Varian Edge加速器射野外辐射剂量水平与铅防护用品的防护效果

目的:研究Varian Edge加速器不同工作状态下射野外辐射剂量水平以及铅防护用品的防护效果。方法:利用实验测量的方法，研究加速器在不同工作能量、不同线束均整状态、使用不同防护用品，测量距射野边缘不同距离及不同深度下辐射剂量水平的变化情况。结果:射野外辐射剂量随距射野边缘距离增加（5~40 cm）近似呈指数规律下降，距射野边缘20 cm范围内低能量射束（6 MV、6 MV FFF）的辐射剂量低于高能射束（10 MV、10 MV FFF）的辐射剂量，且随测量深度增加（1~2 cm）而降低。非均整模式下射野外剂量测量结果低于均整模式射束。在相同能量条件下，铅防护用品的防护效果与线束的均整状态无关。对高能射束的防护效果要优于低能射束且随深度增加防护效果迅速下降。深度为1 cm，射束能量10 MV FFF，距射野边缘5~30 cm条件下，防护效果最强，射野外辐射剂量水平降低50%以上。测量深度为2 cm，射束能量为6 MV FFF，距离射野边缘5~30 cm的条件下，防护效果最差，仅能降低10%以下。结论:在实现临床目标的前提下，治疗过程中若无铅防护用品进行保护，推荐采用低能非均整模式进行计划设计；若使用铅防护用品进行保护，可以采用高能非均整模式射束，此时铅防护用品效果最佳，射野外浅层器官所受剂量最低，可有效降低二次肿瘤发生几率。     

AAA和PDIP算法在非均整模式容积调强放射治疗剂量预测方面的差异

【摘要】目的:探究各项异性算法（AAA）和射野剂量图像预测（PDIP）算法在非均整模式（FFF）容积调强放射治疗计划治疗前验证γ分析中的差异以及计划复杂程度对这种差异的影响,为临床上基于电子射野影像系统（EPID）的剂量预测算法的选择提供依据。方法:选取能量为6 MV FFF的两种测试野和16例头颈部肿瘤治疗计划,利用PDIP和AAA两种算法分别生成预测数据并与EPID实测数据进行γ分析,统计两种算法在不同γ评判标准下的通过率并计算通过率差异（Delta γ）。计算上述病例每个射野的复杂系数,分析不同标准下两种算法的Delta γ与复杂系数的相关性;利用γmean、γsd、γ1和γ通过率共同描述γ分布,并分析其与复杂系数间的相关性。结果:当评判标准为3%/3 mm或2%/2 mm时,不同算法下测试射野的Delta γ较小。当评判标准为1%/1 mm,不同开野的Delta γ变化明显:射野较小时,PDIP算法的通过率低于AAA;当射野增大到（10×10） cm2时,通过率基本一致;当射野继续增大时,PDIP算法的通过率逐渐高于AAA。全部射野的通过率与评判标准的关系类似:在3%/3 mm标准下,两种算法的结果基本一致;随着标准的提高,两种算法的通过率逐渐下降,二者之间的差异也逐渐明显。复杂系数与Delta γ、γmean、γsd和γ1为正相关,与γ通过率为负相关。结论:PDIP算法对于有机械臂支撑的EPID的剂量预测更准确;AAA则适用于无机械臂支撑的EPID或机械臂反散射影响较小的射野。当计划复杂程度或评判标准提高时,两种算法的差异也增大。计划复杂程度对FFF计划验证结果的影响是负面的。上述结果提示临床应针对性地选择计划验证工具来确保治疗的安全有效。     

射野大小对全脑调强放疗计划EPID验证结果的影响

目的:利用电子射野影像系统（EPID）对全脑调强放疗计划进行γ测试,寻找计划设计对测试结果的影响,以此分析如何优化全脑调强计划以及推测EPID在剂量验证方面的局限性。方法:选取67例全脑放疗患者,对其放疗计划用加速器自带的EPID进行计划验证,对于容积旋转调强放疗（VMAT）计划统计并分析X方向射野大小与γ（3 mm/3%）通过率的关系,对于调强放疗（IMRT）对比分析大野调强和分野调强计划γ（3 mm/3%）通过率的差异。结果:VMAT计划验证结果发现X方向小于15 cm的射野γ（3 mm/3%）通过率普遍优于大于等于15 cm的射野,利用SPSS软件进行t检验,发现结果具有统计学意义（t=-3.828, P<0.05）;IMRT验证结果发现,X方向大于等于15 cm的射野会包含两个子野,合野验证时其交叠部分γ（3 mm/3%）通过率较差,而采用分野验证时,由于无交叠则通过率普遍较好。结论:全脑放疗VMAT计划将X方向射野控制在15 cm以内可以提升多叶准直器调节能力,并提高EPID验证的γ（3 mm/3%）通过率;EPID原件对低剂量区的响应偏差会导致全脑IMRT大野调强计划两子野交叠处γ（3 mm/3%）通过率较差,改用分野验证可以显著消除这种影响。     

6 MV光子束有无均整器的剂量学特性研究

目的 研究直线加速器去掉均整器后6 MV光子束的剂量学特性,并与有均整器的情况下比较,为临床应用提供依据。方法 采集瓦里安Trilogy 6 MV光子束在有均整器(FF)(用6X FF表示)和去掉均整器(FFF)(用6X FFF表示)后的剂量学数据,比较两者百分深度剂量(PDD)、离轴剂量分布(Profile)、表面相对剂量(PDD_{1 mm})、射野外相对剂量、总输出因子(S_cp)。结果 6X FF的最大剂量深度(d_max)在3 cm×3 cm至15 cm×15 cm射野时为1.4 cm,在20 cm×20 cm射野后变为1.2 cm,30 cm×30 cm后变为1.0 cm,而6X FFF的d_max为1.2 cm且不随射野大小变化。在大于d_max后,同一深度6X FFF的PDD比6X FF的小,但两者差距随射野和深度变化不明显,在3 cm×3 cm射野5 cm深度处6X FFF的PDD比6X FF的小2.4,在40 cm×40 cm射野30 cm深度处6X FFF的PDD比6X...     

有无均整器模式下臂架与准直器不同角度条件下的剂量学比较

目的:探讨臂架或准直器角度的改变对均整（FF）与非均整（FFF）两种模式的射线剂量的影响。方法:选用Versa HD直线加速器配备的6 MV/10 MV光子束FF/FFF模式4档能量在设定好九点位置的10 cm×10 cm标准射野内进行实验。首先,借助IMF等中心夹具将Mapcheck2固定于治疗机机头,并用Mapcheck2测量相同臂架与准直器角度条件下4种光子束输出的平面剂量值;其次,用Mapcheck2测量在相同臂架角度、不同准直器角度与相同准直器角度、不同臂架角度两种条件下4种光子束的中心轴剂量值;最后,固定准直器为0°,设立两组臂架对穿射野（0°与180°,90°与270°）。拆除Mapcheck2,采用固体水和FC65-G电离室建立一个测量模体来测量4种光子束在两组等中心对穿野的剂量。运用SPSS统计软件对该实验收集到的数据进行对比分析。结果:在相同臂架与准直器角度条件下,4种光子束辐照9个点的平面剂量之间均存在明显统计学差异（P6 MV FF =0.020, P6 MV FFF=0.017, P10 MV FF =0.030, P10 MV FFF=0.016）;而不同臂架角度或不同准直器角度条件下,4种能量光子束的中心轴点剂量值均无统计学差异。在0°与180°的对穿野,4种能量光子束的输出剂量存在统计学差异（P6 MV FF =0.001, P6 MV FFF=0.002, P10 MV FF =0.003, P10 MV FFF=0.001）,而在90°与270°的对穿野无统计学差异。结论:Versa HD直线加速器拥有优良的机械等中心性能。在实际工作时,臂架和准直器的旋转,均不影响光子束的中心轴剂量的准确输出。在FF模式下,射线能量越高,受治疗床影响越小;FFF模式射线由于射线质软,能量越高,更易受到治疗床的衰减作用,在实际中应引起重视。     

加速器多叶准直器半影特性研究

目的:探讨和比较瓦里安(Varian)2100C/D与医科达(Elekta)Synergy直线加速器配备的多叶准直器的半影特性,为临床设野提供参数供参考。方法:利用CRS三维水箱和PTW电离室测量6 MV和10 MV X线的射野离轴比曲线,测量从4 cm×4 cm到20 cm×20 cm各方野的半影;同时针对两种加速器多叶准直器的设计特点,分析利用不同准直器形成射野对半影宽度的影响。结果:(1)对于不同能量和不同大小射野,设野时靶区外扩的范围应综合半影大小等因素;(2)Varian 2100C/D加速器采用准直器三层结构,采用"MLC+JAW"方式形成射野能减少半影,即应注意形成射野时JAW必须跟随MLC;(3)Elekta Synergy加速器形成射野时MLC方向半影较大,重要器官可采用JAW来保护。     

基于非晶硅电子射野影像装置的剂量响应研究

目的：临床条件下研究探讨非晶硅电子射野影像装置（a-Si EPID）的剂量响应特性。方法 ：本实验在Elekta Precise直线加速器上X射线能量分别为6 MV和10 MV,采用PTW电离室、等效固体水和不同厚度铜板条件下实施测量。首先,通过EPID信号和模体中电离室的测量比较,确定出EPID剂量响应的建成厚度。其次,临床条件下利用模体的不同厚度测量分析有关剂量、每脉冲剂量和脉冲重复频率（PRF）函数的EPID信号响应情况。结果：在不增加建成材料、10 cm～60cm空气间隙条件下EPID显示了最大11.6%的过响应信号变化。临床上额外将3 mm铜建成区置于EPID上方,空气间隙大于40 cm条件下EPID响应变化将会降至1%以内。在测量范围内随MU数、PRF和每脉冲剂量变化的EPID信号响应是非线性的,最大信号变化接近于3%。因假峰和图像滞后效应等影响,短时间照射EPID会明显地产生出低剂量响应。结论：采用合适的建成层和实施对每脉冲剂量、PRF等校正,非晶硅EPID剂量响应变化可控制在1%以内,从而建立起较为理想的剂量响应曲线。     

国产外置式自动多叶准直器控制设计及其临床性能测试分析

目的：设计一款高位移精度的国产外置式医学用自动多叶准直器（Multi Leaf Collimator,简称MLC）,同时验证和测试该MLC在放射治疗临床使用中的辐射野精度、半影、漏射率、适形度等重要指标数据。方法：控制设计方面,采用直流电机模糊控制算法、位移精度控制、漏射优化等关键方案实现MLC的核心功能。测试方面采用柏co治疗机作为放射源,国产MLC形成射野,使用三维水箱系统和二维剂量探测器对剂量分布进行测量,并根据测量结果计算MLC性能指标数据。结果：射野数字指示与辐射野精度误差测试中,在叶片运动方向上位移误差在0．2mm～1．6mm之间;漏射率测试中,最大漏射率小于2％;半影测试中,半影随射野增大而增大,在10cm~10Clll射野时,叶片运动方向半影为8．6mm适形度测试中,MLC形成射野和TPS定制适形度值在99．7％以上。结论：国产外置式MLC各项指标符合食药总局检验标准,完全满足临床治疗需求。     

Octavius 4D系统稳定性验证分析

目的:探讨Octavius 4D系统用于容积旋转调强放射治疗(VMAT)三维剂量验证的稳定性。方法:比较分析semiflex电离室和Octavius 729探测器阵列在6 MV、10 MV射束下对射野大小、剂量线性、剂量率线性和射野输出因子的响应。测量观察Octavius 4D系统旋转过程中角度仪示值与机架角的角度偏差。用3%/3 mm标准分析(10×10)cm~2旋转照射计划和VMAT计划机架归零与旋转照射的二维剂量分布;用3%/3 mm gamma分析标准评估VMAT计划。结果:预热剂量大于6 Gy是探测器稳定的必要条件。探测器阵列剂量响应是线性的,不同标称剂量率下剂量测量是稳定的。旋转照射过程中加速器机架角和Octavius 4D模体旋转角度误差在0.4°以内。6 MV和10 MV射束VMAT计划在分析标准为3%/3 mm时,三维剂量分布的平均通过率分别为96.03%和95.56%,满足临床计划剂量验证的标准。结论:Octavius4D模体联合Octavius 729探测器阵列是一套稳定性装置,用于治疗前验证VMAT计划是可靠的。     

三维分析仪与两维矩阵射野测量的比较

目的:应用不同仪器与方法测量加速器6 MV X线射野的特性,比较各方法的优劣和局限性,探讨快速简便检测射野特性的方法。材料与方法:分别采用电离室和半导体探头配合三维射野分析仪测量加速器6 MV X线不同射野大小的百分深度剂量曲线PDD和离轴比曲线OCR,并以二维电离室矩阵测量相同条件的OCR。(1)比较采用电离室和半导体探头测量PDD的差别。(2)比较两维矩阵与电离室半导体探头测量射野的对称性、平坦度、射野大小和半影等的差别。结果:对小于15 cm×15 cm照射野,半导体探头和电离室测量PDD的结果一致性较好,两者偏差小于1.3%。对于20 cm×20 cm照射野,半导体探头的测量结果大于电离室,最大差别3.5%,偏差为2.6%。用半导体探头与电离室测量射野的大小,两者的最大差别为0.6 mm,两者有较好的一致性,二维电离室矩阵测量与前两者比较,最大差别为2.9 mm,最小差别0.5 mm。三种方法测量的射野平坦度差别在1.2%~2.6%,矩阵的测量数值在半导体和电离室测量范围之内。结论:在检测加速器射野性能时,二维矩阵可以快速检测射野平坦度、对称性,但测量射野大小时可能有较大误差,不宜用作验收加速器和收集...     

Correction of pixel sensitivity variation and off-axis response for amorphous silicon EPID dosimetry

The aim of this work is to determine the pixel sensitivity variation and off-axis dose response of an amorphous silicon electronic portal imaging device (EPID), and develop a correction method to improve EPID dosimetry. The uncorrected or raw pixel response of the aS500 amorphous silicon EPID shows differences in response (sensitivity) of individual pixels as well as a large off-axis differential response with respect to an ion chamber in water. Both can be corrected by division of raw images by the flood-field (FF) image. However, this leads to two problems for dosimetry: (1) the beam profile is present in both the raw image and FF image, and hence is "washed out" of the corrected image, and (2) any mismatch of EPID position between dosimetry and FF calibration means that the beam profile and off-axis response in the raw image and FF are misaligned. This causes artifacts in FF division and dosimetric errors. A method was developed to measure the off-axis response and pixel sensitivity variation separately to allow correction of images at any EPID position while retaining beam profile information. The pixel sensitivity variation is applied to the imager plane and is independent of imager position. The off-axis response depends on the imager plane position relative to the beam central axis. The pixel sensitivities were derived from multiple images of the same symmetric field acquired with the detector displaced laterally between each image. The off-axis response was measured by acquiring off-axis raw images (FF correction removed) and dividing out the off-axis beam fluence and previously determined pixel sensitivity differences. The dosimetric errors due to lateral and vertical detector displacement with the conventional FF calibration method were measured and compared to the new method. Corrected EPID profiles were then compared to beam profiles measured with ion chamber in water for open fields. The EPID was found to have a large off-axis differential response with respect to an ion chamber in water, particularly for 6 MV. This increased to 13% at 15 cm off-axis for 6 MV, and 3.5% for 18 MV at the isocenter plane. The dosimetric errors introduced by detector displacement with conventional FF calibration were found to be approximately 1% per centimeter of lateral detector displacement and 0.1% per centimeter of vertical displacement. These were reduced to less than 1% for any position with the new correction method. Corrected EPID images agreed with ion-chamber measurements to within 2% (excluding penumbra and low-dose areas outside the field) for various field sizes. The new correction method gives consistent dosimetry for any EPID position and retains beam profile information in the image.     

Slit x-ray beam primary dose profiles determined by analytical transport of Compton recoil electrons

Accurate measurement of radiation beam penumbras is essential for conformal radiotherapy. For this purpose a detailed knowledge of the dosimeter's spatial response is required. However, experimental determination of detector spatial response is cumbersome and restricted to the specific detector type and beam spectrum used. A model has therefore been developed to calculate in slit beam geometry both dose profiles and detector response profiles. Summations over representative photon beam spectra yield profiles for polyenergetic beams. In the present study the model is described and resulting dose profiles verified. The model combines Compton scattering of incident photons, transport of resulting electrons by Fermi-Eyges small-angle multiple scattering theory, and functions to limit electron transport. This analytic model thus yields line spread kernels of primary dose in a water phantom. It is shown that the spatial response of an ideal point detector to a primary photon beam can be well described by the model; the calculations are verified by measurements with a diamond detector in a telescopic slit geometry in which all dose contributions except for the primary dose can be excluded. Effects of photon detector behavior, source size of the linear accelerator (linac) and detector size are studied. Measurements show that slit dose profiles calculated by means of the kernel are accurate within 0.1 mm of the full-width at half-maximum. For a theoretical point source and point detector combined with a 0.2 mm wide slit, the full-width half-maximum values of the slit beam dose profiles are calculated as 0.37 mm and 0.42 mm in a 6 MV and 25 MV x-ray beam, respectively. The present study shows that the model is adequate to calculate local dose effects that are dominated by approximately mono-directional, primary photon fluence. The analytic model further provides directional electron fluence information and is designed to be applied to various detectors and linac beam spectra.     

Off-axis dose response characteristics of an amorphous silicon electronic portal imaging device

Amorphous silicon (a-Si) electronic portal imaging devices (EPIDs) have typically been calibrated to dose at central axis (CAX). Division of acquired images by the flood-field (FF) image that corrects for pixel sensitivity variation as well as open field energy-dependent off-axis response variation should result in a flat EPID response over the entire matrix for the same field size. While the beam profile can be reintroduced to the image by an additional correction matrix, the CAX EPID response to dose calibration factor is assumed to apply to all pixels in the detector. The aim of this work was to investigate the dose response of the Varian aS500 amorphous silicon detector across the entire detector area. First it was established that the EPID response across the panel became stable (within approximately 0.2%) for MU settings greater than approximately 200 MU. The EPID was then FF calibrated with a high MU setting of approximately 400 for all subsequent experiments. Whole detector images with varying MU settings from 2-500 were then acquired for two dose rates (300 and 600 MU/min) for 6 MV photons for two EPIDs. The FF corrected EPID response was approximately flat or uniform across the detector for greater than 100 MU delivered (within 0.5%). However, the off-axis EPID response was greater than the CAX response for small MU irradiations, giving a raised EPID profile. Up to 5% increase in response at 20 cm off-axis compared to CAX was found for very small MU settings for one EPID, while it was within 2% for the second (newer) EPID. Off-axis response nonuniformities attributed to detector damage were also found for the older EPID. Similar results were obtained with the EPID at 18 MV energy and operating in asynchronous mode (acquisition not synchronized with beam pulses), however the profiles were flatter and more irregular for the small MU irradiations. By moving the detector laterally and repeating the experiments, the increase in response off-axis was found to depend on the pixel position relative to the beam CAX. When the beam was heavily filtered by a phantom the off-axis response variation was reduced markedly to within 0.5% for all MU settings. Independent measurements of off-axis point doses with ion chamber did not show any change in off-axis factor with MUs. Measurements of beam quality (TMR20-10) for MU settings of 2, 5, and 100 at central axis and at 15 cm off-axis could not explain the effect. The response change is unlikely to be significant for clinical IMRT verification with this imaging/acclerator system where MUs are of the order of 100-300, provided the detector does not exhibit radiation damage artifacts.     

Daily monitoring of linear accelerator beam parameters using an amorphous silicon EPID

An amorphous silicon EPID has been investigated to test its suitability as a daily check device for linac output and to provide daily monitoring of beam profile parameters such as flatness, symmetry, field size and wedge factor. Open and wedged 6 and 8 MV photon beams were collected on a daily basis for a period of just over a year and analysed in software to determine daily values of these parameters. Daily output results gave agreement between EPID measured dose and ion chamber measurements with a standard deviation of 0.65%. Step changes in flatness, symmetry and field size were readily detected by the EPID and could be correlated with adjustments made on service days and QC sessions. The results could also be used to assess the long term beam stability. Recalibration of the EPID required new baseline values of the parameters to be set. Wedge factors measured at one collimator angle proved stable but sensitive to changes in beam steering. The EPID proved to be a useful daily check device for linac output which can simultaneously be used for daily monitoring of beam profiles and field sizes.     

Diamond detector versus silicon diode and ion chamber in photon beams of different energy and field size

The aim of this work was to test the suitability of a PTW diamond detector for nonreference condition dosimetry in photon beams of different energy (6 and 25 MV) and field size (from 2.6 cm x 2.6 cm to 10 cm x 10 cm). Diamond behavior was compared to that of a Scanditronix p-type silicon diode and a Scanditronix RK ionization chamber. Measurements included output factors (OF). percentage depth doses (PDD) and dose profiles. OFs measured with diamond detector agreed within 1% with those measured with diode and RK chamber. Only at 25 MV, for the smallest field size, RK chamber underestimated OFs due to averaging effects in a pointed shaped beam profile. Agreement was found between PDDs measured with diamond detector and RK chamber for both 6 MV and 25 MV photons and down to 5 cm x 5 cm field size. For the 2.6 cm x 2.6 cm field size, at 25 MV, RK chamber underestimated doses at shallow depth and the difference progressively went to zero in the distal region. PDD curves measured with silicon diode and diamond detector agreed well for the 25 MV beam at all the field sizes. Conversely, the nontissue equivalence of silicon led, for the 6 MV beam, to a slight overestimation of the diode doses in the distal region, at all the field sizes. Penumbra and field width measurements gave values in agreement for all the detectors but with a systematic overestimate by RK measurements. The results obtained confirm that ion chamber is not a suitable detector when high spatial resolution is required. On the other hand, the small differences in the studied parameters, between diamond and silicon systems, do not lead to a significant advantage in the use of diamond detector for routine clinical dosimetry.     

The impact of MLC transmitted radiation on EPID dosimetry for dynamic MLC beams

The purpose of this study was to experimentally quantify the change in response of an amorphous silicon (a-Si) electronic portal imaging device (EPID) to dynamic multileaf collimator (dMLC) beams with varying MLC-transmitted dose components and incorporate the response into a commercial treatment planning system (TPS) EPID prediction model. A combination of uniform intensity dMLC beams and static beams were designed to quantify the effect of MLC transmission on EPID response at the central axis of 10 x 10 cm2 beams, at off-axis positions using wide dMLC beam profiles, and at different field sizes. The EPID response to MLC transmitted radiation was 0.79 +/- 0.02 of the response to open beam radiation at the central axis of a 10 x 10 cm2 field. The EPID response to MLC transmitted radiation was further reduced relative to the open beam response with off-axis distance. The EPID response was more sensitive to field size changes for MLC transmitted radiation compared to open beam radiation by a factor of up to 1.17 at large field sizes. The results were used to create EPID response correction factors as a function of the fraction of MLC transmitted radiation, off-axis distance, and field size. Software was developed to apply the correction factors to each pixel in the TPS predicted EPID image. The corrected images agreed more closely with the measured EPID images in areas of intensity modulated fields with a large fraction of MLC transmission and, as a result the accuracy of portal dosimetry with a-Si EPIDs can be improved. Further investigation into the detector response function and the radiation source model are required to achieve improvements in accuracy for the general case.     

Matching photon and electron fields with dynamic intensity modulation

A technique was developed to reduce the size and magnitude of the hot and cold spots in the abutting regions of photon and electron fields. The photon and electron fields were set up such that the photon field extended approximately 2 cm into the electron field in the abutting region. The region of the photon beam that overlapped the electron field was modulated using a multileaf collimator, effectively broadening the photon penumbra to make it complimentary to the electron penumbra. The computer calculations were verified using film measurements for abutting a 6 MV photon beam with a 9 MeV electron beam. A uniform dose was achieved at a prespecified depth of 2 cm, and dose uniformity was improved at the specified depth and beyond compared with unmodulated photon beams. A slight increase in dose inhomogeneity was seen at shallower depths. The overall areas of the hot and cold spots were significantly reduced. The technique also reduced the sensitivity of dose homogeneity to setup errors such that the magnitudes of the hot and cold spots were about half of those produced with unmodulated photon beam when an overlap or gap of 4 mm was introduced. The technique was applied to the treatment of a head and neck cancer and a lymphoma involving the right pleura with markedly reduced dose inhomogeneity in the abutting regions.     

A direction-selective flattening filter for clinical photon beams. Monte Carlo evaluation of a new concept

A new concept for the design of flattening filters applied in the generation of 6 and 15 MV photon beams by clinical linear accelerators is evaluated by Monte Carlo simulation. The beam head of the Siemens Primus accelerator has been taken as the starting point for the study of the conceived beam head modifications. The direction-selective filter (DSF) system developed in this work is midway between the classical flattening filter (FF) by which homogeneous transversal dose profiles have been established, and the flattening filter-free (FFF) design, by which advantages such as increased dose rate and reduced production of leakage photons and photoneutrons per Gy in the irradiated region have been achieved, whereas dose profile flatness was abandoned. The DSF concept is based on the selective attenuation of bremsstrahlung photons depending on their direction of emission from the bremsstrahlung target, accomplished by means of newly designed small conical filters arranged close to the target. This results in the capture of large-angle scattered Compton photons from the filter in the primary collimator. Beam flatness has been obtained up to any field cross section which does not exceed a circle of 15 cm diameter at 100 cm focal distance, such as 10 × 10 cm(2), 4 × 14.5 cm(2) or less. This flatness offers simplicity of dosimetric verifications, online controls and plausibility estimates of the dose to the target volume. The concept can be utilized when the application of small- and medium-sized homogeneous fields is sufficient, e.g. in the treatment of prostate, brain, salivary gland, larynx and pharynx as well as pediatric tumors and for cranial or extracranial stereotactic treatments. Significant dose rate enhancement has been achieved compared with the FF system, with enhancement factors 1.67 (DSF) and 2.08 (FFF) for 6 MV, and 2.54 (DSF) and 3.96 (FFF) for 15 MV. Shortening the delivery time per fraction matters with regard to workflow in a radiotherapy department, patient comfort, reduction of errors due to patient movement and a slight, probably just noticable improvement of the treatment outcome due to radiobiological reasons. In comparison with the FF system, the number of head leakage photons per Gy in the irradiated region has been reduced at 15 MV by factors 1/2.54 (DSF) and 1/3.96 (FFF), and the source strength of photoneutrons was reduced by factors 1/2.81 (DSF) and 1/3.49 (FFF).