Pre-treatment verification of intensity modulated photon beams with films and electronic portal imaging--two years of clinical experience

Aim of this report was to summarise clinical experience in the field of pre-treatment dosimetric verification of intensity-modulated photon beams (IRMT). From May 2001 to July 2003, 50 patients were irradiated according to IMRT techniques with 6 MV photon beams produced by a Varian Clinac equipped with a 80 leaves multileaf collimator. Dose plans were computed using commercial treatment planning systems, Nucletron Helax-TMS for static cases and Varian Eclipse-Helios for dynamic cases. Pre-treatment dosimetric verification was carried out on a field-per-field basis measuring 2D absolute dose distributions in solid water at 10 cm depth using films or an electronic portal imaging device (EPID). Verification measurements were compared with expected dose maps, and differences were evaluated by means of both a point-to-point analysis and the Gamma Index. Irradiated target volumes (30 head and neck, 8 breast, 12 other patients) ranged from 111 to 2121 cm3 with a mean of 652 +/- 378 cm3. Twenty-nine dose plans were delivered with dynamic technique and 44 with static technique. On average, 5.9 +/- 1.3 fields were applied per plan, with 12.1 +/- 1.6 segments per field in the static mode. Averaging over the whole number of fields we obtained a mean difference (on a pixel-by-pixel basis and per 100 MU delivered) of -0.22 +/- 0.64 cGy between calculation and measurement, with a standard deviation of 1.93 +/- 0.65 cGy. The mean value for the Gamma Index evaluation was 0.47 +/- 0.10, with a mean standard deviation of 0.35 +/- 0.17. The fraction of pixels lying inside the field and showing a gamma index larger than 1 was 5.7% for the triplet Eclipse-film-dynamic delivery and 9.9% for the triplet Helax-TMS-EPID-static delivery. The employed IMRT treatments proved that this technique is feasible and dosimetrically accurate. Treatment verification stability and dosimetric analysis of treated plans are highly satisfactory and allow the safe introduction of this modality in the spectrum of techniques offered to a large class of patients.     

Dosimetric characteristics of 6 and 10 MV unflattened photon beams

Purpose

To determine dosimetric properties of unflattened megavoltage photon beams.

Materials and methods

Dosimetric data including depth dose, profiles, output factors and phantom scatter factors from three different beam qualities provided by Elekta Precise linacs, operated with and without flattening filter were examined. Additional measurements of leaf transmission, leakage radiation and surface dose were performed. In flattening filter free (FFF) mode a 6-mm thick copper filter was placed into the beam to stabilize it.

Results

Depths of dose maxima for flattened and unflattened beams did not deviate by more than 2 mm and penumbral widths agreed within 1 mm. In FFF mode the collimator exchange effect was found to be on average 0.3% for rectangular fields. Between maximum and minimum field size head scatter factors of unflattened beams showed on average 40% and 56% less variation for 6 and 10 MV beams than conventional beams. Phantom scatter factors for FFF beams differed up to 4% from the published reference data. For field sizes smaller than 15 cm, surface doses relative to the dose at d_max increased for unflattened beams with maximum differences of 7% at 6 MV and 25% at 10 MV for a 5 × 5 cm² field. For a 30 × 30 cm² field, relative surface dose decreased by about 10% for FFF beams. Leaf transmission on the central axis was 0.3% and 0.4% lower for unflattened 6 and 10 MV beams, respectively. Leakage radiation was reduced by 52% for 6 MV and by 65% for 10 MV unflattened beams.

Conclusions

The results of the study were independently confirmed at two radiotherapy centres. Phantom scatter reference data need to be reconsidered for medical accelerators operated without a flattening filter.     

Pre-treatment dosimetric verification by means of a liquid-filled electronic portal imaging device during dynamic delivery of intensity modulated treatment fields.

BACKGROUND AND PURPOSE: Although intensity modulated radiation therapy is characterized by three-dimensional dose distributions which are often superior to those obtained with conventional treatment plans, its routine clinical implementation is partially held back by the complexity of the beam verification. This is even more so when a dynamic multileaf collimator (dMLC) is used instead of a segmented beam delivery. We have therefore investigated the possibility of using a commercially available, liquid-filled electronic portal imaging device (EPID) for the pre-treatment quality assurance of dynamically delivered dose distributions. METHODS AND MATERIALS: A special acquisition mode was developed to optimize the image acquisition speed for dosimetry with the liquid-filled EPID. We investigated the accuracy of this mode for 6 and 18 MV photon beams through comparison with film and ion chamber measurements. The impact of leaf speed and pulse rate fluctuations was quantified by means of dMLC plans especially designed for this purpose. Other factors influencing the accuracy of the dosimetry (e.g. the need for build-up, remanence of the ion concentration in the liquid and bulging of the liquid at non-zero gantry angles) were studied as well. We finally compared dosimetric EPID images with the corresponding image prediction delivered without a patient in the beam. RESULTS: The dosimetric accuracy of the measured dose distribution is approximately 2% with respect to film and ion chamber measurements. The accuracy declines when leaf speed is increased beyond 2 cm/s, but is fairly insensitive to accelerator pulse rate fluctuations. The memory effect is found to be of no clinical relevance. When comparing the acquired and expected distributions, an overall agreement of 3% can be obtained, except at areas of steep dose gradients where slight positional shifts are translated into large errors. CONCLUSIONS: Accurate dosimetric images of intensity modulated beam profiles delivered with a dMLC can be obtained with a commercially available, liquid-filled EPID. The developed acquisition mode is especially suited for fast and accurate pre-treatment verification of the intensity modulated fields.     

电子射野影像系统临床应用的研究进展

电子射野影像系统（EPID）越来越多的被应用、对摆位误差的研究是电子射野影像系统的最初设计目的，利用其进行摆位误差的校正有在线和离线两种形式？随着对电子射野影像系统的剂量学特性的不断了解，用它进行剂量学验证也开始从实验室研究走向临床应用。电子射野影像系统在放疗元件的质量保证中也起到重要的作用，近年来在这方面的研究主要是埘多叶光阑质量保证：本文就电子射野影像系统的临床应用做一简要的概述。     

Linear array measurements of enhanced dynamic wedge and treatment planning system (TPS) calculation for 15 MV photon beam and comparison with electronic portal imaging device (EPID) measurements

INTRODUCTION.: Enhanced dynamic wedges (EDW) are known to increase drastically the radiation therapy treatment efficiency. This paper has the aim to compare linear array measurements of EDW with the calculations of treatment planning system (TPS) and the electronic portal imaging device (EPID) for 15 MV photon energy. MATERIALS AND METHODS.: The range of different field sizes and wedge angles (for 15 MV photon beam) were measured by the linear chamber array CA 24 in Blue water phantom. The measurement conditions were applied to the calculations of the commercial treatment planning system XIO CMS v.4.2.0 using convolution algorithm. EPID measurements were done on EPID-focus distance of 100 cm, and beam parameters being the same as for CA24 measurements. RESULTS: Both depth doses and profiles were measured. EDW linear array measurements of profiles to XIO CMS TPS calculation differ around 0.5%. Profiles in non-wedged direction and open field profiles practically do not differ. Percentage depth doses (PDDs) for all EDW measurements show the difference of not more than 0.2%, while the open field PDD is almost the same as EDW PDD. Wedge factors for 60 deg wedge angle were also examined, and the difference is up to 4%. EPID to linear array differs up to 5%. CONCLUSIONS: The implementation of EDW in radiation therapy treatments provides clinicians with an effective tool for the conformal radiotherapy treatment planning. If modelling of EDW beam in TPS is done correctly, a very good agreement between measurements and calculation is obtained, but EPID cannot be used for reference measurements.     

Detection of organ movement in cervix cancer patients using a fluoroscopic electronic portal imaging device and radiopaque markers

PURPOSE: To investigate the use of a fluoroscopic electronic portal imaging device (EPID) and radiopaque markers to detect internal cervix movement. METHODS AND MATERIALS: For 10 patients with radiopaque markers clamped to the cervix, electronic portal images were made during external beam irradiation. Bony structures and markers in the portal images were registered with the same structures in the corresponding digitally reconstructed radiographs of the planning computed tomogram. RESULTS: The visibility of the markers in the portal images was good, but their fixation should be improved. Generally, the correlation between bony structure displacements and marker movement was poor, the latter being substantially larger. The standard deviations describing the systematic and random bony anatomy displacements were 1.2 and 2.6 mm, 1.7 and 2.9 mm, and 1.6 and 2.7 mm in the lateral, cranial-caudal, and dorsal-ventral directions, respectively. For the marker movement those values were 3.4 and 3.4 mm, 4.3 and 5.2 mm, 3.2 and 5.2 mm, respectively. Estimated clinical target volume to planning target volume (CTV-PTV) planning margins (approximately 11 mm) based on the observed overall marker displacements (bony anatomy + internal cervix movement) are only marginally larger than the margins required to account for internal marker movement alone. CONCLUSIONS: With our current patient setup techniques and methods of setup verification and correction, the required CTV-PTV margins are almost fully determined by internal organ motion. Setup verification and correction using radiopaque markers might allow decreasing those margins, but technical improvements are needed.     

Transit dosimetry with an electronic portal imaging device (EPID) for 115 prostate cancer patients

Purpose: Comparison of predicted portal dose images (PDIs) with PDIs measured with an electronic portal imaging device (EPID) may be used to detect errors in the dose delivery to patients. However, these comparisons cannot reveal errors in the MU calculation of a beam, since the calculated number of MU is used both for treatment (and thus affects the PDI measurement) and for PDI prediction. In this paper a method is presented that enables “in vivo” verification of the MU calculation of the treatment beams. The method is based on comparison of the intended on-axis patient dose at 5 cm depth for each treatment beam, D₅, with D₅ as derived from the portal dose D_p measured with an EPID. The developed method has been evaluated clinically for a group of 115 prostate cancer patients.

Methods and Materials: The patient dose D₅ was derived from the portal dose measured with a fluoroscopic EPID using (i) the predicted beam transmission (i.e., the ratio of the portal dose with and without the patient in the beam) calculated with the planning CT data of the patient, and (ii) an empirical relation between portal doses D_p and patient doses D₅. For each beam separately, the derived patient dose D₅ was compared with the intended dose as determined from the relative dose distribution as calculated by the treatment planning system and the prescribed isocenter dose (2 Gy). For interpretation of observed deviating patient doses D₅, the corresponding on-axis measured portal doses D_p were also compared with predicted portal doses.

Results: For three beams, a total of 7828 images were analyzed. The mean difference between the predicted patient dose and the patient dose derived from the average measured portal dose was: 0.4 ± 3.4% (1 SD) for the anterior–posterior (AP) beam and −1.5 ± 2.4% (1 SD) for the lateral beams. For 7 patients the difference between the predicted portal dose and the average measured portal dose for the AP beam and the corresponding difference in patient dose were both greater than 5%. All these patients had relatively large gas pockets (3–3.5 cm in AP direction) in the rectum during acquisition of the planning CT, which were not present during (most) treatments.

Conclusions: An accurate method for verification of the MU calculation of an x-ray beam using EPID measurements has been developed. The method allows the discrimination of errors that are due to changes in patient anatomy related to appearance or disappearance of gas pockets in the rectum and errors due to a deviating cGy/MU-value.     

In vivo dosimetry for prostate cancer patients using an electronic portal imaging device (EPID); demonstration of internal organ motion.

PURPOSE: To investigate the use of a commercially available video-based EPID for in vivo dosimetry during treatment of prostate cancer patients. METHODS: For 10 prostate cancer patients, the inter-fraction variation within measured portal dose images (PDIs) was assessed and measured PDIs were compared with corresponding predicted PDIs based on the planning CT scan of the patient. RESULTS: For the lateral fields, the average standard deviation in the measured on-axis portal doses during the course of a treatment was 0.9%; for the anterior fields this standard deviation was 2.2%. The difference between the average on-axis measured portal dose and the predicted portal dose was 0.3+/-2.1% (1 SD) for the lateral fields and 0.7+/-3.4% (1 SD) for the anterior fields. Off-axis differences between measured and predicted portal doses were regularly much larger (up to 15%) and were caused by frequently occurring gas pockets inside the rectum of the patients during treatment or during acquisition of the planning CT scan. The detected gas pockets did sometimes extend into the gross tumour volume (GTV) area as outlined in the planning CT scans, implying a shift of the anterior rectum wall and prostate in the anterior direction (internal organ motion). CONCLUSIONS: The developed procedures for measurement and prediction of PDIs allow accurate dosimetric quality control of the treatment of prostate cancer patients. Comparing measured PDIs with predicted PDIs can reveal internal organ motion.     

非晶硅平板电子射野影像用于放射治疗剂量学质量控制检验的应用

背景与目的:非晶硅平板型电子射野影像系统(amorphous silicon electronic portal imaging device,a-Si EPID)具有良好的剂量学品质,作为一种快速的二维剂量测量系统,在常规质量控制、调强照射野验证及实时患者剂量监测等方面具有广阔的应用前景.为将非晶硅平板电子射野影像用于放射治疗的剂量学检验,本研究针对其射野影像建立了修正模型,并应用于加速器照射野的常规质量保证工作.方法:对a-Si EPID常用的图像刻度模式进行剂量刻度改进以用于照射剂量测量:通过一种由若干个小野组合形成"泛野"的方式来克服传统的泛野获取方式的缺陷,从而较准确地修正a-Si EPID各像素单元之间的灵敏度差异;并建立离轴剂量的响应曲线和修正数学模型.以修正后的a-Si EPID射野影像测量照射野的剂量分布并与三维水箱中电离室扫描的结果进行比较验证.结果:经所建立的模型进行剂量刻度和修正后,高剂量区,a-Si EPID与电离室测量结果偏差＜2%,在半影区,a-Si EPID测量的剂量分布曲线比电离室测量结果略为陡峭.结论:非晶硅平板型电子射野影像(a-Si EPID)系统具有良好的物理剂量学品质,可以用作照射野常规质控检验和调强放射治疗射野剂量能量分布的快速工具.     

Dosimetric verification of modulated photon fields by means of compensators for a kernel model.

The approach in treatment planning of applying beam quality correction factors to model compensator-induced depth-hardening effects is investigated and the present work comprises a dosimetric verification of the model for a common compensator material. Lead sheet modulators for four different phantom shapes were designed using a treatment planning system based on the model. The modulators were designed to yield homogeneous dose in a plane. The calculated modulation created by the lead sheets was re-imported into the treatment planning system and applied to a water phantom geometry for verification purposes. Comparing measurements, a total of 31 different geometries were measured, with calculations in this geometry showing good agreement for depth doses, dose profiles and output data with a maximum deviation of 4% except locally in the penumbra region and close to the edges of the cut lead sheets.     

An isocenter position verification device for electronic portal imaging: physical and dosimetrical characteristics

The physical and dosimetrical characteristics of a device, designed to visualize the isocenter position on electronic portal images, were examined. The device, to be mounted on the gantry head of the accelerator, containing five spheric lead markers, was designed in order to visualize the isocenter position on portal images. A quality control device was designed to check the reliability of this technique. The disturbance of the dose distribution by the markers was studied with gel dosimetry. The use of markers resulted in a precise and accurate method to visualize the isocenter on portal images. A maximum underdosage of 11%, due to attenuation by the markers, was observed. The use of markers to visualize the isocenter position on portal images, is a fast and reliable method when analyzing patient setup errors with online electronic portal imaging.     

Dosimetric characteristics of a 6 MV photon beam from a linear accelerator with asymmetric collimator jaws

Dosimetric measurements have been made of a 6 MV photon beam from a linear accelerator equipped with asymmetric jaws. The field size factors for asymmetrically set fields are compared to those for symmetrically set fields. The change of beam quality has been measured as a function of off-axis position of the asymmetric fields to assess its effect on depth dose. Additional measurements include beam penumbra and shape of isodose curves for open and wedge fields as the field opening is moved asymmetrically from the central ray.     

Critical appraisal of treatment techniques based on conventional photon beams, intensity modulated photon beams and proton beams for therapy of intact breast.

PURPOSE: To analyse different treatment techniques with conventional photon beams, intensity modulated photon beams, and proton beams for intact breast irradiation for patients in whom conventional irradiation would cause potentially dangerous lung irradiation. MATERIALS AND METHODS: Five breast cancer patients with highly concave breast tissue volume around the lung were considered at planning level in order to assess the suitability of different irradiation techniques. Three-dimensional dose distributions for conventional two-field tangential photon treatment, two-field intensity modulated radiotherapy (IMRT), three-field non-IMRT, three-field IMRT, and single-field proton treatment were investigated, aiming at assessing the possibility to reduce lung irradiation below risk levels. Analysis of dose-volume histograms and related physical and biological parameters (significant minimum, maximum and mean doses, conformity indexes and equivalent uniform dose (EUD)) for planned target volume (PTV) and lung was carried out. Dose plans were compared with the conventional two-field tangential photon technique. RESULTS: PTV coverage was comparable for non-IMRT and IMRT techniques (EUD from 47.1 to 49.4 Gy), and improved with single-field proton treatment (EUD=49.8 Gy). Lung irradiation was reduced, in terms of mean dose, with three-field (9.5 Gy) and proton technique (3.5 Gy), with respect to the conventional two-field treatment (12.9 Gy); also a reduction of the lung volume irradiated at high doses was observed. Better results could be achieved with protons. In addition, cardiac irradiation was also reduced with those techniques. CONCLUSIONS: Geometrically difficult breast cancer patients could be irradiated with a three-field non-IMRT technique thus reducing the dose to the lung which is proposed as standard for this category of patients. Intensity modulated techniques were only marginally more successful than the corresponding non-IMRT treatments, while protons offer excellent results.     

A simple theoretical verification of monitor unit calculation for intensity modulated beams using dynamic mini-multileaf collimation.

A spreadsheet based program is presented to perform an independent Monitor Unit (MU) calculation verification for the Quality Assurance (QA) of Intensity Modulated Radiation Therapy (IMRT) using Dynamic MultiLeaf Collimation (DMLC). The computed dose value is compared to the planned dose by calculating the percent dose difference per Intensity Modulated Beam (IMB) and absolute dose difference per IMB. The proposed acceptability levels are +/-5.0% or +/-2.0 cGy for the percent dose difference per IMB and the absolute dose difference per IMB, respectively. For percent dose difference per treatment, an acceptability level of +/-2.0% is proposed. The presented program is considered adequate for checking the treatment plans calculated for IMRT treatments using DMLC as a part of the QA procedure.     

基于EPID三维剂量验证系统的物理模型测试及临床应用的初步研究

目的 使用EPID三维剂量验证系统进行物理建模和物理参数优化,并行临床应用前的初步研究。方法 通过EPID采集3、5、10、15、20、25 cm的方野图像建立物理模型,比较在均匀水模体中系统重建的百分深度剂量、射野总散因子及10 cm深度处的离轴比曲线,优化物理模型参数。采用指型电离室和免冲洗胶片,在均匀模体和仿真人模体中测量单野、组合野及IMRT计划点剂量和平面剂量,并与系统重建结果比较。在仿真人模体和 10例不同部位IMRT计划中,比较系统重建和TPS计算的5%/3 mm、3%/3 mm标准下的γ通过率,并对临床病例进行靶区和OAR剂量体积分析。结果 对于单野、组合野以及IMRT计划,系统重建剂量和电离室测量及TPS计算的点剂量平均偏差分别<0.5%和2.0%;在均匀或仿真人模体中以及临床病例中其平面或三维剂量的5%/3 mm、3%/3 mm平均γ通过率均>95%;但临床病例中体现小体积的OAR有较大剂量偏差。结论 通过一系列临床应用前测试,明确了该三维剂量验证系统可有效应用于临床剂量验证,并有较好的临床应用价值。     

Commissioning and periodic quality assurance of a clinical electronic portal imaging device

PURPOSE: An electronic portal imaging device (EPID) was recently installed on our dual-energy linear accelerator. Commissioning and quality assurance techniques were developed for the EPID. METHODS AND MATERIALS: A commissioning procedure was developed consisting of five parts: (a) physical operation and safety; (b) image acquisition, resolution, and sensitivity calibration; (c) image storage, analysis, and handling; (d) reference image acquisition; and (e) clinical operations. RESULTS: The physical operation and safety tests relate to the motions of the unit, stability of the unit supports, safety interlocks, and interlock overrides. Imager contrast and spatial resolutions are monitored by imaging a contrast-detail phantom. The imager calibration procedure consists of a no-radiation image to compensate for signal offsets, as well as a "flat-field image." The flat-field image is taken with 5.0 cm of homogeneous phantom material placed at isocenter to provide some photon scatter and to approximate the presence of a patient. Daily quality assurance procedures consists of safety tests and the acquisition and inspection of images of the contrast-detail phantom. After 1 year, the frequency of the daily procedure was reduced to weekly. Quarterly QA procedures are conducted by the physicist and consist of the same procedures conducted in the weekly test. The annual QA procedure consists of a duplication of the commissioning procedure. CONCLUSION: The procedures discussed in this article were applied to an ionization-chamber device. They have been useful in identifying difficulties with the EPID operation, including the need for recalibrating and monitoring the accelerator output stability.     

应用电子射野影像装置实时纠正鼻咽癌调强放疗摆位误差研究

果 30例患者纠正前所有EPID影像中,各个方向的摆位误差测定值大约88.3%≤3 mm、99.1%≤5 mm.其中16例分别出现某个方向或同时出现2个方向的摆位误差 2 mm,均已进行实时纠正.纠正后的总体系统误差和随机误差均明显低于纠正前水,各个方向的M PTV 值纠正前为3 mm芹右,纠正后约为纠正前的1/2.结论 牙合垫法与EPID结合交时纠正鼻咽癌调强放疗摆位误差可有效降低摆位系统误差和随机误差,从而降低摆位外扩边界值,进而提高摆位精度.