Modelling the dynamic dose response of an nMAG polymer gel dosimeter

Gel dosimetry measures the absorbed radiation dose with high spatial resolution in 3D. However, recently published data show that the response of metacrylic-based polymer gels depends on the segmented delivery pattern, which could potentially be a considerable disadvantage for measurements of modern dynamic radiotherapy techniques. The aim of this study is to design a dynamic compartment model for the response of a gel dosimeter, exposed to an arbitrary irradiation pattern (segmented delivery and intensity modulation), in order to evaluate the associated effects on absorbed dose measurements. The model is based on the separation of the protons affecting the magnetic resonance signal (i.e. the R2 value) into six compartments, described by a set of differential equations. The model is used to calculate R2 values for a number of different segmented delivery patterns between 0-4 Gy over 1-33 fractions. Very good agreement is found between calculated and measured R2 values, with an average difference of 0.3 ± 1.1% (1 SD). The model is also used to predict the behaviour of a gel dosimeter exposed to irradiation according to typical IMRT, VMAT and respiratory gating scenarios. The calculated R2 values are approximately independent of the segmented delivery, given that the same total dose is delivered during the same total time. It is concluded that this study helps to improve the theoretical understanding of the dependence of metacrylic-based polymer gel response to segmented radiation delivery.     

Performance of a commercial optical CT scanner and polymer gel dosimeters for 3-D dose verification

Performance analysis of a commercial three-dimensional (3-D) dose mapping system based on optical CT scanning of polymer gels is presented. The system consists of BANG 3 polymer gels (MGS Research, Inc., Madison, CT), OCTOPUS laser CT scanner (MGS Research, Inc., Madison, CT), and an in-house developed software for optical CT image reconstruction and 3-D dose distribution comparison between the gel, film measurements and the radiation therapy treatment plans. Various sources of image noise (digitization, electronic, optical, and mechanical) generated by the scanner as well as optical uniformity of the polymer gel are analyzed. The performance of the scanner is further evaluated in terms of the reproducibility of the data acquisition process, the uncertainties at different levels of reconstructed optical density per unit length and the effects of scanning parameters. It is demonstrated that for BANG 3 gel phantoms held in cylindrical plastic containers, the relative dose distribution can be reproduced by the scanner with an overall uncertainty of about 3% within approximately 75% of the radius of the container. In regions located closer to the container wall, however, the scanner generates erroneous optical density values that arise from the reflection and refraction of the laser rays at the interface between the gel and the container. The analysis of the accuracy of the polymer gel dosimeter is exemplified by the comparison of the gel/OCT-derived dose distributions with those from film measurements and a commercial treatment planning system (Cadplan, Varian Corporation, Palo Alto, CA) for a 6 cm x 6 cm single field of 6 MV x rays and a 3-D conformal radiotherapy (3DCRT) plan. The gel measurements agree with the treatment plans and the film measurements within the "3%-or-2 mm" criterion throughout the usable, artifact-free central region of the gel volume. Discrepancies among the three data sets are analyzed.     

Initial evaluation of commercial optical CT-based 3D gel dosimeter

We evaluated the OCTOPUS-ONE research laser CT scanner developed and manufactured by MGS Research, Inc. (Madison, CT). The scanner is designed for imaging 3D optical density distributions in BANG gels. The scanner operates in a translate-rotate configuration with a single scanning laser beam. The rotating cylindrical gel phantom is immersed in a refractive index matching solution and positioned at the center of a square tank made of plastic and glass. A stationary polarized He-Ne laser beam (633 nm) is reflected from a mirror moving parallel to the tank wall and scans the gel. Another mirror moves synchronously along the opposite side of the tank and collects the transmitted light and sends it to a single stationary silicon photodetector. A filtered backprojection algorithm is used to reconstruct projection data in a plane. The laser-mirrors-detector assembly is mounted on a horizontal platform that moves vertically for slice selection. We have tested the mechanical and optical setup, projection centering on the axis of rotation, linearity, and spatial resolution. We found the optical detector to respond linearly to transmitted light from control samples. The spatial resolution of the scanner was determined by employing a split field resolution technique. We obtained the horizontal and vertical full widths at half maxima of the laser beam intensity profiles as 0.6 and 0.8 mm, respectively. Dose calibration tests of the gel were performed using a nine-field (2 x 2 cm2 each) dose pattern irradiated at different dose levels. Finally, we compared gel-derived 2D planar dose distribution against radiochromic film measured dose distribution for both the nine-field and a uniform 5 x 5 cm2 field of 6 MV x rays. Very similar dose distributions were observed in gel and radiochromic film except in regions of steep dose gradient and highest dose. A dose normalization of 15.6% was required between the two dosimeters due to differences in overall radiation response. After normalization, analysis using the gamma evaluation showed that the radiochromic film and gel-measured dose distributions differed by a maximum gamma of 1.3 using 5% and 1.5 mm dose difference and distance-to-agreement criteria. The optical CT scanner has great potential as a 3D dosimeter, but a few refinements and further testing are necessary before its routine clinical use.     

基于3D打印的胸部个体化剂量验证体模设计

目的:为更精确地对治疗计划进行验证,利用3D打印设计出反映患者真实情况的个体化体模。方法:依据病人定位时的CT图像重建并进行3D打印得到患者的三维立体结构空壳,然后以CT值为依据选择各组织的辐射等效材料完成填充,即得到体现病人特征的个体化剂量验证体模。结果:将合成的等效材料行CT扫描,骨组织、肺组织、软组织、肿瘤CT值分别为1 100、-747.6、-22、-471 HU,误差均小于22%。结论:最终设计的胸部体模能够较准确地体现个体之间的差异,等效材料辐射等效性较好,可用于实际剂量验证。     

Three-dimensional dose verification for intensity modulated radiation therapy using optical CT based polymer gel dosimetry

Dose distributions generated from intensity-modulated-radiation-therapy (IMRT) treatment planning present high dose gradient regions in the boundaries between the target and the surrounding critical organs. Dose accuracy in these areas can be critical, and may affect the treatment. With the increasing use of IMRT in radiotherapy, there is an increased need for a dosimeter that allows for accurate determination of three-dimensional (3D) dose distributions with high spatial resolution. In this study, polymer gel dosimetry and an optical CT scanner have been employed to implement 3D dose verification for IMRT. A plastic cylinder of 17 cm diameter and 12 cm height, filled with BANG3 polymer gels (MGS Research, Inc., Madison, CT) and modified to optimal dose-response characteristics, was used for IMRT dose verification. The cylindrical gel phantom was immersed in a 24 x 24 x 20 cm water tank for an IMRT irradiation. The irradiated gel sample was then scanned with an optical CT scanner (MGS Research Inc., Madison, CT) utilizing a single He-Ne laser beam and a single photodiode detector. Similar to the x-ray CT process, filtered back-projection was used to reconstruct the 3D dose distribution. The dose distributions measured from the gel were compared with those from the IMRT treatment planning system. For comparative dosimetry, a solid water phantom of 24 x 24 x 20 cm, having the same geometry as the water tank for the gel phantom, was used for EDR2 film and ion chamber measurements. Root mean square (rms) deviations for both dose difference and distance-to-agreement (DTA) were used in three-dimensional analysis of the dose distribution comparison between treatment planning calculations and the gel measurement. Comparison of planar dose distributions among gel dosimeter, film, and the treatment planning system showed that the isodose lines were in good agreement on selected planes in axial, coronal, and sagittal orientations. Absolute point-dose verification was performed with ion chamber measurements at four different points, varying from 48% to 110% of the prescribed dose. The measured and calculated doses were found to agree to within 4.2% at all measurement points. For the comparison between the gel measurement and treatment planning calculations, rms deviations were 2%-6% for dose difference and 1-3 mm for DTA, at 60%-110% doses levels. The results from this study show that optical CT based polymer gel dosimetry has the potential to provide a high resolution, accurate, three-dimensional tool for IMRT dose distribution verification.     

An experimental study of the dose response of polymer gel dosimeters imaged with x-ray computed tomography.

Changes in the linear attenuation coefficient of polymer gel dosimeters post-irradiation enable the imaging of dose distributions by x-ray computed tomography (CT). Various compositions of polymer gel dosimeters manufactured from acrylamide (AA), and N,N'-methylene-bis-acrylamide (BIS) comonomers and gelatin or agarose gelling agents were investigated. This work shows that increasing the comonomer concentration increases the CT-dose sensitivity of the polymer gel dosimeter. This can be further increased by replacing gelatin with agarose. Varying the gelatin concentration however does not significantly change the CT-dose sensitivity. Among the compositions studied, dose resolution (D(delta)95%) was found to be optimal for polymer gel dosimeters comprising 5% gelatin, 3% AA, 3% BIS and 89% water.     

Heterogeneity phantoms for visualization of 3D dose distributions by MRI-based polymer gel dosimetry

Heterogeneity corrections in dose calculations are necessary for radiation therapy treatment plans. Dosimetric measurements of the heterogeneity effects are hampered if the detectors are large and their radiological characteristics are not equivalent to water. Gel dosimetry can solve these problems. Furthermore, it provides three-dimensional (3D) dose distributions. We used a cylindrical phantom filled with BANG-3 polymer gel to measure 3D dose distributions in heterogeneous media. The phantom has a cavity, in which water-equivalent or bone-like solid blocks can be inserted. The irradiated phantom was scanned with an magnetic resonance imaging (MRI) scanner. Dose distributions were obtained by calibrating the polymer gel for a relationship between the absorbed dose and the spin-spin relaxation rate of the magnetic resistance (MR) signal. To study dose distributions we had to analyze MR imaging artifacts. This was done in three ways: comparison of a measured dose distribution in a simulated homogeneous phantom with a reference dose distribution, comparison of a sagittally scanned image with a sagittal image reconstructed from axially scanned data, and coregistration of MR and computed-tomography images. We found that the MRI artifacts cause a geometrical distortion of less than 2 mm and less than 10% change in the dose around solid inserts. With these limitations in mind we could make some qualitative measurements. Particularly we observed clear differences between the measured dose distributions around an air-gap and around bone-like material for a 6 MV photon beam. In conclusion, the gel dosimetry has the potential to qualitatively characterize the dose distributions near heterogeneities in 3D.     

Cosolvent-free polymer gel dosimeters with improved dose sensitivity and resolution for x-ray CT dose response

This study reports new N-isopropylacrylamide (NIPAM) polymer gel recipes with increased dose sensitivity and improved dose resolution for x-ray CT readout. NIPAM can be used to increase the solubility of N, N'-methylenebisacrylamide (Bis) in aqueous solutions from approximately 3% to 5.5% by weight, enabling the manufacture of dosimeters containing up to 19.5%T, which is the total concentration of NIPAM and Bis by weight. Gelatin is shown to have a mild influence on dose sensitivity when gels are imaged using x-ray CT, and a stronger influence when gels are imaged optically. Phantoms that contain only 3% gelatin and 5 mM tetrakis hydroxymethyl phosphonium chloride are sufficiently stiff for dosimetry applications. The best cosolvent-free gel formulation has a dose sensitivity in the linear range (~0.88 H Gy(-1)) that is a small improvement compared to the best NIPAM-based gels that incorporate isopropanol as a cosolvent (~0.80 H Gy(-1)). This new gel formulation results in enhanced dose resolution (~0.052 Gy) for x-ray CT readout, making clinical applications of this imaging modality more feasible.     

Influence of phantom diameter,kVp and scan mode upon computed tomography dose index

The computed tomography (CT) radiation dose to pediatric patients has received considerable attention recently. Moreover, it is important to be able to determine CT radiation doses for various patient sizes ranging from infants to large adults. The current AAPM protocol only measures CT radiation dose using a 16 cm acrylic phantom to represent an adult head and a 32 cm acrylic phantom to represent an adult body. The goal of this paper is to study the dependence of the computed tomography dose index (CTDI) upon the size of the phantom, the kVp selected and the scan mode employed. Our measurements were done on phantom sizes ranging from 6 cm to 32 cm. The x-ray tube potential ranged from 80 to 140 kVp. The scan modes utilized for the measurements included: consecutive axial scans, single-slice helical scans with variable pitch and multislice helical scans with variable pitch. The results were consolidated into simplified equations which related the phantom diameter and kVp to the measured CTDI. Some generalizations were made about the relationship between the scan modes of the various CT units to the measured radiation doses. The CTDI appears to be an exponential function of phantom diameter. For the same kVp and mAs, the radiation doses for smaller phantoms are much greater than for larger sizes. The derived relationship can be used to estimate the radiation doses for a variety of scan conditions and modes from measurements with the two standard reference phantoms. A method was also given for converting axial CT dose measurements to appropriate MSAD values for helical CT scans.     

Preliminary study on the use of an inhomogeneous anthropomorphic Fricke gel phantom and 3D magnetic resonance dosimetry for verification of IMRT treatment plans

An inhomogeneous anthropomorphic phantom of the human thorax including lungs and spine was developed for verification of three-dimensional (3D) intensity-modulated radiotherapy (IMRT). The phantom and spinal cord were filled with undiluted Fricke gel, whereas the lungs were filled with a special low-density Fricke gel. Based on a computed tomography scan of the phantom, an intensity-modulated stereotactic radiotherapy plan for a bronchial carcinoma was calculated using an inverse planning system (KonRad, DKFZ Heidelberg, Germany). The plan consisted of seven beams delivered in a step and shoot technique out of 67 sub-fields. Immediately after irradiation 3D magnetic resonance (MR) imaging of the phantom was performed using a special pulse sequence for T1 relaxometry. From the MR image data maps of the longitudinal relaxation rate R1 = 1/T1 were calculated. The R1 maps were converted to dose-proportional image data and compared to planning data. Measurement and planning show good agreement in regions of standard Fricke gel with an average deviation below 5%. In regions of the low-density Fricke gel, deviations are higher due to a decreased signal-to-noise ratio in the MR measurement. In these areas also a different sensitivity of the dose response was observed as compared to standard Fricke gel. The inhomogeneous thorax phantom has proven to be a useful pre-clinical tool for 3D methodical verifications.     

动态心脏体模在心脏计算机断层扫描成像质量控制中的应用

论述了动态心脏体模作为计算机断层扫描（CT）设备心脏成像质量评价与质量保证工具的重要性;介绍了当前几类动态心脏体模在心脏CT成像质量控制中的应用进展,包括运动型、功能型和仿真型动态心脏体模。分析了现有动态心脏体模在解剖结构、组织材料和运动特性的不足,并提出利用等效材料制作四腔室心脏体模和基于容积-时间曲线控制模拟心脏全周期运动的可行性,以探究研制一种“结构仿生、运动仿真”动态心脏体模的可行性,从而为建立心脏CT成像质量定量评价标准提供理论依据和技术支撑。     

Initial investigation of a novel light-scattering gel phantom for evaluation of optical CT scanners for radiotherapy gel dosimetry

There is a need for stable gel materials for phantoms used to validate optical computerized tomography (CT) scanners used in conjunction with radiation-induced polymerizing gel dosimeters. Phantoms based on addition of light-absorbing dyes to gelatine to simulate gel dosimeters have been employed. However, to more accurately simulate polymerizing gels one requires phantoms that employ light-scattering colloidal suspensions added to the gel. In this paper, we present the initial results of using an optical CT scanner to evaluate a novel phantom in which radiation-exposed polymer gels are simulated by the addition of colloidal suspensions of varying turbidity. The phantom may be useful as a calibration transfer standard for polymer gel dosimeters. The tests reveal some phenomena peculiar to light-scattering gels that need to be taken into account when calibrating polymer gel dosimeters.     

A deformable phantom for 4D radiotherapy verification: design and image registration evaluation

Motion of thoracic tumors with respiration presents a challenge for three-dimensional (3D) conformal radiation therapy treatment. Validation of techniques aimed at measuring and minimizing the effects of respiratory motion requires a realistic deformable phantom for use as a gold standard. The purpose of this study was to develop and study the characteristics of a reproducible, tissue equivalent, deformable lung phantom. The phantom consists of a Lucite cylinder filled with water containing a latex balloon stuffed with dampened natural sponges. The balloon is attached to a piston that mimics the human diaphragm. Nylon wires and Lucite beads, emulating vascular and bronchial bifurcations, were uniformly glued at various locations throughout the sponges. The phantom is capable of simulating programmed irregular breathing patterns with varying periods and amplitudes. A tissue equivalent tumor, suitable for holding radiochromic film for dose measurements was embedded in the sponge. To assess phantom motion, eight 3D computed tomography data sets of the static phantom were acquired for eight equally spaced positions of the piston. The 3D trajectories of 12 manually chosen point landmarks and the tumor center-of-mass were studied. Motion reproducibility tests of the deformed phantom were established on seven repeat scans of three different states of compression. Deformable image registration (DIR) of the extreme breathing phases was performed. The accuracy of the DIR was evaluated by visual inspection of image overlays and quantified by the distance-to-agreement (DTA) of manually chosen point landmarks and triangulated surfaces obtained from 3D contoured structures. In initial tests of the phantom, a 20-mm excursion of the piston resulted in deformations of the balloon of 20 mm superior-inferior, 4 mm anterior-posterior, and 5 mm left-right. The change in the phantom mean lung density ranged from 0.24 (0.12 SD) g/cm3 at peak exhale to 0.19 (0.12 SD) g/cm3 at peak inhale. The SI displacement of the landmarks varied between 94% and 3% of the piston excursion for positions closer and farther away from the piston, respectively. The reproducibility of the phantom deformation was within the image resolution (0.7 x 0.7 x 1.25 mm3). Vector average registration accuracy based on point landmarks was found to be 0.5 (0.4 SD) mm. The tumor and lung mean 3D DTA obtained from triangulated surfaces were 0.4 (0.1 SD) mm and 1.0 (0.8 SD) mm, respectively. This phantom is capable of reproducibly emulating the physically realistic lung features and deformations and has a wide range of potential applications, including four-dimensional (4D) imaging, evaluation of deformable registration accuracy, 4D planning and dose delivery.     

Validation and application of polymer gel dosimetry for the dose verification of an intensity-modulated arc therapy (IMAT) treatment

Polymer gel dosimetry was used to assess an intensity-modulated arc therapy (IMAT) treatment for whole abdominopelvic radiotherapy. Prior to the actual dosimetry experiment, a uniformity study on an unirradiated anthropomorphic phantom was carried out. A correction was performed to minimize deviations in the R2 maps due to radiofrequency non-uniformities. In addition, compensation strategies were implemented to limit R2 deviations caused by temperature drift during scanning. Inter- and intra-slice R2 deviations in the phantom were thereby significantly reduced. This was verified in an investigative study where the same phantom was irradiated with two rectangular superimposed beams: structural deviations between gel measurements and computational results remained below 3% outside high dose gradient regions; the spatial shift in those regions was within 2.5 mm. When comparing gel measurements with computational results for the IMAT treatment, dose deviations were noted in the liver and right kidney, but the dose-volume constraints were met. Root-mean-square differences between both dose distributions were within 5% with spatial deviations not more than 2.5 mm. Dose fluctuations due to gantry angle discretization in the dose computation algorithm were particularly noticeable in the low-dose region.     

3D dose verification in 192Ir HDR prostate monotherapy using polymer gels and MRI

VIPAR polymer gels and 3D MRI techniques were evaluated for their ability to provide experimental verification of 3D dose distributions in a simulation of a 192Ir prostate monotherapy clinical application. A real clinical treatment plan was utilized, generated by post-irradiation, CT based calculations derived from Plato BPS and Swift treatment planning systems. The simulated treatment plan involved the use of 10 catheters and 39 source positions within a glass vessel of appropriate dimensions, homogeneously filled with the VIPAR gel. 3D high resolution MR scanning of the gel produced T2 relaxation time maps, from which 3D dose distributions were derived via an appropriate calibration procedure. Results were compared to corresponding dose distributions obtained from the Plato and Swift treatment planning systems. Quantitative comparison, on a point by point basis, was based on user adopted acceptance criteria of 5% dose-difference and 3 mm distance-to-agreement. Significant deviations between experimental and calculated dose distributions were found for doses lower than 50% due to the reduced dose resolution of the method in the low dose, low dose gradient region. Measurement errors were observed at 1.0-1.5 mm around each catheter due to MR imaging susceptibility artifacts. For most remaining points the acceptance criteria were fulfilled. Systematic offsets of the order of 1-2 mm, observed between measured and corresponding calculated isocontours at specific segments, are attributed to the 1 mm uncertainty in catheter reconstruction and 1 mm uncertainty in the alignment of the MR and CT imaging planes.     

Focusing optics of a parallel beam CCD optical tomography apparatus for 3D radiation gel dosimetry

Optical tomography of gel dosimeters is a promising and cost-effective avenue for quality control of radiotherapy treatments such as intensity-modulated radiotherapy (IMRT). Systems based on a laser coupled to a photodiode have so far shown the best results within the context of optical scanning of radiosensitive gels, but are very slow ( approximately 9 min per slice) and poorly suited to measurements that require many slices. Here, we describe a fast, three-dimensional (3D) optical computed tomography (optical-CT) apparatus, based on a broad, collimated beam, obtained from a high power LED and detected by a charged coupled detector (CCD). The main advantages of such a system are (i) an acquisition speed approximately two orders of magnitude higher than a laser-based system when 3D data are required, and (ii) a greater simplicity of design. This paper advances our previous work by introducing a new design of focusing optics, which take information from a suitably positioned focal plane and project an image onto the CCD. An analysis of the ray optics is presented, which explains the roles of telecentricity, focusing, acceptance angle and depth-of-field (DOF) in the formation of projections. A discussion of the approximation involved in measuring the line integrals required for filtered backprojection reconstruction is given. Experimental results demonstrate (i) the effect on projections of changing the position of the focal plane of the apparatus, (ii) how to measure the acceptance angle of the optics, and (iii) the ability of the new scanner to image both absorbing and scattering gel phantoms. The quality of reconstructed images is very promising and suggests that the new apparatus may be useful in a clinical setting for fast and accurate 3D dosimetry.     

Verification of dynamic radiotherapy: the potential for 3D dosimetry under respiratory-like motion using polymer gel

Following the implementation of advanced treatment procedures in radiotherapy, there is a need for dynamic dose verification in 3D. Gel dosimetry could potentially be used for such measurements. However, recently published data show that certain types of gels have a dose rate and fractionation dependence. The aim of this study was to investigate the feasibility of using a polymer gel dosimeter for dose verification of dynamic radiotherapy. To investigate the influence of dose rate dependence during respiratory-like motion in and out of the beam, a respiration robot together with two types of gel systems (normoxic methacrylic acid gel (nMAG) and normoxic polyacrylamide gel (nPAG)) were used. Reference measurements were obtained using a linear diode array (LDA). Expected results, if there was no influence of the dose rate variation, were calculated by convolving the static irradiated gel data with the motion function controlling the robot. To investigate the fractionation dependence, the gels were irradiated using gated and ungated deliveries. Magnetic resonance imaging was used to evaluate the absorbed dose response of the gel. The measured gel data coincided well with the LDA data. Also, the calculated data agreed well with the measured dynamic gel data, i.e. no dose rate dependence due to motion was observed. The difference in the R2 response for the gels receiving ungated and gated, i.e. fractionated, deliveries was less than 1% for the nPAG and 4% for the nMAG, for absorbed doses up to 2 Gy. The maximum difference was 1.2% for the nPAG and 9% for the nMAG, which occurred at the highest given dose (4 Gy). The investigated gels were found to be feasible detectors for dose measurements under respiratory-like motion. For dose verification of dynamic RT involving gated delivery, e.g. breathing-adapted radiotherapy, relative absorbed dose evaluation should be used in order to minimize the effects of fractionated irradiation.     

胸段椎体转移癌放射治疗二维和三维位置验证的比较分析

目的通过VARIAN—OBI系统提供二维KV—KV和三维锥形束CT（CBCT）位置验证模式,对比分析其在胸段椎体骨转移癌的应用。找寻其最佳图像引导放射治疗（IGRT）方式。方法选择50例胸段椎体骨转移癌患者,其中男性33例,女性17例,中位年龄为57岁。随机分为A、B组,每次治疗前位置验证,A组行二维KV-KV位置验证,图像配准后记录位移偏差值。移动治疗床治疗,治疗结束后评估患者疼痛症状,按照世界卫生组织的疼痛评分标准评分;B组行三维CBCT位置验证,图像配准后,记录位移偏移值（包括旋转偏差）,移动治疗床执行治疗,并记录患者疼痛指数。统计并计算均值和标准差,对比分析两种验证方式的差异。结果A组和B组各获取125组图像,位移偏差：A组在Vertical（Vrt）、Longitudinal（Lng）、Lateral（Lat）的位移偏差分别为（0．02±0．14）cm、（0．02±0．13）cm、（-0．01±0．17）cm;B组为（0．04±0．15）cm、（0．01±0．14）cm、（-O．03±0．16）cm,两组数据比较,差异无统计学意义（P=0．642、0．549、0．996〉0．05）;疼痛指数：A组患者为2．21±0．77,B组患者为3．03±0．80（P=0〈0．05）;验证用时：二维KV—KV配准时间为（3．97±0．63）rain,三维CBCT配准时间为（8．13±0．98）min（P=0〈0．05）。结论二维KV—KV与三维CBCT位置验证在位置移动偏差值的比较无统计学意义。均能满足临床应用需求。二维KV—KV位置验证相对三维CBCT位置验证,整个验证需要时间是后者的1／2～1／3,二维KV—KV位置验证是疼痛症状明显的椎体骨转移患者的首选方式。     

三维剂量验证系统Delta4在容积旋转调强计划剂量验证中的应用

目的：探讨三维剂量验证系统Delta4对容积旋转调强计划进行剂量验证的可行性。方法：利用Delta4对20例患者的容积旋转调强计划进行剂量验证;为了对比,同时用MatriXX电离室矩阵对相同计划的冠状面进行剂量分布验证：利用剂量仪、指形电离室对相同计划进行点剂量的绝对剂量验证;将三种方法的验证结果进行比对。剂量分布验证采用Gamma分析,取剂量3％、距离3mm,通过率≥95％为验证通过标准。结果：三种验证方法所有计划的误差均在允许范围之内。使用Delta4验证治疗计划的Gamma通过率平均为98．90％,各个射野的Gamma通过率平均99．83％;用MatriXX电离室矩阵验证各计划冠状面剂量分布Gamma通过率平均为98．02％;绝对剂量验证平均误差为1．18％。结论：利用Delta4对旋转容积调强计划进行剂量验证是可行的。     

Optimization of multiple spin-echo sequences for 3D polymer gel dosimetry

The overall performance of polymer gel dosimeters for three-dimensional radiation dosimetry is determined by the temporal and spatial stability of the gels, dose sensitivity and image quality with respect to both systematic and stochastic deviations. The dose resolution (D(p)delta) is determined by the dose sensitivity and the signal-to-noise ratio (SNR) in the dose images. The dose sensitivity can be altered by changing the chemical composition of the polymer gel. The SNR is determined by the scanner and the imaging sequence. In the dose verification of conformal radiotherapy treatments the chosen number of slices may reach a number of 10-20. For these experiments, to obtain a sufficient SNR within a reasonable measurement time using a certain MR scanner, the imaging sequence should be optimized. A few other studies have emphasized the importance of optimizing the imaging sequence with respect to dose resolution (D(p)delta) or SNR but do not give quantitative values for the optimal sequence parameters for scanning a polymer gel dosimeter in three dimensions. In this paper, it is proved that a multiple spin-echo sequence is preferable to a single spin-echo sequence. It is also shown that when using a multiple spin-echo sequence it is not the inter-echo time that should be optimized but the number of echoes. An algebraical expression is derived for the dose resolution in terms of sequence parameters. A mathematical formalism and look-up tables are provided that can be used to optimize both a single and a slice-selective multiple spin-echo sequence to acquire a set of dose images at various locations. The use of the optimization protocol is illustrated by some examples. The optimization protocol enables the user to derive the optimal sequence parameters to acquire a set of dose maps obtained by quantitative T2 imaging for each polymer gel dosimeter within the shortest time possible.