Exradin A12 Research Articles

Sci‐Sat AM: Radiation Dosimetry and Practical Therapy Solutions ‐ 03: Energy dependence of a clinical probe‐format calorimeter and its pertinence to absolute photon and electron beam dosimetry

Purpose:To evaluate the intrinsic and absorbed‐dose energy dependence of a small‐scale graphite calorimeter probe (GPC) developed for use as a routine clinical dosimeter. The influence of charge deposition on the response of the GPC was also assessed by performing absolute dosimetry in clinical linac‐based electron beams.Methods:Intrinsic energy dependence was determined by performing constant‐temperature calorimetry dose measurements in a water‐equivalent solid phantom, under otherwise reference conditions, in five high‐energy photon (63.5 < %dd(10)X < 76.3), and five electron (2.3 cm < R50 < 8.3 cm) beams. Reference dosimetry was performed for all beams in question using an Exradin A19 ion chamber with a calibration traceable to national standards. The absorbed‐dose component of the overall energy dependence was calculated using the EGSnrc egs_chamber user code.Results:A total of 72 measurements were performed with the GPC, resulting in a standard error on the mean absorbed dose of better than 0.3 % for all ten beams. For both the photon and electron beams, no statistically‐significant energy dependence was observed experimentally. Peak‐to‐peak, variations in the relative response of the GPC across all beam qualities of a given radiation type were on the order of 1 %. No effects, either transient or permanent, were attributable to the charge deposited by the electron beams.Conclusions:The GPC's apparent energy‐independence, combined with its well‐established linearity and dose rate independence, make it a potentially useful dosimetry system capable measuring photon and electron doses in absolute terms at the clinical level.

SU-G-BRB-12: Polarity Effects in Small Volume Ionization Chambers in Small Fields

Purpose: Dosimetric quantities such as the polarity correction factor (Ppol) are important parameters for determining the absorbed dose and can influence the choice of dosimeter. Ppol has been shown to depend on beam energy, chamber design, and field size. This study is to investigate the field size and detector orientation dependence of Ppol in small fields for several commercially available micro-chambers. Methods: We evaluate the Exradin A26, Exradin A16, PTW 31014, PTW 31016, and two prototype IBA CC-01 micro-chambers in both horizontal and vertical orientations. Measurements were taken at 10cm depth and 100cm SSD in a Wellhofer BluePhantom2. Measurements were made at square fields of 0.6, 0.8, 1.0, 1.2, 1.4, 2.0, 2.4, 3.0, and 5.0 cm on each side using 6MV with both ± 300VDC biases. PPol was evaluated as described in TG-51, reported using −300VDC bias for Mraw. Ratios of PPol measured in the clinical field to the reference field are presented. Results: A field size dependence of Ppol was observed for all chambers, with increased variations when mounted vertically. The maximum variation observed in PPol over all chambers mounted horizontally was <1%, and occurred at different field sizes for different chambers. Vertically mounted chambers demonstrated variations as large as 3.2%, always at the smallest field sizes. Conclusion: Large variations in Ppol were observed for vertically mounted chambers compared to horizontal mountings. Horizontal mountings demonstrated a complicated relationship between polarity variation and field size, probably relating to differing details in each chambers construction. Vertically mounted chambers consistently demonstrated the largest PPol variations for the smallest field sizes. Measurements obtained with a horizontal mounting appear to not need significant polarity corrections for relative measurements, while those obtained using a vertical mounting should be corrected for variations in PPol.

SU‐F‐T‐413: Calculation Accuracy of AAA and Acuros Using Cerrobend Blocks for TBI at 400cm

Purpose:It is essential to assess the lung dose during TBI to reduce toxicity. Here we characterize the accuracy of the AAA and Acuros algorithms when using cerrobend lung shielding blocks at an extended distance for TBI.Methods:We positioned a 30×30×30 cm3 solid water slab phantom at 400 cm SSD and measured PDDs (Exradin A12 and PTW parallel plate ion chambers). A 2 cm thick, 10×10 cm2 cerrobend block was hung 2 cm in front of the phantom. This geometry was reproduced in the planning system for both AAA and Acuros. In AAA, the mass density of the cerrobend block was forced to 9.38 g/cm3 and in Acuros it was forced to 8.0 g/cm3 (limited to selecting stainless steel). Three different relative electron densities (RED) were tested for each algorithm; 4.97, 6.97, and 8.97.Results:PDDs from both Acuros and AAA underestimated the delivered dose. AAA calculated that depth dose was higher for RED of 4.97 as compared to 6.97 and 8.97 but still lower than measured. There was no change in the percent depth dose with changing relative electron densities for Acuros.Conclusion:Care should be taken before using AAA or Acuros with cerrobend blocks as the planning system underestimates dose. Acuros limits the ability to modify RED when compared to AAA.

TH‐AB‐201‐06: Examining the Influence of Humidity On Reference Ion Chamber Performance

Purpose:International dosimetry protocols require measurements made with a vented ionization chamber to be corrected for the influence of air density by using the standard temperature‐pressure correction factor. The effect of humidity, on the other hand, is generally ignored with the provision that the relative humidity is between certain limits (15% to 80%). However, there is little experimental data in the published literature as to the true effect of humidity on modern reference‐class ion chambers. This investigation used two different radiation beams – a Co‐60 irradiator and a Sr‐90 check source – to examine the effect of humidity on several versions of the standard Farmer‐type ion chamber.Methods:An environmental cabinet controlled the humidity. For the Co‐60 beam, the irradiation was external, whereas for the Sr‐90 measurements, the source itself was placed within the cabinet. Extensive measurements were carried out to ensure that the experimental setup provided reproducible readings. Four chamber types were investigated: IBA FC65‐G (×2), IBA FC65‐P, PTW30013 &amp; Exradin A19. The different wall materials provided potentially different mechanical responses (i.e., in terms of expansion/contraction) to the water content in the air. The relative humidity was varied between 8 % and 97 % and measurements were made with increasing and decreasing humidity to investigate possible hysteresis effects.Results:Measurements in Co‐60 were consistent with the published data obtained with primary standard cavity chambers in ICRU Report 31. Ionization currents with Sr‐90 showed no dependence with the relative humidity, within the measurement uncertainties. Very good repeatability of the ionization current was obtained over successive wet/dry cycles, no hysteresis was observed, and there was no dependence on chamber type.Conclusion:This null result is very encouraging as it indicates that humidity has no significant effect on these particular types of ionization chambers, consistent with recommendations in current megavoltage dosimetry protocols.

SU‐F‐T‐557: Evaluation of Detector Response in Rectangular Small Field Dosimetry

Purpose:As stereotactic treatment modalities grow towards becoming the standard of care, the need for accurate dose computation in small fields is becoming increasingly essential. The purpose of this study is to evaluate the response of different detectors, intended for small field dosimetry, in jaw defined small rectangular fields by analyzing output factors from a stereotactic clinical accelerator.Methods:Two Dosimeters, the Exradin A26 Microionization Chamber (Standard Imaging) and Edge Diode Detector (Sun Nuclear) were used to measure output factors taken on the Varian Edge Stereotactic Linear accelerator. Measurements were taken at 6MV and 6FFF at 10cm depth, 100cm SSD in a 48×48×40cm3 Welhoffer BluePhantom2 (IBA) with X and Y jaws set from 0.6 to 2.0cm. Output factors were normalized to a 5×5cm2 machine‐specific reference field. Measurements were made in the vertical orientation for the A26 and horizontal orientation for both the A26 and Edge. Output factors were measured as: OFFS = MFS/Mref where MFS and Mref are the measured signals for the clinical field and the reference field, respectively. Measured output factors were then analyzed to establish relative responses of the detectors in small fields.Results:At 6MV the Edge detector exhibited a variation in output factors dependent on jaw positioning (X‐by‐Y vs Y‐by‐X) of 5.7% of the 5×5cm reference output and a variation of 3.33% at 6FFF. The A26 exhibited variation of output factor dependent on jaw positioning of upto 7.7% of the 5×5cm reference field at 6MV and upto 5.33% at 6FFF.Conclusion:Both the Edge detector and A26 responded as expected at small fields however a dependence on the jaw positioning was noted. At 6MV and 6FFF the detector response showed an increased dependence on the positioning of the X jaws as compared to the positioning of the Y jaws.

SU-F-T-577: Comparison of Small Field Dosimetry Measurements in Fields Shaped with Conical Applicators On Two Different Accelerating Systems

Purpose: To investigate small field dosimetry measurements and associated uncertainties when conical applicators are used to shape treatment fields from two different accelerating systems. Methods: Output factor measurements are made in water in beams from the CyberKnife radiosurgery system, which uses conical applicators to shape fields from a (flattening filter-free) 6 MV beam, and in a 6 MV beam from the Elekta Precise linear accelerator (with flattening filter) with BrainLab external conical applicators fitted to shape the field. The measurements use various detectors: (i) an Exradin A16 ion chamber, (ii) two Exradin W1 plastic scintillation detectors, (iii) a Sun Nuclear Edge diode, and (iv) two PTW microDiamond synthetic diamond detectors. Profiles are used for accurate detector positioning and to specify field size (FWHM). Output factor measurements are corrected with detector specific correction factors taken from the literature where available and/or from Monte Carlo simulations using the EGSnrc code system. Results: Differences in measurements of up to 1.7% are observed with a given detector type in the same beam (i.e., intra-detector variability). Corrected results from different detectors in the same beam (inter-detector differences) show deviations up to 3 %. Combining data for all detectors and comparing results from the two accelerators results in a 5.9% maximum difference for the smallest field sizes (FWHM=5.2–5.6 mm), well outside the combined uncertainties (∼1% for the smallest beams) and/or differences among detectors. This suggests that the FWHM of a measured profile is not a good specifier to compare results from different small fields with the same nominal energy. Conclusion: Large differences in results for both intra-detector variability and inter-detector differences suggest potentially high uncertainties in detector-specific correction factors. Differences between the results measured in circular fields from different accelerating systems provide insight into sources of variability in small field dosimetric measurements reported in the literature.

SU‐C‐BRC‐01: A Monte Carlo Study of Out‐Of‐Field Doses From Cobalt‐60 Teletherapy Units Intended for Historical Correlations of Dose to Normal Tissue

Purpose:Innovations in radiotherapy treatments, such as dynamic IMRT, VMAT, and SBRT/SRS, result in larger proportions of low‐dose regions where normal tissues are exposed to low doses levels. Low doses of radiation have been linked to secondary cancers and cardiac toxicities. The AAPM TG Committee No.158 entitled, ‘Measurements and Calculations of Doses outside the Treatment Volume from External‐Beam Radiation Therapy’, has been formed to review the dosimetry of non‐target and out‐of‐field exposures using experimental and computational approaches. Studies on historical patients can provide comprehensive information about secondary effects from out‐of‐field doses when combined with long‐term patient follow‐up, thus providing significant insight into projecting future outcomes of patients undergoing modern‐day treatments.Methods:We present a Monte Carlo model of a Theratron‐1000 cobalt‐60 teletherapy unit, which historically treated patients at the University of Florida, as a means of determining doses located outside the primary beam. Experimental data for a similar Theratron‐1000 was obtained at the University of Wisconsin's ADCL to benchmark the model for out‐of‐field dosimetry. An Exradin A12 ion chamber and TLD100 chips were used to measure doses in an extended water phantom to 60 cm outside the primary field at 5 and 10 cm depths.Results:Comparison between simulated and experimental measurements of PDDs and lateral profiles show good agreement for in‐field and out‐of‐field doses. At 10 cm away from the edge of a 6×6, 10×10, and 20×20 cm2 field, relative out‐of‐field doses were measured in the range of 0.5% to 3% of the dose measured at 5 cm depth along the CAX.Conclusion:Out‐of‐field doses can be as high as 90 to 180 cGy assuming historical prescription doses of 30 to 60 Gy and should be considered when correlating late effects with normal tissue dose.

Determination of small field output factors in 6 and 10 MV flattening filter free photon beams using various detectors

The study aimed to determine appropriate detectors for output factor measurement of small fields in 6 and 10 MV flattening filter free photon beams using five different detectors. Field sizes were varied between 0.6 × 0.6 and 4.0 × 4.0 cm2. An indirect method (daisy-chaining) was applied to normalize the output factors. For the smallest field size, the variations of output factors compared among the detectors were 13%. Exradin A16 had the lowest output factor and increasing in sequence with CC01, microDiamond, microLion and EDGE detectors, respectively, for both energies. The similarity between CC01 and microDiamond output factor values were within 1.6% and 1% for all field sizes of 6 and 10 MV FFF, respectively. EDGE and microLion presented the highest values while ExradinA16 gave lowest values. In conclusion, IBACC01, Exradin A16, microLion, microDiamond and EDGE detectors seem to be the detectors of choices for small field output factor measurement of FFF beams down to 1.6 × 1.6 cm2. However, we could not guarantee which detector is the most suitable for output factor measurement in small field less than 1.6 × 1.6 cm2 of FFF beams. Further studies are required to provide reference information for validation purposes.

Comparison of dosimeter response: ionization chamber, TLD, and Gafchromic EBT2 film in 3D-CRT, IMRT, and SBRT techniques for lung cancer

This research was conducted by measuring point dose in the target area (lungs), heart, and spine using four dosimeters (PTW N30013, Exradin A16, TLD, and the Gafchromic EBT2 film). The measurement was performed in CIRS 002LFC thorax phantom. The main objective of this study was to compare the dosimetry of those different systems. Dose measurements performed only in a single fraction of irradiation. The measurements result shown that TLD has the least accuracy and precision. As the effect of volume averaging, ionization chamber reaches the discrepancy value up to -13.30% in the target area. EBT2 film has discrepancy value of <1% in the 3D-CRT and IMRT techniques. This dosimeter is proposed to be an appropriate alternative dosimeter to be used at point dose verification.

A.187 - Characterization of exradin A26 micro ionization chamber in unflattened photon beams

A.187 - Characterization of exradin A26 micro ionization chamber in unflattened photon beams

Direct measurement of electron beam quality conversion factors using water calorimetry.

In this work, the authors describe an electron sealed water calorimeter (ESWcal) designed to directly measure absorbed dose to water in clinical electron beams and its use to derive electron beam quality conversion factors for two ionization chamber types. A functioning calorimeter prototype was constructed in-house and used to obtain reproducible measurements in clinical accelerator-based 6, 9, 12, 16, and 20 MeV electron beams. Corrections for the radiation field perturbation due to the presence of the glass calorimeter vessel were calculated using Monte Carlo (MC) simulations. The conductive heat transfer due to dose gradients and nonwater materials was also accounted for using a commercial finite element method software package. The relative combined standard uncertainty on the ESWcal dose was estimated to be 0.50% for the 9-20 MeV beams and 1.00% for the 6 MeV beam, demonstrating that the development of a water calorimeter-based standard for electron beams over such a wide range of clinically relevant energies is feasible. The largest contributor to the uncertainty was the positioning (Type A, 0.10%-0.40%) and its influence on the perturbation correction (Type B, 0.10%-0.60%). As a preliminary validation, measurements performed with the ESWcal in a 6 MV photon beam were directly compared to results derived from the National Research Council of Canada (NRC) photon beam standard water calorimeter. These two independent devices were shown to agree well within the 0.43% combined relative uncertainty of the ESWcal for this beam type and quality. Absorbed dose electron beam quality conversion factors were measured using the ESWcal for the Exradin A12 and PTW Roos ionization chambers. The photon-electron conversion factor, kecal, for the A12 was also experimentally determined. Nonstatistically significant differences of up to 0.7% were found when compared to the calculation-based factors listed in the AAPM's TG-51 protocol. General agreement between the relative electron energy dependence of the PTW Roos data measured in this work and a recent MC-based study are also shown. This is the first time that water calorimetry has been successfully used to measure electron beam quality conversion factors for energies as low as 6 MeV (R50=2.25 cm).

Technical Note: Influence of Compton currents on profile measurements in small-volume ion chambers.

This work is to evaluate the effects of Compton current generation in three small-volume ionization chambers on measured beam characteristics for electron fields. Beam scans were performed using Exradin A16, A26, and PTW 31014 microchambers. Scans with varying chamber components shielded were performed. Static point measurements, output factors, and cable only irradiations were performed to determine the contribution of Compton currents to various components of the chamber. Monte Carlo simulations were performed to evaluate why one microchamber showed a significant reduction in Compton current generation. Beam profiles demonstrated significant distortion for two of the three chambers when scanned parallel to the chamber axis, produced by electron deposition within the wire. Measurements of ionization produced within the cable identified Compton current generation as the cause of these distortions. The size of the central collecting wire was found to have the greatest influence on the magnitude of Compton current generation. Microchambers can demonstrate significant (>5%) deviations from properties as measured with larger volume chambers (0.125 cm(3) and above). These deviations can be substantially reduced by averaging measurements conducted at opposite polarities.

SU‐E‐T‐623: Polarity Effects for Small Volume Ionization Chambers in Cobalt‐60 Beams

Purpose:To investigate the polarity effects for small volume ionization chambers in 60Co gamma‐ray beams using the Leksell Gamma Knife Perfexion.Methods:Measurements were made for 7 small volume ionization chambers (a PTW 31016, an Exradin A14, 2 Capintec PR0‐5P, and 3 Exradin A16) using a PTW UNIDOSwebline Universal Dosemeter and an ELEKTA solid water phantom with proper inserts. For each ion chamber, the temperature/pressure corrected electric charge readings were obtained for 16 voltage values (±50V, ±100V, ±200V, ±300V, ±400V, ±500V, ±600V, ±700V). For each voltage, a five‐minute leakage charge reading and a series of 2‐minute readings were continuously taken during irradiation until 5 stable signals (less than 0.05% variation) were obtained. The average of the 5 reading was then used for the calculation of the polarity corrections at the voltage and for generating the saturation curves.Results:The polarity effects are more pronounced at high or low voltages than at the medium voltages for all chambers studied. The voltage dependence of the 3 Exradin A16 chambers is similar in shape. The polarity corrections for the Exradin A16 chambers changes rapidly from about 1 at 500V to about 0.98 at 700V. The polarity corrections for the 7 ion chambers at 300V are in the range from 0.9925 (for the PTW31016) to 1.0035 (for an Exradin A16).Conclusion:The polarity corrections for certain micro‐chambers are large even at normal operating voltage.

SU‐E‐T‐204: Comparison of Absorbed‐Dose to Water in High‐Energy Photon Beams Based On Addendum AAPM TG‐51, IAEA TRS‐398, and JSMP 12

Purpose:Several clinical reference dosimetry protocols for absorbed‐dose to water have recently been published: The American Association of Physicists in Medicine (AAPM) published an Addendum to the AAPM's TG‐51 (Addendum TG‐51) in April 2014, and the Japan Society of Medical Physics (JSMP) published the Japan Society of Medical Physics 12 (JSMP12), a clinical reference dosimetry protocol, in September 2012. This investigation compared and evaluated the absorbed‐dose to water of high‐energy photon beams according to Addendum TG‐51, International Atomic Energy Agency Technical Report Series No. 398 (TRS‐398), and JSMP12.Methods:Differences in the respective beam quality conversion factors with Addendum TG‐51, TRS‐398, and JSMP12 were analyzed and the absorbed‐dose to water using 6‐ and 10‐MV photon beams was measured according to the protocols recommended in Addendum TG‐51, TRS‐398, and JSMP12. The measurements were conducted using two Farmer‐type ionization chambers, Exradin A12 and PTW 30013.Results:The beam quality conversion factors for both the 6‐ and 10‐MV photon beams with Addendum TG‐51 were within 0.6%, in agreement with the beam quality conversion factors with TRS‐398 and JSMP12. The Exradin A12 provided an absorbed‐dose to water ratio from 1.003 to 1.006 with TRS‐398 / Addendum TG‐51 and from 1.004 to 1.005 with JSMP 12 / Addendum TG‐51, whereas the PTW 30013 provided a ratio of 1.001 with TRS‐398 / Addendum TG‐51 and a range from 0.997 to 0.999 with JSMP 12 / Addendum TG‐51.Conclusion:Despite differences in the beam quality conversion factor, no major differences were seen in the absorbed‐dose to water with Addendum TG‐51, TRS‐398, and JSMP12. However, Addendum TG‐51 provides the most recent data for beam quality conversion factors based on Monte Carlo simulation and greater detail for the measurement protocol. Therefore, the absorbed‐dose to water measured with Addendum TG‐51 is an estimate with less uncertainty.

SU‐E‐T‐336: Dosimetric Properties of a New Solid Water High Equivalency Phantom for High‐Energy Photon Beams

Purpose:To investigate dosimetric properties in high‐energy photon beams for a Solid Water High Equivalency (SWHE, SW557) phantom (Gammex) which was newly developed as water mimicking material.Methods:The mass density of SWHE and SWHE/water electron density ratio are 1.032 g/cm3 and 1.005 according to the manufacturer information, respectively. SWHE is more water equivalent material in physical characteristics and uniformity than conventional SW457. This study calculated the relative ionization ratio of water and SWHE as a function of depth from the cavity dose in PTW30013 and Exradin A19 Farmer‐type ionization chambers using Monte Caro simulations. The simulation was performed with a 10 × 10 cm2 field at SAD of 100 cm for 4, 6, 10, 15, and 18 MV photons. The ionization ratio was also measured with the PTW30013 chamber for 6 and 15 MV photons. In addition, the overall perturbation factor of both chambers was calculated for both phantoms.Results:The relative ionization ratio curves for water and SWHE was in good agreement for all photon energies. The ionization ratio of water/SWHE for both chambers was 0.999–1.002, 0.999–1.002, 1.001–1.004, 1.004–1.007, and 1.006–1.010 at depths of over the buildup region for 4, 6, 10, 15, and 18 MV photons, respectively. The ionization ratio of water/SWHE increased up to 1% with increasing the photon energy. The measured ionization ratio of water/SWHE for 6 and 15 MV photons agreed well with calculated values. The overall perturbation factor for both chambers was 0.983–0.988 and 0.978–0.983 for water and SWHE, respectively, in a range from 4 MV to 18 MV.Conclusion:The depth scaling factor of water/SWHE was equal to unity for all photon energies. The ionization ratio of water/SWHE at a reference depth was equal to unity for 4 and 6 MV and larger up to 0.7% than unity for 18 MV.

SU‐E‐T‐804: Verification of the BJR‐25 Method of KQ Determination for CyberKnife Absolute Dosimetry

Purpose:Absolute calibration of the CyberKnife is performed using a 6cm‐diameter cone defined at 80cm SAD. Since kQ is defined using PDD values determined using 10×10 cm fields at 100cm SSD, the PDD must be corrected in order to correctly apply the quality conversion factor. The accepted method is based on equivalent field‐size conversions of PDD values using BJR25. Using the new InCise MLC system, the CK is capable of generating a rectangular field equivalent to 10×10 cm square field. In this study, a comparison is made between kQ values determined using the traditional BJR25 method and the MLC method introduced herein.Methods:First, kQ(BJR) is determined: a PDD is acquired using a 6cm circular field at 100cm SSD, its field size converted to an equivalent square, and PDD converted to a 10×10cm field using the appropriate BJR25 table. Maintaining a consistent setup, the collimator is changed, and the MLC method is used. Finally, kQ is determined using PDDs acquired with a 9.71×10.31cm at 100cm SSD. This field is produced by setting the field to a size of 7.77×8.25cm (since it is defined at 80cm SAD). An exact 10×10cm field since field size is relegated to increments of its leaf width (0.25cm). This comparison is made using an Exradin A1SL, IBA CC08, IBA CC13, and an Exradin A19. For each detector and collimator type, the beam injector was adjusted to give 5 different beam qualities; representing a range of clinical systems.Results:Averaging across all beam qualities, kQ(MLC) differed from kQ(BJR) by less than 0.15%. The difference between the values increased with detector volume.Conclusion:For CK users with standard cone collimators, the BJR25 method has been verified. For CK users the MLC system, a technique is described to determine kQ.Primary author is the President/Owner of Spectrum Medical Physics, LLC, a company which maintains contracts with Siemens Healthcare and Standard Imaging, Inc.

SU‐C‐304‐07: Are Small Field Detector Correction Factors Strongly Dependent On Machine‐Specific Characteristics?

Purpose:The current small field dosimetry formalism utilizes quality correction factors to compensate for the difference in detector response relative to dose deposited in water. The correction factors are defined on a machine‐specific basis for each beam quality and detector combination. Some research has suggested that the correction factors may only be weakly dependent on machine‐to‐machine variations, allowing for determinations of class‐specific correction factors for various accelerator models. This research examines the differences in small field correction factors for three detectors across two Varian Truebeam accelerators to determine the correction factor dependence on machine‐specific characteristics.Methods:Output factors were measured on two Varian Truebeam accelerators for equivalently tuned 6 MV and 6 FFF beams. Measurements were obtained using a commercial plastic scintillation detector (PSD), two ion chambers, and a diode detector. Measurements were made at a depth of 10 cm with an SSD of 100 cm for jaw‐defined field sizes ranging from 3×3 cm2 to 0.6×0.6 cm2, normalized to values at 5×5cm2. Correction factors for each field on each machine were calculated as the ratio of the detector response to the PSD response. Percent change of correction factors for the chambers are presented relative to the primary machine.Results:The Exradin A26 demonstrates a difference of 9% for 6×6mm2 fields in both the 6FFF and 6MV beams. The A16 chamber demonstrates a 5%, and 3% difference in 6FFF and 6MV fields at the same field size respectively. The Edge diode exhibits less than 1.5% difference across both evaluated energies. Field sizes larger than 1.4×1.4cm2 demonstrated less than 1% difference for all detectors.Conclusion:Preliminary results suggest that class‐specific correction may not be appropriate for micro‐ionization chamber. For diode systems, the correction factor was substantially similar and may be useful for class‐specific reference conditions.

SU‐E‐T‐207: Comparison of Integrated Tissue Air Ratio (ITAR) to Traditional TAR for Kilovoltage Pencil‐Beams

Purpose:Clinically viable depth dose determination in kilovoltage pencil‐beams is a great challenge that resulted in a published dosimetry method called ITAR, which involves measurement of air kerma and attenuation with a detector in a low scatter environment coupled with MCNP scatter calculations. The objective of this work is to compare ITAR to traditional TAR using inherently water‐proof microchambers that have only recently become commercially available.Methods:An Exradin A26 microchamber was centered 150 mm from a 100 kVp x‐ray source with 2 mm aluminum HVL. Depth dose in water from 16 to 24 mm in 2 mm increments was determined by: (1) placing blocks of Plastic Water LR near the source to minimize scatter and using previously published conversion coefficients [ITAR method] and (2) submerging the detector in a water tank with 2 mm thick Plastic Water LR walls and jogging the tank with motor controllers while keeping the detector position fixed [traditional TAR method]. Each method was repeated four to five times. For each repetition, dose was measured free in‐air to normalize the data for exponential regression.Results:Traditional TAR indicated higher depth dose than ITAR; differences ranged from 2.1% at 24 mm depth to 2.5% at 16 mm depth. However, the results of traditional TAR did not include a correction for Pq,cham because it is unknown for this detector type in these conditions. It is estimated that the component of Pq,cham due to the effect of water displacement alone is ∼0.94, but Pq,cham is likely several percent larger than 0.94 due to the energy dependency of the microchamber in the presence of low energy scatter that is not present during in‐air calibration.Conclusion:The ITAR method remains preferable for clinical depth dose determination in kilovoltage pencil‐beams due to Pq,cham being unknown for suitable detectors in relevant conditions.All four of the authors are either current full time employees, which include stock option grants, or consultants of Oraya Therapeutics Inc.

WE-AB-BRB-07: Alanine and Monte Carlo Determined Beam Quality Corrections for Nonstandard Fields of the Varian TrueBeam Accelerator

Purpose: To determine beam quality correction factors for flattened and unflattened beams of the Varian TrueBeam™ accelerator using alanine measurements and Monte Carlo calculations. Methods: Measurements were performed using L-α-alanine pellets encased in Virtual Water™ paddles. These were irradiated in liquid water using various field sizes of the 6MV and 10MV beam energies of the TrueBeam™, with both flattening-filter-free (FFF) and flattened beam modes. Measurements were also performed in a 10×10 cm2 60Co field. Pellets were read out by the National Physical Laboratory (NPL, Teddington, UK) using electron paramagnetic resonance (EPR) spectrometry, and results were corrected for energy dependence using graphite calorimetry. Beam quality correction factors (kQ, kQmsr,Q, and kQclin,Qmsr) were determined for three ionization chambers: an Exradin A12 Farmer-type chamber, an Exradin A1SL scanning chamber, and an Exradin A26 microchamber. All chambers were modeled in the egs_chamber user code of EGSnrc. Sources were created using Varian-supplied IAEA-compliant phase spaces and the BEAMnrc user code, and were validated by comparing measured and simulated dose profiles. Results: Measured and calculated kQ values agreed within uncertainties, showing the chamber models to be reliable. A comparison of measured and calculated kQmsr,Q results with TG-51-based values showed that TG-51 adequately accounts for variations in beam quality between flattened and unflattened 10×10 cm2 fields. Small field corrections, kQclin,Qmsr, were determined to be up to 5%, referenced to a 10×10 cm2 field of the same energy and mode. Conclusion: Beam quality corrections were determined for several beam energies of the TrueBeam™ accelerator using Monte Carlo calculations and were validated using measurements with alanine. Values of kQ determined using TG-51 were found to be adequate for FFF reference fields. However, small field corrections are necessary for both flattened and unflattened beams to ensure greater treatment planning accuracy.

Sci-Fri AM: Mountain - 01: Validation of a new formulism and the related correction factors on output factor determination for small photon fields

Small field dosimetry measurements including output factors are difficult due to lack of charged-particle equilibrium, occlusion of the radiation source, the finite size of detectors, and non-water equivalence of detector components. With available detectors significant variations could be measured that will lead to incorrect delivered dose to patients. IAEA/AAPM have provided a framework and formulation to correct the detector response in small photon fields. Monte Carlo derived correction factors for some commonly used small field detectors are now available, however validation has not been performed prior to this study. An Exradin A16 chamber, EDGE detector and SFD detector were used to perform the output factor measurement for a series of conical fields (5–30mm) on a Varian iX linear accelerator. Discrepancies up to 20%, 10% and 6% were observed for 5, 7.5 and 10 mm cones between the initial output factors measured by the EDGE detector and the A16 ion chamber, while the discrepancies for the conical fields larger than 10 mm were less than 4%. After the application of the correction, the output factors agree well with each other to within 1%. Caution is needed when determining the output factors for small photon fields, especially for fields 10 mm in diameter or smaller. More than one type of detector should be used, each with proper corrections applied to the measurement results. It is concluded that with the application of correction factors to appropriately chosen detectors, output can be measured accurately for small fields.