Investigating the effect of dose rate and maximum allowable MLC leaf velocity in dynamic IMRT

The purpose of this study is to analyze the effect of various dose rates (DR) and maximum allowable MLC leaf velocities (MLV) in dynamic Intensity Modulated Radiotherapy (IMRT) planning and delivery of head and neck patients. Five head and neck patients were retrospectively included in this study. The initial dynamic IMRT ‘reference plans’ were created for all these patients, using a DR of 400 MU/min and MLV of 2.5 cm/s. Additional plans were generated by varying the DR and MLV values. The DR value was varied from 100 to 600 MU/min, in increments of 100 MU/min, for a MLV of 2.5 cm/s. Also the MLV was varied from 0.5 to 3 cm/s, in increments of 0.5 cm, for a DR of 400 MU/min. In order to maintain the prescribed dose to the PTV, the DR was allowed to vary (‘beam hold or DR modulation’ during delivery) when the MLV was changed and the MLV was allowed to vary when the DR was changed. The mean doses to the PTV as well as parotids, maximum dose of spinal cord and total MU were recorded for analysis. The effect of DR and MLV on treatment delivery was analyzed using the portal dosimetry for all the above plans. The predicted portal dose fluences of the TPS were compared with the measured EPID fluences using gamma evaluation criteria of 2% dose difference and 2 mm distance to agreement. A small proportional increase in OAR doses with DR was observed. Increases to MLV value resulted in decreases of the OAR doses and this effect was considerable for values below 1.5 cm/s. DR and MLV both resulted in no appreciable dose variation to the target. The total MU to deliver the plan increases with increasing DR and decreasing MLV. When comparing portal images derived from the treatment plans with portal images obtained by delivering the treatments, it was observed that the treatments was most reliably delivered when the DRs were set to lower values and when the MLVs were set to higher values.     

Software tool for portal dosimetry research

This paper describes a software tool developed for research into the use of an electronic portal imaging device (EPID) to verify dose for intensity modulated radiation therapy (IMRT) beams. A portal dose image prediction (PDIP) model that predicts the EPID response to IMRT beams has been implemented into a commercially available treatment planning system (TPS). The software tool described in this work was developed to modify the TPS PDIP model by incorporating correction factors into the predicted EPID image to account for the difference in EPID response to open beam radiation and multileaf collimator (MLC) transmitted radiation. The processes performed by the software tool include; i) read the MLC file and the PDIP from the TPS, ii) calculate the fraction of beam-on time that each point in the IMRT beam is shielded by MLC leaves, iii) interpolate correction factors from look-up tables, iv) create a corrected PDIP image from the product of the original PDIP and the correction factors and write the corrected image to file, v) display, analyse, and export various image datasets. The software tool was developed using the Microsoft Visual Studio.NET framework with the C# compiler. The operation of the software tool was validated. This software provided useful tools for EPID dosimetry research, and it is being utilised and further developed in ongoing EPID dosimetry and IMRT dosimetry projects.     

Incorporation of gantry angle correction for 3D dose prediction in intensity-modulated radiation therapy

Pretreatment dose verification with beam-by-beam analysis for intensity-modulated radiation therapy (IMRT) is commonly performed with a gantry angle of 0° using a 2D diode detector array. Any changes in multileaf collimator (MLC) position between the actual treatment gantry angle and 0° may result in deviations from the planned dose. We evaluated the effects of MLC positioning errors between the actual treatment gantry angles and nominal gantry angles. A gantry angle correction (GAC) factor was generated by performing a non-gap test at various gantry angles using an electronic portal imaging device (EPID). To convert pixel intensity to dose at the MLC abutment positions, a non-gap test was performed using an EPID and a film at 0° gantry angle. We then assessed the correlations between pixel intensities and doses. Beam-by-beam analyses for 15 prostate IMRT cases as patient-specific quality assurance were performed with a 2D diode detector array at 0° gantry angle to determine the relative dose error for each beam. The resulting relative dose error with or without GAC was added back to the original dose grid for each beam. We compared the predicted dose distributions with or without GAC for film measurements to validate GAC effects. A gamma pass rate with a tolerance of 2%/2 mm was used to evaluate these dose distributions. The gamma pass rate with GAC was higher than that without GAC (P = 0.01). The predicted dose distribution improved with GAC, although the dosimetric effect to a patient was minimal.     

EBT2 radiochromic film for quality assurance of complex IMRT treatments of the prostate: micro-collimated IMRT,RapidArc, and TomoTherapy

In response to the clinical need for a dosimetry system with both high resolution and minimal angular dependence, this study demonstrates the utility of Gafchromic EBT2 radiochromic dosimetry film for the quality assurance of micro-collimated IMRT, RapidArc and TomoTherapy treatments. Firstly, preliminary measurements indicated that the dose response of EBT2 film does not appreciably vary with either the angle of incidence of the radiation beam or the depth in water at which the film is placed. Secondly, prostate treatment plans designed for delivery using static-beam IMRT (collimated using the BrainLab m3 microMLC), RapidArc and TomoTherapy were investigated by comparing dose planes obtained from treatment planning calculations with EBT2 film measurements. For all treatment plans, the proportion of dose points agreeing with the film measurements to within γ (3%,3 mm) was found to be above 95%, with all points agreeing within 5%. The film images provided sufficient information to verify that the treatments could be delivered with an acceptable level of accuracy, while also providing additional information on low-level dose variations that were not predicted by the treatment planning systems. This information included: the location and extent of dose from inter-leaf leakage (in the RapidArc plan) and helical field junctioning (in the TomoTherapy plan), as well as the existence of small regions where the treatment planning system under-predicted the dose from very small treatment segments (in the micro-collimated IMRT plan).     

Long-term two-dimensional pixel stability of EPIDs used for regular linear accelerator quality assurance

The long-term stability of three clinical electronic portal imaging devices (EPIDs) was studied to determine if longer times between calibrations can be justified. This would make alternatives to flood-field calibration of EPIDs clinically feasible, allowing for more effective use of EPIDs for dosimetry. Images were acquired monthly for each EPID as part of regular clinical quality assurance over a time period of approximately 3 years. The images were analysed to determine (1) the long-term stability of the EPID positioning system, (2) the dose response of the central pixels and (3) the long term stability of each pixel in the imager. The position of the EPID was found to be very repeatable with variations less than 0.3 pixels (0.27 mm) for all imagers (1 standard deviation). The central axis dose response was found to reliably track ion chamber measurements to better than 0.5%. The mean variation in pixel response (1 standard deviation), averaged over all pixels in the EPID, was found to be at most 0.6% for the three EPIDs studied over the entire period. More than 99% of pixels in each EPID showed less than 1% variation. Since the EPID response was found to be very stable over long periods of time, an annual calibration should be sufficient in most cases. More complex dosimetric calibrations should be clinically feasible.     

The prediction of transmitted dose distributions using a 3D treatment planning system

Patient dose verification is becoming increasingly important with the advent of new complex radiotherapy techniques such as conformal radiotherapy (CRT) and intensity-modulated radiotherapy (IMRT). An electronic portal imaging device (EPID) has potential application for in vivo dosimetry. In the current work, an EPID has been modelled using a treatment planning system (TPS) to predict transmitted dose maps. A thin slab of RW3 material used to initially represent the EPID. A homogeneous RW3 phantom and the thin RW3 slab placed at a clinical distance away from the phantom were scanned using a CT simulator. The resulting CT images were transferred via DICOM to the TPS and the density of the CT data corresponding to the thin RW3 slab was changed to 1 g/cm3. Transmitted dose maps (TDMs) in the modelled EPID were calculated by the TPS using the collapsed-cone (C-C) convolution superposition (C/S) algorithm. A 6 MV beam was used in the simulation to deliver 300 MU to the homogenous phantom using an isocentric and SSD (source-to-surface) technique. The phantom thickness was varied and the calculated TDMs in the modelled EPID were compared with corresponding measurements obtained from a calibrated scanning liquid-filled ionisation chamber (SLIC) EPID. The two TDMs were compared using the gamma evaluation technique of Low et al. The predicted and measured TDMs agree to within 2 % (averaged over all phantom thicknesses) on the central beam axis. More than 90 % of points in the dose maps (excluding field edges) produce a gamma index less than or equal to 1, for dose difference (averaged over all phantom thicknesses), and distance-to-agreement criteria of 4 %, 3.8 mm, respectively. In addition, the noise level on the central axis in the predicted dose maps is less than 0.1 %. We found that phantom thickness changes of approximately 1 mm, which correspond to dose changes on the central beam axis of less than 0.6 %, can be detected in the predicted transmitted dose distributions.     

Study of X-ray field junction dose using an a-Si electronic portal imaging device

Field junctions between megavoltage photon beams are important in modern radiotherapy for treatments such as head and neck and breast cancer. An electronic portal imaging device (EPID) may be used to study junction dose between two megavoltage X-ray fields. In this study, the junction dose was used to determine machine characteristics such as jaw positions and their reproducibility, collimator rotation and the effect of gantry rotation. All measurements were done on Varian linear accelerators with EPID (Varian, Palo Alto, CA). The results show reproducibility in jaw positions of approximately 0.3 mm for repeated jaw placement while EPID readings were reproducible within a standard deviation of 0.4% for fixed jaw positions. Junction dose also allowed collimator rotation error of 0.1° to be observed. Dependence of junction dose on gantry rotation due to gravity was observed; the gravity effect being maximum at 180° gantry angle (beam pointing up). EPIDs were found to be reliable tools for checking field junctions, which in turn may be used to check jaw reproducibility and collimator rotation of linacs.     

Mapcheck在调强放疗验证中的应用

目的：研究二维半导体剂量矩阵（Mapcheck）的剂量学特性及其在调强验证中的应用。方法：在6MVX射线条件下检测Mapcheck的剂量重复性、线性等基本特性,并用Mapcheck验证头颈部肿瘤的调强放疗计划,比较绝对剂量和相对剂量与治疗计划系统剂量分布差异。再通过电离室,将Mapcheck测量点的绝对剂量与计划计算进行比较。结果：Mapcheck在高能光子照射下剂量重复性和线性均满足临床要求,而Mapcheck所测的绝对剂量和相对剂量分布,与计划剂量分布的比较显示,也在可接受的误差范围内。结论：Mapcheck提供了绝对剂量和相对剂量的验证,测量方法不复杂,是调强放疗很好的验证工具。     

多层放射性铬胶片剂量验证系统的研制

目的：采用放射性铬胶片（RCF）快速准确地验证调强放射治疗（IMRT）形成的复杂剂量分布,研制多层RCF剂量验证系统。方法：以RCF为载体,利用测量模体、RCF、胶片扫描仪及验证软件的剂量验证系统实施测量及分析;模体模拟人体外形,包含多种专用模块,软件包含二维、三维无标记点配准及验证分析功能。结果：模体凸凹槽结构结合软件无标记点自动定位功能,可快速完成胶片的固定及其与计划数据的配准以及RCF的免冲洗自动显影,减少了调强放射治疗测量和分析的不确定因素,减少物理师工作量。患者调强验证以剂量偏差3%和3 mm距离偏差为控制标准,6个临床病例的伽马分析通过率均＞90%。结论：多层RCF剂量验证系统是调强适形放射治疗剂量验证和常规质量保证的多用途工具,具有方便、准确、真实、海量信息及多用途等特点,可用于对直线加速器、伽玛刀、射波刀、后装机及粒子植入等放射治疗设备质量验证和调强治疗患者剂量的二维、三维验证。     

电子射野影像系统的质量保证及应用

目的 详述电子影像射野系统(EPID)的质量保证和质量控制,让读者全面系统地掌握EPID质量保证和质量控制的方法;通过EPID在放射治疗中的应用,保障患者治疗的准确性,从而得到精确放疗。方法 从EPID位置设置、EPID采集系统验证、门户剂量测定法选择验证、IsoCal选项验证方面全面系统介绍EPID质量保证和质量控制的方法。从位置验证、剂量验证、加速器的常规质量保证工作方面介绍EPID在放射治疗中的应用。结果 让读者了解了怎样全面系统地对EPID进行质量保证和质量控制;怎样利用EPID进行位置验证、剂量验证、加速器的常规质量保证工作。结论 充分了解EPID的质量保证和质量控制方法,严谨的质量保证措施可以保证EPID功能的充分发挥,在放射治疗中合理利用EPID,可确保患者治疗的准确,使精确放射治疗有的放矢。     

Plan evaluation and dosimetric comparison of IMRT using AAPM TG119 test suites and recommendations

In order to verify intensity modulated radiotherapy quality assurance procedure and to establish the practical base line commissioning, American Association of Physicists in Medicine-Task Group 119 test suite DICOM-RT images and structure were downloaded for planning and dosimetric comparison. The square slab phantom of water equivalent plastic was used for the measurement. This phantom can permit point dose measurement with ionization chamber by placing the chamber at 7.5 cm depth in the slab phantom. The planar dose measurements were carried out by positioning the Matrixx detector at 10 cm depth. The planning and measurements were performed as per AAPM TG119 guidelines. The test suite includes AP:PA field, band test, multitarget, prostate, head and neck and C-shape. The ion chamber measurements were within 3% of the planned dose for target and avoidance structure region. The ion chamber measurement results are in good agreement with the TG119 recommendation of ±3% for all the test suites. The planar dose measurements were performed with Matrixx for individual fields at the planned gantry angle. The results show that the pass criteria for γ ≤ 1 were between 93 to 97% for all the test cases. Our results are in good agreement with the TG119 recommendation. The present study aimed to compare the measured dose with the planned dose using computer planning system. The test suites were used to assess the planning and delivery systems so as to provide the basis for IMRT commissioning and QA.     

鼻咽癌调强放疗计划的剂量学比较研究

目的：分析鼻咽癌调强放疗（IMRT）不同计划中靶区和周围正常组织器官的剂量学特点。方法：对25例鼻咽癌患者进行动态调强放疗,针对每例患者按照不同方案设计A、B、C3个计划。方案A：5野均匀分布;方案B：前5野（每30°为1野）;方案C：非共面5野（床角90°,机架330°和30°）;所有计划均用同样的物理目标函数和约束条件进行优化,分别比较3个计划的靶区和危机器官剂量分布、剂量体积直方图（DVH）、靶区适形度指数（CI）等指标。结果：3个计划的靶区剂量分布情况基本相同;方案B比方案A、C分别降低了颞叶D5剂量的3.5%、0.86%,脑干D5剂量的2.8%、0.14%;方案A、C比方案B降低了口腔黏膜平均剂量Dmean的31.3%和21.3%;同时在后颈软组织处的剂量A方案大于B、C方案。结论：调强放疗5野可以满足鼻咽癌靶区的剂量要求,应根据靶区和危机器官位置及体积的不同,采用最优的计划设计方式,提高治疗增益比。     

A study into the relationship between the measured penumbra and effective source size in the modeling of the Pinnacle RTPS

The effect of detector size in the broadening of the penumbra on the model in the Pinnacle RTPS is investigated. A second order polynomial was devised to correlate the source size parameter with the RTPS-calculated penumbra. The optimal source size parameter was calculated for penumbra measurements based on the diamond detector and a standard ionization chamber (IC). This work was done for Jaw fields, MLC fields with a leaf end radius of 8 cm, and MLC fields with a leaf end radius of 12 cm. The optimum source size of the 8 cm MLC fields matched the jaw fields, and an average (based on field sizes studied) of 1.1 mm for the diamond detector data and 2.4 mm for the ionization chamber was established. The effect of this overestimation of the source size parameter based on detector-induced penumbra broadening was considered for a clinical IMRT prostate plan by using two models (diamond and IC). There were differences in the DVH of the PTV and of OARs but these effects were of negligible clinical significance. Dose difference distributions showed dose difference areas to be in penumbra regions of the segments, with larger dose differences where penumbras intersected and/or there was a significant weighting on the segment. Gamma analysis was also performed between the two plans, and was found to increase the amount of fail rates significantly for both 2%/2 mm and 3%/3 mm criteria. This decreases the sensitivity of IMRT QA in the detection of systematic errors.     

乳腺癌保乳术后动态调强放疗与常规放疗的剂量学比较

目的 比较早期乳腺癌保乳术后动态调强放疗(intensity modulated radiotherapy,IMRT)与常规放疗(conventional radiotherapy,CRT)的剂量学分布,为IMRT在乳腺癌保乳术后放疗中的应用提供依据。方法 选取12例保乳术后乳腺癌患者,针对每个患者分别设计IMRT计划和CRT计划,处方剂量均为50 Gy/25次,利用剂量体积直方图(DVH图)比较两种计划的靶区和危及器官的剂量学差异。结果 两种计划中95%等剂量曲线包绕计划靶区的体积相似,分别为98.22%和98.76%(P > 0.05);IMRT计划计划靶区的V_105%、V_110%分别为12.86%、1.02%低于CRT计划的44.56%、6.37%(P < 0.05);IMRT计划计划靶区的CI值由0.55提高至0.68,HI值由1.15降至1.09(P < 0.05)。与CRT计划相比,IMRT计划患侧肺V₃₀、V₂₀分别降低了3.68%和5.75%,同时肺平均剂量(MLD)由9.24 Gy下降至7.12 Gy(P < 0.05);IMRT计划将左侧乳腺癌患者心脏V₃₀、V₄₀和V₅₀分别降低了5.99%、5.68%和1.68%(P < 0.05)。结论 IMRT相比CRT在保证靶区覆盖率的同时,减少了高剂量的照射,改善了靶区剂量分布的均匀性,同时降低了周围正常器官的照射剂量。     

Assessment of flatness and symmetry of megavoltage x-ray beam with an electronic portal imaging device (EPID)

The input/output characteristics of the Wellhofer BIS 710 electronic portal imaging device (EPID) have been investigated to establish its efficacy for periodic quality assurance (QA) applications. Calibration curves have been determined for the energy fluence incident on the detector versus the pixel values. The effect of the charge coupled device (CCD) camera sampling time and beam parameters (such as beam field size, dose rate, photon energy) on the calibration have been investigated for a region of interest (ROI) around the central beam axis. The results demonstrate that the pixel output is a linear function of the incident exposure, as expected for a video-based electronic portal imaging system. The field size effects of the BIS 710 are similar to that of an ion chamber for smaller field sizes up to 10 x 10 cm2. However, for larger field sizes the pixel value increases more rapidly. Furthermore, the system is slightly sensitive to dose rate and is also energy dependent The BIS 710 has been used in the current study to develop a QA procedure for measurements of flatness and symmetry of a linac x-ray beam. As a two-dimensional image of the radiation field is obtained from a single exposure of the BIS 710, a technique has been developed to calculate flatness and symmetry from a defined radiation area. The flatness and symmetry values obtained are different from those calculated conventionally from major axes only (inplane, crossplane). This demonstrates that the technique can pick up the "cold" and "hot" spots in the analysed area, providing thus more information about the radiation beam. When calibrated against the water tank measurements, the BIS 710 can be used as a secondary device to monitor the x-ray beam flatness and symmetry.     

两种算法在非小细胞肺癌调强放疗中的剂量学比较

目的 比较两种算法在非小细胞肺癌（NSCLC）调强放疗（IMRT）中的剂量学差异,为非小细胞肺癌IMRT计划设计提供数据参考。方法 选择确诊为NSCLC的患者10例,分别制定采用Sliding Wnd（SLD）和Smart Sequence（SMT）算法的IMRT计划,比较两种算法在IMRT计划中的剂量分布、机器跳数、治疗时间以及二维剂量验证结果。结果 SLD算法的计划靶区D_2%、D_98%、D_mean、CI、HI值均优于SMT算法（P<0.05）,差异有统计学意义;SLD算法双肺的D_mean、V₂₀、V₃₀、V₄₀、V₅₀均优于SMT算法,差异有统计学意义;心脏、脊髓等其它参数相差不大,差异无统计学意义。SMT算法的机器跳数与SLD算法相比减少了48 MU,治疗时间减短了7.2 s,差异有统计学意义。SLD算法和SMT算法的IMRT计划二维剂量验证3%/3 mm的γ通过率分别为（99.2±0.5）%和（99.3±0.6）%,2%/2 mm的γ通过率分别为（95.1±1.5）%和（96.4±1.8）%（P>0.05）,差异无统计学意义。结论 两种算法的靶区剂量分布、危及器官受量以及二维剂量验证通过率均能满足临床要求,可应用于非小细胞肺癌IMRT计划设计。SLD算法的靶区剂量分布以及对双肺的保护均优于SMT算法,在NSCLC调强放疗中可优先考虑SLD算法。     

Depth scaling of solid phantom for intensity modulated radiotherapy beams

To reduce the uncertainty of absorbed dose for high energy photon beams, water has been chosen as a reference material by the dosimetry protocols. However, solid phantoms are used as media for absolute dose verification of intensity modulated radiotherapy (IMRT). For the absorbed dose measurement, the fluence scaling factor is used for converting an ionization chamber reading in a solid phantom to absorbed dose to water. Furthermore the depth scaling factor is indispensable in determining the fluence scaling factor. For IMRT beams, a photon energy spectrum is varied by transmitting through a multileaf collimator and attenuating in media. However, the effects of spectral variations on depth scaling have not been clarified yet. In this study, variations of photon energy spectra were determined using the EGS Monte Carlo simulation. The depth scaling factors for commercially available solid phantoms were determined from effective mass attenuation coefficients using photon energy spectra. The results clarified the effect of spectral variation on the depth scaling and produced an accurate scaling method for IMRT beams.     

Dosimetric impact of systematic MLC positional errors on step and shoot IMRT for prostate cancer: a planning study

The positional accuracy of multileaf collimators (MLC) is crucial in ensuring precise delivery of intensity-modulated radiotherapy (IMRT). The aim of this planning study was to investigate the dosimetric impact of systematic MLC positional errors on step and shoot IMRT of prostate cancer. A total of 12 perturbations of MLC leaf banks were introduced to six prostate IMRT treatment plans to simulate MLC systematic positional errors. Dose volume histograms (DVHs) were generated for the extraction of dose endpoint parameters. Plans were evaluated in terms of changes to the defined endpoint dose parameters, conformity index (CI) and healthy tissue avoidance (HTA) to planning target volume (PTV), rectum and bladder. Negative perturbations of MLC had been found to produce greater changes to endpoint dose parameters than positive perturbations of MLC (p < 0.01). Negative and positive asynchronised MLC perturbations of −1 mm resulted in median changes in D₉₅ of −1.2 and 0.9% respectively. Negative and positive synchronised MLC perturbations of 1 mm in one direction resulted in median changes in D₉₅ of −2.3 and 1.8% respectively. Doses to rectum were generally more sensitive to systematic MLC errors compared to bladder (p < 0.01). Negative and positive synchronised MLC perturbations of 1 mm in one direction resulted in median changes in endpoint dose parameters of rectum and bladder from 1.0 to 2.5%. Maximum reduction of −4.4 and −7.3% were recorded for conformity index (CI) and healthy tissue avoidance (HTA) respectively due to synchronised MLC perturbation of 1 mm. MLC errors resulted in dosimetric changes in IMRT plans for prostate.     

Evaluation of triple channel correction acquisition method for radiochromic film dosimetry

The purpose of this study was to evaluate the triple channel correction acquisition (TCCA) method for radiochromic film dosimetry performed with a flatbed scanner. The study had two parts: a fundamental and a clinical examination. In the fundamental examination, we evaluated the accuracy of calibration curves for Gafchromic EBT2 (EBT2). The films were calibrated using a field-by-field method with 13 dose steps. Seven calibration curves obtained by TCCA were compared with those produced by a single channel acquisition (SCA) method. For the clinical examination, we compared relative dose distributions obtained by TCCA and SCA for four cases of intensity-modulated radiation therapy (IMRT) and intensity-modulated arc therapy (IMAT). The fundamental examination showed that the consistency of the calibration curves was better for TCCA than for SCA, particularly for the dose range between 0.25 Gy and 1.00 Gy. The clinical examination showed that the dose differences between the measured and calculated doses in high-gradient regions were smaller with TCCA than with SCA. The average pass rates in gamma analysis for the TCCA and SCA methods were 97.2 ± 0.8% (n = 20) and 93.0 ± 1.2% (n = 20), respectively. In conclusion, TCCA can acquire accurate average dose values when creating the calibration curve. The potential advantage of TCCA for EBT2 film dosimetry was seen in high-gradient regions in clinically relevant IMRT and IMAT cases. TCCA is useful to verify dose distribution.