Build-up Cap Research Articles

SU‐FF‐T‐348: Beam Attenuation and Beam Spoiling Properties of An Electromagnetic Array Used for Patient Localization and Tumor Tracking

Purpose: To investigate the beam attenuation and beam spoiling properties of the Calypso electromagnetic array used for patient localization and tumor tracking during the radiation delivery Method and Materials: One of the main components of the Calypso 4D system is an electromagnetic array placed above patients who are implanted with transponders. We measured both the narrow and board beam attenuation properties of the array including the effect of beam angle (0 to 90 degree). A photon diode placed in cylindrical graphite buildup cap was used for narrow geometry, field size of 1 cm ×1 cm, and a 0.6 cc cylindrical farmer chamber placed in a polystyrene buildup cap for broad geometry, field size of 10 cm × 10 cm. For broad beam geometry, distances of 2, 5, and 10 cm between the chamber and the array were used. Measurements were performed using a Varian Clinac IX linear accelerator with 6 MV and 15 MV photon beams. Beam spoiling properties of the array was studied by placing the array above a water equivalent phantom with an imbedded Markus parallel plate chamber and measuring depth doses. Depth doses were measured for both 6 MV and 15 MV photon beams with 10 cm × 10 cm field size for distances of 2, 5, and 10 cm between array and phantom surface. Results: Narrow beam geometry attenuation is 1.5 % and 1.3% for 6 MV and 15 MV beam respectively. Beam angle dependence is more pronounced at angles greater than 70 degrees with attenuation greater than 5%. Broad beam attenuation, in clinical cases, is nonexistent due to scatter contribution. Spoiling properties could be appreciable. Conclusion: Attenuation properties of the array may be ignored for treatment delivery for most clinical cases. Spoiling effect could be appreciable depending on array distance to patient surface.

SU‐FF‐T‐341: Small Field Dosimetry for Electron Beams

Purpose: This study examines the accuracy of dose calculation for electron beam in small circular fields for Varian accelerators using an empirical method based on dose per incident fluence. Method and Materials: Fractional Depth doses (FDD) at SSD = 100 cm for open beam and small circular cutouts (r = 0.5, 1.0, 1.5, 2.0, 3.0, 4.0, 5.0 and 5.6cm) are renormalized to surface. The resulting depth dose is then normalized to the broad beam FDD to determine a fractional FDD, FDDf for.all electron energies (4, 6, 9, 12, 16, and 20MeV). The head scatter factor (H), defined as the ratio of electron energy fluence at point of interest to open cone at otherwise the same geometry, is measured using a diode in air without buildup cap as a function of cutout radius for all electron energies and cone sizes (6, 10, 15, 20 and 25cm2). The phantom scatter factor, PSF=BF/H, defined as the ratio of blocking factor in water at reference depth and is measured as a function of radius, electron energy, and SSD (100, 120 and 140cm). Results: It is found that H is a single function of renormalized radius, rN = r*100/(SSD+d) and is independent of SSD for a given electron energy and cone size. Normally, the radius dependence of H is a function of both the SSD and cone size. The extrapolated value of PSF from percent depth dose (PDD) renormalized to the surface is remarkably close to the measured PSF data, to within 2%. The calculated FDDf has a bowl‐shaped dependence on the depth with the height increasing with increasing electron energy. Conclusion: By normalizing the Fractional depth dose (FDD) to the surface, most characteristics of small field electron dosimetry can be simplified and adequately described by H, a quantity describing the incident electron energy fluence.

TU-D-BRB-07: Determination of Neutron Dose Due to a Therapeutic Proton Beam Incident On a Closed Tungsten MLC Using the Dual Hydrogenous/non-Hydrogenous Ionization Chamber Method

Purpose: To measure neutron dose around a tungsten MLC used for proton beam therapy using the dual ionization chamber (IC) method. Method and Materials: ICs were constructed with large 10cc volumes to enable low dose measurements in the mGy range. The chambers are identical aside from wall, electrode, and guard-ring material with one constructed of A150 tissue-equivalent plastic (T) and filled with methane based tissue-equivalent gas and the other magnesium (U) and filled with argon gas. IC(T) has approximately the same sensitivity to neutrons and photons while the non-hydrogenous IC(U) exhibits a lower sensitivity to neutrons. The dose from neutrons and photons can be expressed as DN=(RT-RU)/(1-kU) and DG=(RU- kURT)/(1-kU) where RU,RT are IC readings and kU is the relative sensitivity of IC(U) to neutrons which varies with energy. Measurements were made with 230MeV proton beams incident on a closed MLC with the ICs on the beam axis 21cm from the MLC. Open field measurements were made with attenuation by 0, 0.5, 2, 5, 9, and 10cm of solid water. The ICs were calibrated against a Farmer chamber in air with a 6MV beam and suitable buildup caps. Results: Initial analysis of the measurements suggests an average neutron energy of 2.6MeV corresponding to kU=0.033. Measured neutron dose with the MLC closed and detector geometry stated above is 1.67mGy/Gy where IC(T) was used to determine dose in the open field. Conclusion: Initial measurements of neutron dose from a proton therapy beam incident on a MLC were performed which indicate the validity of the dual chamber method for determining neutron dose around a proton beam. This work was supported by the US Army Medical Research and Materiel Command under Contract Agreement No. DAMD17-W81XWH-07-2-0121. Opinions, interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the US Army.

Sci-Fri PM: Planning-01: Measured electron and x-ray angular distribution data for benchmarking Monte Carlo codes.

Monte Carlo (MC) studies of the output of medical linear accelerators have demonstrated that in-air profiles are useful in the beam commissioning process. A recent investigation of x-ray profiles (Tonkopi et al, Med. Phys 32 (9), 2005) showed very good agreement between measurement and EGSnrc calculations but to achieve this level of agreement the beam linac spot size, energy and angular divergence had to be treated as variables. In this project we carried out measurements and MC calculations for an electron accelerator for which the initial beam parameters are well known. Two sets of investigations were carried out. In the first we measured electron scatter distributions for a range of scattering foils and electron energies of 13 and 20 MeV. The profiles were parameterised and compared to EGSnrc Monte Carlo calculations. It was found that generally the EGSnrc calculations gave agreement with the measurements within 1.5 %. In the second investigation, which is on-going, in-air profiles were obtained for photon beams produced using different targets (from beryllium to lead). Measured angular distributions were obtained using ion chambers with different build-up caps (low and high-Z) and the sensitivity of the data to small changes in geometry (e.g., moving the x-ray target) was investigated. The photon energy fluence was calculated using EGSnrc and preliminary indications are that the measured and calculated distributions agree to better than 5 %. Work supported in part by NIH grant R01 CA104777-01A2.

SU‐GG‐T‐449: Peripheral Dose Investigation of a Corpus Collosotomy by Gamma Knife, Model C

Purpose: Corpus collosotomy by Gamma Knife Radiotherapy is a treatment for medically refractory epilepsy. Due to the long treatment duration (3–6 hours), the peripheral dose received by patients becomes a concern. In this study, peripheral doses for a posterior corpus collosotomy by Gamma Knife, Model C were investigated. Method and Materials: A Rando‐Phantom was used to measure the peripheral doses. Three types of dosimetry systems were used, which included ionization chambers, TLDs (TLD‐100 3×3×1mm chips), and OneDose™ MOSFET dosimeters. The dosimeters were placed on the surface of the Rando‐Phantom under 0.5 cm of build up at distances of 5.0, 17.5, 30.0, 42.5, 54.0, 70.0 and 84cm from the center of the treatment volume. Seven cylindrical ionization chambers with buildup caps were placed at the same distances, one chamber at each position. The leakage of the ionization chambers was measured before irradiation and recorded during treatment sessions. The phantom was irradiated using a clinical treatment plan for a posterior corpus collosotomy. The prescription dose was 130Gy (max dose), and the delivery dose was reduced to one third of the prescription dose for measurement purposes. The treatment time was 99.38 minutes. Results: The measured doses by TLD were 17.9, 9.9, 5.8, 2.9, 2.4, 1.5, and 1.1cGy for the seven positions respectively. The measured doses by ionization chambers were 16.9, 12.0, 5.2, 2.9, 2.8, 2.2, and 2.0cGy. The results obtained with the OneDose™ were not consistent due to the decay of the signal over the period of measurement. The doses measured by TLD and ionization chamber for the same position were averaged. The total doses to the measurement positions were 52.2, 33.0, 16.5, 8.7, 7.8, 5.7, and 4.8cGy, respectively, for the full prescription dose of 130Gy. Conclusion: The peripheral doses were significant, especially in the head and neck region.

SU‐GG‐T‐304: Comparison of Three Commercial MOSFET Dosimeters for Use in Total Body Irradiation

Purpose: To determine the efficacy of three MOSFET dosimeters to perform in vivo measurements of both entrance and exit dose in total body irradiation. Method and Materials: Three commercially available OSFET dosimeters including the Thomson Nielsen Mobile Mosfet (Best Medical) OneDose and OneDose Plus (both from Sicel) were tested under TBI conditions. Measurements were made with two phantoms: a 30cm × 30cm × 21cm solid water phantom and an anthropomorphic phantom. The midlines of the phantoms were placed at 492 cm from the source, the standard TBI condition in our clinic. TBI procedures in our clinic involve irradiation of the patient decubitis with two opposed 6 MV photon beam. To ensure equilibrium, 1.5cm of bolus was used for the Thomson Nielsen MOSFET as well as OneDose. The OneDose Plus system possesses an internal buildup cap. For the solid water phantom, measurements were made at five positions beginning with the central axis. Measurement positions on the anthropomorphic phantom included the head, neck, chest, and umbillicus. Entrance and exit doses were recorded and normalized to an ion chamber reading. Results: All three MOSFET detector measurements were within 2% of the ion chamber measurements when measuring entrance dose on the solid water phantom. When measuring exit dose, OneDose Plus recorded a 40% over response while the other detectors were within tolerance. Entrance and exit dose measurements using the anthropomorphic phantom saw more variation with results deviating from the ion chamber reading as much as 5%. Conclusion: The MOSFET dosimeters provide a convenient method for in vivo dosimetry for total body irradiation. Entrance and exit doses under ideal conditions were accurate, but degraded slightly when placed on an anthropomorphic phantom. OneDose Plus showed an over response when measuring exit dose.

Photon spectral characteristics of dissimilar linear accelerators

This work measures and compares the energy spectra of four dosimetrically matched beams, generated from four physically different linear accelerators. The goal of this work is twofold. First, this study determines whether the spectra of dosimetrically matched beams are measurably different. This study also demonstrates that the spectra of clinical photon beams can be measured as a part of the beam data collection process for input to a three‐dimensional (3D) treatment planning system. The spectra of beams that are dosimetrically matched for clinical use were studied to determine if the beam spectra are similarly matched. Each of the four accelerators examined had a standing waveguide, but with different physical designs. The four accelerators were two Varian 2100C/Ds (one waveguide and one waveguide), one Varian with a vertically mounted waveguide and no bending magnet, and one Siemens MD 6740 with a waveguide. All four accelerators had percent depth dose curves for the beam that were matched within 1.3%. Beam spectra were determined from narrow beam transmission measurements through successive thicknesses of pure aluminum along the central axis of the accelerator, made with a graphite Farmer ion chamber with a Lucite buildup cap. An iterative nonlinear fit using a Marquardt algorithm was used to find each spectrum. Reconstructed spectra show that all four beams have similar energy distributions with only subtle differences, despite the differences in accelerator design. The measured spectra of different beams are similar regardless of accelerator design. The measured spectra show excellent agreement with those found by the auto‐modeling algorithm in a commercial 3D treatment planning system that uses a convolution dose calculation algorithm. Thus, beam spectra can be acquired in a clinical setting at the time of commissioning as a part of the routine beam data collection.

Effect of scatter material on diode detector performance for in vivo dosimetry

The accuracy of Scanditronix EDE, EDP10 and EDP20 diodes for entrance dosimetry of complex fields has been assessed using static and dynamic multileaf collimator test fields on phantoms. Specifically, surrounding scatter material size and composition have been investigated. The EDP10 and EDP20 diodes incorporate steel build-up caps. The effect of varying the diameter of the wax scatter discs on the dosimeter response showed a systematic detector under-response for the smaller discs with errors up to 4.1% relative to full scatter conditions. In static fields, all diodes over-respond at a field size of 1 × 1 cm2. Diodes with non-water-equivalent build-up material exhibit over-response of up to 10.8%. In dynamic fields, diodes over-respond when there is an increased contribution from phantom scatter and under-respond in shielded regions due to low dose rate and beam hardening. For high dose regions, all diodes over-respond with the greatest over-response of 3.8% observed with a 6 mm sliding window field. The EDE diode with a 6 cm scatter disc correlated best with the reference dosimeter. A diode design with minimal non-water-equivalent components and the addition of a 6 cm diameter water-equivalent disc for scatter material is recommended for the in vivo dosimetry of 6 MV complex fields.

Correction factors for low perturbation in vivo diodes used in the determination of entrance doses in high energy photon beams

Low perturbation diodes, with thin buildup caps, can be used to reduce perturbations to the delivered dose. The literature states that additional correction factors are required for low perturbation diodes, however, there are few reported studies into their use. This report measured the dose perturbations and correction factors for diodes with varying buildup cap thicknesses. Scanditronix EDP15, EDD5, and EDD2 diodes were investigated. Dose perturbations and correction factors for field size, source-surface distance (SSD), obliquity, and wedge were measured in megavoltage photon beams. EDP15 produces a 6% dose perturbation. EDD5 produces a perturbation between 1% and 2%. EDD2 perturbation is negligible. The variation of correction factors for the full buildup EDP15 diode is small and consistent with the literature. The low perturbation diode EDD2 has large correction factors. The field size correction factor varies from 1.38 to 0.87 for 10 MV. The SSD correction factor varies from 0.92 to 1.09 for 10 MV. At the maximum angle measured, the obliquity correction factor is 0.73 for 10 MV. Intermediate results were observed for the EDD5 diode. It is expected that it will be very difficult to achieve accurate in vivo dosimetry using the EDD2 diode. The EDD5 diode may represent a reasonable compromise between EDD2 and the full buildup EDP15. The EDD5 dose perturbation is small and the correction factors are not as large as for EDD2, so accurate in vivo dosimetry may be possible as long as the obliquity is below 45 degrees.

SU‐FF‐T‐98: Characterization of Buildup Effects in OneDosePlus MOSFET‐Based Dosimeters

Purpose: Evaluate the buildup cap used on OneDosePlus™ MOSFET dosimeters and study the field size, angular, and SSD dependency. Method and Materials: OneDosePlus (ODP) dosimeters are single use, pre‐calibrated MOSFETs used to measure entrance patient dose. ODP has an integrated 1.2 mm thick tin cap. Custom dosimeters were created with buildup caps of varied thicknesses to determine the approximate tin dmax thickness for 6, 15, and 18 MV. Non‐calibrated ODP dosimeters were evaluated to assess the effect on response of the dosimeter as a function of field size, SSD, and anisotropy for different energies. Results: PDD curves for 6, 15 and 18MV show broad maximum responses centered near cap thicknesses of 1.2, 2.6 and 2.8 mm for 6, 15, and 18 MV, respectively. Evaluation of the field size response showed that higher energies have an over‐ and under‐response for larger/smaller field sizes as compared to 6 MV response, suggesting an increase in electron contamination dose contribution. Results of extended SSDs studies, typical of TBI treatments, show enhanced measurement precision, supporting these findings.For patient use, the dosimeter response is pre‐calibrated and adjusted such that the reading corresponds to Dmax in‐tissue for each specific energy. In addition, the dosimeters are corrected for the 6 MV field size and SSD dependency. Therefore, for higher energies, a slight over‐ or under‐response can be observed for larger/smaller field sizes but not in excess of the reported dosimeter accuracy (± 5%,2σ). Conclusion: The buildup cap for ODP provides a partial buildup effect with results within the expected accuracy for different dosimetric parameters. The dosimeter factory calibration corrects the response from the partial buildup effect seen from the buildup cap. Extended SSD data suggests that ODP could be used to measure dose for TBI treatments.This research was sponsored by Sicel Technologies, Inc.

SU-FF-T-281: Leakage Radiation of Elekta Synergy-S LINAC Machine

Purpose: To evaluate leakage radiation through treatment head of Elekta Synergy-S linear accelerator, which has a new “beam modulator” MLC design. Method and Materials: As part of the acceptance test of a newly installed Elekta Synergy-S, Kodak XV films were used to encompass the entire treatment head to locate potential radiation hot spots. After evaluating film results, an ion chamber (with appropriate buildup caps) was used to measure the leakage radiation for 6MV and 15MV x-ray beams with MLC leaves fully closed. The chamber was placed in a plane perpendicular to the beam axis and which contained the machine isocenter, at distances ranging from 13–100 cm away from the machine isocenter in three directions (inline, crossline and diagonal). A 4.5 cm lead block placed on the blocking tray was used to evaluate the scattered radiation contribution from closed MLC leaves to ion chamber measurements at selected locations near the secondary collimator edges. Results: Leakage radiation was greater than 0.1% of the dose at isocenter for the original beam modulator treatment head design. A Beam Modulator Head Shielding Kit was installed and measurements repeated. Leakage radiation was only slightly reduced. A second version of Beam Modulator Head Shielding Kit was installed, and subsequent measurements showed that head leakage radiation was finally reduced below 0.1%. Contributions to ion chamber readings from scattered radiation generated from closed MLC leaves can be as high as 60% at close proximity to the secondary collimator edge. Conclusions: Thorough measurement of head leakage is necessary during acceptance testing of a LINAC machine, especially for new type machines such as the Synergy-S, to ensure patient and staff safety. It is recommended leakage radiation measurements be made by blocking the scattered radiation contribution from the closed MLC leaves.

SU-FF-T-346: Photon Spectral Characteristics of Dissimilar 6MV Linear Accelerators

Purpose: To determine and compare the energy spectra of four 6MV beams, generated from four physically different accelerators. Also, to demonstrate that the spectra of clinical photon beams can be measured as a part of the beam data collection process for input to a 3D treatment planning system. Methods and Materials: The spectra of 6MV beams that are dosimetrically matched were studied to determine if the beam spectra are similarly matched. Each of the four accelerators examined had different physical designs. The four accelerators were two Varian 2100C/Ds (one 6MV/18MV waveguide and one 6MV/10MV waveguide), one Varian 600C with a vertically mounted waveguide and no bending magnet, and one Siemens MD 67/40 with a 6MV/10MV waveguide. All four accelerators had Percent Depth Dose curves for 6MV that were matched. Spectral measurements were performed by narrow beam transmission measurements through pure aluminum along the central axis of the accelerator, made with a graphite Farmer ion chamber with a Lucite buildup cap. Results: Reconstructed spectra show that all four beams have similar energy distributions with only subtle differences, despite the differences in accelerator design. Comparisons of the measured spectra to the spectra found by the auto‐modeling algorithm in a commercial treatment planning system also showed excellent agreement. Plots of the measured spectra are shown. Conclusions: The measured spectra of different 6MV beams are similar, regardless of accelerator design. The measured spectra agree with those derived by the treatment planning system. Thus, beam spectra can be acquired in a clinical setting at the time of commissioning as a part of the routine beam data collection.

Irradiation dose control of chicken meat processing with alanine/ESR dosimetric system

Irradiation of foodstuff is a well-known food preservation technique. In Brazil spices are already irradiated for sanitary and preservation reasons. Chicken meat is an important commodity; Brazil is the second largest world producer and the largest world exporter. The shelf-life of chicken meat is limited by the presence of micro-organisms and enzyme activity and together with other preservation techniques irradiation seems to be an attractive option. In this study the dose delivered to frozen chicken cuts was measured and compared with the prescribed value. Chicken breast cuts were analyzed for 39 days for their microbiological activity, chemical and organoleptic properties. Cylindrical dosimeters were prepared using the weight composition of 80% of DL-alanine (Sigma Co), used without any further treatment except drying, and 20% of paraffin. The dosimeters having 4.7 mm diameter and 12 mm length were inserted in a build-up cap. Dosimeters were placed inside cardboard boxes containing frozen chicken breast cuts, packed in styrofoam trays wrapped with plastic film. The boxes were irradiated in an industrial 60Co irradiator (Nordion JS 7500) with a dose rate of 4 kGy/h. First derivative ESR signals were obtained in a VARIAN E-4 spectrometer operating at X-band ( ν ≈ 9 GHz ) and equipped with a rectangular cavity (TE-102, model E-231). The cavity was constantly purged with dry nitrogen and modulated at 100 KHz with 0.5 mT peak to peak. A calibration curve was made for a few dosimeters from the same batch and used to obtain the dose from the ESR signal intensity. A batch of six boxes was irradiated at each experiment with prescribed doses of 1.5, 3.0 and 7.0 kGy. Considering that the larger the radiation dose the greater is the probability of finding a product with its sensorial characteristics altered (odor of burned meat), we conclude that a dose of 3 kGy would be more adequate, taking into account the microbiological and sensorial aspects.

Monte Carlo simulation of MOSFET detectors for high-energy photon beams using the PENELOPE code

The aim of this work was the Monte Carlo (MC) simulation of the response of commercially available dosimeters based on metal oxide semiconductor field effect transistors (MOSFETs) for radiotherapeutic photon beams using the PENELOPE code. The studied Thomson&Nielsen TN-502-RD MOSFETs have a very small sensitive area of 0.04 mm2 and a thickness of 0.5 µm which is placed on a flat kapton base and covered by a rounded layer of black epoxy resin. The influence of different metallic and Plastic water™ build-up caps, together with the orientation of the detector have been investigated for the specific application of MOSFET detectors for entrance in vivo dosimetry. Additionally, the energy dependence of MOSFET detectors for different high-energy photon beams (with energy >1.25 MeV) has been calculated. Calculations were carried out for simulated 6 MV and 18 MV x-ray beams generated by a Varian Clinac 1800 linear accelerator, a Co-60 photon beam from a Theratron 780 unit, and monoenergetic photon beams ranging from 2 MeV to 10 MeV. The results of the validation of the simulated photon beams show that the average difference between MC results and reference data is negligible, within 0.3%. MC simulated results of the effect of the build-up caps on the MOSFET response are in good agreement with experimental measurements, within the uncertainties. In particular, for the 18 MV photon beam the response of the detectors under a tungsten cap is 48% higher than for a 2 cm Plastic water™ cap and approximately 26% higher when a brass cap is used. This effect is demonstrated to be caused by positron production in the build-up caps of higher atomic number. This work also shows that the MOSFET detectors produce a higher signal when their rounded side is facing the beam (up to 6%) and that there is a significant variation (up to 50%) in the response of the MOSFET for photon energies in the studied energy range. All the results have shown that the PENELOPE code system can successfully reproduce the response of a detector with such a small active area.

In vivo prostate IMRT dosimetry with MOSFET detectors using brass buildup caps

The feasibility of using dual bias metal oxide semiconductor field effect transistor (MOSFET) detectors with the new hemispherical brass buildup cap for in vivo dose measurements in prostate intensity‐modulated radiotherapy (IMRT) treatments was investigated and achieved. In this work, MOSFET detectors with brass buildup caps placed on the patient's skin surface on the central axis of the individual IMRT beams are used to determine the maximum entrance dose (Dmax) from the prostate IMRT fields. A general formalism with various correction factors taken into account to predict Dmax entrance dose for the IMRT fields with MOSFETs was developed and compared against predicted dose from the treatment‐planning system (TPS). We achieved an overall accuracy of better than ±5% on all measured fields for both 6‐MV and 10‐MV beams when compared to predicted doses from the Philips Pinnacle 3 and CMS XiO TPSs, respectively. We also estimate the total uncertainty in estimation of MOSFET dose in the high‐sensitivity mode for IMRT therapy to be 4.6%.PACS numbers: 87.53Xd, 87.56Fc

SU‐DD‐A1‐04: In Vivo Prostate IMRT Dosimetry With MOSFET Detectors Using Brass Build‐Up Caps

Purpose: To develop a general formalism with various correction factors to predict dmax entrance dose with the new hemispherical brass buildup caps to be used with MOSFET detectors in anterior prostate IMRT fields and thereby integrate in vivo IMRT dose measurement as part of routine QA process in IMRT radiotherapy Method and Materials: We have used the new wide energy hemispherical build‐up caps for this study. Due to its high density and high atomic number it provides the minimal amount of metal needed to achieve full build‐up at Dmax for a range of photon energies. We have developed a general formalism to predict Dmax entrance dose by applying necessary correction factors after studying the response of MOSFET with brass build up caps for energy, dose rate, dose reproducibility, SSD and patient specific IMRT correction factor. Results: In vivo Prostate IMRT dose measurements with MOSFET detectors using brass buildup caps was performed and compared against dose predicted by two different treatment planning systems. We used both 6 MV and 10 MV for this study and compared the in vivo MOSFET detector reading with dose predicted by Philips Pinnacle (6 MV) and CMS XiO (10 MV) treatment planning systems respectively. We achieved a overall accuracy of better than ± 5% on measured patient doses. Conclusion: Routine IMRT QA in most institutions today only involves verifying the optimized fluence map delivered to the patient in a test phantom at a certain preset depth. Based on our work here, we believe adding in vivo IMRT dosimetry with MOSFET detectors using the new brass build up caps along with routine fluence map verification in phantoms and MLC quality assurance offers greater accuracy and confidence in actual dose delivered to the patient.

Influence of ion chamber response on in‐air profile measurements in megavoltage photon beams

This article presents an investigation of the influence of the ion chamber response, including buildup caps, on the measurement of in-air off-axis ratio (OAR) profiles in megavoltage photon beams using Monte Carlo simulations with the EGSnrc system. Two new techniques for the calculation of OAR profiles are presented. Results of the Monte Carlo simulations are compared to measurements performed in 6, 10 and 25 MV photon beams produced by an Elekta Precise linac and shown to agree within the experimental and simulation uncertainties. Comparisons with calculated in-air kerma profiles demonstrate that using a plastic mini phantom gives more accurate air-kerma measurements than using high-Z material buildup caps and that the variation of chamber response with distance from the central axis must be taken into account.

TH-C-T-617-09: Wide Energy Metallic Build-Up Caps for MOSFET Dosimeters: Monte Carlo Simulation and Experimental Study of Dose Correction Factors at Dmax

Purpose: Current MOSFET dosimeters use large size (2cm) solid water build-up caps at high energies for patient dose measurements at Dmax, less practical for routine use. Dense metallic materials have been studied to reduce the amount of build-up needed to achieve electronic equilibrium at Dmax, and extend the MOSFET clinical use. Method and Materials: The caps studied were hemispherical, with a groove to secure the MOSFET detector. The radiation tests were performed on brass, aluminum and plastic-water equivalent caps. Monte Carlo simulations of depth dose curves in the central axis of the geometry were performed to evaluate the cap effect on the dose measurements. Correction factors to scale the dose value at Dmax in water were also measured for each cap at 6 and 23 MV photon beams. Linearity and angular responses were also evaluated. Results: At 6 MV and 10×10 cm2 field (100 cm SSD), the correction factors were for aluminum (R 10 mm), brass (R 6.35 mm) and solid water (R 2 cm) caps, 0.96, 0.9 and 0.97 respectively. At 23MV and for similar caps, the correction factors were 0.97, 1.19 and 0.84 respectively. The brass cap is the only cap to achieve electronic equilibrium at both energies for the smallest size possible (6.35 mm). Its slight over response at 23MV is linked to its high atomic number. Its hemispherical shape ensures isotropic response up to 40°, and only 3 % SD was observed for angles close to 90° around the central axis at 6 MV. Conclusion: The smallest brass build-up cap is highly suitable to create a full build-up for MOSFET measurements at high energies. Correction factors can be obtained for all photon energies, making routine in-vivo dosimetry easily feasible for different beams. Conflict of Interest: Research supported by Thomson Nielsen Inc.

TU‐EE‐A2‐04: Helical Tomotherapy Leakage Radiation Half and Tenth Value Layers in Concrete and Lead

Purpose: According to NCRP Report 51, if the leakage TVL is not measured, the primary radiation TVL should be used for the shielding design. Varian measured its own 90° leakage TVL in 1995. To determine which TVL value is most suitable for Hi‐Art II Tomotherapy, its leakage radiation half and tenth value layer in concrete and lead are measured. Methods & Materials: Film dosimetry is used to identify the high leakage region on the Hi‐Art II Tomotherapy machine. The entire surface area of the machine was covered with Kodak XV2 films. After identifying the high leakage region, an ion chamber was used to measure the leakage. The ion chamber (Exardin A12) was placed near the high leakage region, and located 1.13m above floor level and 2m away from the radiation isocenter. To allow for enough build up for high energy photons a 1cm bolus was added to the existing buildup cap. The leakage radiation was measured by adding blocks of concrete and lead in front of the ion chamber. The exposure data was recorded using a PTW Unidose electrometer. The half and tenth value layers of concrete and lead were calculated. All measurements were obtained under normal treatment conditions (800 MU/min, 360° rotation) for a period of 7 minutes. Results: The measured leakage radiation HVL and TVL values of the Hi‐Art II helical Tomotherapy machine are 0.011m (0.44″) and 0.044m (1.74″) in lead, and 0.066m (2.61″) and 0.282m (11.11″) in concrete, respectively. Conclusion: The leakage radiation TVL in concrete and lead obtained for Hi‐Art II helical Tomotherapy machine are close to published data for 90 degree leakage radiation of Varian (1995), but significantly lower than the NCRP (1997) data for primary radiation.

In-air calibration of an HDR 192Ir brachytherapy source using therapy ion chambers

The Gammamed Plus 192Ir high dose rate brachytherapy sources were calibrated using the therapy level ionization chambers (0.1 and 0.6 cc) and the well-type chamber. The aim of the present study was to assess the accuracy and suitability of use of the therapy level chambers for in-air calibration of brachytherapy sources in routine clinical practice. In a calibration procedure using therapy ion chambers, the air kerma was measured at several distances from the source in a specially designed jig. The room scatter correction factor was determined by superimposition method based on the inverse square law. Various other correction factors were applied on measured air kerma values at multiple distances and mean value was taken to determine the air kerma strength of the source. The results from four sources, the overall mean deviation between measured and quoted source strength by manufacturers was found -2.04% (N = 18) for well-type chamber. The mean deviation for the 0.6 cc chamber with buildup cap was found -1.48 % (N = 19) and without buildup cap was 0.11% (N = 22). The mean deviation for the 0.1 cc chamber was found -0.24% (N = 27). Result shows that probably the excess ionization in case of 0.6 cc therapy ion chamber without buildup cap was estimated about 2.74% and 1.99% at 10 and 20 cm from the source respectively. Scattered radiation measured by the 0.1 cc and 0.6 cc chamber at 10 cm measurement distance was about 1.1% and 0.33% of the primary radiation respectively. The study concludes that the results obtained with therapy level ionization chambers were extremely reproducible and in good agreement with the results of the well-type ionization chamber and source supplier quoted value. The calibration procedure with therapy ionization chambers is equally competent and suitable for routine calibration of the brachytherapy sources.