宫颈癌调强放疗和三维适形放疗剂量对比研究

目的:研究宫颈癌调强放疗(IMRT)和三维适形放疗(3D-CRT)时靶区及其周围正常组织受照剂量的差异.材料方法:用拓能公司生产的WiMRT三维适形调强放疗计划系统分别进行6～9个照射角度的3D-CRT和IMRT计划设计,肿瘤量45Gy,计算出正常组织和靶区的剂量—体积直方图以及所需照射的总跳数.用Siemens生产的Primart电子直线加速器(X射线能量6MV,MLC叶片29对)实施放疗计划,测量出10 cm×10cm射野外漏射线和散射线剂量率,估算放疗时正常组织所受辐射剂量随距离的变化关系.结果:照射野数和照射角度一致,IMRT时膀胱、直肠、阴道所受平均剂量分别只有3D-CRT时的19.5％(29.3/150.3)、64.5％(538.0/833.0)和61.0％(1553.6/2546.3),靶区平均受照剂量略高于3D-CRT.IMRT病人正常组织所受散射线和漏射线剂量约为3D-CRT病人的1.5倍.结论:宫颈癌IMRT剂量分布优于3D-CRT.     

头颈部肿瘤静态调强中强度分级数的研究

目的:研究在头颈部肿瘤静态调强放疗中强度分级数对计划结果的影响,确定在临床调强放疗中最佳的强度分级参数。方法:利用商用的放疗计划系统,分别对5个临床头颈部肿瘤病例作静态调强放疗计划。在强度矩阵离散化时,设置离散化等级分别为3、4、5、6、7、8、10、15和20。比较和分析强度分级数对放疗计划结果的影响。结果:从总的趋势来看,随着强度分级数的提高,PTV的最小剂量增大,最大剂量减小,剂量标准差减小;危及器官受到的剂量照射降低;总的子野数目增加;总的机器跳数降低。结论:提高强度分级数,意味着可能提高IMRT计划的质量。综合强度分级数对PTV、OAR和治疗时间的影响,建议在计划设计时,将强度分级数控制在5~10级范围内。     

Pinnacle计划系统静态调强治疗计划导入方法的探讨

目的:探讨Pinnacle计划系统导入静态调强治疗计划的方法。材料和方法:在Matlab软件中读取XiO计划系统通过Dicom传输到Pinnacle7.6c计划系统上的调强计划DicomRT文件,利用现有公开的Pinnacle脚本语言资料,用Matlab程序生成Pinnacle计划系统静态调强治疗计划导入脚本,在Pinnacle系统中运行脚本,验证脚本是否正确导入静态调强治疗计划。结果:解决了诸如控制点的安排,治疗计划系统间坐标系的转换,加速器MLC叶片附值次序等等问题后,成功将XiO计划系统产生的静态调强计划导入Pinnacle治疗计划系统。结论:本研究显示,通过Matlab编程生成脚本的方法方法,将其他治疗计划系统产生的调强计划导入Pinnacle系统是完全可行的。     

不同能量X线胸段食管癌调强放疗计划的剂量学研究

目的:研究不同能量X射线治疗胸段食管癌调强放疗(IMRT)计划的剂量学差异。方法:选择l2例胸段食管癌患者,在ADAC Pinnacle3三维治疗计划系统(TPS)中分别采用6 MV、10 MV和15 MV X线给每位患者设计三个调强放疗计划,在规定计划靶区(PTV)至少达到95%处方剂量的前提下,根据剂量体积直方图(DVH)比较三种计划的靶区剂量分布及脊髓、肺、心脏等正常组织受照射剂量的差异。结果:三种计划中靶区的最大剂量、最小剂量、平均剂量及靶区适形度指数、均匀性指数均无明显差异,但15 MV计划高剂量覆盖程度大于6 MV和10 MV计划,脊髓、双肺及心脏受照剂量都在可耐受的范围内,差异也无统计学意义(P>0.05)。结论:6 MV、10 MV、15 MV X射线都能满足胸段食管癌临床调强放疗需求。     

影响调强放疗绝对剂量的主要因素及应对措施

目的:验证调强放射治疗的绝对剂量误差,探索影响调强放疗绝对剂量的因素及其应对措施.方法:将20例准备实施调强放疗病人的实际治疗计划,用标准水模体进行计划移植,生成验证计划并计算体模内电离室测量点的计划剂量,执行验证计划的照射,用电离室进行实际物理绝对剂量测量,计算实际测量剂量值和计划剂量值的百分相对误差.分析影响调强放疗绝对剂量误差的主要因素,采取相应改进措施,验证另80例调强放疗的绝对剂量,比较前20例与改进后80例调强放疗绝对剂量验证结果.结果:前20例调强放疗绝对剂量百分相对误差分布范围是-8.00%～5.00%,平均误差为-2.01%,标准差为3.55%.采取相应改进措施后,80例调强放疗绝对剂量百分相对误差全部在4.4%以内,分布范围缩小到-4.4%～2.5%,平均误差为-1.49%,比前20例平均误差下降25.9%,标准差为1.40%,比前20例下降60.6%.结论:分析影响调强放疗绝对剂量的因素,采取必要的应对措施,能够有效提高调强放射治疗绝对剂量的准确性.     

调强放射治疗“end to end”质量控制核查中应用针尖电离室测量小野剂量的初步研究

目的:在调强放射治疗“end to end”质量核查中,探讨应用针尖电离室对调强放射治疗小野照射进行绝对剂量测量的研究。方法:选择3省20家医院,将放有热释光剂量计TLD（距模体表面距离约7.5 cm）和胶片的国际原子能机构（IAEA）模体进行CT扫描,图像导入放射治疗计划系统（TPS）中,设计治疗计划,进行7野等中心调强照射,MLC照射野大小>2 cm×2 cm且<4 cm×4 cm。同时针尖电离室（0.015 cc）放在固体水模体距模体表面7.5 cm下进行点剂量绝对剂量验证:（1）将治疗计划中射野角度归零平移到固体水模体中进行剂量验证;（2）治疗计划射野角度不归零时为实际治疗照射方向,平移到固体水模体中进行绝对剂量验证。结果:在调强放射治疗多叶光栅小野照射的固体水模体中,用针尖电离室测量的绝对剂量与TPS计算得到的绝对剂量比较,7野照射方向归为零度时,比较偏差<5%;实际照射方向时,比较偏差<5%。验证后的计划,在IAEA模体上进行实际7野调强治疗,模体中的高剂量靶区胶片（Gafchromic EBT3 film）绝对剂量通过率均≥90%（Gamma分析:3%, 3 mm）,TLD偏差<7%。均符合IAEA提出的标准。结论:在调强放射治疗多叶光栅小野照射时,可以应用针尖电离室作为绝对剂量验证的一个方法。     

鼻咽癌动态调强与静态调强放疗的比较

目的：比较鼻咽癌动态调强与静态调强放疗计划设计与执行及剂量分布的区别：方法：记录从计划到实施的全过程以比较动态调强与静态调强实施难易程度及耗时长短。分别比较两者的靶区及危及器官的剂量分布差异。分别对两者进行剂量学验证，比较两者剂量验证结果的差异。结果：两者从计划到实施的方法、难易程度均相同：动态调强各个靶区的适形度、均匀度均优于或等于静态调强，静态调强的子野越多适形度、均匀度越好。各个危及器官受量大部分相同，但两侧腮腺平均剂量静态调强略小于动态调强，且静态调强的子野越少腮腺平均剂量越低。两者的剂量学验证结果基本一致。静态调强治疗时间略长于动态调强。结论：动态调强靶区剂量分布优于静态调强，静态调强子野越多靶区剂量分布越好。两者的危及器官受量大体相当，静态调强似乎更有利于保护腮腺等正常器官。总体上讲，鼻咽癌动态凋强放疗略优于静态调强。     

直肠癌术后容积旋转调强和静态调强放疗的剂量学研究

目的:比较直肠癌术后患者瓦里安容积旋转调强(RapidARC)和静态调强(Intensity Modulated Radiotherapy,IMRT)两种调强放疗技术的剂量学差异。方法:应用ADAC Pinnacle V9.2放疗计划系统,对20例直肠癌术后患者分别设计Rapid Arc和IMRT放射治疗计划,处方量统一为50.4 Gy分28次照射,比较这两种治疗计划中靶区和危及器官的剂量学差异,以及这两种计划的机器跳数(MU)和治疗时间。结果:两组治疗计划靶区的Dmean、Dmin、Dmax、Dmean、V95%、CI、HI相近,无统计学意义(P>0.05);膀胱和小肠的V50,小肠Dmax,以及左右股骨头的V50、Dmean两种计划比较差异无统计学意义(P>0.05);Rapid ARC计划的膀胱和小肠的Dmean、总机器跳数和治疗时间明显低于IMRT,差异有统计学意义(P<0.05)。结论 :在直肠癌术后患者的放射治疗中,RapidArc和IMRT放射治疗计划的靶区剂量相近。RapidArc减少了膀胱和小肠的平均剂量、总的MU,缩短了治疗时间。     

非共面野在脑胶质瘤调强放射治疗中的剂量学优势

目的:探讨非共面射野在脑胶质瘤调强放疗中的应用特点及剂量学优势,为非共面野应用于脑胶质瘤调强放疗临床应用提供理论依据。材料方法:对6例脑胶质瘤患者分别设计5野共面调强计划和在此之上增加一个非共面野的6野调强计划,利用剂量体积直方图评价两种计划方法的正常组织照射剂量、适形度指数以及不均匀指数。结果:6野非共面计划比5野共面计划有更好适形性和均匀性;同时减少了眼球、晶体、视神经照射剂量。结论:在脑胶质瘤的调强放疗中,加入非共面野计划比共面计划有更明显的剂量学优势。     

多叶光栅叶片宽度对鼻咽癌剂量学的影响

目的：评估加速器治疗机多叶光栅（MLC）叶片宽度在鼻咽癌调强放疗中的剂量学影响。方法：建立两个直线加速器治疗机模型：两个模型的剂量学参数完全一致,将多叶光栅宽度分别设置为10 mm和5 mm,其余机械参数也完全一致;将这两个治疗机模型应用于7例鼻咽癌患者的调强放疗计划设计,比较不同的多叶光栅叶片宽度对鼻咽癌调强放疗计划带来的剂量学影响。结果：使用10 mm MLC叶片宽度的模型和5 mm MLC叶片宽度的模型制定调强放疗计划,其脑干最大剂量分别为54.5 Gy±3.4 9 Gy和53.5 Gy±3.67 Gy,左侧视神经最大剂量分别为43.6 Gy±15.5 Gy和42.5 Gy±15.3 Gy,右侧视神经最大剂量分别为40.8 Gy±16.3 Gy和39.6 Gy±16.4 Gy,左侧颞叶大于65 Gy的体积分别为0.58 cm^3±0.57cm^3和0.48 cm^3±0.46 cm^3。对于几例脑干受侵和视神经视交叉受侵的鼻咽癌病人,使用5 mm MLC叶片宽度相比10 mm MLC叶片宽度的机器模型,能够明显的降低其对应受侵部位的高剂量,且能带来更好的靶区剂量均匀性。结论：5 mm MLC叶片宽度治疗机相对于10 mm MLC叶片宽度治疗机,更适合对危及器官有高剂量要求的鼻咽癌的放射治疗。     

Delivery time comparison for intensity-modulated radiation therapy with/without flattening filter: a planning study

The treatment delivery time of intensity-modulated radiation therapy (IMRT) with a multileaf collimator (MLC) is generally longer than that of conventional radiotherapy. In theory, removing the flattening filter from the treatment head may reduce the beam-on time by enhancing the output dose rate, and then reduce the treatment delivery time. And in practice, there is a possibility of delivering the required fluence distribution by modulating the unflattened non-uniform fluence distribution. However, the reduction of beam-on time may be discounted by the increase of leaf-travel time and (or) verification-and-recording (V&R) time. Here we investigate the overall effect of flattening filter on the treatment delivery time of IMRT with MLCs implemented in the step and shoot method, as well as with compensators on six hybrid machines. We compared the treatment delivery time with/without flattening filter for ten nasopharynx cases and ten prostate cases by observing the variations of the ratio of the beam-on time, segment number, leaf-travel time and the treatment delivery time with dose rate, leaf speed and V&R time. The results show that, without the flattening filter, the beam-on time reduces for both static MLC and compensator-based techniques: the number of segments and the leaf-travel time increase slightly for the static MLC technique; the relative IMRT treatment delivery time decreases more with lower dose rate, higher leaf speed and shorter V&R overhead time. The absolute treatment delivery time reduction depends on the fraction dose. It is not clinically significant at a fraction dose of 2 Gy for the technique of removing the flattening filter, but becomes significant when the fraction dose is as high as that for radiosurgery.     

Real-time motion-adaptive delivery (MAD) using binary MLC: II. Rotational beam (tomotherapy) delivery

Intra-fraction target motion hits the fundamental basis of IMRT where precise target positions are assumed. Real-time motion compensation is necessary to ensure that the same dose is delivered as planned. Strategies for conventional IMRT delivery for moving targets by dynamic multi-leaf collimators (MLC) tracking are well published. Binary MLC-based IMRT,such as TomoTherapy , requires synchronized motion of MLC, the couch and the gantry, which suggests a unique motion management strategy. Thanks to it sultra-fast leaf response and fast projection rate, real-time motion compensation for binary MLC-based IMRT is feasible. Topotherapy is a new IMRT delivery technique, which can be implemented in commercial helical TomoTherapy machines using only fixed gantry positions. In this paper, we present a novel approach for TopoTherapy delivery that adjusts for moving targets without additional hardware and control requirement. This technique uses the planned leaf sequence but rearranges the projection and leaf indices. It does not involve time-consuming operations, such as reoptimization. Unlike gating or breath hold-based methods, this technique can achieve nearly a 100% duty cycle with little breath control. Unlike dynamic MLC-based tracking methods, this technique requires neither the whole target motion trajectory nor the velocity of target motion. Instead, it only requires instantaneous target positions, which greatly simplifies the system implementation. Extensive simulations, including the worst-case scenarios, validated the presented technique to be applicable to relatively regular or mild irregular respirations. The delivered dose conforms well to the target, and significant margin reduction can be achieved provided that accurate, real-time tumor localization is available.     

Enhancement of IMRT delivery through MLC rotation

Multileaf collimator (MLC) based intensity modulated radiation therapy (IMRT) techniques are well established but suffer several physical limitations. Dosimetric spatial resolution is limited by the MLC leaf width; interleaf leakage and tongue-and-groove effects degrade dosimetric accuracy and the range of leaf motion limits the maximum deliverable field size. Collimator rotation is used in standard radiation therapy to improve the conformity of the MLC shape to the target volume. Except for opposed orthogonal fields, collimator rotation has not been exploited in IMRT due to the complexity of deriving the MLC leaf configurations for rotated sub-fields. Here we report on a new way that MLC-based IMRT is delivered which incorporates collimator rotation, providing an extra degree of freedom in deriving leaf sequences for a desired fluence map. Specifically, we have developed a series of unique algorithms that are capable of determining rotated MLC segments. These IMRT fields may be delivered statically (with the collimator rotating to a new position in between sub-fields) or dynamically (with the collimator rotating and leaves moving simultaneously during irradiation). This introductory study provides an analysis of the rotating leaf motion calculation algorithms with focus on radiation efficiency, the range of collimator rotation and number of segments. We then evaluate the technique by characterizing the ability of the algorithms to generate rotating leaf sequences for desired fluence maps. Comparisons are also made between our method and conventional sliding window and step-and-shoot techniques. Results show improvements in spatial resolution, reduced interleaf effects and maximum deliverable field size over conventional techniques. Clinical application of these enhancements can be realized immediately with static rotational delivery although improved dosimetric modelling of the MLC will be required for dynamic delivery.     

A high-speed scintillation-based electronic portal imaging device to quantitatively characterize IMRT delivery

We have developed an electronic portal imaging device (EPID) employing a fast scintillator and a high-speed camera. The device is designed to accurately and independently characterize the fluence delivered by a linear accelerator during intensity modulated radiation therapy (IMRT) with either step-and-shoot or dynamic multileaf collimator (MLC) delivery. Our aim is to accurately obtain the beam shape and fluence of all segments delivered during IMRT, in order to study the nature of discrepancies between the plan and the delivered doses. A commercial high-speed camera was combined with a terbium-doped gadolinium-oxy-sulfide (Gd2O2S:Tb) scintillator to form an EPID for the unaliased capture of two-dimensional fluence distributions of each beam in an IMRT delivery. The high speed EPID was synchronized to the accelerator pulse-forming network and gated to capture every possible pulse emitted from the accelerator, with an approximate frame rate of 360 frames-per-second (fps). A 62-segment beam from a head-and-neck IMRT treatment plan requiring 68 s to deliver was recorded with our high speed EPID producing approximately 6 Gbytes of imaging data. The EPID data were compared with the MLC instruction files and the MLC controller log files. The frames were binned to provide a frame rate of 72 fps with a signal-to-noise ratio that was sufficient to resolve leaf positions and segment fluence. The fractional fluence from the log files and EPID data agreed well. An ambiguity in the motion of the MLC during beam on was resolved. The log files reported leaf motions at the end of 33 of the 42 segments, while the EPID observed leaf motions in only 7 of the 42 segments. The static IMRT segment shapes observed by the high speed EPID were in good agreement with the shapes reported in the log files. The leaf motions observed during beam-on for step-and-shoot delivery were not temporally resolved by the log files.     

IMAT-SIM: a new method for the clinical dosimetry of intensity-modulated arc therapy (IMAT)

Dynamic-gantry multi-leaf collimator (MLC)-based, intensity-modulated radiotherapy (IMAT) has been proposed as an alternative to tomotherapy. In contrast to fixed-gantry, MLC-based intensity-modulated radiotherapy (IMRT), where commercial treatment planning systems (TPS) or dosimetric analysis software currently provide many automatic tools enabling two-dimensional (2D) detectors (matrix or electronic portal imaging devices) to be used as measurement systems, for the planning and delivery of IMAT these tools are generally not available. A new dosimetric method is proposed to overcome some of these limitations. By converting the MLC files of IMAT beams from arc to fixed gantry-angle modality, while keeping the leaf trajectories equal, IMAT plans can be both simulated in the TPS and executed as fixed-gantry, sliding-window DMLC treatments. In support of this idea, measurements of six IMAT plans, in their double form of original arcs and converted fixed-gantry DMLC beams (IMAT-SIM), have been compared among themselves and with their corresponding IMAT-SIM TPS calculations. Radiographic films and a 2D matrix ionization chamber detector rigidly attached to the accelerator gantry and set into a cubic plastic phantom have been used for these measurements. Finally, the TPS calculation-algorithm implementations of both conformal dynamic MLC arc (CD-ARC) modalities, used for clinical IMAT calculations, and DMLC modalities (IMAT-SIM), proposed as references for validating IMAT plan dose-distributions, have been compared. The comparisons between IMAT and IMAT-SIM delivered beams have shown very good agreement with similar shapes of the measured dose profiles which can achieve a mean deviation (+/-2sigma) of (0.35+/-0.16) mm and (0.37+/-0.14)%, with maximum deviations of 1.5 mm and 3%. Matching the IMAT measurements with their corresponding IMAT-SIM data calculated by the TPS, these deviations remain in the range of (1.01+/-0.28) mm and (-1.76+/-0.42)%, with maximums of 3 mm and 5%, limits generally accepted for IMRT plan dose validation. Differences in the algorithm implementations have been found, but by correcting CD-ARC calculations for the leaf-end transmission offset (LTO) effect the IMAT and IMAT-SIM simulations agree well in terms of final dose distributions. The differences found between IMAT and the IMAT-SIM beam measurements are due to the different controls of leaf motion (via electron gun delay in the latter) that cannot be used in the former to correct possible speed variations in the rotation of the gantry. As the IMAT delivered beams are identical to what the patient will receive during the treatment, and the IMAT-SIM beam calculations made by the TPS reproduce exactly the treatment plans of that patient, the accuracy of this new dosimetric method is comparable to that which is currently used for static IMRT. This new approach of 2D-detector dosimetry, together with the commissioning, quality-assurance, and preclinical dosimetric procedures currently used for IMRT techniques, can be applied and extended to any kind of dynamic-gantry MLC-based treatment modality either CD-ARC or IMAT.     

An exact approach to direct aperture optimization in IMRT treatment planning

We consider the problem of intensity-modulated radiation therapy (IMRT) treatment planning using direct aperture optimization. While this problem has been relatively well studied in recent years, most approaches employ a heuristic approach to the generation of apertures. In contrast, we use an exact approach that explicitly formulates the fluence map optimization (FMO) problem as a convex optimization problem in terms of all multileaf collimator (MLC) deliverable apertures and their associated intensities. However, the number of deliverable apertures, and therefore the number of decision variables and constraints in the new problem formulation, is typically enormous. To overcome this, we use an iterative approach that employs a subproblem whose optimal solution either provides a suitable aperture to add to a given pool of allowable apertures or concludes that the current solution is optimal. We are able to handle standard consecutiveness, interdigitation and connectedness constraints that may be imposed by the particular MLC system used, as well as jaws-only delivery. Our approach has the additional advantage that it can explicitly account for transmission of dose through the part of an aperture that is blocked by the MLC system, yielding a more precise assessment of the treatment plan than what is possible using a traditional beamlet-based FMO problem. Finally, we develop and test two stopping rules that can be used to identify treatment plans of high clinical quality that are deliverable very efficiently. Tests on clinical head-and-neck cancer cases showed the efficacy of our approach, yielding treatment plans comparable in quality to plans obtained by the traditional method with a reduction of more than 75% in the number of apertures and a reduction of more than 50% in beam-on time, with only a modest increase in computational effort. The results also show that delivery efficiency is very insensitive to the addition of traditional MLC constraints; however, jaws-only treatment requires about a doubling in beam-on time and number of apertures used. Finally, we showed the importance of accounting for transmission effects when assessing or, preferably, optimizing treatment plan quality.     

A method for photon beam Monte Carlo multileaf collimator particle transport

Monte Carlo (MC) algorithms are recognized as the most accurate methodology for patient dose assessment. For intensity-modulated radiation therapy (IMRT) delivered with dynamic multileaf collimators (DMLCs), accurate dose calculation, even with MC, is challenging. Accurate IMRT MC dose calculations require inclusion of the moving MLC in the MC simulation. Due to its complex geometry, full transport through the MLC can be time consuming. The aim of this work was to develop an MLC model for photon beam MC IMRT dose computations. The basis of the MC MLC model is that the complex MLC geometry can be separated into simple geometric regions, each of which readily lends itself to simplified radiation transport. For photons, only attenuation and first Compton scatter interactions are considered. The amount of attenuation material an individual particle encounters while traversing the entire MLC is determined by adding the individual amounts from each of the simplified geometric regions. Compton scatter is sampled based upon the total thickness traversed. Pair production and electron interactions (scattering and bremsstrahlung) within the MLC are ignored. The MLC model was tested for 6 MV and 18 MV photon beams by comparing it with measurements and MC simulations that incorporate the full physics and geometry for fields blocked by the MLC and with measurements for fields with the maximum possible tongue-and-groove and tongue-or-groove effects, for static test cases and for sliding windows of various widths. The MLC model predicts the field size dependence of the MLC leakage radiation within 0.1% of the open-field dose. The entrance dose and beam hardening behind a closed MLC are predicted within +/- 1% or 1 mm. Dose undulations due to differences in inter- and intra-leaf leakage are also correctly predicted. The MC MLC model predicts leaf-edge tongue-and-groove dose effect within +/- 1% or 1 mm for 95% of the points compared at 6 MV and 88% of the points compared at 18 MV. The dose through a static leaf tip is also predicted generally within +/- 1% or 1 mm. Tests with sliding windows of various widths confirm the accuracy of the MLC model for dynamic delivery and indicate that accounting for a slight leaf position error (0.008 cm for our MLC) will improve the accuracy of the model. The MLC model developed is applicable to both dynamic MLC and segmental MLC IMRT beam delivery and will be useful for patient IMRT dose calculations, pre-treatment verification of IMRT delivery and IMRT portal dose transmission dosimetry.     

Comparative analysis of 60Co intensity-modulated radiation therapy

In this study, we perform a scientific comparative analysis of using (60)Co beams in intensity-modulated radiation therapy (IMRT). In particular, we evaluate the treatment plan quality obtained with (i) 6 MV, 18 MV and (60)Co IMRT; (ii) different numbers of static multileaf collimator (MLC) delivered (60)Co beams and (iii) a helical tomotherapy (60)Co beam geometry. We employ a convex fluence map optimization (FMO) model, which allows for the comparison of plan quality between different beam energies and configurations for a given case. A total of 25 clinical patient cases that each contain volumetric CT studies, primary and secondary delineated targets, and contoured structures were studied: 5 head-and-neck (H&N), 5 prostate, 5 central nervous system (CNS), 5 breast and 5 lung cases. The DICOM plan data were anonymized and exported to the University of Florida optimized radiation therapy (UFORT) treatment planning system. The FMO problem was solved for each case for 5-71 equidistant beams as well as a helical geometry for H&N, prostate, CNS and lung cases, and for 3-7 equidistant beams in the upper hemisphere for breast cases, all with 6 MV, 18 MV and (60)Co dose models. In all cases, 95% of the target volumes received at least the prescribed dose with clinical sparing criteria for critical organs being met for all structures that were not wholly or partially contained within the target volume. Improvements in critical organ sparing were found with an increasing number of equidistant (60)Co beams, yet were marginal above 9 beams for H&N, prostate, CNS and lung. Breast cases produced similar plans for 3-7 beams. A helical (60)Co beam geometry achieved similar plan quality as static plans with 11 equidistant (60)Co beams. Furthermore, 18 MV plans were initially found not to provide the same target coverage as 6 MV and (60)Co plans; however, adjusting the trade-offs in the optimization model allowed equivalent target coverage for 18 MV. For plans with comparable target coverage, critical structure sparing was best achieved with 6 MV beams followed closely by (60)Co beams, with 18 MV beams requiring significantly increased dose to critical structures. In this paper, we report in detail on a representative set of results from these experiments. The results of the investigation demonstrate the potential for IMRT radiotherapy employing commercially available (60)Co sources and a double-focused MLC. Increasing the number of equidistant beams beyond 9 was not observed to significantly improve target coverage or critical organ sparing and static plans were found to produce comparable plans to those obtained using a helical tomotherapy treatment delivery when optimized using the same well-tuned convex FMO model. While previous studies have shown that 18 MV plans are equivalent to 6 MV for prostate IMRT, we found that the 18 MV beams actually required more fluence to provide similar quality target coverage.     

A dose verification method using a monitor unit matrix for dynamic IMRT on Varian linear accelerators

Dosimetry verification is an important step during intensity modulated radiotherapy treatment (IMRT). The verification is usually conducted with measurements and independent dose calculations. However, currently available independent dose calculation methods were developed for step-and-shoot beam delivery methods, and their uses for dynamic multi-leaf collimator (MLC) delivery methods are not efficient. In this study, a dose calculation method was developed to perform independent dose verifications for a dynamic MLC-based IMRT technique for Varian linear accelerators. This method extracts the machine delivery parameters from the dynamic MLC (DMLC) files generated by the IMRT treatment planning system. Based on the parameters a monitor unit (MU) matrix was separately calculated as two terms: direct exposure from the open MLC field and leakage contributions, where the leaf-end leakage contribution becomes more important in higher dose gradient regions. The MU matrix was used to compute the primary dose and the scattered dose with a modified Clarkson technique. The doses computed using the method were compared with both measurement and treatment planning for 14 and 25 plans respectively. An average of less than 2% agreement was observed and the standard deviation was about 1.9%.