Use of electronic portal imaging devices for electron treatment verification

This study aims to help broaden the use of electronic portal imaging devices (EPIDs) for pre-treatment patient positioning verification, from photon-beam radiotherapy to photon- and electron-beam radiotherapy, by proposing and testing a method for acquiring clinically-useful EPID images of patient anatomy using electron beams, with a view to enabling and encouraging further research in this area. EPID images used in this study were acquired using all available beams from a linac configured to deliver electron beams with nominal energies of 6, 9, 12, 16 and 20 MeV, as well as photon beams with nominal energies of 6 and 10 MV. A widely-available heterogeneous, approximately-humanoid, thorax phantom was used, to provide an indication of the contrast and noise produced when imaging different types of tissue with comparatively realistic thicknesses. The acquired images were automatically calibrated, corrected for the effects of variations in the sensitivity of individual photodiodes, using a flood field image. For electron beam imaging, flood field EPID calibration images were acquired with and without the placement of blocks of water-equivalent plastic (with thicknesses approximately equal to the practical range of electrons in the plastic) placed upstream of the EPID, to filter out the primary electron beam, leaving only the bremsstrahlung photon signal. While the electron beam images acquired using a standard (unfiltered) flood field calibration were observed to be noisy and difficult to interpret, the electron beam images acquired using the filtered flood field calibration showed tissues and bony anatomy with levels of contrast and noise that were similar to the contrast and noise levels seen in the clinically acceptable photon beam EPID images. The best electron beam imaging results (highest contrast, signal-to-noise and contrast-to-noise ratios) were achieved when the images were acquired using the higher energy electron beams (16 and 20 MeV) when the EPID was calibrated using an intermediate (12 MeV) electron beam energy. These results demonstrate the feasibility of acquiring clinically-useful EPID images of patient anatomy using electron beams and suggest important avenues for future investigation, thus enabling and encouraging further research in this area. There is manifest potential for the EPID imaging method proposed in this work to lead to the clinical use of electron beam imaging for geometric verification of electron treatments in the future.     

A nearly mono-energetic 6-7 MeV photon calibration source

A photon source has been developed which delivers about 85% of its photon dose equivalent from photons with energies of 6.1,6.9 and 7.1 MeV produced in the 19F(p, alpha gamma)16O reaction. The source uses up to 50 muA of 2.7 MeV protons incident on a 6 mg/cm2 target of CaF2. It produces a photon field with a dose equivalent rate of up to 6 mSv/h (600 mrem/h) over a large area 100 cm from the target. The field can be calibrated in terms of photon fluence to within +/- 5%. In common with other high-energy photon sources, there is considerable contamination of the field by knock-on electrons and scattered photons. Experiments with various filter materials and detailed Monte-Carlo calculations with the EGS electron-photon transport code have been done to investigate the importance of these contaminants.     

Investigation of photon beam models in heterogeneous media of modern radiotherapy

This study investigates the performance of photon beam models in dose calculations involving heterogeneous media in modern radiotherapy. Three dose calculation algorithms implemented in the CMS FOCUS treatment planning system have been assessed and validated using ionization chambers, thermoluminescent dosimeters (TLDs) and film. The algorithms include the multigrid superposition (MGS) algorithm, fast Fourier Transform Convolution (FFTC) algorithm and Clarkson algorithm. Heterogeneous phantoms used in the study consist of air cavities, lung analogue and an anthropomorphic phantom. Depth dose distributions along the central beam axis for 6 MV and 10 MV photon beams with field sizes of 5 cm x 5 cm and 10 cm x 10 cm were measured in the air cavity phantoms and lung analogue phantom. Point dose measurements were performed in the anthropomorphic phantom. Calculated results with three dose calculation algorithms were compared with measured results. In the air cavity phantoms, the maximum dose differences between the algorithms and the measurements were found at the distal surface of the air cavity with a 10 MV photon beam and a 5 cm x 5 cm field size. The differences were 3.8%. 24.9% and 27.7% for the MGS. FFTC and Clarkson algorithms. respectively. Experimental measurements of secondary electron build-up range beyond the air cavity showed an increase with decreasing field size, increasing energy and increasing air cavity thickness. The maximum dose differences in the lung analogue with 5 cm x 5 cm field size were found to be 0.3%. 4.9% and 6.9% for the MGS. FFTC and Clarkson algorithms with a 6 MV photon beam and 0.4%. 6.3% and 9.1% with a 10 MV photon beam, respectively. In the anthropomorphic phantom, the dose differences between calculations using the MGS algorithm and measurements with TLD rods were less than +/-4.5% for 6 MV and 10 MV photon beams with 10 cm x 10 cm field size and 6 MV photon beam with 5 cm x 5 cm field size, and within +/-7.5% for 10 MV with 5 cm x 5 cm field size, respectively. The FFTC and Clarkson algorithms overestimate doses at all dose points in the lung of the anthropomorphic phantom. In conclusion, the MGS is the most accurate dose calculation algorithm of investigated photon beam models. It is strongly recommended for implementation in modern radiotherapy with multiple small fields when heterogeneous media are in the treatment fields.     

The peripheral dose outside the applicator in electron beams of an Elekta linear accelerator

Peripheral doses out of field could have short and long terms biological effects on patients treated with electron beams. In this study, peripheral dose outside the applicator was measured using the 6, 10 and 18 MeV beams of an Elekta synergy linac. For these beams dose profiles were measured using EBT3 film at various depths within a solid water phantom. Measurements were performed using 6?×?6, 10?×?10, 14?×?14 and 20?×?20 cm² applicators at gantryangles of 0°, 10° and 20° and depths of 0, 0.5, 1 cm and depth of Dmax (maximum dose) for each energy. The peripheral dose profiles were normalized to the distance of 2 cm from the edge of each field. The largest peak of the peripheral dose was observed for 18 MeV 3 cm from the outer edge of the applicator. Peak dose increased with increasing energy. Peak dose at 18 MeV electron beam was 1.6% at the surface of phantom and at the distance of 2 cm from the outer edge of the applicator when the applicator of 20?×?20 cm² was used. Peak dose at 6 MeV electron beam was 1.15% at the same distance in the same applicator size. It was found that the peak dose decreased with increasing depth and increased with increase in field size. Also, the peak dose moved towards CAX with increase in gantry angle.In general dose to tissue out of field could be reduced using appropriate shielding for each applicator and beam energy.     

不同模体材料对加速器绝对剂量校准的影响与分析

目的:探讨使用不同的模体材料如水、固体水校准加速器绝对剂量时对校准结果的影响程度,分析固体水模在不同能量挡时代替水模体的可行性。方法:分别采用水及PTW固体水测量Varian 21EX各个能量挡,测量探头板测量点深度为0.7 cm;6、9 MeV电子线测量深度为1 cm,需放置0.3 cm的固体水;12、16、20 MeV电子线测量深度为2 cm,需放置1.3 cm的固体水;6 MV X线测量深度为5 cm,需放置4.3 cm的固体水;15 MV X线测量深度为10 cm,需放置9.3 cm的固体水。结果:6、9、12、16、20 MeV电子线测量模体为固体水时比水分别小5.6%、2.8%、1.9%、0.7%、0.1%,6、15 MV X线测量模体为固体水时与水的差别分别为3.7%、3.6%。结论:校准12、16、20 MeV电子线及6、15 MV X线时固体水模可代替水模体,差别较小;6、9 MeV电子线因测量深度较浅,加之固体水是多个小单位固体水的叠加,测量差别较大,不易代替水测量。     

An investigation of central axis depth dose distribution perturbation due to an air gap between patient and bolus for electron beams

Electron beams can be used for the radiotherapy treatment of superficial cancers. In many cases of electron beam radiotherapy, tissue equivalent bolus material placed on the skin is to be used to enhance skin dose. An air gap might be present between the bolus and the skin due to variation in the patient contour. The impact of semi-infinite air gaps under bolus material on central axis depth dose distributions for electron beams was investigated in this study. Semi-infinite air gaps were introduced between bolus and the surface of a water phantom for air gap sizes up to 20.0 mm and for bolus thicknesses of 5, 10 and 15 mm. The electron beams studied had nominal energies of 6, 10 and 14 MeV and circular fields of 3, 5, 7 and 9 cm diameter. Depth dose measurements were carried out in the water phantom with a Scanditronix p-Si electron diode. It was found that the impact of an air gap is dependent on beam energy, field size, air gap size and bolus thickness used. The impact of the air gap on central axis depth dose distribution increased with decreasing field size, increasing air gap size, decreasing electron beam energy and increasing bolus thickness. For 15 mm bolus, 3 cm diameter circular field, 6 MeV beam and the 20 mm air gap, the maximum dose and the surface dose was reduced by approximately 60% and the depth of dose maximum shifted 3.5 mm. An air gap between bolus and a patient should be avoided to ensure that there is no impact on the treatment. The measured data in this study can be used to determine the likely degree of impact on the treatment, of unavoidable air gaps between bolus and the patient.     

Comparison of skin dose between conventional radiotherapy and IMRT

The purpose of this study was to measure skin dose using radiochromic film for two step-and-shoot IMRT fields and compare the results to the skin dose for a conventional open field. All exposures were made using a 6 MV photon beam produced by a Varian 21EX linear accelerator (Varian Medical Systems, CA, USA) equipped with a Millennium 120 leaf MLC. Three different field configurations were used, these were an open field, a step-and-shoot IMRT field and a clinical IMRT field. The mean ratio of the skin dose to dose at d(max) for an open 10 x 10 cm2 field at 100 cm SSD was 0.178 +/- 0.003. The step-and-shoot IMRT field consisted of 1 cm wide strips of decreasing intensity that were delivered using a step-and-shoot technique across a 10 x 10 cm2 field. The ratio of skin dose to dose at d(max) ranged from 0.180 to 0.257, with the low intensity steps having a higher relative skin dose compared to the high intensity steps. A model was derived that attributed these variations to the electron contamination from both the adjacent and more distant high intensity steps. The clinical field consisted of a 25 segment 9.8 x 10.0 cm2 beam arrangement. The ratio of skin dose to dose at d(max) for the clinical IMRT field ranged from 0.093 to 0.284. The results indicated that an IMRT field produced only minor changes in the relative skin dose, with variations potentially attributable to fluctuations in the electron contamination produced by neighbouring regions of different intensity. The use of an individual IMRT field does not significantly increase the skin dose above that of a conventional photon field.     

Determining superficial dosimetry for the internal canthus from the Monte Carlo simulation of kV photon and MeV electron beams

This paper presents the findings of an investigation into the Monte Carlo simulation of superficial cancer treatments of an internal canthus site using both kilovoltage photons and megavoltage electrons. The EGSnrc system of codes for the Monte Carlo simulation of the transport of electrons and photons through a phantom representative of either a water phantom or treatment site in a patient is utilised. Two clinical treatment units are simulated: the Varian Medical Systems Clinac® 2100C accelerator for 6 MeV electron fields and the Pantak Therapax SXT 150 X-ray unit for 100 kVp photon fields. Depth dose, profile and isodose curves for these simulated units are compared against those measured by ion chamber in a PTW Freiburg MP3 water phantom. Good agreement was achieved away from the surface of the phantom between simulated and measured data. Dose distributions are determined for both kV photon and MeV electron fields in the internal canthus site containing lead and tungsten shielding, rapidly sloping surfaces and different density interfaces. There is a relatively high level of deposition of dose in tissue-bone and tissue-cartilage interfaces in the kV photon fields in contrast to the MeV electron fields. This is reflected in the maximum doses in the PTV of the internal canthus field being 12 Gy for kV photons and 4.8 Gy for MeV electrons. From the dose distributions, DVH and dose comparators are used to assess the simulated treatment fields. Any indication as to which modality is preferable to treat the internal canthus requires careful consideration of many different factors, this investigation provides further perspective in being able to assess which modality is appropriate.     

Evaluation of the effect of tooth and dental restoration material on electron dose distribution and production of photon contamination in electron beam radiotherapy

The aim of this study is to evaluate the effect of tooth and dental restoration materials on electron dose distribution and photon contamination production in electron beams of a medical linac. This evaluation was performed on 8, 12 and 14 MeV electron beams of a Siemens Primus linac. MCNPX Monte Carlo code was utilized and a 10 × 10 cm² applicator was simulated in the cases of tooth and combinations of tooth and Ceramco C3 ceramic veneer, tooth and Eclipse alloy and tooth and amalgam restoration materials in a soft tissue phantom. The relative electron and photon contamination doses were calculated for these materials. The presence of tooth and dental restoration material changed the electron dose distribution and photon contamination in phantom, depending on the type of the restoration material and electron beam’s energy. The maximum relative electron dose was 1.07 in the presence of tooth including amalgam for 14 MeV electron beam. When 100.00 cGy was prescribed for the reference point, the maximum absolute electron dose was 105.10 cGy in the presence of amalgam for 12 MeV electron beam and the maximum absolute photon contamination dose was 376.67 μGy for tooth in 14 MeV electron beam. The change in electron dose distribution should be considered in treatment planning, when teeth are irradiated in electron beam radiotherapy. If treatment planning can be performed in such a way that the teeth are excluded from primary irradiation, the potential errors in dose delivery to the tumour and normal tissues can be avoided.     

An experimental study of recombination and polarity effect in a set of customized plane parallel ionization chambers

Plane parallel ionization chambers are an important tool for dosimetry and absolute calibration of electron beams used for radiotherapy. Most dosimetric protocols require corrections for recombination and polarity effects, which are to be determined experimentally as they depend on chamber design and radiation quality. Both effects were investigated in electron beams from a linear accelerator (Varian 21CD) for a set of four tissue equivalent plane parallel ionization chambers customized for the present research by Standard Imaging (Madison WI). All four chambers share the same design and air cavity dimensions, differing only in the diameter of their collecting electrode and the corresponding width of the guard ring. The diameters of the collecting electrodes were 2 mm, 4 mm, 10 mm and 20 mm. Measurements were taken using electron beams of nominal energy 6 to 20 MeV in a 10 cm x 10 cm field size with a SSD of 100 cm at various depths in a Solid Water slab phantom. No significant variation of recombination effect was found with radiation quality, depth of measurement or chamber design. However, the polarity effect exceeded 5% for the chambers with small collecting electrode for an effective electron energy below 4 MeV at the point of measurement. The magnitude of the effect increased with decreasing electron energy in the phantom. The polarity correction factor calculated following AAPM protocol TG51 ranged from approximately 1.00 for the 20.0 mm chamber to less than 0.95 for the 2 mm chamber at 4.1 cm depth in a electron beam of nominally 12 MeV. By inverting the chamber it could be shown that the polarity effect did not depend on the polarity of the electrode first traversed by the electron beam. Similarly, the introduction of an air gap between the overlying phantom layer and the chambers demonstrated that the angular distribution of the electrons at the point of measurement had a lesser effect on the polarity correction than the electron energy itself. The magnitude of the absolute difference between charge collected at positive and negative polarity was found to correlate with the area of the collecting electrode which is consistent with the explanation that differences in thickness of the collecting electrodes and the number of electrons stopped in them contribute significantly to the polarity effect. Overall, the polarity effects found in the present study would have a negligible effect on electron beam calibration at a measurement depth recommended by most calibration protocols. However, the present work tested the corrections under extreme conditions thereby aiming at greater understanding of the mechanism underlying the correction factors for these chambers. This may lead to better chamber design for absolute dosimetry and electron beam characterization with less reliance on empirical corrections.     

放疗中用热塑成型模固定病人时剂量建成的评价

目的研究使用四种热塑成型膜在6MV光子束和6MeV,9MeV电子束时皮肤剂量改变情况。方法使用瓦里安23EX直线加速器,TM23343-3381平行板电离室。测量条件：100Mu,10cm×10cm照射野,100cm源皮距。分别放置不同塑型膜,测量0.1mm和1mm等效水厚度物质下剂量,并归一于相同测量条件无塑型膜时剂量。结果 6MV光子时,使用四种塑型膜均提高了基底细胞层（0.1mm）和真皮细胞层（1mm）剂量,其中延时型膜提高基底细胞层剂量最高达1.9%。已涂和未涂膜在电子束时能降低基底细胞层剂量达0.5%;其余实验结果都增加了皮肤剂量最高为6MeV延时型膜为9.6%。结论使用热塑成型膜能增加皮肤表面剂量,但在电子束下使用已涂和未涂膜能轻微降低皮肤基底细胞层剂量。     

Radiotherapy x-ray dose distribution beyond air cavities.

Radiation oncologists are particularly concerned about tumours growing on the surface of air cavities in the head and neck regions, which involve treatment with small x-ray fields. An inhomogeneous dose distribution exists within and beyond the cavity. This is caused by the loss of electron equilibrium and the attenuation of both the primary and scattered photons is altered. The scatter function photon beam models for tissue inhomogeneity, such as the ETAR correction algorithm, currently implemented in commercial treatment planning systems do not predict the dose distribution accurately in many situations where lateral electron equilibrium does not exist. Using a Markus ionization chamber and different solid water slabs to simulate different air cavities, it is found that internal body cavities, depending upon their sizes, experience underdose or overdose on the distal surfaces of the cavities when compared with the results predicted by an ETAR correction method for 6 MV and 18 MV x-ray beams. For an infinitely long air passage of dimensions 2 cm x 2 cm, the error in the ETAR correction for a 6 MV x-ray beam is 4.8%, 0.5% and 1.1% for the field size of 5 cm x 5 cm, 7 cm x 7 cm and 10 cm x 10 cm respectively. The ETAR correction is accurate to within 1.6% for a 6 MV x-ray beam provided that the field size is 5 cm across the cavity and greater than 7 cm along it.     

Characterisation of mega-voltage electron pencil beam dose distributions: viability of a measurement-based approach

The concept of electron pencil-beam dose distributions is central to pencil-beam algorithms used in electron beam radiotherapy treatment planning. The Hogstrom algorithm, which is a common algorithm for electron treatment planning, models large electron field dose distributions by the superposition of a series of pencil beam dose distributions. This means that the accurate characterisation of an electron pencil beam is essential for the accuracy of the dose algorithm. The aim of this study was to evaluate a measurement based approach for obtaining electron pencil-beam dose distributions. The primary incentive for the study was the accurate calculation of dose distributions for narrow fields as traditional electron algorithms are generally inaccurate for such geometries. Kodak X-Omat radiographic film was used in a solid water phantom to measure the dose distribution of circular 12 MeV beams from a Varian 21EX linear accelerator. Measurements were made for beams of diameter, 1.5, 2, 4, 8, 16 and 32 mm. A blocked-field technique was used to subtract photon contamination in the beam. The "error function" derived from Fermi-Eyges Multiple Coulomb Scattering (MCS) theory for corresponding square fields was used to fit resulting dose distributions so that extrapolation down to a pencil beam distribution could be made. The Monte Carlo codes, BEAM and EGSnrc were used to simulate the experimental arrangement. The 8 mm beam dose distribution was also measured with TLD-100 microcubes. Agreement between film, TLD and Monte Carlo simulation results were found to be consistent with the spatial resolution used. The study has shown that it is possible to extrapolate narrow electron beam dose distributions down to a pencil beam dose distribution using the error function. However, due to experimental uncertainties and measurement difficulties, Monte Carlo is recommended as the method of choice for characterising electron pencil-beam dose distributions.     

Electron contamination in 4 MV and 10 MV radiotherapy x-ray beams

A thin window parallel-plate ionization chamber was constructed for dose measurement in the build-up region of high energy radiotherapy photon beams. The chamber is an integral part of a perspex block. The entrance window is 12 microns Melinex foil with a thin aluminium surface. Cavity thickness is 1.45 mm. Surface doses for varying field sizes were found to increase almost linearly with the side length of a square field. The surface dose for a 10x10 cm 4 MV photon beam is 12.1% for an open field and this increases to 14.1% with a polycarbonate block tray in the beam. Similarly for a 10 MV photon beam the surface dose is 10.6% for an open field and this increases to 12.4% with a polycarbonate block tray. The difference between the dose for an open field and a field with a polycarbonate block tray inserted becomes more significant for larger field sizes. Electron contamination depth dose curves are determined for a 4 MV and 10 MV photon beam. This is achieved by subtracting a pure photon beam build-up curve generated by an EGS4 Monte Carlo simulation from the experimental build-up curve. The EGS4 curve is a theoretical, electron contamination free curve. The electron contamination curve (of the 10 MV photon beam) has depth dose characteristics similar to that of a broad low energy electron beam.     

Water equivalence evaluation of PRESAGE® formulations for megavoltage electron beams: a Monte Carlo study

To investigate the radiological water equivalency of three different formulations of the radiochromic, polyurethane based dosimeter PRESAGE^® for three dimensional (3D) dosimetry of electron beams. The EGSnrc/BEAMnrc Monte Carlo package was used to model 6–20 MeV electron beams and calculate the corresponding doses delivered in the three different PRESAGE^® formulations and water. The depth of 50 % dose and practical range of electron beams were determined from the depth dose calculations and scaling factors were calculated for these electron beams. In the buildup region, a 1.0 % difference in dose was found for all PRESAGE^® formulations relative to water for 6 and 9 MeV electron beams while the difference was negligible for the higher energy electron beams. Beyond the buildup region (at a depth range of 22–26 mm for the 6 MeV beam and 38 mm for the 9 MeV beam), the discrepancy from water was found to be 5.0 % for the PRESAGE^® formulations with lower halogen content than the original formulation, which was found to have a discrepancy of up to 14 % relative to water. For a 16 MeV electron beam, the dose discrepancy from water increases and reaches about 7.0 % at 70 mm depth for the lower halogen content PRESAGE^® formulations and 20 % at 66 mm depth for the original formulation. For the 20 MeV electron beam, the discrepancy drops to 6.0 % at 90 mm depth for the lower halogen content formulations and 18 % at 85 mm depth for the original formulation. For the lower halogen content PRESAGE^®, the depth of 50 % dose and practical range of electrons differ from water by up to 3.0 %, while the range of differences from water is between 6.5 and 8.0 % for the original PRESAGE^® formulation. The water equivalent depth scaling factor required for the original formulation of PRESAGE^® was determined to be 1.07–1.08, which is larger than that determined for the lower halogen content formulations (1.03) over the entire beam energy range of electrons. All three of the PRESAGE^® formulations studied require a depth scaling factor to convert depth in PRESAGE^® to water equivalent depth for megavoltage electron beam dosimetry. Compared to the original PRESAGE^® formulation, the lower halogen content formulations require a significantly smaller scaling factor and are thus recommended over the original PRESAGE^® formulation for electron beam dosimetry.     

Determination of field size limitations in stereotactic and intensity modulated radiotherapy

For dose measurement in small photon fields, different detectors are currently in use: TLD, semiconductor, diamond-detector, film, etc. But for absolute dosimetry, ionization chambers show the most advantages. To meet the basic dosimetrical requirements for lateral electron equilibrium the field size F must not remain under specified values: i.e. 5.2 x 5.2 cm2 for 15 MeV X-bremsstrahlung. As well as increasing the focus-chamber-distance, changing the physical density of the build-up material in the close vicinity of the chamber will be helpful to determine the output factor OF for smaller fields. By means of a correction factor, k(mat), this is taken into account. For a 6 MeV X and a 15 MeV X-bremsstrahlung of linear accelerators the lower limit of the field size F is determined: F > or = 0.8 cm. This value is mainly dependent on the diameter of the focal-spot (phi = 3 mm) of the treatment unit including design characteristics of the treatment head. Beside the dosimetrical aspects, some geometrical parameters have to be considered, when accuracy of dose application should remain on the same level as in medium and large field treatment (4 cm < or = F < or = F(max)). To keep dose-volume errors as low as +/- 10 % (diameter of PTV: 20 mm), the mean total error delta of CT-scanning (delta(p)), planning (delta(pl)), patient positioning (delta(x)), and treatment unit instabilities (delta(m)) should not exceed +/- 0.8 mm.     

6MeV电子线和铱192治疗瘢痕疙瘩的剂量学比较

目的 比较6 MeV电子线和铱192(192Ir)高剂量率(HDR)三维后装近距离敷贴两种放射治疗技术在瘢痕疙瘩术后平整表面靶区、曲面靶区、超长靶区(长度>25 cm)模体中的剂量学差异.方法 在胸部仿真模体上,采用激光灯标记拟照射瘢痕疙瘩术后位置(包括平整表面和曲面),固体水模体模拟超长靶区,分别获取电子线放射治疗C...     

QA BeamChecker Plus的质量保证

目的：对新购进QA BeamChecker Plus射野分析仪进行全面质保以评价其性能。方法：分别照射X线和电子束测量面12次,X线时使用PTW参考电离室,计算标准差以评价各电离室重复性。使用QA BeamChecker Plus测得原始数据,使用自编软件利用相关计算公式计算各项射野评价指标与Communication software 2.1.2软件显示值比较,验证其算法的准确性。在对QA BeamChecker Plus加上不同厚度材料和不加任何材料2种情况分别测量分析射野,并比较差异,以检验其抽样深度与使用公式的合理性,并验证推荐标准（3%和5%）的合理性。结果：5个分析射野平板电离室重复性测量均值,中心轴电离室最大标准差为±0.102%（16MeV电子线）,上、下、右、左电离室最大标准差分别为±0.104%（6MVX线）,±0.090%（6MeV电子线）,±0.162%（20MeV电子线）,±0.120%（6MVX线）。应用原始数据计算的平坦度、对称性、中心轴重复性与软件显示值符合性为100%。对于X线,3.5cm平面与10cm平面抽样点差别最大为15MVG-T方向对称性2.8（2.2）;对于电子束,1.5深度平面与1/2R85深度平面差别最大者为16MeV-EG-T对称性0.8（1.3）。结论：QA BeamChecker Plus射野分析仪所使用平行板电离室重复性达到临床使用标准,所选择的测量深度抽样点及计算公式能反应射野输出指标相关变化。且使用快捷方便,是作为直线加速器射野日常质保检测的理想工具。     

Measurement of photoneutron dose produced by wedge filters of a high energy linac using polycarbonate films

Radiotherapy represents the most widely spread technique to control and treat cancer. To increase the treatment efficiency, high energy linacs are used. However, applying high energy photon beams leads to a non-negligible dose of neutrons contaminating therapeutic beams. In addition, using conventional linacs necessitates applying wedge filters in some clinical conditions. However, there is not enough information on the effect of these filters on the photoneutrons produced. The aim of this study was to investigate the change of photoneutron dose equivalent due to the use of linac wedge filters. A high energy (18 MV) linear accelerator (Elekta SL 75/25) was studied. Polycarbonate films were used to measure the dose equivalent of photoneutrons. After electrochemical etching of the films, the neutron dose equivalent was calculated using Hp(10) factor, and its variation on the patient plane at 0, 5, 10, 50 and 100 cm from the center of the X-ray beam was determined. By increasing the distance from the center of the X-ray beam towards the periphery, the photoneutron dose equivalent decreased rapidly for the open and wedged fields. Increasing of the field size increased the photoneutron dose equivalent. The use of wedge filter increased the proportion of the neutron dose equivalent. The increase can be accounted for by the selective absorption of the high energy photons by the wedge filter.     

Tin foil as bolus material for therapeutic electron beams from the Varian Clinac 2100C/D

Tin foils of sub-millimetre thickness have been investigated as bolus material for therapeutic electron beams from the Varian Clinac 2100C/D linear accelerator. Measurements with ionisation chamber and radiographic film in Plastic Water or water were performed under tin foil bolus to determine surface dose, therapeutic ranges, output factor correction, penumbra and dose outside the field edge. Appropriate thicknesses of tin foil for 90% dose at the surface were found to be approximately 0.3 mm for 6 MeV, and 0.4 mm for 9 MeV and 12 MeV. Enhanced therapeutic interval with tin foil bolus over water-equivalent bolus has previously been reported, but was found not to be evident for 12 MeV and for a small (4 x 4 cm2) 9 MeV field. The penumbra width of fields with tin foil and water-equivalent bolus were found to be within 2 mm, while the doses at 1 cm outside the field edge were within 1.5% of peak dose. Output factor corrections for fields with tin foil were measured as within 2% of unity. Air gaps between the tin foil and phantom surface up to 5 mm were observed to have minimal effect on output correction factor, relative surface dose, and therapeutic range.