电子束不对称照射野输出因子的实验研究

目的 探讨电子束不对称照射野输出因子的变化规律.方法 用电离室法对不同大小限光筒不同能量下3个特定照射野按照偏离中心轴不同距离实测其输出因子,计算不对称照射野输出因子与对称照射野输出因子的偏差.结果 不对称照射野的输出因子随着偏轴距离的变化规律与所对应限光筒标准方野离轴比的变化基本上是一致的,提示不对称照射野的输出因子与标准方野的离轴比相关.限光筒和铅挡野大小对输出因子偏差值的影响不明显.结论 电子束不对称照射野输出因子的变化主要与标准方野的离轴比有关,是否修正可根据标准方野离轴比的变化情况决定.     

电子束照射野面积对中心轴剂量和输出因子的影响

目的探讨电子束照射野挡块对中心轴剂量和输出因子的影响.方法用瑞典Scaditronix公司生产的RFA-300型三维水箱及P型硅半导体探头对瓦里安2100C和2300C/D直线加速器的多种能量电子束进行了中心轴百分深度剂量(PDD)扫描,并测量了照射野输出因子.结果测得的PDD数据表明,电子束深度剂量对照射野铅挡大小有某种程度的依赖性,一般倾向是当照射野减小时表面剂量增大,治疗深度减小,最大剂量深度(R100)向表面移.这些变化在高能时最为明显.输出因子的测量结果说明,对不同能量电子束在不同限光筒条件下,输出因子随照射野铅挡大小改变的情况不尽相同.结论临床治疗时使用的限光筒大小要尽量与实际照射野面积接近,在使用铅挡构成很小的照射野(如<6cm×6cm)时,应实际测量输出因子,以减少剂量误差.     

电子束窄条野剂量特性的测定与分析

目的探讨电子柬窄条野的剂量特性。方法用辐射野扫描系统对瓦里安2300C／D直线加速器多种能量电子束窄条野进行中心轴深度剂量扫描，分析R100、R90、R50及Rp等参数的变化规律，并与标准方野进行比较。用胶片法测不同窄条野不同能量下射野中心轴平面等剂量分布，分析等剂量曲线特点。用NE Farmer 2570剂量仪和PTW 0．1 cm^3电离室测量窄条野输出因子，并与方根式法计算值比较。结果窄条野使高值剂量深度（R100、R90）移向表面，且当窄条野短边变小和在高能时尤为明显；而对低值剂量深度（R50）和电子射程（Rp）影响不大，窄条野长边对百分深度剂量影响很小。90％等剂量曲线宽度随深度的增加由窄变宽，至最大后向内收缩，底部为弧形；低值等剂量曲线随着深度的增加向两侧扩展。方根式法计算窄条野输出因子的偏差随窄条野短边减小而增大。结论窄条野与方野相比治疗深度变浅，并在一定深度处90％等剂量曲线变宽；方根式法计算电子束窄条野输出因子有偏差，临床应用建议实际测量。     

电子束不对称照射野剂量分布特性的测定与分析

临床应用电子束放疗时常常会遇到不对称野,如乳腺癌术后胸壁电子线照射、鼻咽癌后上颈电子线补量照射等.不对称野通常是指限光筒上铅挡野中心偏离标准限光筒射束中心,临床上在进行射野设计、能量选择和剂量计算时需了解不同大小、不同偏轴位置的不对称野的电子束特性.     

医用直线加速器矩形野散射因子的简便计算方法

 目的　建立一种适合于临床应用的加速器散射因子 Sc.p的简便计算方法。方法　 ( 1 )通过测量方野的散射因子 ,建立方野散射因子与射野边长的拟合公式 ;( 2 )利用 Kim的经验公式 ,计算矩形野的等效方边长 ;( 3)应用已拟合的方野散射因子公式计算矩形野的散射因子 ,并与矩形野的测量结果进行比较。结果　采用该方法计算医用加速器散射因子的最大误差在 0 .5%以下 ,而采用面积 -周长比原理确定的等效方边长 ,最大误差达到 3%左右。结论　该方法可用于简便快速地确定方野和矩形野的散射因子 ,精确度高 ,完全可应用于临床.     

电子束空气间隙对输出剂量的影响

目的 研究电子线放射治疗中由于空气间隙导致的输出剂量偏差。方法 采用实测的方式,分别测试：①8MEV电子线不同射野下的有效源皮距；②不同的能量、不同规格的限光筒、不同距离的空气间隙所引起的输出剂量的改变；③对电子束的空气间隙修正，将理论值与实测值数据进行对比、分析。结果 ①对于同一规格的限光筒,各个不同能量、不同的筒皮距下,引起的输出剂量衰减百分比，对大规格限光筒而言近似相同，偏差低于1%，而对于较小规格的限光筒，则偏差较大，最大可达6%；②用有效源皮距的概念来通过计算修正电子束空气间隙导致的剂量偏差只对较大射野适用。结论 对每台加数器都应采用实测的方式得到所需的空气间隙导致的剂量修正参数。     

移动式术中放疗Mobetron加速器的测试

目的 测试移动式术中放疗Mobetron加速器,分析它的电子束剂量学特点.方法 测量移动式术中放疗Mobetron加速器电子束的剂量学特点,并与西门子常规加速器电子束进行比较.Mobetron加速器配置有4、6、9、12 MeV电子束.测量项目包括垂直于水模体表面的中心轴百分深度剂量和平行于水模体表面的射野离轴比、输出因子、限光简外漏射剂量、铅挡块对电子束的衰减、输出量校准.使用的测量仪器包括三维水箱、静电计、0.6 cm3Farmer电离室、平行板电离室和固体水.测量时将不同端面和直径限光筒依次与加速器机头连接,并使端面与模体表面相切.结果 除12 MeV外其他能量的表面剂量均低于90%,相同能量下术中加速器表面剂量明显高于常规加速器剂量.对10 cm直径、0°倾斜角的限光筒四档能量的最大剂量深度依次为0.7、1.3、2.0、2.2 cm,治疗深度依次为1.0、1.8、2.7、3.6 cm;对0°限光筒治疗时只需选直径比瘤床大1cm的筒即可.由于斜端面的限光筒照射野平坦度和对称性明显变差,限光筒尺寸的选择要依据等剂量分布图.四档能量的限光筒外1 cm处漏射线分别为1.2%、5.1%、10.0%、9.1%,全挡时铅挡厚度分别为1.5、3.0、4.5、6.0 mm.结论 通过测试了解了Mobetron加速器性能特点并获得了临床应用和日常质量保证所需数据.     

卧位全身电子线照射技术

对于表浅的恶性皮肤淋巴瘤等疾病,采用电子线全身照射能得到较好的疗效。从五十年代开始到现在已发展多种照射技术。我们采用一种简单的病人卧位的全身照射方法,用4MeV能量的电子束照射病人,治疗中光阑开到最大而且不加任何限光筒,SSD=158cm。病人采用仰卧和俯卧两种体位,每种体位机架从45°和315°两个方向照射,人体从头侧到足侧按光野结合,根据病人身长用了3～4个照射野相接。治疗常规是     

多叶准直器射野处方剂量的快速计算

建立一种计算多叶准直器 (MLC)不规则射野处方剂量的快速方法。方法　考虑到MLC射野的边缘是台阶状、叶片位置坐标可以由治疗计划系统或与MLC配套的叶片位置产生器打印输出这两个特点 ,首先由叶片位置坐标计算出射野周长和面积 ,然后由面积周长比法计算出不规则野的等效方野边长 ,最后按照处理常规方野的方法计算处方剂量。结果　作为例子 ,计算了采用SSD技术的全脑全脊髓照射野和采用SAD技术的鼻咽癌面颈联合野的处方剂量。结论　这种方法适合于靶区参考点位于射野中心区域未被遮挡的情况。与Clarkson积分等其它方法相比 ,它具有快速简单的特点。     

鼻咽癌放疗面颈联合颈后电子束野铅模制作问题的研究

目的探讨鼻咽癌放疗面颈联合颈后电子束野铅模制作问题。方法对5例不同等中心点深度的鼻咽癌面颈联合野放疗患者,按照分野前后的灯光野投影,在患者固定面罩上画出颈后电子束照射野。应用几何关系计算出所给定源片距的校正源片距,分别按照实际源片距和校正后源片距制作电子束铅模,到加速器下治疗摆位,测量源片距校正前后铅挡野与体表野的符合性。结果应用几何关系计算出所给定源片距的校正源片距,均大于给定源片距。源片距校正前的铅挡野与体表野偏差明显较大,最大偏差达1.25 cm;源片距校正后铅挡野与体表野偏差较小,最大偏差为0.22 cm。结论鼻咽癌面颈联合野后程分野颈后电子束野铅模制作时,应用首程等中心照射影像资料制作出来的铅模,偏差较大,通过源片距校正方法,铅挡野与体表照射野符合性较好。     

Impact of simple tissue inhomogeneity correction algorithms on conformal radiotherapy of lung tumours

Background and purpose: Conformal radiotherapy requires accurate dose calculation at the dose specification point, at other points in the planning target volume (PTV) and in organs at risk. To assess the limitations of treatment planning of lung tumours, errors in dose values, calculated by some simple tissue inhomogeneity correction algorithms available in a number of currently applied treatment planning systems, have been quantified.

Materials and methods: Single multileaf collimator-shaped photon beams of 6, 8, 15 and 18 MV nominal energy were used to irradiate a 50 mm diameter spherical solid tumour, simulated by polystyrene, which was located centrally inside lung tissue, simulated by cork. The planned dose distribution was made conformal to the PTV, which was a 15 mm three-dimensional expansion of the tumour. Values of both the absolute dose at the International Commission on Radiation Units and Measurement (ICRU) reference point and relative dose distributions inside the PTV and in the lung were calculated using three inhomogeneity correction algorithms. The algorithms investigated in this study are the pencil beam algorithm with one-dimensional corrections, the modified Batho algorithm and the equivalent path length algorithm. The calculated data were compared with measurements for a simple beam set-up using radiographic film and ionization chambers.

Results: For this specific configuration, deviations of up to 3.5% between calculated and measured values of the dose at the ICRU reference point were found. Discrepancies between measured and calculated beam fringe values (distance between the 50 and 90% isodose lines) of up to 14 mm have been observed. The differences in beam fringe and penumbra width (20–80%) increase with increasing beam energy. Our results demonstrate that an underdosage of the PTV up to 20% may occur if calculated dose values are used for treatment planning. The three algorithms predict a considerably higher dose in the lung, both along the central beam axis and in the lateral direction, compared with the actual delivered dose values.

Conclusions: The dose at the ICRU reference point of such a tumour in lung geometry is calculated with acceptable accuracy. Differences between calculated and measured dose distributions are primarily due to changes in electron transport in the lung, which are not adequately taken into account by the simple tissue inhomogeneity correction algorithms investigated in this study. Particularly for high photon beam energies, clinically unacceptable errors will be introduced in the choice of field sizes employed for conformal treatments, leading to underdosage of the PTV. In addition, the dose to the lung will be wrongly predicted which may influence the choice of the prescribed dose level in dose-escalation studies.     

Accuracy of central axis dose calculations for photon external radiotherapy beams in Finland: the quality of local beam data and the use of averaged data.

PURPOSE: The accuracy of central axis dose calculation was evaluated for 48 photon beams from 28 linear accelerators at nine centres in Finland. In addition, inter-accelerator consistency of beam data was evaluated for Varian Clinac 600 CDs and 2100 CDs, and averaged data sets were generated for output factors (OFs) and percentage depth doses (PDDs). The averaged data sets obtained were used to identify potential dosimetry reasons for local errors. MATERIALS AND METHODS: Agreement between measured and calculated doses was determined at isocentre at 10 cm depth in water for nine different sized open square and rectangular fields. Averaged OFs were determined for nominal energies of 4, 6, 10, 15 and 18 MV both at d(max) and at a 10-cm depth. In order to develop a function for the OF data, OFs for square fields were parameterised through empirical model fitting. The feasibility of a simple equivalent square collimator formula was also evaluated for the presentation of OFs for rectangular fields. Averaged PDDs were determined at a 10-cm depth. RESULTS: The difference between measured and calculated doses exceeded +/-3%, +/-2% and +/-1% for 3, 6 and 35 of the investigated 48 beams, respectively. The differences were due to errors observed in both OFs and depth dose data. When the agreement between dose calculation and measurement was within +/-1%, inter-accelerator differences in OFs were within +/-1.0% at both the depth of dose maximum and at 10 cm for Clinac 600 CDs and also for 2100 CDs. Differences in PDDs were within +/-1.2%. CONCLUSIONS: The importance of quality control for beam data was demonstrated by showing significant errors in measured data. For Clinac 600 and 2100 CDs, the quality control can be accurately performed by comparing local data to averaged reference data. Robust averaged data sets were obtained for 6, 15 and 18 MV beams of Clinac 2100 CDs.     

Scatter dose summation for irregular fields: speed and accuracy study

Using program IRREG as a standard, we have compared speed and accuracy of several algorithms that calculate the scatter dose in an irregular field. All the algorithms, in some manner, decompose the irregular field into component triangles and obtain the scatter dose as the sum of the contributions from those triangles. Two of the algorithms replace each such component triangle with a sector of a certain "effective radius": in one case the average radius of the triangle, in the other the radius of the sector having the same area as the component triangle. A third algorithm decomposes each triangle further into two right triangles and utilizes the precalculated "equivalent radius" of each, to find the scatter contribution. For points near the center of a square field, all the methods compare favorably in accuracy to program IRREG, with less than a 1% error in total dose and with approximately a factor of 3-5 savings in computation time. Even for extreme rectangular fields (2 cm X 30 cm), the methods using the average radius and the equivalent right triangles agree to within 2% in total dose and approximately a factor of 3-4 savings in computation time.     

Comparison of the ESTRO formalism for monitor unit calculation with a Clarkson based algorithm of a treatment planning system and a traditional "full-scatter" methodology

The ESTRO formalism for monitor unit (MU) calculations was evaluated and implemented to replace a previous methodology based on dosimetric data measured in a full-scatter phantom. This traditional method relies on data normalised at the depth of dose maximum (Zm), as well as on the utilisation of the BJR 25 table for the conversion of rectangular fields into equivalent square fields. The treatment planning system (TPS) was subsequently updated to reflect the new beam data normalised at a depth ZR of 10 cm. Comparisons were then carried out between the ESTRO formalism, the Clarkson-based dose calculation algorithm on the TPS (with beam data normalised at Zm and ZR), and the traditional "full-scatter" methodology. All methodologies, except for the "full-scatter" methodology, separated head-scatter from phantom-scatter effects and none of the methodologies; except for the ESTRO formalism, utilised wedge depth dose information for calculations. The accuracy of MU calculations was verified against measurements in a homogeneous phantom for square and rectangular open and wedged fields, as well as blocked open and wedged fields, at 5, 10, and 20 cm depths, under fixed SSD and isocentric geometries for 6 and 10 MV. Overall, the ESTRO Formalism showed the most accurate performance, with the root mean square (RMS) error with respect to measurements remaining below 1% even for the most complex beam set-ups investigated. The RMS error for the TPS deteriorated with the introduction of a wedge, with a worse RMS error for the beam data normalised at Zm (4% at 6 MV and 1.6% at 10 MV) than at ZR (1.-9% at 6 MV and 1.1% at 10 MV). The further addition of blocking had only a marginal impact on the accuracy of this methodology. The "full-scatter" methodology showed a loss in accuracy for calculations involving either wedges or blocking, and performed worst for blocked wedged fields (RMS errors of 7.1% at 6 MV and 5% at 10 MV). The origins of these discrepancies were quantified and the shortcomings of these MU calculation methodologies are discussed in the paper.     

Monitor tables for electron beams in radiotherapy

The application of electron beams in radiotherapy is still based on tables of monitor units, although 3-D treatment planning systems for electron beams are available. This have several reasons: The need for 3-D treatment planning is not recognized; there is no confidence in the calculation algorithm; Monte-Carlo algorithms are too time-consuming; and the effort necessary to measure basic beam data for 3-D planning is considered disproportionate. However, the increasing clinical need for higher dosimetric precision and for more conformal electron beams leads to the requirement for more sophisticated tables of monitor units. The present paper summarizes and discusses the main aspects concerning the preparation of tables of monitor units for electron beams. The measurement equipment and procedures for measuring basic beam data needed for tables of monitor units for electron beams are described for a standard radiation therapy linac. The design of tables of monitor units for standard electron applicators is presented; this design can be extended for individual electron inserts, to variable applicator surface distances, to oblique beam incidence, and the use of bolus material. Typical data of an Elekta linac are presented in various tables.     

Experimental validation tests of fast Fourier transform convolution and multigrid superposition algorithms for dose calculation in low-density media.

BACKGROUND AND PURPOSE: Modern conformal radiotherapy treatments require accurate dose calculation in any relevant clinical situation. One of these situations is the treatment of lung tumors, where irradiation has to be planned under challenging conditions for dose calculation. In this study we assess the errors in dose values predicted by fast Fourier transform convolution (FFTC) and multigrid superposition (MGS) algorithms implemented in a commercial treatment planning system (TPS). MATERIALS AND METHODS: FFTC and MGS algorithms were used in a FOCUS 3.0.0 (Computerized Medical Systems, USA) to calculate doses in treatment plans using photon beams of 6 and 25 MV nominal energy from a Saturne 43 linac (GE Medical Systems, USA). A 10x10-cm beam irradiating a mediastinum-lung and a thoracic wall-lung-thoracic wall modeled geometry was assessed. The calculated data were compared with measurements performed with radiographic films and ionization chamber. RESULTS: FFTC algorithm leads to an average deviation from ionometric dose measurements of over 10%. Discrepancies between measured and calculated beam fringe values (distance between 50 and 90% isodose lines) of up to 8 mm were observed. For MGS algorithm, all the points assessed in both geometries fulfilled the 3%-3 mm accuracy criteria and the average deviation of absolute dose was about 1%. A maximum of 3 mm deviation in the beam fringe for any depth was found and was within 2 mm beyond the buildup region. Deviations between ionometric and film measurements were within 3%. CONCLUSIONS: MGS algorithm assesses with reasonable accuracy dose distributions and absolute dose in inhomogeneous regions like the lung region. Therefore, and respecting the inhomogeneity dose calculation, the system could be used in routine clinical practice and in dose-escalation programs. This is not true in the case of FFTC algorithm which leads to errors greater than 10% in the absolute dose calculation and underestimates the beam fringe by up to 8 mm.     

Dosimetric verification of radiotherapy treatment planning systems in Serbia: national audit

ABSTRACT: BACKGROUND: Independent external audits play an important role in quality assurance programme in radiation oncology. The audit supported by the IAEA in Serbia was designed to review the whole chain of activities in 3D conformal radiotherapy (3D-CRT) workflow, from patient data acquisition to treatment planning and dose delivery. The audit was based on the IAEA recommendations and focused on dosimetry part of the treatment planning and delivery processes. METHODS: The audit was conducted in three radiotherapy departments of Serbia. An anthropomorphic phantom was scanned with a computed tomography unit (CT) and treatment plans for eight different test cases involving various beam configurations suggested by the IAEA were prepared on local treatment planning systems (TPSs). The phantom was irradiated following the treatment plans for these test cases and doses in specific points were measured with an ionization chamber. The differences between the measured and calculated doses were reported. RESULTS: The measurements were conducted for different photon beam energies and TPS calculation algorithms. The deviation between the measured and calculated values for all test cases made with advanced algorithms were within the agreement criteria, while the larger deviations were observed for simpler algorithms. The number of measurements with results outside the agreement criteria increased with the increase of the beam energy and decreased with TPS calculation algorithm sophistication. Also, a few errors in the basic dosimetry data in TPS were detected and corrected. CONCLUSIONS: The audit helped the users to better understand the operational features and limitations of their TPSs and resulted in increased confidence in dose calculation accuracy using TPSs. The audit results indicated the shortcomings of simpler algorithms for the test cases performed and, therefore the transition to more advanced algorithms is highly desirable.