Use of BANG polymer gel for dose measurements in a 68 MeV proton beam

BANG polymer gel dosimetry using magnetic resonance imaging (MRI) was applied to an ophthalmologic 68 MeV proton beam. The object was to examine the use of BANG gel for the verification of proton fields in eye tumor therapy and to explore the applicability of polymer gel dosimetry in proton therapy under practical aspects. The gel phantoms were irradiated with monoenergetic and modulated proton beams. MRI analysis was carried out at clinical 1.5 and 3 T MR scanners. At constant LET, results show a linear relationship between spin-spin relaxation rates and dose. However, depth dose curves in BANG gel reveal a quenching of the Bragg maximum due to LET effects. The dose response of the gel for monoenergetic protons and spread-out depth dose distributions can be calculated based on ionization chamber measurements. Experiment and calculations show good agreement and indicate that BANG polymer gels might become a valuable tool in proton therapy quality assurance.     

A dosimetric system for quantitative cell irradiation experiments with laser-accelerated protons

An integrated dosimetry and cell irradiation system (IDOCIS) with laser-accelerated proton beams was developed, characterized, calibrated and successfully used for systematic in vitro experiments. Due to the broad exponentially shaped energy spectrum, the low-energy range of the protons (<20 MeV) and the high pulse dose, the absolute dosimetry for this beam quality is challenging. Therefore, a dedicated Faraday cup is used as an energy and dose rate independent absolute dosimeter that has been calibrated consistently with three independent methods. A transmission ionization chamber providing online relative dose information is cross-calibrated against the Faraday cup. Providing both online and absolute dose information, the IDOCIS allows for quantitative dosimetric and radiobiological studies at current low-energy laser-accelerated proton beams. Finally, first dosimetric characterizations of a laser-accelerated proton beam with the IDOCIS are presented.     

The properties of the ultramicrocylindrical ionization chamber for small field used in stereotactic radiosurgery

Accurate dosimetry of small-field photon beams tends to be difficult to perform due to the presence of lateral electronic disequilibrium and steep dose gradients. In stereotactic radiosurgery (SRS), small fields of 6-30 mm in diameter are used. Generally thermoluminescence dosimetry chips, Farmer, Thimble ion chamber, and film dosimetry are not adequate to measure dose in SRS beams. These techniques generally do not provide the required precision due to their energy dependence and/or poor resolution. It is necessary to construct a small, accurate detector with high spatial resolution for the small fields used in SRS. The ultramicrocylindrical ionization chamber (UCIC) with a gold wall of 2.2 mm in diameter and 4.0 mm in length has dual sensitive volumes of air (8.0 mm3) and borosilicate (2.6 mm3) cavity. Reproducibility, linearity, and radiation damage with respect to absorbed dose, beam profile of small beam, and independence of dose rate of the UCIC are tested by the dose measurements in high energy photon (5, 15 MV) and electron (9 MeV) beams. The UCIC with a unique supporting system in the polystyrene phantom is demonstrated to be a suitable detector for the dose measurements in a small beam size.     

An application of GafChromic MD-55 film for 67.5 MeV clinical proton beam dosimetry

The purpose of this study is to explore the use of GafChromic MD-55 (RC) film for 67.5 MeV clinical proton beam dosimetry at the Crocker Nuclear Laboratory, University of California, Davis. Several strips of RC film 6 cm x 6 cm in dimension were irradiated at a depth of 18.2 mm corresponding to the middle of a 24 mm spread-out Bragg peak (SOBP). The films were irradiated to a proton dose in the range of 0.5 Gy to 100 Gy. The beam profiles were also measured at the middle of the 24 mm SOBP. The Bragg peak was measured by using a wedge shaped phantom made of Lucite. The Bragg peak measured with RC film was compared with diode and ionization chamber measurements. After background subtraction, the calibration of the dose response of RC film showed, to a maximum deviation of 10%, a linear increase of optical density (OD) with dose from 0.5 to 100 Gy. The uniformity of OD over a single sheet of film showed a variation of +/-6%. The distal-fall off between 90% and 20% measured with GafChromic film for the Bragg peak was 1.3 mm as compared to 1.1 mm for a diode measurement and 1.4 mm for an ionization chamber measurement. The FWHM of the Bragg peak was 7.5 mm when measured with GafChromic film, 5.3 mm when measured with a diode and 8.1 mm as measured by an ionization chamber. The peak/plateau ratio with GafChromic film was 3.3 as compared to 3.7 with a diode and 3.2 with an ionization chamber. In conclusion, GafChromic MD-55 film may be a useful and convenient detector for dose measurement and quality assurance programmes of proton beams.     

Investigations of a flat-panel detector for quality assurance measurements in ion beam therapy

Increased accuracy in radiation delivery to a patient provided by scanning particle beams leads to high demands on quality assurance (QA). To meet the requirements, an extensive quality assurance programme has been implemented at the Heidelberg Ion Beam Therapy Center. Currently, high-resolution radiographic films are used for beam spot position measurements and homogeneity measurements for scanned fields. However, given that using this film type is time and equipment demanding, considerations have been made to replace the radiographic films in QA by another appropriate device. In this study, the suitability of the flat-panel detector RID 256 L based on amorphous silicon was investigated as an alternative method. The currently used radiographic films were taken as a reference. Investigations were carried out for proton and carbon ion beams. The detectors were irradiated simultaneously to allow for a direct comparison. The beam parameters (e.g. energy, focus, position) currently used in the daily QA procedures were applied. Evaluation of the measurements was performed using newly implemented automatic routines. The results for the flat-panel detector were compared to the standard radiographic films. Additionally, a field with intentionally decreased homogeneity was applied to test the detector's sensitivities toward possible incorrect scan parameters. For the beam position analyses, the flat-panel detector results showed good agreement with radiographic films. For both detector types, deviations between measured and planned spot distances were found to be below 1% (1 mm). In homogeneously irradiated fields, the flat-panel detector showed a better dose response homogeneity than the currently used radiographic film. Furthermore, the flat-panel detector is sensitive to field irregularities. The flat-panel detector was found to be an adequate replacement for the radiographic film in QA measurements. In addition, it saves time and equipment because no post-exposure treatment and no developer and darkroom facilities are needed.     

Dosimetry techniques for narrow proton beam radiosurgery

Characterization of narrow beams used in proton stereotactic radiosurgery (PSRS) requires special efforts, since the use of finite size detectors can lead to distortion of the measured dose distributions. Central axis depth doses, lateral profiles and field size dependence factors are the most important beam characteristics to be determined prior to dosimetry calculations and beam modelling for PSRS. In this paper we report recommendations for practical dosimetry techniques which were developed from a comparison of beam characteristics determined with a variety of radiation detectors for 126 and 155 MeV narrow proton beams shaped with 2-30 mm circular brass collimators. These detectors included small-volume ionization chambers, a diamond detector, an Hi-p Si diode, TLD cubes, radiographic and radiochromic films. We found that both types of film are suitable for profile measurements in narrow beams. Good agreement between depth dose distributions measured with ionization chambers, diamond and diode detectors was demonstrated in beams with diameters of 20-30 mm. The diode detector can be used in smaller beams, down to 5 mm diameter. For beams with diameters less than 5 mm, reliable depth dose data may be obtained only with radiochromic film. The tested ionization chambers are appropriate for calibration of beams with diameters of 20-30 mm. TLD cubes and diamond detectors are useful to determine relative dose in beams with diameters of 10-20 mm. Field size factors for smaller beams should be obtained with diode and radiochromic film. We conclude that dosimetry characterization of proton beams down to several millimetres in diameter can be performed using the described procedures.     

Comparison of Dosimetric Performance among Commercial Quality Assurance Systems for Verifying Pretreatment Plans of Stereotactic Body Radiotherapy Using Flattening-Filter-Free Beams

The purpose of this study was to compare the performance of different commercial quality assurance (QA) systems for the pretreatment verification plan of stereotactic body radiotherapy (SBRT) with volumetric arc therapy (VMAT) technique using a flattening-filter-free beam. The verification for 20 pretreatment cancer patients (seven lung, six spine, and seven prostate cancers) were tested using three QA systems (EBT3 film, I’mRT MatriXX array, and MapCHECK). All the SBRT-VMAT plans were optimized in the Eclipse (version 11.0.34) treatment planning system (TPS) using the Acuros XB dose calculation algorithm and were delivered to the Varian TrueBeam^® accelerator equipped with a high-definition multileaf collimator. Gamma agreement evaluation was analyzed with the criteria of 2% dose difference and 2 mm distance to agreement (2%/2 mm) or 3%/3 mm. The highest passing rate (99.1% for 3%/3 mm) was observed on the MapCHECK system while the lowest passing rate was obtained on the film. The pretreatment verification results depend on the QA systems, treatment sites, and delivery beam energies. However, the delivery QA results for all QA systems based on the TPS calculation showed a good agreement of more than 90% for both the criteria. It is concluded that the three 2D QA systems have sufficient potential for pretreatment verification of the SBRT-VMAT plan.     

电离室矩阵用于加速器质量保证：X射线

目的：探讨利用二维电离室矩阵MatriXX进行医用直线加速器质量保证的方法及可靠性。方法：在点剂量的质量保证中。将MatriXX系统在特制水箱中测得的数据与O．6CC电离室在标准水箱中的剂量测量值作比较,从而定出Ma．triXX的校准系数．以此来校准射线的输出量和能量。在面剂量的质量保证中,用MatriXX测量医用加速器射线野的对称性、平坦度以及辐射野的大小及中心轴偏离．将测量结果与RFA300三维水箱扫描结果及控制台显示数值进行比较分析。结果：用MatriXX加有机玻璃板可准确快捷地完成对射线输出量和能量的校准。用MatriXX所测射野的对称性和平坦度及射线野大小与RFA300水箱的测量数据一致。结论：MatriXX系统进行加速器的质量保证简便可靠。     

An Active Matrix Flat Panel Dosimeter (AMFPD) for in-phantom dosimetric measurements

An a-Si Active Matrix Flat Panel Imager (AMFPI) prototype developed in-house has been modified to function as an in-phantom dosimetry system providing high resolution two-dimensional (2-D) data. This Active Matrix Flat Panel Dosimeter (AMFPD) system can be used as a replacement device for standard in-phantom dosimeters, such as scanning ion chambers in water, or film in solid water. The initial characterization of the device demonstrates a wide dynamic range (up to 160 cGy), a stable calibration curve (less than 1.5% variation over 1 year), dose rate independence (less than 1%), and excellent agreement of output factors with ion chamber measurements for a range of field sizes (less than 2%). The device also compares well to film for 2-D planar dose distributions. It is expected that the AMFPD system will be useful for beam commissioning, algorithm verification test data, and routine IMRT quality assurance dosimetry.     

Transit dosimetry in IMRT with an a-Si EPID in direct detection configuration

In this study an amorphous silicon electronic portal imaging device (a-Si EPID) converted to direct detection configuration was investigated as a transit dosimeter for intensity modulated radiation therapy (IMRT). After calibration to dose and correction for a background offset signal, the EPID-measured absolute IMRT transit doses for 29 fields were compared to a MatriXX two-dimensional array of ionization chambers (as reference) using Gamma evaluation (3%, 3 mm). The MatriXX was first evaluated as reference for transit dosimetry. The accuracy of EPID measurements was also investigated by comparison of point dose measurements by an ionization chamber on the central axis with slab and anthropomorphic phantoms in a range of simple to complex fields. The uncertainty in ionization chamber measurements in IMRT fields was also investigated by its displacement from the central axis and comparison with the central axis measurements. Comparison of the absolute doses measured by the EPID and MatriXX with slab phantoms in IMRT fields showed that on average 96.4% and 97.5% of points had a Gamma index<1 in head and neck and prostate fields, respectively. For absolute dose comparisons with anthropomorphic phantoms, the values changed to an average of 93.6%, 93.7% and 94.4% of points with Gamma index<1 in head and neck, brain and prostate fields, respectively. Point doses measured by the EPID and ionization chamber were within 3% difference for all conditions. The deviations introduced in the response of the ionization chamber in IMRT fields were<1%. The direct EPID performance for transit dosimetry showed that it has the potential to perform accurate, efficient and comprehensive in vivo dosimetry for IMRT.     

Performance of a fluorescent screen and CCD camera as a two-dimensional dosimetry system for dynamic treatment techniques

A two-dimensionally position sensitive dosimetry system has been tested for different dosimetric applications in a radiation therapy facility with a scanning proton beam. The system consists of a scintillating (fluorescent) screen, mounted at the beam-exit side of a phantom and it is observed by a charge coupled device (CCD) camera. The observed light distribution at the screen is equivalent to the two-dimensional (2D)-dose distribution at the screen position. It has been found that the dosimetric properties of the system, measured in a scanning proton beam, are equal to those measured in a proton beam broadened by a scattering system. Measurements of the transversal dose distribution of a single pencil beam are consistent with dose measurements as well as with dose calculations in clinically relevant fields made with multiple pencil beams. Measurements of inhomogeneous dose distributions have shown to be of sufficient accuracy to be suitable for the verification of dose calculation algorithms. The good sensitivity and sub-mm spatial resolution of the system allows for the detection of deviations of a few percent in dose from the expected (intended or calculated) dose distribution. Its dosimetric properties and the immediate availability of the data make this device a useful tool in the quality control of scanning proton beams.     

Monte Carlo study of si diode response in electron beams

Silicon semiconductor diodes measure almost the same depth-dose distributions in both photon and electron beams as those measured by ion chambers. A recent study in ion chamber dosimetry has suggested that the wall correction factor for a parallel-plate ion chamber in electron beams changes with depth by as much as 6%. To investigate diode detector response with respect to depth, a silicon diode model is constructed and the water/silicon dose ratio at various depths in electron beams is calculated using EGSnrc. The results indicate that, for this particular diode model, the diode response per unit water dose (or water/diode dose ratio) in both 6 and 18 MeV electron beams is flat within 2% versus depth, from near the phantom surface to the depth of R50 (with calculation uncertainty <0.3%). This suggests that there must be some other correction factors for ion chambers that counter-balance the large wall correction factor at depth in electron beams. In addition, the beam quality and field-size dependence of the diode model are also calculated. The results show that the water/diode dose ratio remains constant within 2% over the electron energy range from 6 to 18 MeV. The water/diode dose ratio does not depend on field size as long as the incident electron beam is broad and the electron energy is high. However, for a very small beam size (1 X 1 cm(2)) and low electron energy (6 MeV), the water/diode dose ratio may decrease by more than 2% compared to that of a broad beam.     

Verification of absolute ionization chamber dosimetry in a proton beam using carbon activation measurements

Reference ionization chamber dosimetry implemented in a clinical proton beam and based on the ICRU 59 recommendations has been verified with an independent carbon activation method. The 12C(p,pn)11C nuclear reaction was used to measure the beam fluence and entrance dose. A method to transfer from the entrance dose to the dose at the ion chamber calibration position has been developed. Measurements performed in a monochromatic 200 MeV beam show that the ratio of absolute doses measured using the carbon activation and the ion chamber methods is 1.017 +/- 0.03 (type A uncertainty). This result is within the uncertainties of both methods employed, which are estimated at +/- 4.3% (carbon activation) and +/- 2.7% (ion chamber calibration).     

A scintillating gas detector for 2D dose measurements in clinical carbon beams

A two-dimensional position sensitive dosimetry system based on a scintillating gas detector has been developed for pre-treatment verification of dose distributions in hadron therapy. The dosimetry system consists of a chamber filled with an Ar/CF(4) scintillating gas mixture, inside which two cascaded gas electron multipliers (GEMs) are mounted. A GEM is a thin kapton foil with copper cladding structured with a regular pattern of sub-mm holes. The primary electrons, created in the detector's sensitive volume by the incoming beam, drift in an electric field towards the GEMs and undergo gas multiplication in the GEM holes. During this process, photons are emitted by the excited Ar/CF(4) gas molecules and detected by a mirror-lens-CCD camera system. Since the amount of emitted light is proportional to the dose deposited in the sensitive volume of the detector by the incoming beam, the intensity distribution of the measured light spot is proportional to the 2D hadron dose distribution. For a measurement of a 3D dose distribution, the scintillating gas detector is mounted at the beam exit side of a water-bellows phantom, whose thickness can be varied in steps. In this work, the energy dependence of the output signal of the scintillating gas detector has been verified in a 250 MeV/u clinical (12)C ion beam by means of a depth-dose curve measurement. The underestimation of the measured signal at the Bragg peak depth is only 9% with respect to an air-filled ionization chamber. This is much smaller than the underestimation found for a scintillating Gd(2)O(2)S:Tb ('Lanex') screen under the same measurement conditions (43%). Consequently, the scintillating gas detector is a promising device for verifying dose distributions in high LET beams, for example to check hadron therapy treatment plans which comprise beams with different energies.     

Dose verification of helical tomotherapy intensity modulated radiation therapy planning using 2D-array ion chambers

Purpose:

To investigate the clinical usage of dose verification of Helical Tomotherapy plans by using 2D-array ion chambers, and to develop an efficient way to validate the dose delivered for the patients during treatments.

Materials and Methods:

A pixel-segmented ionisation chamber device, IMRT MatriXX™ and Multicube™ phantom from IBA were used on ten selected Tomotherapy IMRT/IGRT head and neck plans in this study. The combined phantom was set up to measure the dose distribution from coronal and sagittal planes. The setup of phantom was guided for verifying the correction position by pre-treatment Tomotherapy MVCT images. After the irradiation, the measured dose distributions of coronal and sagittal planes were compared with those from calculation by the planning system for cross verification. The results were evaluated by the absolute and relative doses as well as Gamma (γ) function. The feasibility of the different measuring methods was studied for this rotational treatment technique.

Results:

The dose distributions measured by the MatriXX 2D array were in good agreements with plans calculated by Tomotherapy planning system. The discrepancy between the measured dose and predicted dose in the selected points was within ±3%. In the comparison of the pixel-segmented ionisation chamber versus treatment planning system using the 3 mm/3% γ-function criteria, the mean passing rates of 2 mm dose grid with γ-parameter ≤1 were 97.37% and 96.91%, in two orthogonal planes (coronal and sagittal directions), respectively.

Conclusion:

MatriXX with Multicube is a new system created for rotational delivery quality assurance (QA) and found to be reliable to measure both absolute dose and relative dose distributions, simultaneously. It achieves the goal of an efficient and accurate dosimetry validation method of the helical delivery pattern for the Helical Tomotherapy IMRT planning.     

Independent verification of transferred delivery sinogram between two dosimetrically matched helical tomotherapy machines: a protocol for patient-specific quality assurance

The purpose of this study was to independently verify the transferred delivery sinogram between two dosimetrically matched helical tomotherapy machines with the goal of eliminating redundant quality assurance (QA) measurements on the second machine. The equivalence of the two machines was evaluated based on both geometric and dosimetric beam characteristics, including measuring open field per cent depth doses (PDD), longitudinal and transverse profiles and helical delivery of clinical patient treatment plans measured in phantoms. QA of 56 patient plans was studied. The delivery sinogram on the secondary machine was computed by accounting for the differences in the MLC characteristics of the two machines. Computed sinograms were compared against the transferred sinograms by tomotherapy's data management system for the same 56 patient plans. The PDD, transverse and longitudinal dose profiles agreed within ±1% between the two machines. Ionization chamber and planar dose measurements with the Iba MatriXX device on both machines for the 56 patients were found to be within ±3% of the doses computed by the tomotherapy treatment planning system. For all 56 patients, the differences between computed sinograms and DMS-converted sinograms were within ±2%. The matched tomotherapy machines had similar beam characteristics. The sinogram-based QA was validated using point and planar dose measurements and found to be acceptable for clinical use.     

Calculation of the replacement correction factors for ion chambers in megavoltage beams by Monte Carlo simulation

This article describes four methods of calculating the replacement correction factor, P(repl0 (or the product p(cav)P(dis) in the IAEA's notation), for a plane-parallel chamber in both electron and photon beams, and for a Farmer chamber in photon beams, by using the EGSnrc Monte Carlo code. The accuracy of underlying assumptions and relative merits of each technique are assessed. With careful selection of parameters it appears that all four methods give reasonable answers although the direct methods are more intellectually satisfying and more accurate in some cases. The direct methods are shown to have an accuracy of 0.11% when appropriate calculation parameters are selected. The depth dependence of P(repl) for the NACP02 plane-parallel chamber has been calculated in both 6 and 18 MeV electron beams. At the reference depth (0.6R50-0.1 cm) P(repl) is 0.9964 for the 6 MeV beam and 1.0005 for the 18 MeV beam for this well-guarded chamber; at the depth of maximum dose for the 18 MeV beam, P(repl) is 1.0010. P(repl) is also calculated for the NACP02 chamber and a Farmer chamber (diameter 6 mm) at a depth of 5 cm in a 60Co photon beam, giving values of 1.0063 and 0.9964, respectively. For the Farmer chamber, P(repl) is about half a percent higher than the value (0.992) recommended by the AAPM dosimetry protocol. It is found that the dosimetry protocols may have adopted an incorrect value of P(repl) for cylindrical chambers in photon beams. The nonunity values of P(repl) for plane-parallel chambers in lower energy electron beams imply a variety of values used in dosimetry protocols must be reassessed.     

The characterization of the Advanced Markus ionization chamber for use in reference electron dosimetry in the UK

The IPEM Code of Practice (IPEM 2003) for electron dosimetry for radiotherapy beams recommends design requirements for parallel-plate ionization chambers used to determine absorbed dose to water in an electron beam. The Classic Markus design has been found not to meet these requirements. The Advanced Markus ionization chamber has been designed to rectify the problems associated with the Classic Markus ionization chamber. The response of three Advanced Markus ionization chambers was investigated and compared to the designated chamber types. Absorbed dose to water calibration factors were derived at the National Physical Laboratory (NPL) for each ionization chamber at seven electron energies in the range nominally 4-19 MeV. Investigations were carried out into chamber settling, polarity effects, ion recombination and the chamber perturbation. The response of the ionization chambers in a clinical beam was also investigated. In general all three Advanced Markus ionization chambers showed the same energy response. The magnitude of the polarity effect was typically 5% at a nominal energy of 4 MeV. There was discrepancy between the polarity measurements made at the NPL and in the clinic. The recommendation of this study is that this chamber type is not suitable for reference dosimetry in electron beams.     

Patient specification quality assurance for glioblastoma multiforme brain tumors treated with intensity modulated radiation therapy

The aim of this study was to evaluate the significance of performing patient specification quality assurance for patients diagnosed with glioblastoma multiforme treated with intensity modulated radiation therapy. The study evaluated ten intensity modulated radiation therapy treatment plans using 10 MV beams, a total dose of 60 Gy (2 Gy/fraction, five fractions a week for a total of six weeks treatment). For the quality assurance protocol we used a two-dimensional ionization-chamber array (2D-ARRAY). The results showed a very good agreement between the measured dose and the pretreatment planned dose. All the plans passed >95% gamma criterion with pixels within 5% dose difference and 3 mm distance to agreement. We concluded that using the 2D-ARRAY ion chamber for intensity modulated radiation therapy is an important step for intensity modulated radiation therapy treatment plans, and this study has shown that our treatment planning for intensity modulated radiation therapy is accurately done.