 2D detectors. PRESAGE 3D polymer dosimeters were used to measure the output factors for the 4 mm and 8 mm collimators of the Leksell Perfexion Gamma Knife radiosurgery treatment system. Discrepancies between the planned and measured distance between shot centers were also investigated. A Gamma Knife head frame was mounted onto an anthropomorphic head phantom. Special inserts were machined to hold 60 mm diameter, 70 mm tall cylindrical PRESAGE dosimeters. The phantom was irradiated with one 16 mm shot and either one 4 mm or one 8 mm shot, to a prescribed dose of either 3 Gy or 4 Gy to the 50% isodose line. The two shots were spaced between 30 mm and 60 mm apart and aligned along the central axis of the cylinder. The Presage dosimeters were measured using the DMOS-RPC optical CT scanning system. Five independent 4 mm output factor measurements fell within 2% of the manufacturer’s Monte Carlo simulation-derived nominal value, as did two independent 8 mm output factor measurements. The measured distances between shot centers varied by ±0.8 mm with respect to the planned shot displacements. On the basis of these results, we conclude that PRESAGE dosimetry is excellently suited to quantify the difficult-to-measure Gamma Knife output factors.

Design and experimental testing of air slab caps which convert commercial electron diodes into dual purpose, correction-free diodes for small field dosimetry.

Two diodes which do not require correction factors for small field relative output measurements are designed and validated using experimental methodology. This was achieved by adding an air layer above the active volume of the diode detectors, which canceled out the increase in response of the diodes in small fields relative to standard field sizes. Due to the increased density of silicon and other components within a diode, additional electrons are created. In very small fields, a very small air gap acts as an effective filter of electrons with a high angle of incidence. The aim was to design a diode that balanced these perturbations to give a response similar to a water-only geometry. Three thicknesses of air were placed at the proximal end of a PTW 60017 electron diode (PTWe) using an adjustable "air cap". A set of output ratios (ORDet (fclin) ) for square field sizes of side length down to 5 mm was measured using each air thickness and compared to ORDet (fclin) measured using an IBA stereotactic field diode (SFD). kQclin,Qmsr (fclin,fmsr) was transferred from the SFD to the PTWe diode and plotted as a function of air gap thickness for each field size. This enabled the optimal air gap thickness to be obtained by observing which thickness of air was required such that kQclin,Qmsr (fclin,fmsr) was equal to 1.00 at all field sizes. A similar procedure was used to find the optimal air thickness required to make a modified Sun Nuclear EDGE detector (EDGEe) which is "correction-free" in small field relative dosimetry. In addition, the feasibility of experimentally transferring kQclin,Qmsr (fclin,fmsr) values from the SFD to unknown diodes was tested by comparing the experimentally transferred kQclin,Qmsr (fclin,fmsr) values for unmodified PTWe and EDGEe diodes to Monte Carlo simulated values. 1.0 mm of air was required to make the PTWe diode correction-free. This modified diode (PTWeair) produced output factors equivalent to those in water at all field sizes (5-50 mm). The optimal air thickness required for the EDGEe diode was found to be 0.6 mm. The modified diode (EDGEeair) produced output factors equivalent to those in water, except at field sizes of 8 and 10 mm where it measured approximately 2% greater than the relative dose to water. The experimentally calculated kQclin,Qmsr (fclin,fmsr) for both the PTWe and the EDGEe diodes (without air) matched Monte Carlo simulated results, thus proving that it is feasible to transfer kQclin,Qmsr (fclin,fmsr) from one commercially available detector to another using experimental methods and the recommended experimental setup. It is possible to create a diode which does not require corrections for small field output factor measurements. This has been performed and verified experimentally. The ability of a detector to be "correction-free" depends strongly on its design and composition. A nonwater-equivalent detector can only be "correction-free" if competing perturbations of the beam cancel out at all field sizes. This should not be confused with true water equivalency of a detector.

Correction-less dosimetry of nonstandard photon fields: a new criterion to determine the usability of radiation detectors

In the IAEA-AAPM dosimetry formalism, detector measurements in general nonstandard conditions are corrected using the factor . This factor needs to be evaluated on a case-by-case basis which is difficult to accomplish in practice. The present paper aims to provide a method that allows neglecting correction factors for small and composite IMRT fields by first determining a radiation detector’s usability in these fields. Detailed models of nine radiation detectors are built: four ionization chambers (NE2571, A12, A1SL, A14), three small field detectors (PTW31018 microLion, PTW60003 natural diamond, PTW60012 unshielded diode) and two near water-equivalent detectors (alanine, W1 scintillating fiber). Using the egs_chamber Monte Carlo code, dose response functions at 6 MV and 25 MV are sampled for each detector and their corresponding volume of water. These functions are then used with a newly derived criterion to evaluate an upper bound on the variable if no field collimation/modulation occurs over a given perturbation zone. The variable is defined as the absolute value of the relative deviation from unity of a nonstandard field quality correction factor . Using the same criterion, perturbation zones are evaluated by finding the smallest field size allowed for correction-less dosimetry with a given tolerance . For composite fields, the sensitivity of detectors to the non-uniformity of virtual symmetric collapsed beams over regions of interest specified by the criterion is studied to estimate an upper bound on for a given beam flatness. Finally, a newly defined perturbation function is used to minimize the perturbations of the microLion chamber through density compensation. The theoretical criterion shows good agreement with full Monte Carlo simulations of . Perturbation zones are shown to be sensitive to both the energy of the beam and the orientation of the detector. The density-compensated microLion shows significantly improved response in both axial and radial orientations in small and composite IMRT fields. Finally, the new Exradin W1 scintillator is shown to have values under 1% in small fields. The methods presented in this work theoretically show that correction-less dosimetry of nonstandard field can be accomplished by knowing the limit of usability of radiation detectors in these conditions. Potential applications include small field output factor measurements and absolute absorbed dose to water verification in the QA of clinical IMRT fields.

SU‐E‐T‐456: Development of An EPID‐Based Output Measurement and Dosimetric Verification Tool for Electron Beam Therapy

Purpose:To develop an efficient and robust method for output and absolute dosimetric measurements of electron beam therapy by using a high spatialresolution and high frame‐rate amorphous silicon flat‐panel electronic portal imaging device (EPID).Methods:A previously‐established EPID dosimetry system was extended to measure the output factors of electron beams of various sizes and their planar dose distributions at reference depths. Specific EPID responses to different electron energies were derived from Monte Carlo (MC) simulations to de‐convolve the EPID raw images to the incident electron fluence map. To reconstruct the 2D water‐based dose distribution map at reference depths for different energies, the fluence map was further convolved with a MC simulated pencil beam kernel. Different energies of 6, 9, 12, 16, and 20 MeV from a Varian C‐series linac were tested. Standard square fields ranging from 2×2 to 15×15 cm2 were measured and validated against film/ion chamber measurements. Small fields of different patient cases with irregular cutout shapes were also tested.Results:The EPIDmeasured output factors for 2×2, 3×3, 6×6, and 10×10 cm2 fields with all electron energies agree with the film/ion chamber data within 2.7%. The average discrepancy between EPID and film/ion chamber measurements was 1.4%. The differences for the three patient cases were 0.1%, 2.1%, and 1.6%. Deliveries with different monitor units were tested and the results exhibited good linearity. Measurements with different dose rate showed that the dose rate dependence was less than 1%. Using the proposed method, 2D absolute dose maps were created from the EPID raw images and the results were consistent with film measurements.Conclusion:The efficient data readout and portability of the newly developed EPID system provides an efficient and accurate solution for electron beam therapy. It addresses an important unmet clinical need for a fast and reliable small field output factor measurement.

SU‐E‐T‐298: Small Field Total Scatter Factors Using a Commercial Scintillator Detector: Calibration Parameters Are Not Independent of Field Size

Purpose:To evaluate the accuracy in total scatter factor (Sc,p) determination for small fields using commercial plastic scintillator detector (PSD). The manufacturer's spectral discrimination method to subtract Cerenkov light from the signal is discussed.Methods:Sc,p for field sizes ranging from 0.5 to 10 cm were measured using PSD Exradin (Standard Imaging) connected to two channel electrometer measuring the signals in two different spectral regions to subtract the Cerenkov signal from the PSD signal. A Pinpoint ionisation chamber 31006 (PTW) and a non‐shielded semiconductor detector EFD (Scanditronix) were used for comparison. Measures were performed for a 6 MV X‐ray beam. The Sc,p are measured at 10 cm depth in water for a SSD=100 cm and normalized to a 10' 10 cm2 field size at the isocenter. All detectors were placed with their symmetry axis parallel to the beam axis.We followed the manufacturer's recommended calibration methodology to subtract the Cerenkov contribution to the signal as well as a modified method using smaller field sizes. The Sc,p calculated by using both calibration methodologies were compared.Results:Sc,p measured with the semiconductor and the PinPoint detectors agree, within 1.5%, for field sizes between 10' 10 and 1' 1 cm2. Sc,p measured with the PSD using the manufacturer's calibration methodology were systematically 4% higher than those measured with the semiconductor detector for field sizes smaller than 5' 5 cm2. By using a modified calibration methodology for smalls fields and keeping the manufacturer calibration methodology for fields larger than 5' 5cm2 field Sc,p matched semiconductor results within 2% field sizes larger than 1.5 cm.Conclusion:The calibration methodology proposed by the manufacturer is not appropriate for dose measurements in small fields. The calibration parameters are not independent of the incident radiation spectrum for this PSD. This work was partially financed by grant 2012 of Barcelona board of the AECC.

SU-E-T-553: Characterization of Plastic Scintillator Detectors for Radiation Therapy

Purpose: To characterize basic performance of plastic scintillator detectors (PSD) designed for dosimetry of radiation therapy. Methods: The Exradin W1 Scintillator is a plastic scintillating fiber-based detector designed for highly accurate measurement of small radiotherapy fields used in patient plan verification and machine commissioning and QA procedures. The Cerenkov emissions were corrected using spectral separation. The optical signal was converted to electronic signal with a photodiode. We measured its dosimetry performance, including percentage depth dose, output factor, dose and dose rate linear response. We compared the dosimetry results with reference ion chamber measurements. Results: The dosimetry results of PSD agree well with reference ion chamber measurements. For percentage depth dose, the differences between PSD and ion chamber results are on average 1.7±1.1% and 0.8±0.8% with a maximum of 3.5% and 2.7% for 6MV and 15MV beams, respectively. For the output factors, PSD measurements are within 2% from ion chamber results. The dose linear response is within 1% when dose is larger than 20 MU for both 6 MV and 15 MV. The dose rate linear response is within 1% for the entire dose rate used (100 MU/min to 600MU/min). Conclusions: The current design of PSD is feasible for the dosimtry measurement in radiation therapy. This combination of PSD and photodiode system could be extended to multichannel array detection of dose distribution. It might as well be used as range verification in proton therapy. The work is partially supported by: DOD (W81XWH-09-2-0174) and American Cancer Society (IRG-78-002-28)

SU‐E‐T‐17: A Mathematical Model for PinPoint Chamber Correction in Measuring Small Fields

Purpose:For small field dosimetry, such as measuring the cone output factor for stereotactic radiosurgery, ion chambers often result in underestimation of the dose, due to both the volume averaging effect and the lack of electron equilibrium. The purpose of this work is to develop a mathematical model, specifically for the pinpoint chamber, to calculate the correction factors corresponding to different type of small fields, including single cone‐based circular field and non‐standard composite fields.Methods:A PTW 0.015cc PinPoint chamber was used in the study. Its response in a certain field was modeled as the total contribution of many small beamlets, each with different response factor depending on the relative strength, radial distance to the chamber axis, and the beam angle. To get these factors, 12 cone‐shaped circular fields (5mm,7.5mm, 10mm, 12.5mm, 15mm, 20mm, 25mm, 30mm, 35mm, 40mm, 50mm, 60mm) were irradiated and measured with the PinPoint chamber. For each field size, hundreds of readings were recorded for every 2mm chamber shift in the horizontal plane. These readings were then compared with the theoretical doses as obtained with Monte Carlo calculation. A penalized‐least‐square optimization algorithm was developed to find out the beamlet response factors. After the parameter fitting, the established mathematical model was validated with the same MC code for other non‐circular fields.Results:The optimization algorithm used for parameter fitting was stable and the resulted response factors were smooth in spatial domain. After correction with the mathematical model, the chamber reading matched with the Monte Carlo calculation for all the tested fields to within 2%.Conclusion:A novel mathematical model has been developed for the PinPoint chamber for dosimetric measurement of small fields. The current model is applicable only when the beam axis is perpendicular to the chamber axis. It can be applied to non‐standard composite fields. Further validation with other type of detectors is being conducted.

N-isopropylacrylamide gel dosimeter to evaluate clinical photon beam characteristics

The introduction of beam intensity control concept in current radiotherapy techniques has increased treatment planning complexity. Thus, small-field dose measurement has become increasingly vital. Polymer gel dosimetry method is widely studied. It is the only dose measurement tool that provides 3D dose distribution. This study aims to use an N-isopropylacrylamide (NIPAM) gel dosimeter to conduct beam performance measurements of percentage depth dose (PDD), beam flatness, and symmetry for photon beams with field sizes of 3×3 and 4×4cm2. Computed tomography scans were used to readout the gel dosimeters. In the PDD measurement, the NIPAM gel dosimeter and GafchromicTM EBT3 radiochromic film displayed high consistency in the region deeper than the build-up region. The gel dosimeter dose profile had 3% lower flatness and symmetry measurement at 5cm depth for different fields compared with that of the GafchromicTM EBT3 film. During gamma evaluation under 3%/3mm dose difference/distance-to-agreement standard, the pass rates of the polymer gel dosimeter to the TPS and EBT3 film were both higher than 96%. Given that the gel is tissue equivalent, it did not exhibit the energy dependence problems of radiochromic films. Therefore, the practical use of NIPAM polymer gel dosimeters is enhanced in clinical dose verification.

Monte Carlo simulated correction factors for output factor measurement with the CyberKnife system—results for new detectors and correction factor dependence on measurement distance and detector orientation

A previous study of the corrections needed for output factor measurements with the CyberKnife system has been extended to include new diode detectors (IBA SFD and Exradin D1V), an air filled microchamber (Exradin CC01) and a scintillation detector (Exradin W1). The dependence of the corrections on detector orientation (detector long axis parallel versus perpendicular to the beam axis) and source to detector distance (SDD) was evaluated for these new detectors and for those in our previous study. The new diodes are found to over-respond at the smallest (5 mm) field size by 2.5% (D1V) and 3.3% (SFD) at 800 mm SDD, while the CC01 under-responds by 7.4% at the same distance when oriented parallel to the beam. Corrections for all detectors tend to unity as field size increases. The W1 corrections are <0.5% at all field sizes. Microchamber correction factors increase substantially if the detector is oriented perpendicular to the beam (by up to 23% for the PTW 31014). Corrections also vary with SDD, with the largest variations seen for microchambers in the perpendicular orientation (up to 13% change at 650 mm SDD versus 800 mm) and smallest for diodes (∼1% change at 650 mm versus 800 mm). The smallest and most stable corrections are found for diodes, liquid filled microchambers and scintillation detectors, therefore these should be preferred for small field output factor measurements. If air filled microchambers are used, then the parallel orientation should be preferred to the perpendicular, and care should be taken to use corrections appropriate to the measurement SDD.

PD-0037: Small field output factor measurement using various active detectors and OSLD

PD-0037: Small field output factor measurement using various active detectors and OSLD

PD-0036: Monte Carlo output correction factors for Cyberknife and Synergy small fields for the Exradin W1 Scintillator

Materials and Methods: The Exradin W1 Scintillator is a plastic scintillating fiber-based detector designed for highly accurate measurement of small radiotherapy fields. This new near-water equivalent plastic scintillation detector (PSD) is being manufactured by Standard Imaging, Inc. The Exradin W1 Scintillator sensitive element is composed of a plastic scintillating fiber enclosed inside a 2.8 mm diameter by 42 mm long water-equivalent robust housing probe. The dimensions of the plastic scintillating fiber (detector sensitive element) are 1.0 mm in diameter and 3.0 mm long. The physical density of the plastic scintillating fiber (PSD) is 1.05 g/cm. This PSD is optically coupled to a clear multi-clad optical fiber made of PMMA of a 1.0mm diameter core and 3 m in length protected with a 2.2 mm diameter polyethylene jacket. The scintillation light produced in the PSD is transmitted through this 3 m long optical fiber to a photodiode enclosed in a light tight small housing. The Monte Carlo method used to calculate

Detector comparison for small field output factor measurements in flattening filter free photon beams

PurposeThe applicability of various detectors for small field dosimetry and whether there are differences in the detector response when irradiated with FF- and FFF-beams was investigated. Materials and methodsOutput factors of 6 and 10MV FF- and FFF-beams were measured with 14 different online detectors using field sizes between 10×10 and 0.6×0.6cm2 at a depth of 5cm of water in isocentric conditions. Alanine pellets with a diameter of 5 and 2.5mm were used as reference dosimeters for field sizes down to 1.2×1.2 and 0.6×0.6cm2, respectively. The ratio of the relative output measured with the online detectors to the relative output measured with alanine was evaluated (referred to as dose response ratio). ResultsThe dose response ratios of two different shielded diodes measured with 10MV FF-beams deviated substantially by 2–3% compared to FFF-beams at a field size of 0.6×0.6cm2. This difference was less pronounced for 6MV FF- and FFF-beams. For all other detectors the dose response ratios of FF- and FFF-beams showed no significant difference. ConclusionThe dose response ratios of the majority of the detectors agreed within the measurement uncertainty when irradiated with FF- and FFF-beams. Of all investigated detectors, the microDiamond and the unshielded diodes would require only small corrections which make them suitable candidates for small field dosimetry in FF- and FFF-beams.

Radiotherapy electron beams collimated by small tubular applicators: characterization by silicon and diamond diodes

High-energy electron beams generated by linear accelerators, typically in the range 6 to 20 MeV, are used in small field sizes for radiotherapy of localized superficial tumors. Unshielded silicon diodes (Si-D) are commonly considered suitable detectors for relative dose measurements in small electron fields due to their high spatial resolution. Recently, a novel synthetic single crystal diamond diode (SCDD) showed suitable properties for standard electron beams and small photon beams dosimetry. The aim of the present study is twofold: to characterize 6 to 15 MeV small electron beams shaped by using commercial tubular applicators with 2, 3, 4 and 5 cm diameter and to assess the dosimetric performance under such irradiation conditions of the novel SCDD dosimeter by comparison with commercially available dosimeters, namely a Si-D and a plane–parallel ionization chamber. Percentage depth dose curves, beam profiles and output factors (OFs) were measured. A good agreement among the dosimeters was observed in all of the performed measurements. As for the tubular applicators, two main effects were evidenced: (i) OFs larger than unity were measured for a number of field sizes and energies, with values up to about 1.3, that is an output 30% greater than that obtained at the 10 × 10 cm2 reference field; (ii) for each diameter of the tubular applicator a noticeable increase of the OF values was observed with increasing beam energy, up to about 100% in the case of the smaller applicator. This OF behavior is remarkably different from what typically observed for small blocked fields having the same size and energy as those used in this study. OFs for tubular applicators depend considerably on the field size, so interpolation is unadvisable to predict the linear accelerator output for such applicators whereas reliable high-resolution detectors, as the silicon and diamond diodes used in this work allow OF measurements with uncertainties of about 1%.

Commissioning and validation of BrainLAB cones for 6X FFF and 10X FFF beams on a Varian TrueBeam STx

Small field dosimetry is a challenging task. The difficulties of small field measurements, particularly stereotactic field size measurements, are highlighted by the large interinstitution variability that can be observed for circular cone collimator commissioning measurements. We believe the best way to improve the consistency of small field measurements is to clearly document and share the results of small field measurements. In this work we report on the commissioning and validation of a BrainLAB cone system for 6 MV and 10 MV flattening filter‐free (FFF) beams on a Varian TrueBeam STx. Commissioning measurements consisted of output factors, percent depth dose, and off‐axis factor measurements with a diode. Validation measurements were made in a polystyrene slab phantom at depths of 5 cm, 10 cm, and 15 cm using radiochromic film. Output factors for the 6xFFF cones are 0.689, 0. 790, 0.830, 0.871, 0.890, and 0.901 for 4 mm, 6 mm, 7.5 mm, 10 mm, 12.5 mm, and the 15 mm cones, respectively. Output factors for the 10xFFF cones are 0.566, 0. 699, 0.756, 0.826, 0.864, and 0.888 for 4 mm, 6 mm, 7.5 mm, 10 mm, 12.5 mm, and the 15 mm cones, respectively. The full width half maximum values of the off‐axis factors agreed with the nominal cone size to within 0.5 mm. Validation measurements showed an agreement of absolute dose between calculation and plan of ≤ 3.6%, and an agreement of field sizes of ≤ 0.3 mm in all cases. Radiochromic film validation measurements show reasonable agreement with beam models for circular collimators based on diode commissioning measurements.PACS numbers: 87.53.Ly, 87.53.Bn, 87.56.nk, 87.55.D‐, 87.55.km

Design and performance analysis of three superconducting magnetic sensors for the measurement of small fields

We analyse here the sensing performance of a magnetometer which is based on the linear dc-field dependence of the second harmonic signal generated by high-Tc polycrystalline superconductors. Experiments realized in a series of polycrystalline REBa2Cu3O7−δ (RE = Y, Sm and Nd) superconductors have shown that some parameters (e.g. Hc1J and H*) peculiar to the studied superconductors change depending on the RE ions, and those determine the sensitivity limit of the magnetometer. The lowest measurable magnetic fields are 0.1, 5 and 38 nT for RE = Sm, Y and Nd, respectively at the functioning temperature of 77 K. With an upper limit above the Earth's field the magnetometer has attracted the attention of manufacturers in the industry.

Fricke dosimeter gel measurements of the profiles of shielded fields

In radiation therapy, the shielding of normal tissue can be made using Cerrobend® blocks or a multileaf collimator. In this work, profiles of shielded fields collimated by Cerrobend blocks were obtained through the Fricke Xylenol Gel (FXG) dosimeter irradiated with 6MV photon beams. The results show that the FXG system can be used in profile measurements of small fields in radiotherapy.

Evaluation of the EDGE detector in small-field dosimetry

This study evaluates a new diode detector design for small-field dosimetry. An accurate detector that has a small volume are necessary to compile data for stereotactic radiosurgery (SRS), stereotactic body radiotherapy (SBRT), and intensity-modulated radiotherapy (IMRT). Two semiconductor diode detectors and one ionization chamber were used to measure the profiles, percent depth doses (PDDs), and relative output factors (OFs) of a Novalis 6-MV SRS beam. Profiles and PDD data were collected using 5.0-, 10.0-, 15.0-, 20.0-, 30.0-, and 50.0-mm micro multileaf collimators (mMLCs) at small fields and a 98.0 × 98.0-mm2 reference field. OFs were collected for each of the mMLCs. The EDGE diode detector, the diode detector, and the ion chamber (0.007 cc) were used in the study. Detector measurements were performed using the 3D water phantom with a source-to-surface distance of 100-cm at a depth of 1.5-cm. The measurements were analyzed using the IBA OmniPro Accept 7th version software. In addition, all data were compared to Monte Carlo simulations. The semiconductor diodes had similar OFs and PDDs for each of the mMLCs used. The Dmax values of the EDGE diode detector, measured from the PDD, ranged from 8.5 to 14.0-mm with an average of 12.4-mm. The field widths of the EDGE diode detector were found to have similar values. The performance of the EDGE diode detector was comparable for all small-field measurements. Additionally, no evidence of an energy response was observed for the EDGE detectors for a field of 98 × 98-mm2. This is particularly important when measuring the relative OF for small fields or gathering larger-sized field data for the commissioning of a treatment planning system.

Small fields output factors measurements and correction factors determination for several detectors for a CyberKnife® and linear accelerators equipped with microMLC and circular cones

The use of small photon fields is now an established practice in stereotactic radiosurgery and radiotherapy. However, due to a lack of lateral electron equilibrium and high dose gradients, it is difficult to accurately measure the dosimetric quantities required for the commissioning of such systems. Moreover, there is still no metrological dosimetric reference for this kind of beam today. In this context, the first objective of this work was to determine and to compare small fields output factors (OF) measured with different types of active detectors and passive dosimeters for three types of facilities: a CyberKnife(®) system, a dedicated medical linear accelerator (Novalis) equipped with m3 microMLC and circular cones, and an adaptive medical linear accelerator (Clinac 2100) equipped with an additional m3 microMLC. The second one was to determine the kQclin,Qmsr (fclin,fmsr) correction factors introduced in a recently proposed small field dosimetry formalism for different active detectors. Small field sizes were defined either by microMLC down to 6 × 6 mm(2) or by circular cones down to 4 mm in diameter. OF measurements were performed with several commercially available active detectors dedicated to measurements in small fields (high resolution diodes: IBA SFD, Sun Nuclear EDGE, PTW 60016, PTW 60017; ionizing chambers: PTW 31014 PinPoint chamber, PTW 31018 microLion liquid chamber, and PTW 60003 natural diamond). Two types of passive dosimeters were used: LiF microcubes and EBT2 radiochromic films. Significant differences between the results obtained by several dosimetric systems were observed, particularly for the smallest field size for which the difference in the measured OF reaches more than 20%. For passive dosimeters, an excellent agreement was observed (better than 2%) between EBT2 and LiF microcubes for all OF measurements. Moreover, it has been shown that these passive dosimeters do not require correction factors and can then be used as reference dosimeters. Correction factors for the active detectors have then been determined from the mean experimental OF measured by the passive dosimeters. Four sets of correction factors needed to apply the new small field dosimetry formalism are provided for several active detectors. A protocol for small photon beams OF determination based on passive dosimeters measurements has been recently proposed to French radiotherapy treatment centers.

SU‐E‐T‐436: Lung Material Phantom for Small Field Monte Carlo Dose Validation

Purpose: Commissioning Monte Carlo beam data for clinical use within a treatment planning system (TPS) for Stereotactic Body Radiation Therapy (SBRT) should require validation both in homogeneous and in heterogeneous phantom materials. Compounding this task is the difficulty in accurately measuring small field sizes. This work outlines the use of a new lung phantom in conjunction with a new scintillation detector designed for small field measurement to verify the dose calculated in the TPS. Methods: The measurements were performed on a CyberKnife VSI. The Stereotactic Dose Verification Phantom (SDVP) from Standard Imaging (SI) was fitted with a newly designed 12cm lung material insert with a mass density of 0.28 g/cc. The scintillating detector was placed in a 1.5 cm diameter water‐equivalent insert, simulating the central dose measurement of a small tumor, which was then placed in the SDVP lung phantom and CT scanned. The new SI W1 pinpoint scintillator is nearly water equivalent but the 1mm diameter by 3mm long scintillating fiber was visible and was contoured in the MultiPlan TPS. After dose cross‐calibration with the SI A19 ion chamber in the basic SDVP water equivalent phantom, measurements were taken with the 5, 7.5, 10, 12.5, 15, 20, 30, and 60mm cones. For cross‐calibration to known larger field conditions, the SI A16 ion chamber was used to remeasure the 20, 30, and 60mm cones. Results: The measured dose matched the Monte Carlo dose within 2% for the three collimator sizes that were used with the A16. All measurements with the W1 matched Monte Carlo dose as well, with the exception of the 30mm cone, which was 3.18% deviant. Conclusion: The SDVP lung material inserts combined with the W1 scintillator Result in clinically acceptable correlation to the Monte Carlo dose calculated in the TPS for all CyberKnife fixed collimator sizes. Dr. Grimm received a consulting fee from Standard Imaging for guiding the research.