Exradin A12 Research Articles

WE‐D‐BRB‐04: Evaluation of MatriXX in Peripheral Dose Regions for IMRT and VMAT Dose Verifications

Purpose: To evaluate MatriXX and to develop a MatriXX correction technique for IMRT and VMAT dose verifications in peripheral (low) dose regions. Method and Materials: We compare a 2D ion chamber array — MatriXX with a reference Exradin A12 ion chamber in solid water phantoms. Comparisons were performed for a number of cases that involve different gantry and MLC dynamics including open beam, sweeping MLC gap mimicking IMRT, oscillating sweeping MLC gap mimicking VMAT, as well as clinical IMRT and RapidArc plans. Results: We have identified four types of error associated with MatriXX dose in peripheral regions: (1) Pixel‐dependent positive bias which is independent of dose level, cumulative over time, and dependent on operation parameters; (2) Over‐response to scattered beams by 2.0% on average for regions 2 – 15 cm from the field edge where patient and collimator scatters dominate; (3) Angular dependence up to 11% and 8% for primary and scattered (peripheral) radiations, respectively; (4) Round‐off error due to insufficient precision in conversion of raw signals to MatriXX software data for very small doses. In order to extend the application of MatriXX to peripheral regions, we have developed a method to correct for these errors. With corrections, MatriXX shows good agreement with A12 in all cases involving different gantry and/or MLC dynamics as well as the clinical plans. For both primary and peripheral doses, MatriXX shows good dose linearity down to about 1 – 2 cGy with an accuracy of within 1% of the local doses. Conclusion: Inherent dose errors of MatriXX were identified and thoroughly studied. A correction method was developed and successfully tested by a series of experimental and clinical plans. Therefore, we have demonstrated that, with proper corrections, MatriXX can be used for accurate 2D peripheral (low) dose measurements.

Investigation of three radiation detectors for accurate measurement of absorbed dose in nonstandard fields

To establish accurate experimental dosimetry techniques for reference dose measurements in nonstandard composite fields. A cylindrical PMMA phantom filled with water was constructed, at the center of which reference absorbed dose to water for a head and neck IMRT delivery was measured. Based on the proposed new formalism for reference dosimetry of nonstandard fields [Alfonso et al., Med. Phys. 35, 5179-5186 (2008)], a candidate plan-class specific reference (pcsr) field for a typical head and neck IMRT delivery was created on the CT images of the phantom. The absorbed dose to water in the pcsr field normalized to that in a reference 10 x 10 cm2 field was measured using three radiation detectors: Gafchromic EBT films, a diamond detector, and a guarded liquid-filled ionization chamber developed in-house (GLIC-03). Pcsr correction factors [formula in text] were determined for five different types of air-filled ionization chambers (Exradin A12, NE-2571, Exradin A1SL, Exradin A14, and PinPoint 31006) in a fully rotated delivery and in a delivery with the same MLC settings and weights but from a single gantry angle (a collapsed delivery). The combined standard uncertainty in measuring the correction factor [formula in text] using the three dosimetry techniques was 0.3%. For all the air-filled ionization chambers and the pcsr field tested, the correction factor was not different from unity by more than +/- 0.8%. For the fully rotated delivery, the correction factors were in a narrow range of 0.9955-0.9986, while in the collapsed delivery, they were in a slightly broader range of 0.9922-1.0048. In the collapsed delivery, the Farmer-type chambers (Exradin A12 and NE2571) had very similar correction factors (0.9922 and 0.9931, respectively), whereas the correction factors for the smaller chambers showed more distinct chamber-type dependence. The authors have established three experimental dosimetry techniques that allow reference measurements of nonstandard field correction factors [formula in text] for air-filled ionization chambers at the 0.3% 1sigma uncertainty level. These techniques can be used to determine criteria for the selection of plan-class specific reference fields and ultimately improve clinical reference dosimetry of nonstandard fields.

Ionization chamber gradient effects in nonstandard beam configurations

For the purpose of nonstandard beam reference dosimetry, the current concept of reporting absorbed dose at a point in water located at a representative position in the chamber volume is investigated in detail. As new nonstandard beam reference dosimetry protocols are under development, an evaluation of the role played by the definition of point of measurement could lead to conceptual improvements prior to establishing measurement procedures. The present study uses the current definition of reporting absorbed dose to calculate ionization chamber perturbation factors for two cylindrical chamber models (Exradin A12 and A14) using the Monte Carlo method. The EGSnrc based user-code EGS_chamber is used to calculate chamber dose responses of 14 IMRT beams chosen to cause considerable dose gradients over the chamber volume as previously used by Bouchard and Seuntjens ["Ionization chamber-based reference dosimetry of intensity modulated radiation beams," Med. Phys. 31(9), 2454-5465 (2004)]. The study shows conclusively the relative importance of each physical effect involved in the nonstandard beam correction factors of 14 IMRT beams. Of all correction factors involved in the dosimetry of the beams studied, the gradient perturbation correction factor has the highest magnitude, on average, 11% higher compared to reference conditions for the Exradin A12 chamber and about 5% higher for the Extradin A14 chamber. Other perturbation correction factors (i.e., Pwall, Pstem, and Pcel) are, on average, less than 0.8% different from reference conditions for the chambers and beams studied. The current approach of reporting measured absorbed dose at a point in water coinciding with the location of the centroid of the chamber is the main factor responsible for large correction factors in nonstandard beam deliveries (e.g., intensity modulated radiation therapy) reported in literature. To reduce or eliminate the magnitude of perturbation correction factors in nonstandard beam reference dosimetry, two possible ways to report absorbed dose are suggested: (1) Reporting average dose to the sensitive volume of the chamber filled with water, combined with removing the reference field implicit gradient effect when measuring output factors, and (2) reporting average dose to the chamber itself during output factor verifications. The first option could be adopted if clinical beam correction factors are negligible. The second option could simplify quality assurance procedures when correction factors are not negligible and have to be calculated using Monte Carlo simulations.

TU-D-BRB-02: Advanced Techniques to Determine Plan-Class Specific Reference Field Correction Factors for Accurate Dosimetry of Nonstandard Beams

Purpose: To establish experimental reference dosimetry techniques for measuring plan-class specific reference field correction factors for the proposed new formalism for reference dosimetry in nonstandard fields. Methods and Materials: A Lucite cylindrical phantom filled with water was constructed in the center of which reference absorbed dose to water for an IMRT delivery was measured. A plan-class specific reference (pcsr) field for a typical head and neck IMRT delivery was created on CT images of the phantom. The absorbed dose in the pcsr field normalized to that in a 10×10 cm2 field was measured using three reference dosimeters: Gafchromic® EBT films, a diamond detector, and an in-house developed guarded liquid ionization chamber (GLIC-03). Pcsr correction factors were determined for five air-filled ionization chambers (Exradin A1SL, Exradin A12, Exradin A14, PinPoint® 31006, and NE2571) in a fully-rotated delivery and in a delivery from a single angle (collapsed delivery). Results: The relative dose measurement accuracy of the three dosimeters was 0.56%, 0.10%, and 0.29% for the film, diamond detector, and GLIC-03, respectively. The combined relative standard uncertainty in measuring using the three techniques was 0.3%. For all chambers and the pcsr field selected, was unity to within ±1% and in the range of 0.990–0.993 and 0.990–1.003 in the fully-rotated and collapsed deliveries, respectively. The correction factors were the same for the chambers in the fully-rotated delivery. In the collapsed delivery, the Farmer-type chambers (Exradin A12 and NE2571) had a larger but consistent correction factor (0.990 and 0.991, respectively). The correction factors for the smaller chambers were close to unity but showed chamber type dependence. Conclusions: Our techniques provide a potential to improve the dosimetric accuracy in suitable plan-class specific reference fields. The techniques of determining the correction factor will be extremely valuable for other nonstandard field deliveries.

SU-FF-T-678: Commissioning the 3D-Line MicroMLC Controller in the Pinnacle TPS

Purpose: To prepare and commission a parametric Collapsed Cone Convolution Superposition model in the Pinnacle Treatment Planning System in order to accurately describe the output of an Elekta Precise series Linear Accelerator fitted with the 3D‐Line micro MLC controller at two clinical photon energies of 6MV and 10MV. The model is intended to be used for clinical treatment planning purposes for Stereotactic RadioSurgury, Fractionated SRS, and IMAT/VMAT. Method and Materials: Data was collected using an Exradin A16 0.007cc microChamber in a 3D water phantom. Scans of the In‐plane and Cross‐plane dose profiles at Dmax, 5, 10, and 15cm, and depth dose curves, were acquired for field sizes from 0.581×0.581 cm2 to the maximum open field size, and two rectangular fields; all at 90cm and 100cm SSD. The data were used, along with custom optimization sequence, to adjust model parameters in the TPS. Output factor measurements were made with an ADCL traceable Electrometer and the microchamber, with a final cross calibration from open field mMLC to 10×10cm2 using a 0.1cc Farmer Chamber, all at 10cm depth. Results: Comparison between the measurements and the model output for the measured field geometries indicates good agreement at field edges, and for depth dose curves, with larger error apparent at the smallest field sizes. The output factors computed for the model were within 6% for all field sizes, indicating no large discrepancies with the model. Monitor unit comparisons for several field sizes also demonstrated agreement with the previously commissioned Ergo TPS. Conclusion: These results demonstrate the feasibility of commissioning the 3D‐Line mMLC in a TPS other than the Ergo TPS, which provides for the opportunity to maintain a single TPS in sites using Pinnacle. Future research will directly compare plans generated in both Pinnacle, and Ergo, to delivered machine fluence measurements.

WE‐E‐BRD‐05: EGSnrc Benchmarking Against High‐Precision Lateral Distributions of X‐Rays Produced in Thick Targets

Purpose: The EGSnrc Monte Carlo code was benchmarked against newly measured angular distributions of x‐rays produced by electron beams stopping in thick targets. Method and Materials: The electron beam was generated using the National Research Council Canada (NRC) Vickers linear accelerator. Five materials: Be, Al, Cu, Ta and Pb, were used as targets and measurements were made at 20 MeV. The absolute absorbed dose was measured at different lateral positions with various ionization chambers (PTW 30013, Exradin A19, and NE2505/3) using various build‐up caps made of PMMA, Brass, Cu, Sn and W alloy. The BEAMnrc code was used to simulate the experimental setup. The geometry, beam and material properties were implemented in BEAMnrc, while the “cavity” user code was used to score the absorbed dose in the ionization chamber air cavity per incident electron up to angles of 25°. Results: For medium‐Z targets the shape agreement between measured and simulated distributions is better than 1 % for the entire scanning range. In terms of absolute dose, it was found that there are differences of up to 4 % related to the build‐up caps used for the ionization chamber. When considering the five targets, a trend with Z of the target material is noticed. For medium‐Z targets the ratio of calculated to measured dose has a variation below 1 %, for the Be target the ratio increases 12 % with the scanning position, while for the Pb target the ratio decreases 5 %. Conclusion: The present study shows that the EGSnrc code predicts x‐ray angular distributions with shapes in agreement with the measured data at the 1 % level for medium‐Z targets. There are still unresolved discrepancies in the absolute dose related to the build‐up cap used for the ionization chamber and the target material.Partial support from NIH R01 CA104777‐01A2.

SU‐GG‐T‐445: Dosimetric Analysis of Attenuation in the Leksell Gamma Knife PERFEXION Calibration Phantom Adaptor

Purpose: The purpose of this study was to evaluate the attenuation of 16 mm beams caused by the ELEKTA calibration phantom adaptor used for the new Leksell Gamma knife PERFEXION (PFX). Method and Materials: For the calibration of PFX, Elekta AB provides a 160 mm diameter polystyrene phantom with an aluminum adaptor of C‐shape which has stainless steel fixation screws for holding the phantom in the irradiation position in PFX. An Exradin A16 ion chamber (collecting volume 0.007 cm3) and a Standard Imaging MAX 4000 electrometer were used to measure radiation output at the center of the 160 mm diameter polystyrene phantom for each of eight identical sectors and 16 mm collimator. Results: While studying the uniformity of output from each of the 8 sectors it was noticed that for sector No. 3 (270 degree) and sector No. 7 (90 degree), the outputs measured were lower by about 2% as compared to other sectors. On detailed investigation with Kodak EDR2 film, we found that for these two sectors, there were 2 beams of 16 mm diameter passing through the aluminum adaptor and the stainless steel fixation screws which caused attenuation of these two beams and hence a reduction in output. Conclusion: The attenuation by phantom adaptor introduces a systematic error in output calibration of PFX. The overall reduction in output measured for all 8 sectors combined is about 0.5%. We suggest that until Elekta AB redesigns a new phantom adaptor for PFX calibration, physicists should be aware of the attenuation of 16 mm beams by the adaptor and incorporate this effect in the output calibration of the PERFEXION.

An experimental and computational investigation of the standard temperature‐pressure correction factor for ion chambers in kilovoltage x rays

For ion chambers with cavities open to the surrounding atmosphere, the response measured at a given temperature and pressure must be corrected using the standard temperature-pressure correction factor (P(TP)). A previous paper based solely on Monte Carlo simulations [D. J. La Russa and D. W. O. Rogers, Med. Phys. 33, 4590-4599 (2006)] pointed out the shortcomings of the P(TP) correction factor when used to correct the response of non-air-equivalent chambers for low-energy x-ray beams. This work presents the results of several experiments that corroborate these calculations for a number of ion chambers. Monte Carlo simulations of the experimental setup revealed additional insight into the various factors affecting the extent of the breakdown of P(TP), including the effect of impurities and the sensitivity to chamber dimensions. For an unfiltered 60 kV beam, the P(TP)-corrected response of an NE 2571 ion chamber measured at 0.7 atm was 2.5% below the response measured at reference conditions. In general, Monte Carlo simulations of the experimental setup using EGSnrc were within 0.5% of measured values. EGSnrc-calculated values of air kerma calibration coefficients (N(K)) at low x-ray energies are also provided as a means of estimating the level of impurities in the chambers investigated. Calculated values of N(K) normalized to the value measured for a 250 kV beam were obtained for three chambers and were within 1% of experiment with one exception, the Exradin A12 in a 50 kV beam.

Characterization of the ADII‐33 diamond detector

Dosimetry characteristics of the ADII-33 diamond detector were investigated. Sensitivity, stability, bias voltage, hardening, dose rate, energy dependence, and spatial resolution were examined. Current generated in the detector was found to be proportional to the bias voltage applied to the detector with stability degrading as a function of the bias voltage. The average current increased with increasing bias voltage. The statistical fluctuation in current was less than 0.3% regardless of the applied bias voltage. The optimal bias voltage, at which the current uncertainty is negligible, was found to be 125+/-25 V. The detector was hardened up to 85 kGy without significant degradation in output signal. A sub-linearity in the current as a function of dose rate was observed when the dose rate varied from 600.0 to 11.1 cGy/min. A fitting parameter of delta=0.978 was observed in the power relationship of IinfinityDdelta. When comparing the diamond detector readings to corrected ion chamber readings over a wide energy range of electron beams, differences of only 0.2% were observed suggesting no energy dependence for electron beam. PDD curves for a 10 x 10 cm2 field for 6 and 20 MV photon beams measured with the diamond detector and the farmer type ion chamber were also compared and they closely agreed up to a depth of 14 cm. Beyond a depth of 14 cm, diamond detector starts to overestimate the PDD curve reaching difference of 1.90% and 1.0% at 18 cm depth for 6 and 20 MV, respectively, to those values measured with the ion chamber. The diamond detector presents slightly better spatial resolution than the Exradin A16 microchamber. We conclude that at the optimal bias voltage, this new diamond detector is stable and the uncertainties in the current will not affect its suitability for clinical use. If compared against a calibrated ion chamber to correct any energy and dose rate dependence and considering any radiation damage effect, this diamond detector can be used to measure absolute and relative dose.

TU‐C‐224C‐07: A Comprehensive Dosimetric Protocol for the Cyberknife Radiosurgery System

Purpose: To propose dosimetric guidelines specifically designed for the Cyberknife radiosurgery system. Non‐availability of 10×10cm2 field and use of small circular collimators (5mm to 60mm) pose serious problems, that have been faced in this study by means of 8 different detectors and Monte Carlo simulation. This work is oriented to measurement of total scatter factors (Sc,p) and to reference dosimetry, though indications will also be given in view of a comprehensive guideline. Method and Materials: PTW PinPoint 31014, Exradin A16 and T14P microchambers, TN 502RDM micromosfet, PTW 30008 diode and TM60003 diamond, MD55 and EBT radiochromic films were used to measure Sc,p. Monte Carlo simulations (BEAMnrc) were used to produce phase space descriptions at the exit plane of each collimator, to calculate: 1) theoretical Sc,p values in water, and 2) correction factors to be applied to Sc,p as measured by 5 detectors (PinPoint, A16, T14P, diode, diamond), obtained by simulating shape and chemical composition of each detector. BEAMnrc was also used to calculate stopping power ratios and chamber correction factors for the Cyberknife linac, to decide whether values of kQ from the IAEA398 protocol could be applied without using a 10×10cm2 field. Results: Sc,p of the 5mm collimator as measured by simulated detectors averaged 0.653 − 9%+14%. Variation for larger collimators was smaller. After Monte Carlo correction, Sc,p of the 5mm collimator became 0.686 −2%+1%. Pure Monte Carlo calculation gave Sc,p=0.715 +/−1%. Calculation of correction factors showed that kQ values for the investigated chambers could be chosen when using IAEA398, introducing +/−0.2% uncertainty. Conclusion: Pure Monte Carlo calculation gave higher values of Sc,p compared to Monte Carlo‐corrected measurement. The latter is to be preferred because correction factors are less sensitive to beam parameters than pure calculation of Sc,p. For determination of Sc,p use of microchambers and Monte Carlo correction is recommended.

WE-D-224C-01: Water Equivalent Multichannel Dosimeter Arrays for External Beam Radiotherapy

Introduction: Scintillation dosimeters consisting of a single scintillating fiber coupled to an optical fiber and read with a CCD camera have shown accurate dose measurement in high-energy external beam radiotherapy. This work presents the next step, which is to investigate the development of a multi-channel dosimeter array in conjunction with a CCD camera for photodetection. Methods and Materials: The light collection of the CCD camera (Apogee Alta U2000c) was first studied to evaluate the maximum number of detectors that can be read simultaneously. We then looked at possible sources of dose perturbation with a single-fiber detector surrounded by other optical fibers. We constructed a prototype array with 10 detectors. Finally, depth-dose and cross-plane profiles were compared with measurements taken with small ion chambers (Exradin models A14 and A16). Results: We found up to 3000 individual fibers could be detected simultaneously with the full CCD chip. No dose perturbations were seen when a plastic optical fiber was used to transmit the light from the scintillating fiber to the CCD. Depth dose curves measured in water with a single scintillation dosimeter with up to 75 plastic optical fiber in the beam showed no discrepancy to within 0.3% when compared to the same curve taken with an ion chamber and without the plastic fibers. The ten-fiber prototype allowed precise evaluation of profile and depth dose curves in a single irradiation. Conclusion: This work has shown that use of a multi-channel scintillation dosimeter is feasible. The prototype of 10 detectors produced excellent results and could be extended up to 3000 detectors in the near future.

SU-FF-T-26: A Modified TG-51 Formalism for Gamma Knife Dose Rate Calibration

Purpose: The first Lekshell Gamma Knife was developed in 1968. Currently there is no national standard calibration protocol developed specifically for gamma knife dose calibration. Many gamma knife centers perform calibration using TG-21 protocol, while other centers use TG-51 calibration factor ND,W. The purpose of this study is: to exam TG-21 and TG-51 calibration formalism; to compare result of gamma knife dose calibration using these two methods; and to determine if a simplified formula can be applied to gamma knife dose rate calibration. Method and Materials: Exradin A16 ion chamber (wall material C552) was placed in a 16cm spherical polystyrene phantom in our Lekshell Gamma Knife 4C. Dose rate can be calculated using Ngas or ND,W: where M is corrected charge reading; k an unknown correction for polystyrene phantom. Left hand side of the equations is dose rate to water per unit charge. For a given chamber, the factors on the right side of equations are all known except k, which can be determined by comparing above equations. Results: For our A16 chamber: Even if we ignore the unknown correction factor k, the difference between TG-21 and TG-51 calculated dose rate is 1.9%. Thus we estimated k to be in the order of 2% for this A16 ion chamber. Conclusion: A “modified TG-51” formula, , can be used to calibrate gamma knife dose rate. Correction factor k can be determined from the values of ND,W, Ngas, and other known factors found in TG-21. Difference between “modified TG-51” and TG-21 calculated dose rate is expected to be less than 2%.

Absorbed dose to water reference dosimetry using solid phantoms in the context of absorbed‐dose protocols

For reasons of phantom material reproducibility, the absorbed dose protocols of the American Association of Physicists in Medicine (AAPM) (TG-51) and the International Atomic Energy Agency (IAEA) (TRS-398) have made the use of liquid water as a phantom material for reference dosimetry mandatory. In this work we provide a formal framework for the measurement of absorbed dose to water using ionization chambers calibrated in terms of absorbed dose to water but irradiated in solid phantoms. Such a framework is useful when there is a desire to put dose measurements using solid phantoms on an absolute basis. Putting solid phantom measurements on an absolute basis has distinct advantages in verification measurements and quality assurance. We introduce a phantom dose conversion factor that converts a measurement made in a solid phantom and analyzed using an absorbed dose calibration protocol into absorbed dose to water under reference conditions. We provide techniques to measure and calculate the dose transfer from solid phantom to water. For an Exradin A12 ionization chamber, we measured and calculated the phantom dose conversion factor for six Solid Water phantoms and for a single Lucite phantom for photon energies between 60Co and 18 MV photons. For Solid Water of certified grade, the difference between measured and calculated factors varied between 0.0% and 0.7% with the average dose conversion factor being low by 0.4% compared with the calculation whereas for Lucite, the agreement was within 0.2% for the one phantom examined. The composition of commercial plastic phantoms and their homogeneity may not always be reproducible and consistent with assumed composition. By comparing measured and calculated phantom conversion factors, our work provides methods to verify the consistency of a given plastic for the purpose of clinical reference dosimetry.

SU-FF-T-244: Investigation of Anomalous Recombination Behaviour in Cylindrical Ionization Chambers

Purpose: Measurements using air-filled ionization chambers must be corrected for the effect of ion recombination. In recent years a number of authors have presented data for cylindrical and parallel chambers that do not agree with the standard Boag theory. An experimental investigation was carried out to look at the possible mechanisms that have been proposed for these deviations. Method and Materials: Four different Farmer-type chambers - PTW30001, NE2571, NE2581, Exradin A12 — were used to look at how the recombination behaviour depended on: i) chamber type; ii) continuous or pulsed radiation; iii) dose per pulse; iv) modality (photons or electrons). Three radiation sources were used: i) Sr-90 check source; ii) Co-60 beam; iii) Elekta Precise linac. Results: It was found that all four chamber types showed similar non-linear behaviour at polarizing voltages > 250 V using a low-doserate Sr-90 check source. This was a little surprising considering the significant differences in chamber design for these types. Also, all chamber types showed a difference in the shape of the 1/I vs 1/V plot for opposite polarities — the chamber signal was larger when collecting negative charge - indicating that the effect may be due to free-electron collection and/or charge multiplication. In the linac measurements no difference in behaviour was seen between photon and electron beams (as expected) but the non-linear curvature and difference between polarities was only seen at low dose per pulse values (< 0.02 cGy). Stem and cable irradiation were also investigated and found to be insignificant. Conclusion: Deviations from classical Boag recombination theory have been observed for “well-designed” Farmer chambers in a range of photon and electron beams. Measurements to date cannot single out one mechanism as the primary effect but indicate that insulator/stem irradiation and free-electron collection are not significant.

WE‐D‐T‐617‐05: Evaluation of the New Exradin A16 Micro‐Chamber for the Purpose of Water Measurements of Small SRS Collimator by Comparison to Film Dosimetry and Monte‐Carlo Calculations

Purpose: Direct water measurements of small fields in general and small SRS collimators in specific have been a long known problem. The typical ion chambers, which are used with 3D water phantoms have a volume of about 0.125cc, which is not suitable for measuring small fields. The recently introduce Exradin A16 chamber, with a volume of 0.007cc was designed to overcome the problem by allowing accurate measurements of small fields in water. The purpose of this work is to validate the suitability of this chamber for small fields water measurements. Method and Materials: In order to evaluate the suitability of the A16, depth dose curves and a set of profiles were taken with this ionization chamber using SRS collimators ranging from 1cm to 4cm in diameter, in steps of 0.25cm increment. The scans were then compared with film dosimetry of the same collimators and Monte‐Carlo simulations. The films used were the Kodak XDR extended range films in Perspex phantom, which were scanned and analyzed using the Vidar 16 scanner and the RIT software. The Monte‐Carlo simulation was done using the BEAMnrc code. For the larger collimators (2.5cm and up) a set of water measurement were performed also using the standard Wellhoffer chambers (0.125cc) Results: The result showed excellent matching between all evaluations media (water measurements with the A16 and the standard chamber, film dosimetry and Monte‐Carlo simulation). The only discrepancy was with deeper depths in the films, which was due to the phantom used. Conclusion: The A16 is suitable and should be used for small field water measurements.

SU-FF-T-238: Dosimetry of Small Beams Used in Radiosurgery: A Comparison Between Different Detectors and Monte Carlo Simulation

Purpose: To compare experimental results from different detectors to Monte Carlo simulation in the dosimetry of small fields employed in the Cyberknife radiosurgery system. The Monte Carlo code has also been used to decide whether protocols that require a 10×10 cm2 field can be used for dosimetry under reference conditions. Method and Materials: Total scatter factors (sc,p) have been measured by means of the PTW PinPoint, Exradin A16 and T14P microchambers, Thomson and Nielsen micromosfet, PTW diamond and diode detectors, and MD 55-2 radiochromic films. A Monte Carlo calculation (BEAM code) was used to simulate the Cyberknife linac and to calculate sc,p and correction factors to be applied to raw data for various detectors. Two Farmer-type chambers were also simulated in order to calculate the ratio between kQ factors for the Cyberknife and of for a standard 6MV linac. Results: Monte Carlo simulations gave results in agreement within ±1% with experimental results for sc,p, PDD and profiles for collimators above 10 mm. Results for sc,p of smaller collimators (5 to 10 mm) agreed with radiochromic film measurements within ± 2%, while results with ionization chambers and diamond suffer from a systematic underestimation, resolved after application of the Monte Carlo correction factor. Micro-mosfet detectors showed good results as compared to Monte Carlo and radiochromic films, but suffered from poor reproducibility. The PTW diode detector seemed to systematically overestimate sc,p values; for the 5 mm collimator, a value of 0.746 has been obtained versus values in the range 0.67–0.71 obtained with Monte Carlo, radiochromic films, ionization chambers and diamond. The ratio between kQ factors at different beam qualities is 1.002 ± 0.5%. Conclusion: Monte Carlo can be used in the dosimetry of small beams to help solve experimental problems due to the finite dimension and to non-water equivalence of the detectors.

SU-FF-T-60: A Dosimetric Comparison of Three Commercially Available Carbon Fiber Table Extension Boards

Purpose: To measure and compare the changes in dose distributions produced by the presence of a carbon fiber extension board (used to provide necessary patient extension beyond the gun end of the treatment couch). The tested carbon fiber extension boards are designed to be “radiotranslucent” so that the transmitted radiation beam is not significantly altered. We measured the attenuation due to the boards being placed in the beam's path, compared the relative dose distributions obtained using different boards, and also examined the effect of “build‐up”. Method and Materials: The three boards evaluated were the Med‐Tec Type‐S system, the Brainscan H&N Tx system, and the Orfit HP long baseplate for IMRT.Dosimetric data were obtained for 6 MV photons with an Exradin A12, 0.65 cm3 Farmer ionization chamber, with Kodak EDR film, and with an Exradin P11, 0.62 cm3 plane parallel chamber (for measuringdose buildup in a Gammex RMI solid water phantom). Results: The Orfit board's maximum attenuation was 1.2%. The Med‐Tec Type‐S board's maximum attenuation was 2.3%. The Brainscan board's maximum attenuation was 13.7%. The isodose distribution and profiles for each board will be presented. Conclusion: We determined that the Orfit board did not significantly alter the radiation beam and resulting dose distribution. Its adaptability to various tables was favorable compared to the Med‐Tec system, which is well suited for the Varian Exact couch. The Med‐Tec board is also well suited for therapeuticradiation, but had a higher change in attenuation than the Orfit system. The Brainscan board was sub‐optimal due to non‐air equivalent material used as structural support in the board's interior, which caused significant beam attenuation. Conflict of Interest: Research was supported by equipment loan from Orfit Industries and Med‐Tec, Inc.

TU-C-T-6E-04: Clinical Reference Dosimetry of a “Hi-Art II” Helical Tomotherapy Machine

Purpose: To develop a procedure for calibrating “Hi-Art II” helical tomotherapy machines using TG-51 and analyze the uncertainties associated with minor departures from the TG-51 protocol. Method and Materials: Physical limitations of the tomotherapy unit (85cm SAD) restrict the user from performing a rigorous TG-51 calibration. The Radiological Physics Center determined the ionization ratio (IR) using a water phantom and Exradin A12 ion chamber for a 40cm × 5cm field (8.92 eq.sq.). The IR was converted to %dd(10)x using a published relationship between %dd(10)x and IR. The beam quality conversion factor, kQ, was determined, Pion was measured and Ppol was assumed to be unity. The ion chamber center electrode was placed at dmax, instead of the recommended 10cm depth. The output/min was calculated using TG-51 and verified using a newly designed TLD jig. Results: The measured IR indicates a beam energy slightly greater than that for a 4 MV x-ray beam. The reference calibration resulted in a dose rate of 882.7 cGy/min at dmax for a 40cm × 5cm field at 85cm SAD that was within 1% of the dose rate (890.6 cGy/min) set by the factory where 0.4% of the difference was due to ADCL calibration differences and an un-flattened beam. The TLD/ion-chamber dose ratio was 0.993 (std.dev.= 0.007). An analysis of uncertainties associated with measuring IR and conversion to %dd(10)x, use of a 40 × 5 cm2 field instead of 10 × 10 cm2 and calibration at dmax instead of 10 cm results in a possible increased uncertainty of 0.5% in the final reference calibration. We believe this to be negligible. Conclusion: A procedure was developed to calibrate “Hi-Art II” tomotherapy machines using TG-51 with a minimal increase in the uncertainty of the final calculated dose rate. Work supported by PHS grant CA10953 awarded by NCI, DHHS.