Clinical Electron Beams Research Articles

Erratum: “Recommendations for clinical electron beam dosimetry: Supplement to the recommendations of Task Group 25” [Med. Phys. 36, 3239–3279 (2009)

Calculating Req based on either Ē0 or Ep,0 gives very similar values. There is very little difference in the calculated value of Req using either Ē0 or Ep,0, and the difference is between 0.5 and 1.5 mm for the most common clinical electron energies. The use of Ep,0 produces slightly larger values of Req, the minimum radius of a circular field required for lateral scatter equilibrium. We recommend using the original equation from the historical literature based on Ep,0.

Small field electron beam dosimetry using MOSFET detector

The dosimetry of very small electron fields can be challenging due to relative shifts in percent depth‐dose curves, including the location of dmax, and lack of lateral electronic equilibrium in an ion chamber when placed in the beam. Conventionally a small parallel plate chamber or film is utilized to perform small field electron beam dosimetry. Since modern radiotherapy departments are becoming filmless in favor of electronic imaging, an alternate and readily available clinical dosimeter needs to be explored. We have studied the performance of MOSFET as a relative dosimeter in small field electron beams. The reproducibility, linearity and sensitivity of a high‐sensitivity microMOSFET were investigated for clinical electron beams. In addition, the percent depth doses, output factors and profiles have been measured in a water tank with MOSFET and compared with those measured by an ion chamber for a range of field sizes from 1 cm diameter to 10 cm× 10 cm for 6, 12, 16 and 20 MeV beams. Similar comparative measurements were also performed with MOSFET and films in solid water phantom. The MOSFET sensitivity was found to be practically constant over the range of field sizes investigated. The dose response was found to be linear and reproducible (within ±1% for 100 cGy). An excellent agreement was observed among the central axis depth dose curves measured using MOSFET, film and ion chamber. The output factors measured with MOSFET for small fields agreed to within 3% with those measured by film dosimetry. Overall results indicate that MOSFET can be utilized to perform dosimetry for small field electron beam.PACS number: 87.55.Qr

Extraction of depth‐dependent perturbation factors for parallel‐plate chambers in electron beams using a plastic scintillation detector

This work presents the experimental extraction of the overall perturbation factor PQ in megavoltage electron beams for NACP-02 and Roos parallel-plate ionization chambers using a plastic scintillation detector (PSD). The authors used a single scanning PSD mounted on a high-precision scanning tank to measure depth-dose curves in 6, 12, and 18 MeV clinical electron beams. The authors also measured depth-dose curves using the NACP-02 and PTW Roos chambers. The authors found that the perturbation factors for the NACP-02 and Roos chambers increased substantially with depth, especially for low-energy electron beams. The experimental results were in good agreement with the results of Monte Carlo simulations reported by other investigators. The authors also found that using an effective point of measurement (EPOM) placed inside the air cavity reduced the variation of perturbation factors with depth and that the optimal EPOM appears to be energy dependent. A PSD can be used to experimentally extract perturbation factors for ionization chambers. The dosimetry protocol recommendations indicating that the point of measurement be placed on the inside face of the front window appear to be incorrect for parallel-plate chambers and result in errors in the R50 of approximately 0.4 mm at 6 MeV, 1.0 mm at 12 MeV, and 1.2 mm at 18 MeV.

Monte Carlo commissioning of clinical electron beams using large field measurements

Monte Carlo simulation can accurately calculate electron fluence at the patient surface and the resultant dose deposition if the initial source electron beam and linear accelerator treatment head geometry parameters are well characterized. A recent approach used large electron fields to extract these simulation parameters. This method took advantage of the absence of lower energy, widely scattered electrons from the applicator resulting in more accurate data. It is important to validate these simulation parameters for clinically relevant fields. In the current study, these simulation parameters are applied to fields collimated by applicators and inserts to perform a comprehensive validation. Measurements were performed on a Siemens Oncor linear accelerator for 6 MeV, 9 MeV, 12 MeV, 15 MeV, 18 MeV and 21 MeV electron beams and collimators ranging from an open 25 × 25 cm2 applicator to a 10 × 10 cm2 applicator with a 1 cm diameter cerrobend insert. Data were collected for inserts placed in four square applicators. Monte Carlo simulations were performed using EGSnrc/BEAMnrc. Source and geometry parameters were obtained from previous measurements and simulations with the maximum field size (40 × 40 cm2). The applicators were modelled using manufacturer specifications, confirmed by direct measurements. Cerrobend inserts were modelled based on calliper measurements. Monte Carlo-calculated percentage depth dose and off-axis profiles agreed with measurements to within the least restrictive of 2%/1 mm in most cases. For the largest applicator (25 × 25 cm2), and 18 MeV and 21 MeV beams, differences in dose profiles of 3% were observed. Calculated relative output factors were within 2% of those measured with an electron diode for fields 1.5 cm in diameter or larger. The disagreement for 1 cm diameter fields was up to 5%. For open applicators, simulations agreed with parallel plate chamber-measured relative output factors to 1%. This work has validated a recent methodology used to extract data on the electron source and treatment head from large electron fields, resulting in a reduction in the number of unknown parameters in treatment head simulation. Applicator and insert collimated electron fields were accurately simulated without adjusting these parameters. Results demonstrate that commissioning of electron beams based on large electron field measurements is a viable option.

Sci-Sat AM(1): Planning - 10: Evaluation of a New Commercial Monte-Carlo Treatment Planning System for Electrons

It has long been understood that the Monte Carlo (MC) method is the most effective means for accurately computing the dose delivered by clinical electron beams. Every commercial implementation of the MC method involves design compromises and the possibility of error. It is important, therefore, that each implementation is independently validated under conditions similar to those found in the clinic. In this abstract, we present the initial stages of validation for the XiO electron Monte Carlo (XiO eMC) software, a new treatment planning system for electron beams developed and commercialized by CMS incorporated. In this abstract we present a limited set of comparisons of calculated and experimental data for homogeneous water phantoms and for a 3D heterogeneous phantom meant to approximate the geometry of a trachea and spine. All Monte Carlo calculated and measured output factors agree within the estimated standard error for standard and extended SSD for open applicators and cerrobend cutouts with the exception of the smallest cutout size (2×2cm2) for 17 MeV at extended SSD. We also found good agreement between calculated and experimental depth dose curves and dose profiles. Dose calculations in heterogeneous phantoms are also in a very good agreement with measurements, given an estimated positional uncertainty of ±0.1cm in the depth direction, and provided that appropriate calculation voxel sizes are used for a given geometry. Acknowledgment: The authors would like to acknowledge the excellent technical support provided by Dr. J C Satterthwaite of Elekta CMS Software.

SU‐GG‐T‐411: Monte‐Carlo‐Based Cavity Correction Factors for Ion Chambers in Clinical Electron Beams

Purpose: To calculate the cavity correction factor, Pcav at various depths for cylindrical chambers and plane‐parallel chambers, NACP‐02 and ROOS, in high‐energy electron beams by means of the EGSnrc/Cavity code. Method and Materials: The chamber cavity was simulated in detail by the EGSnrc/Cavity code. Pcav was calculated at various depths which include a reference depth, dref, and a half‐value depth, R50, for 6, 9, 15, and 18 MeV electron beams. The effective point of the calculation was at a point shifted 0.5r upstream from the cavity center for cylindrical chambers and was the front face of the cavity for plane‐parallel chambers. Pcav for the cylindrical chamber cavities were calculated with combinations of the diameters of 2, 4, and 6 mm and the lengths of 5, 10, and 20 mm. The results were compared with the values recommended by TRS‐398 and other published data. Results: Pcav values increased as a function of a depth. Especially, the Pcav values at dref for cylindrical chambers were 4–10% higher than those at R50 in 6 MeV. Pcav values were lower with increasing the cavity diameter and decreasing the cavity length. This is because the lateral scattered electrons entering into the chamber cavity from the surrounding water increase. The effect is significant in lower‐energy beams. Similarly, the Pcav values are 0.993 at dref and 1.030 at R50 for NACP‐02 and 1.001 and 1.030 for ROOS in 6 MeV. Pcav for NACP‐02 varies depending on depths (electron mean energy). The variation in Pcav for ROOS values is smaller than NACP‐02 because its guard‐ring is larger. Conclusion: The calculated Pcav value was sufficiently different from TRS‐398 data. The Pcav values vary depending on electron mean energy and the chamber cavity size. For plan‐parallel chambers, the sufficient guard‐ring width needs to decrease the cavity correction.

SU‐GG‐T‐383: Investigation of Water Equivalency of PRESAGE Dosimeters for Electron Beam Radiotherapy

Purpose: To evaluate water equivalency of PRESAGE dosimeters for electron beam radiotherapy. Method and Materials: In this work, three different PRESAGE polymer dosimeters were evaluated: the currently available PRESAGE as well as two with a new chemical formulation. Total mass stopping powers were calculated over the energy range 10keV‐20MeV using the NIST ESTAR database for the three PRESAGE dosimeters and water. Monte Carlo modeling was used to determine the differences in depth doses between the PRESAGE dosimeters and water for a clinical 12MeV electron beam with a 10×10 cm2 field size. The Monte Carlo calculations were performed using the EGSnrc/BEAMnrc package. The BEAMnrc user code was used to calculate the phase space file for the electron beam. The phase space file generated by BEAMnrc were then used to calculate depth dose curves for each of the three PRESAGE dosimeters and water using the DOSXYZnrc user code. Results: Above 10MeV, total mass stopping power of the existing PRESAGE dosimeter has the highest differences with the value of water due to having the highest effective atomic number. For the calculated relative dose versus water equivalent depth, new PRESAGE dosimeters were in better agreement with the depth doses in water. For depths of less than 2cm water equivalent, the relative dose in water is up to 2% higher than the new PRESAGE dosimeters and 3% higher than the existing PRESAGE dosimeter. This can be attributed to higher collisional stopping power of water than the PRESAGE dosimeters due to its lower effective atomic number than the PRESAGE dosimeters. The maximum energy deposition in the three PRESAGE dosimeters is approximately in the same depth as water (2.7cm depth). Conclusion: The PRESAGE dosimeters with the new chemical formulations are more water equivalent than the existing PRESAGE dosimeter and are suitable for dosimetry of electron beams.

SU‐GG‐T‐374: Use of a Rad‐Hard Si Diode in Clinical Electron Beam Dosimetry

Purpose:The preliminary results obtained with a MCz silicon diode as online clinical electron beam dosimeter are presented in this work. Method and Materials: The diode was manufactured by Okmetic Oyj (Finland) and processed by the Microelectronics Center of Helsinki University of Technology in the framework of the CERN RD50 Collaboration. The n+‐p‐p+ junction device used (area of 25 mm2) was housed in a PMMA probe designed to operate without bias voltage in the direct current mode. During all measurements, the diode was held in a PMMA plate, placed at Zref and centered in a radiation field of 10 cm × 10 cm, with the SSD kept in 100 cm. The device's dosimetric response was evaluated for the 6, 9, 12, 15, 18 e 21 MeV electron beams from a Siemens KD2 Accelerator. The radiation induced current in the diode, for ach electron beam energy, was registered as a function of the exposure time during 60 s for a fixed 300 MU. To study the instantaneous repeatability of the MCZ diode, 5 consecutive current signals were registered for a same radiation dose around of 300 cGy. Measurements were performed with an average dose rate of 5.0 cGy/s for the electron beam energies of 6,9,12,15,18 and 21 MeV. Results: The results indicated a linear dose response for doses up to 93,28 Gy for electron beams energies of 6, 9, 12, 15, 18 e 21 MeV with very good instantaneous repeatability (CV better than 2,76 %). Conclusion: It worth noting that these results are preliminar and still remains to be investigated this energy dependence, the central axis depth dose response, the long term stability and the radiation hardness of this diode for absorbed doses higher than investigated in this work. All these studies are under way.

TH‐C‐BRB‐10: Extraction of Perturbation Factors for Parallel‐Plate Chambers in Electron Beams Using a Plastic Scintillation Detector

Purpose: To present a method to experimentally extract the overall perturbation factor pQ (the product of pwall and prepl) in megavoltage electron beams for NACP and Roos parallel plate chambers using a perturbation‐free plastic scintillation detector (PSD). Method and Materials: We used a single scanning PSD mounted on a high precision scanning stage to measure depth dose curves in 6 12 and 18 MeV clinical electron beams. We also measured depth doses using NACP and Roos parallel plate chambers. Perturbation factors were extracted by comparing PDDs measured using the Roos and NACP chamber to PDDs measured using the PSD. Monte‐Carlo simulations using EGSnrc were performed to validate that the PSD is a perturbation‐free detector in electron beams. Results: Our experimental results show excellent concordance with the Monte‐Carlo simulation results of various authors on the subject. We show that the perturbation factors for the NACP and Roos chambers increase substantially with depth especially for low energy electron beams. We show that using an effective point of measurement (EPOM) placed inside the air cavity reduces the variation of perturbation factors with depth. The protocol recommendations suggesting to place the point of measurement at the inside face of the front window appear to be incorrect for parallel plate chambers and lead to errors in the R50 of approximately 0.4 mm at 6 MeV 1 mm at 12 MeV and 1.2 mm at 18 MeV. Conclusion: The results indicate that PSDs can be used as perturbation‐free detectors in reference dosimetry and that PSDs can be used to experimentally extract ionization chamber perturbation factors.

Dosimetric Characteristics of Standard and Micro MOSFET Dosimeters as In-vivo Dosimeter for Clinical Electron Beam

The metal oxide semiconductor field effect transistor (MOSFET) dosimeters have recently become commercially available. The purpose of this study was to investigate the fundamental dosimetric characteristics of MOSFET dosimeters for clinical electron beams and to compare standard MOSFET and micro MOSFET dosimeters. In this study, five identical standard MOSFET (TN-502-RD) and micro MOSFET (TN-502-RDM) dosimeters were used for measurements. All measurements, with the exception of angular dependence, were performed in a slab-shaped PMMA phantom. For determining the angular dependence of MOSFET dosimeters, a cylindrically shaped PMMA phantom was used. Both MOSFET dosimeters showed excellent linearity against doses measured in the dose range of 50-600 cGy for a electron beam of 9 MeV. The reproducibility of all MOSFETs, excepted one standard MOSFET, was less than ±2 %. The dose-rate dependence of the two types MOSFET was within ±3 %. However, for the angular dependence, standard and micro MOSFETs show remarkable differences relative to gantry angles. This study shows the dosimetric characteristic of standard and micro MOSFET dosimeters for clinical electron beams. Both MOSFET dosimeters are suitable for dosimetry of electron beams in the energy range of 6 −20 MeV. However, the dose verification of radiation therapy using multidirectional electron beam treatments allows for better use of micro MOSFETs which have a reduced directional dependence compared to standard MOSFETs.

Treatment head disassembly to improve the accuracy of large electron field simulation

The purposes of this study are to improve the accuracy of source and geometry parameters used in the simulation of large electron fields from a clinical linear accelerator and to evaluate improvement in the accuracy of the calculated dose distributions. The monitor chamber and scattering foils of a clinical machine not in clinical service were removed for direct measurement of component geometry. Dose distributions were measured at various stages of reassembly, reducing the number of geometry variables in the simulation. The measured spot position and beam angle were found to vary with the beam energy. A magnetic field from the bending magnet was found between the exit window and the secondary collimators of sufficient strength to deflect electrons 1 cm off the beam axis at 100 cm from the exit window. The exit window was 0.05 cm thicker than manufacturer's specification, with over half of the increased thickness due to water pressure in the channel used to cool the window. Dose distributions were calculated with Monte Carlo simulation of the treatment head and water phantom using EGSnrc, a code benchmarked at radiotherapy energies for electron scatter and bremsstrahlung production, both critical to the simulation. The secondary scattering foil and monitor chamber offset from the collimator rotation axis were allowed to vary with the beam energy in the simulation to accommodate the deflection of the beam by the magnetic field, which was not simulated. The energy varied linearly with bending magnet current to within 1.4% from 6.7 to 19.6 MeV, the bending magnet beginning to saturate at the highest beam energy. The range in secondary foil offset used to account for the magnetic field was 0.09 cm crossplane and 0.15 cm inplane, the range in monitor chamber offset was 0.14 cm crossplane and 0.07 cm inplane. A 1.5%/0.09 cm match or better was obtained to measured depth dose curves. Profiles measured at the depth of maximum dose matched the simulated profiles to 2.6% or better at doses of 80% or more of the dose on the central axis. The profiles along the direction of MLC motion agreed to within 0.16 cm at the edge of the field. There remained a mismatch for the lower beam energies at the edge of the profile that ran parallel to the direction of jaw motion of up to 1.4 cm for the 6 MeV beam, attributed to the MLC support block at the periphery of the field left out of the simulation and to beam deflection by the magnetic field. The possibility of using these results to perform accurate simulation without disassembly is discussed. Phase-space files were made available for benchmarking beam models and other purposes. The match to measured large field dose distributions from clinical electron beams with Monte Carlo simulation was improved with more accurate source details and geometry details closer to manufacturer's specification than previously achieved.

Detection of electron beam energy variations using a computed radiography system

A method to evaluate the electron beam energy constancy by employing the computed radiography (CR) system has been developed. In this method, a right triangular plastic wedge is used to produce a curve of the CR storage phosphor plate signal versus the wedge thickness. The curve, which resembles the percentage depth ionization curve of the clinical electron beams, can be used to derive the energy constancy metric EC50. The sensitivity of the method was tested using polystyrene sheets of variable thicknesses. For electron energies up to 12 MeV, energy changes induced by 1.5 mm thick polystyrene can be detected, while a 2.3 mm thick polystyrene sheet is required for higher energies. The measurements were carried out over a two‐year period. The results showed a good reproducibility with the use of the same CR plate and cassette, and without the requirement of calibration procedures. The two‐year range of the EC50 was within the 99% confidence intervals, and the standard deviation of the EC50 was measured to be from 0.3 to 0.4 mm for different beam energies. This technique provides an efficient and accurate method to perform the electron beam energy check and could be used by centers equipped with the CR system without requiring additional detection devices.PACS number: 87.56.Fc

Poster — Wed Eve—24: Dosimetry of Lead Shield in Electron Radiotherapy: A Monte Carlo Evaluation

We investigated the dosimetric characteristics of the beam profile, relative dose at the central beam axis (CAX) and penumbra width at the depth of maximum dose (), varying with different sizes and thicknesses of a piece of lead (Pb) layer using different clinical electron beams. Monte Carlo simulations (EGSnrc‐based code) validated by measurements were used to calculate beam profiles at for different Pb shields and electron beams with energies of 4, 9 and 16 MeV. Pb layers with different thicknesses (2–8 mm) and sizes () were placed at the center of an electron field on a Solid Water phantom. Beam profiles were determined at , and dosimetry under the Pb layer was studied. We found that 2 mm of Pb layer is adequate to provide 5 half‐value‐layer attenuation for the 4 MeV electron beams. However, for the 9 and 16 MeV electron beams, the relative dose at CAX and depends on the thickness and size of the Pb layer. The dosimetry of the beam profile under the Pb layer at depends on the penumbra at the edge of the layer, and beam attenuation varies with the thickness of the layer. The dosimetry of the profile also depends on the electron side‐scatter contributing to the CAX, and photon contamination produced by the Pb layer. The dosimetric data calculated by Monte Carlo simulations in this study provides useful information in selecting the Pb shield suitable for the protection of critical tissue in electron radiotherapy.

Recommendations for clinical electron beam dosimetry: Supplement to the recommendations of Task Group 25

The goal of Task Group 25 (TG-25) of the Radiation Therapy Committee of the American Association of.Physicists in Medicine (AAPM) was to provide a methodology and set of procedures for a medical physicist performing clinical electron beam dosimetry in the nominal energy range of 5-25 MeV. Specifically, the task group recommended procedures for acquiring basic information required for acceptance testing and treatment planning of new accelerators with therapeutic electron beams. Since the publication of the TG-25 report, significant advances have taken place in the field of electron beam dosimetry, the most significant being that primary standards laboratories around the world have shifted from calibration standards based on exposure or air kerma to standards based on absorbed dose to water. The AAPM has published a new calibration protocol, TG-51, for the calibration of high-energy photon and electron beams. The formalism and dosimetry procedures recommended in this protocol are based on the absorbed dose to water calibration coefficient of an ionization chamber at 60Co energy, N60Co(D,w), together with the theoretical beam quality conversion coefficient k(Q) for the determination of absorbed dose to water in high-energy photon and electron beams. Task Group 70 was charged to reassess and update the recommendations in TG-25 to bring them into alignment with report TG-51 and to recommend new methodologies and procedures that would allow the practicing medical physicist to initiate and continue a high quality program in clinical electron beam dosimetry. This TG-70 report is a supplement to the TG-25 report and enhances the TG-25 report by including new topics and topics that were not covered in depth in the TG-25 report. These topics include procedures for obtaining data to commission a treatment planning computer, determining dose in irregularly shaped electron fields, and commissioning of sophisticated special procedures using high-energy electron beams. The use of radiochromic film for electrons is addressed, and radiographic film that is no longer available has been replaced by film that is available. Realistic stopping-power data are incorporated when appropriate along with enhanced tables of electron fluence data. A larger list of clinical applications of electron beams is included in the full TG-70 report available at http://www.aapm.org/pubs/reports. Descriptions of the techniques in the clinical sections are not exhaustive but do describe key elements of the procedures and how to initiate these programs in the clinic. There have been no major changes since the TG-25 report relating to flatness and symmetry, surface dose, use of thermoluminescent dosimeters or diodes, virtual source position designation, air gap corrections, oblique incidence, or corrections for inhomogeneities. Thus these topics are not addressed in the TG-70 report.

Monte Carlo calculations of correction factors for plastic phantoms in clinical photon and electron beam dosimetry

The purpose of this study is to calculate correction factors for plastic water (PW) and plastic water diagnostic-therapy (PWDT) phantoms in clinical photon and electron beam dosimetry using the EGSnrc Monte Carlo code system. A water-to-plastic ionization conversion factor k(pl) for PW and PWDT was computed for several commonly used Farmer-type ionization chambers with different wall materials in the range of 4-18 MV photon beams. For electron beams, a depth-scaling factor c(pl) and a chamber-dependent fluence correction factor h(pl) for both phantoms were also calculated in combination with NACP-02 and Roos plane-parallel ionization chambers in the range of 4-18 MeV. The h(pl) values for the plane-parallel chambers were evaluated from the electron fluence correction factor phi(pl)w and wall correction factors P(wall,w) and P(wall,pl) for a combination of water or plastic materials. The calculated k(pl) and h(pl) values were verified by comparison with the measured values. A set of k(pl) values computed for the Farmer-type chambers was equal to unity within 0.5% for PW and PWDT in photon beams. The k(pl) values also agreed within their combined uncertainty with the measured data. For electron beams, the c(pl) values computed for PW and PWDT were from 0.998 to 1.000 and from 0.992 to 0.997, respectively, in the range of 4-18 MeV. The phi(pl)w values for PW and PWDT were from 0.998 to 1.001 and from 1.004 to 1.001, respectively, at a reference depth in the range of 4-18 MeV. The difference in P(wall) between water and plastic materials for the plane-parallel chambers was 0.8% at a maximum. Finally, h(pl) values evaluated for plastic materials were equal to unity within 0.6% for NACP-02 and Roos chambers. The h(pl) values also agreed within their combined uncertainty with the measured data. The absorbed dose to water from ionization chamber measurements in PW and PWDT plastic materials corresponds to that in water within 1%. Both phantoms can thus be used as a substitute for water for photon and electron dosimetry.

SU‐FF‐T‐304: Calculation of the Cavity Correction Factor, Pcav, for Cylindrical Ionization Chambers in Clinical Electron Beams by Monte Carlo Simulation

Purpose: To calculate the cavity correction factor, Pcav, for the cylindrical ionization chambers in clinical electron beams by Monte Carlo simulation. Methods and materials: The Pcav values for cylindrical chambers were calculated with the EGSnrc C++ user code cavity for 6, 9. 12, 15, and 18 MeV electron beams. The air cavity for cylindrical chambers was replaced with “low density water (LDW)” material. LDW is an artificial material that has all the dosimetric properties of nominal water except its density is equal to that of air. The dose to water was calculated with a 0.1 mm thick slab at a reference depth, dref. The point of measurement for the LDW cavity was taken to be 0.5r (r is the radius of the chamber's cavity) deeper than dref. Pcav was calculated for the cavities with diameters of 7 mm, 6 mm, 5 mm, 4 mm, 3 mm, and 2 mm and lengths of 20 mm, 10 mm, and 5 mm. The calculated Pcav values were compared with those from IAEA TRS‐398. Results: Pcav values calculated for cylindrical chambers at dref in clinical electron beams were typically higher than those of TRS‐398, which are based on experimental data of Johansson et al. For a Farmer chamber with the cavity of diameter 6 mm and length 20 mm, calculated Pcav values were higher from 2% to 0.8% than the TRS‐398's data, in a range of 6 MeV to 18 MeV. The dependence on the cavity lengths for Pcav was found at grater or equal to a diameter of 5 mm, but it was not sufficient at diameters of 4 mm and 3 mm. Conclusions: It was found that the calculated Pcav values of cylindrical chambers in electron beams are sufficiently different from the TRS‐398 data.

TU‐A‐BRC‐01: Monte Carlo Treatment Planning: Implementation of Clinical Systems

Monte Carlo (MC) based treatment planning systems are already commercially available for both photon and electron beams and more users are implementing them in a clinical setting. Therefore it is important that strategies and paradigms for clinical commissioning and implementation of such systems be formulated and discussed. The purpose of commissioning tests for MC based treatment planning systems is not only to evaluate the accuracy of the system, but also to define the optimum calculation parameters, such as number of histories, calculation voxel size, etc., which will be applied in clinical use of the system. This in turn requires that medical physicists responsible for clinical implementation of such systems are well educated in principles of Monte Carlo algorithms.We provide a brief review of AAPM Task Group Report No. 105 (Med. Phys. 34 (2007) 4818–4853), a document which outlines the important aspects of a MC‐based dose calculation algorithm, from the basic aspects of the use of the MC method for radiation transport to the application of this approach in routine clinical photon and electron beam treatment planning. We also describe possible clinical implementation issues, including dose‐to‐medium vs. dose‐to‐water differences and give comparison of computation times for typical patient anatomies and calculation parameters.Learning Objectives:1. To provide an educational review of the physics of the MC method.2. To discuss the factors associated with MC dose calculation within the patient‐specific geometry, such as statistical uncertainties, approximations of the underlying physics model, CT‐number to material density assignments, and reporting of dose‐to‐medium versus dose‐to‐water.3. To briefly review the vendor transport codes currently used for clinical treatment planning.4. To discuss the issues associated with experimental verification of MC algorithms.5. To briefly review the potential clinical implications of MC calculated dose distributions.6. To provide example timing comparisons of the major vendor MC codes in the clinical setting.

SU‐FF‐T‐339: Small Field Electron Beam Dosimetry Using MOSFET Detector

Purpose: The dosimetry of very small electron fields can be challenging due to relative shifts in percent depth‐dose curves, including the location of dmax, and lack of lateral electronic equilibrium in an ion chamber when placed in the beam. Conventionally, a small parallel plate chamber or film is utilized to perform small field electron beam dosimetry. With the advent of electronic imaging, modern radiotherapy departments are moving to a filmless environment and an alternate clinical dosimeter is required for routine clinical dosimetry. In this work, we have studied the performance of MOSFET as a relative dosimeter for small field electron beams. Methods and materials: The reproducibility, linearity and sensitivity of a high sensitivity micro MOSFET were investigated for clinical electron beams. In addition, the percent depth doses, output factors and profiles have been measured in a water tank with MOSFET and compared with those measured by an ion chamber and film for a range of field sizes from 1 cm diameter to 10×10 cm2 for 6, 12, 16 and 20 MeV beams. Similar comparative measurements were also performed with MOSFET as well as with films in solid water phantom. Results: The MOSFET sensitivity was found to be stable over the range of field sizes investigated. The dose response was found to be linear and reproducible (within ±1% for 100 cGy). For 1, 1.5, 2, 2.5, 3, 4, 5, 6 cm circular and 10 cm square field, excellent agreement (within ±2% and ±2mm) was observed among the central axis depth dose curves measured using MOSFET, film and ion chamber. The output factors measured with MOSFET for small fields agreed to within 2% with those measured by film dosimetry. Conclusion: Overall results indicate that MOSFET can be utilized to perform dosimetry of small electron fields for routine clinical use.

SU‐FF‐T‐397: Positioning Recommendations for Parallel‐Plate Ionization Chambers in Clinical Electron Beams According to Different Dosimetry Protocols ‐ a Monte Carlo Study for Three Different Chamber Types

Purpose: Current dosimetry protocols recommend the use of plane‐parallel ionization chambers for the dosimetry of clinical electron beams. Regarding the allowance for the non‐water equivalence of the entrance window and therefore the exact positioning of the parallel‐plate chambers, the recommendations given in these protocols are not unique, resulting in positioning deviations in the range of tenth of millimeter. Method and Materials: The dose and the spectral fluences inside the active volume of three parallel‐plate chamber types (Roos‐, Advanced‐ Markus‐ and NACP‐chamber) were calculated in a water phantom in the reference depth zRef and the half‐value depth R50 for three electron energies (6, 11 and 21 MeV) using the EGSnrc Monte Carlo code. From these results the perturbation correction pwall was calculated as pwall = Dcav/Ddet where Ddet is the dose inside the active volume of the chamber and Dcav the dose inside the air cavity with walls entirely made of water. To inspect the consequences of the different positioning recommendations given in the dosimetry protocols (AAPM TG‐51, IAEA TRS‐398, DIN 6800‐2), the chamber's reference point was shifted by an amount Δz to fulfill the recommendations given in the different protocols. Results: Although the recommended shifts Δz are in the range of tenth of millimeters only, the spectral fluence inside the active volume of the chambers changed strongly with Δz. At the depth zref these spectral fluence changes had no impact on pwall. For all chambers its numerical value is larger than 1, the recommended value given in all protocols. Contrary, at the depth R50 the different positioning recommendations resulted in deviations of pwall up to 15%. Conclusion: The different positioning recommendations have no impact on the results of dose measurements at the clinical relevant depth zref. The resulting deviations in pwall at the depth R50may be of academic interest.

SU‐FF‐T‐404: Dosimetric Effect of a Small Air Cavity for Clinical Electron Beams: A Monte Carlo Study

Purpose: This study investigated dosimetric changes in a water phantom when a small air cavity was presented at the central axis of a clinical electron beam. 6, 9 and 16 MeV electron beams with a 10×10 cm2 applicator and cutout produced by a Varian 21EX linear accelerator were used. Methods and Materials: Percentage depth doses (PDDs) for different depths (0.5–7 cm), thicknesses (2–10 mm) and widths (1–5 cm) of air cavities were calculated using Monte Carlo simulations (EGSnrc code) validated by film measurements. Results: When the depth or thickness of the air cavity was changed, the PDD below the cavity was shifted with a distance equal to the thickness of cavity. However, when the width of the air cavity was changed, both the PDD and its slope within and below the cavity were changed. A larger width of the air cavity resulted in a shallower PDD within the cavity. The dependence of the depth dose on the width of the air cavity is due to the contribution of the electron side‐scattering in the water surrounding the cavity. Neglecting an air cavity of 1 cm thickness in the build up region of a 6 MeV electron beam resulted in a delivered dose 10%–12% larger than the original prescription. Delivered doses 3% and 6% higher than the prescribed dose were observed when doses were prescribed at R80 for a 16 MeV electron beam. These results were obtained by neglecting air cavities with thicknesses equal to 2 and 4 mm at a depth of 5 cm, respectively. Conclusions: The dosimetry data in this study can help radiotherapy staff estimate and predict the deviation between the prescribed and delivered dose, when inevitablely neglecting the small air cavity or inhomogeneous correction in the dose calculation and output measurement.