一种新型质子治疗剂量递送系统的设计研究及模拟验证

目的:针对激光等离子体加速的质子束流特性,设计用于剂量递送的新型紧凑治疗头系统,并通过模拟计算验证该方法的有效性与适用性。方法:基于实验上已实现的激光质子束流参数,利用散射体设计软件NEU（Nozzles with Everything Upstream）进行流线型散射体设计。通过散角选择和能散调制进一步优化剂量递送效率,并利用蒙特卡罗模拟计算软件TOPAS（TOol for PArticle Simulation）及底层的Geant4（GEometry ANd Tracking）计算引擎分析并验证激光质子通过此剂量递送方法后水模体中的剂量分布。结果:在直径6 cm、高5 cm的圆柱形靶区内,深度剂量分布平坦度在±1%以内,横向剂量分布在±3%以内。结论:此剂量递送方法及系统适用于现阶段激光质子束流特性,水模体靶区内剂量递送均匀、高效且稳定。     

利用蒙特卡罗模拟评估空气间隙对点扫描质子治疗的剂量影响

目的:利用蒙特卡罗模拟探究空气间隙对点扫描质子治疗的剂量影响。方法:利用通用蒙特卡罗程序Geant4平台构建使用射程移位器的治疗头末端的点扫描质子束流模型,并进行验证。模拟计算不同能量、不同射程移位器、不同束斑尺寸、不同束斑数目在不同空气间隙条件下的质子束流在水模体中的剂量沉积,并通过获得的积分深度剂量生成剂量修正因子对剂量的差异进行比较。结果:不同空气间隙会造成剂量损失,随空气间隙增大而增大,随水模体中深度增加而减小。对于能量更高的射束和使用水等效厚度更薄的射程移位器,剂量损失越大。束斑尺寸改变和束斑数目增加较少时造成的剂量损失与同条件下单一束流无显著差别。结论:当使用射程移位器、肿瘤位置较浅、空气间隙较大时,建议建立剂量修正因子数据库应用于治疗计划系统对剂量进行修正。     

基于GAMOS的蒙特卡罗方法模拟放疗电子线照射的剂量学研究

目的:探讨基于GAMOS的蒙特卡罗（MC）方法模拟电子线放疗的剂量精确性。方法:运用GAMOS MC程序,建立Varian Rapidarc加速器3档能量（6、9和12 MeV）及3种限光筒［（6×6）、（10×10）和（15×15） cm2］的束流模型,模拟束流在水模体中的剂量分布,并与测量得到的百分深度剂量和等平面剂量分布比较,评估GAMOS软件模拟电子线照射的精确性和运算效率。结果:模拟的粒子数越多,模拟与测量结果的误差越小;当模拟粒子的数量达到5×108时,各个能量的电子线射程（Rp）和50%剂量深度（R50）的模拟结果与测量结果一致;除建成区外,百分深度剂量模拟和测量的结果误差在2%以内;等平面剂量分布模拟和测量的结果误差也在2%以内,模拟的照射野大小与测量结果一致。运算效率中,能量越大,限光筒尺寸越大,并行同步模拟所用的时间越多,模拟时间的变化越大。结论:基于GAMOS的MC方法可准确地模拟放疗电子线照射剂量的分布,粒子数的增加可提高模拟的精确性,并行同步计算可提高模拟的效率。     

基于微剂量学蒙特卡罗模拟的重离子生物有效剂量精确计算方法

目的:在蒙特卡罗（MC）模拟当中引入理想组织等效正比计数器（Ideal TEPC）,并结合微剂量动力学模型（MKM）精确计算重离子生物有效剂量。方法:采用Ideal TEPC与MC模拟方法对能量为330 MeV/u具有6 cm展宽Bragg峰（SOBP）的碳离子束生物有效剂量进行计算。结果:优化得到MKM模型的参数为:[α0=0.12 Gy-1],rd=0.39 μm,Rn=3.7 μm。对于能量为330 MeV/u具有6 cm SOBP碳离子束生物有效剂量的计算显示:基于MKM模型计算的生物有效剂量与碳离子放射治疗计划系统（ciPlan）中的生物有效剂量具有较好的一致性,二者偏差随深度的增加而增大,在坪区、SOBP前端、SOBP中点、SOBP后端、尾区的偏差分别为0.3%、1.7%、2.7%、4.9%、10.3%。结论:Ideal TEPC结合MC模拟能够准确计算重离子的生物有效剂量,有效避免TEPC壁引起的辐射场畸变,结构材料所产生的[δ]电子对线能谱的影响,以及实验中位置的偏差,具有良好的移植性。     

扫描治疗头的蒙特卡罗模型研究

目的:评估质子治疗中扫描治疗头对束流品质的影响。方法:通过扫描治疗头的蒙特卡罗模型研究深度剂量曲线的变化,计算射程移位器对束斑截面的影响以及分析扫描磁场对单质子束的偏转情况。结果:随着能量的增加,质子在水中的射程增加,同时散射也越严重,最终布拉格峰变宽,尾端变胖。相比于直接入射水模,通过治疗头后质子在水中的射程缩短了约0.6 cm,但布拉格峰形基本保持不变;将4 cm厚度聚乙烯射程移位器放置于距离水模表面0、10、20、30、40和50 cm分别进行独立计算,发现与水模距离越远,质子的散射越大,因此治疗过程中射程移位器应尽量靠近患者;当扫描磁铁加载磁场后,束斑将偏离束流中心。设置纵向扫描磁场Bx=0.1 T,横向扫描磁场By=0.3 T,180 MeV质子束在Y方向偏离了2.693 cm,横向扫描磁场使质子在-X方向上偏离了8.427 cm。当束流有偏转的时候,要求射程移位器横截面足够大以满足宽扫描场的需要。结论:扫描治疗头的蒙特卡罗模型将有助于理解质子治疗这一新兴的放疗方法以及熟悉扫描治疗的束流特性,在调试和质量保证中提供参考。     

半月裂孔的解剖学分型及临床意义研究

目的 观察半月裂孔（HS）的解剖特征并划分类型，为内镜治疗鼻旁窦疾病提供参考依据。方法 50例结构完整的人鼻腔外侧壁标本，测量中鼻道前端宽度（AW）、中间宽度（MW）、后端宽度（PW）及长度；观察HS的形态及变异，测量HS的垂直部长轴与水平部长轴角度α，并对HS划分解剖学类型。结果 （1）男性左、右两侧中鼻道AW(P=0.875)、MW(P=0.703)、PW(P=0.268)及长度（P=0.675）比较，差异均无统计学意义（P>0.05）。（2）中鼻道AW(4.53±2.27)mm,MW(4.39±1.64)mm,PW(5.69±1.67)mm;PW显著高于AW与MW(P=0.001,F=7.196)。（3）根据HS形态与α值，HS分为：(1)经典型16例，即HS呈月牙形，α介于100°～160°；(2)U型11例，HS的垂直部与水平部长度相当，水平部末端略翘起，α介于100°～160°；(3)J型8例，HS垂直部明显比水平部更长；(4)直线型14例，α>160°；(5)L型1例，α介于80°～100°。结论 对中鼻道、HS的定量研究及分型，是对既往研究的补充。利用鼻内窥...     

利用纳米颗粒的光核反应实现原位活化治疗的可行性研究

目的:利用蒙特卡罗程序Geant4模拟13.5 MeV和6 MeV X射线照射细胞内的纳米颗粒，分析其光核反应的剂量贡献份额。方法:以纳米金颗粒（GNP）为例，分别模拟6 MeV和13.5 MeV照射细胞内的GNP，给出各自条件下由GNP造成的剂量贡献。创建水模体（0.426 mm×0.426 mm×0.426 mm），包含1 103个细胞，作为GNP的载体。在6 MeV和13.5 MeV下分别模拟细胞中包含和不包含GNP所造成的剂量沉积。结果:13.5 MeV X射线照射，其由GNP造成的剂量贡献为5.12 cGy，细胞总能量沉积为25.37 cGy，由GNP引起的剂量贡献占20.19%；6 MeV X射线照射，其由GNP造成的剂量贡献为2.87 cGy，细胞总能量沉积为23.05 cGy，由GNP造成剂量贡献约为12.46%。与6 MeV相比，13.5 MeV下由GNP光核反应造成的剂量贡献占7.7%。结论:对于细胞模型内纳米金的研究表明，GNP确实能引起额外的剂量贡献。由于GNP光核反应引起的剂量贡献很低，难以作为能够被原位激活的放射源使用。     

基于蒙特卡罗模拟的直肠癌术前容积调强放射治疗计划剂量验证

目的: 通过蒙特卡罗模拟评价基于各向异性解析算法（AAA）直肠癌术前容积调强放射治疗计划（VMAT）的剂量计算精度。方法: 选取20例基于AAA算法和RapidPlan模型优化的直肠癌术前VMAT计划,通过对比蒙卡模拟与治疗计划计算结果的平均DVH、靶区适形度（CI）、靶区均匀性（HI）和Gamma 3D通过率等参数,评估基于AAA算法的VMAT治疗计划剂量计算精度。结果: 两种Gamma 3D评估策略通过率的均值与标准差分别为97.58±0.47%（Max Dose）、92.46±1.76%（Local Dose）,且差异具有统计学意义（P<0.05）;对PTV和PGTV的CI、DMin、膀胱的D50%、DMean等不符合正态分布的参数做相关样本非参数检验,除PGTV的CI和Dmin外,差异均具有统计学意义（P < 0.05）;其他服从正态分布的参数做配对样本T检验,差异均具有统计学意义（P < 0.05）。结论: Rapidplan模型计划在低剂量区通过率较低,说明AAA算法射野边缘低剂量区计算偏差较大;靶区Dmin与算法的精度较为相关,CI和HI参数相对于蒙卡模拟结果有一定差异;AAA算法在股骨头和膀胱的D50%、DMean相对蒙卡模型有不同程度的低估。     

鼻咽癌放疗中T形口腔固定器的研制及其摆位误差和剂量影响研究

【摘要】目的:探讨自制T形口腔固定器在鼻咽癌放疗中对摆位误差，以及对靶区和危及器官受照剂量的影响。方法:选择40例鼻咽癌放疗患者，随机分成两组，每组20例。A组为常规热塑膜固定器组，B组为热塑膜联合自制T形口腔固定器组，应用锥形束CT（CBCT）比较两组头部和颈部的平移误差及旋转误差；将误差带入计划系统重新计算模拟计划，得到靶区和危及器官体积剂量参数，与原始计划比较。结果:两组平移误差接近，而旋转误差明显减少，其中颈部≤2°的误差，A组在Cor、Sag、Tra方向上分别占88.7%、83.4%、80.5%；B组占98.4%、95.3%、96.9%，且具有统计学意义（P均<0.01）。靶区体积剂量百分比，A组的GTVnx-D98%、GTVnd-D98%、CTV1-D95%、CTV2-D95%在±3%内占87.5%、88.3%、98.5%、98.5%，B组占100%、96.8%、100%、100%，B组剂量变化范围明显变小且全部具有统计学意义（P均<0.01）。结论:热塑膜联合自制T形口腔固定器可有效减少摆位误差，提高摆位重复性，提高靶区剂量准确性，尤其对于颈部，保障调强放疗的疗效，可在临床中推广应用。     

基于CT三维重建的肩胛切迹解剖分型及临床意义

目的 探讨基于CT三维重建的肩胛切迹解剖形态学分型及临床意义。 方法 收集来自西南医科大学附属中医医院300例因肩部疾病就诊患者的肩胛骨,通过CT三维技术重建肩胛骨图像,并进行肩胛切迹形态学分型及几何数据测量。 结果 我们将收集的肩胛切迹分为5种类型,√-形称为Ⅰ型共138例,约占46%;U-形称为Ⅱ型共125例约占41.7%;Ⅴ-形称为Ⅲ型,共20例,约占6.7%;O-形称为Ⅳ型,共10例约占3.3%;Ω-形称为Ⅴ型,共7例约占2.3%（另外,发现W-形、双O-形各1例,因数量较少暂未纳入分型）;左侧肩胛切迹平均深度、宽度分别为（5.58±1.42、10.22±3.24）mm,右侧肩胛切迹平均深度、宽度分别为（6.02±1.87、10.81±3.35）mm,左右对比差异有统计学意义（P<0.05）;Ⅰ和Ⅱ型的切迹宽度较其他3种类型宽度更宽,分别为（12.46±3.20、9.95±2.68）mm,且P<0.05差异有统计学意义;另外,不同类型中肩胛切迹最低点到肩胛冈基底部的垂直距离长度有所不同,其中Ⅰ型最短的为（12.52±2.56）mm,Ⅲ型最长的为（14.48±4.29）mm,Ⅰ型和Ⅲ型比较差异有统计学意义（P<0.05）。 结论 基于CT三维重建结果,将肩胛切迹分为5型,分别为√-形、U-形、Ⅴ-形、O-形、Ω-形。其中Ⅴ-形和O-形发生肩胛上神经卡压症的几率较大,而√-形及U-形的卡压几率则较小。     

Improvement of spread-out Bragg peak flatness for a carbon-ion beam by the use of a ridge filter with a ripple filter

We have developed a novel design method of ridge filters for carbon-ion therapy using a broad-beam delivery system to improve the flatness of a biologically effective dose in the spread-out Bragg peak (SOBP). So far, the flatness of the SOBP is limited to about ±5% for carbon beams since the weight control of component Bragg curves composing the SOBP is difficult. This difficulty arises from using a large number of ridge-bar steps (e.g. about 100 for a SOBP width of 60 mm) required to form the SOBP for the pristine Bragg curve with an extremely sharp distal falloff. Instead of using a single ridge filter, we introduce a ripple filter to broaden the Bragg peak so that the number of ridge-bar steps can be reduced to about 30 for SOBP with of 60 mm for the ridge filter designed for the broadened Bragg peak. Thus we can manufacture the ridge filter more accurately and then attain a better flatness of the SOBP due to well-controlled weights of the component Bragg curves. We placed the ripple filter on the same frame of the ridge filter and arranged the direction of the ripple-filter-bar array perpendicular to that of the ridge-filter-bar array. We applied this method to a 290 MeV u(-1) carbon-ion beam in Heavy Ion Medical Accelerator in Chiba and verified the effectiveness by measurements.     

Degradation of the Bragg peak due to inhomogeneities

The rapid fall-off of dose at the end of range of heavy charged particle beams has the potential in therapeutic applications of sparing critical structures just distal to the target volume. Here we explored the effects of highly inhomogeneous regions on this desirable depth-dose characteristic. The proton depth-dose distribution behind a lucite-air interface parallel to the beam was bimodal, indicating the presence of two groups of protons with different residual ranges, creating a step-like depth-dose distribution at the end of range. The residual ranges became more spread out as the interface was angled at 3 degrees, and still more at 6 degrees, to the direction of the beam. A second experiment showed little significant effect on the distal depth-dose of protons having passed through a mosaic of teflon and lucite. Anatomic studies demonstrated significant effects of complex fine inhomogeneities on the end of range characteristics. Monoenergetic protons passing through the petrous ridges and mastoid air cells in the base of skull showed a dramatic degradation of the distal Bragg peak. In beams with spread out Bragg peaks passing through regions of the base of skull, the distal fall-off from 90 to 20% dose was increased from its nominal 6 to well over 32 mm. Heavy ions showed a corresponding degradation in their ends of range. In the worst case in the base of skull region, a monoenergetic neon beam showed a broadening of the full width at half maximum of the Bragg peak to over 15 mm (compared with 4 mm in a homogeneous unit density medium). A similar effect was found with carbon ions in the abdomen, where the full width at half maximum of the Bragg peak (nominally 5.5 mm) was found to be greater than 25 mm behind gas-soft-tissue interfaces. We address the implications of these data for dose computation with heavy charged particles.     

A fourier analysis on the maximum acceptable grid size for discrete proton beam dose calculation

We developed an analytical method for determining the maximum acceptable grid size for discrete dose calculation in proton therapy treatment plan optimization, so that the accuracy of the optimized dose distribution is guaranteed in the phase of dose sampling and the superfluous computational work is avoided. The accuracy of dose sampling was judged by the criterion that the continuous dose distribution could be reconstructed from the discrete dose within a 2% error limit. To keep the error caused by the discrete dose sampling under a 2% limit, the dose grid size cannot exceed a maximum acceptable value. The method was based on Fourier analysis and the Shannon-Nyquist sampling theorem as an extension of our previous analysis for photon beam intensity modulated radiation therapy [J. F. Dempsey, H. E. Romeijn, J. G. Li, D. A. Low, and J. R. Palta, Med. Phys. 32, 380-388 (2005)]. The proton beam model used for the analysis was a near monoenergetic (of width about 1% the incident energy) and monodirectional infinitesimal (nonintegrated) pencil beam in water medium. By monodirection, we mean that the proton particles are in the same direction before entering the water medium and the various scattering prior to entrance to water is not taken into account. In intensity modulated proton therapy, the elementary intensity modulation entity for proton therapy is either an infinitesimal or finite sized beamlet. Since a finite sized beamlet is the superposition of infinitesimal pencil beams, the result of the maximum acceptable grid size obtained with infinitesimal pencil beam also applies to finite sized beamlet. The analytic Bragg curve function proposed by Bortfeld [T. Bortfeld, Med. Phys. 24, 2024-2033 (1997)] was employed. The lateral profile was approximated by a depth dependent Gaussian distribution. The model included the spreads of the Bragg peak and the lateral profiles due to multiple Coulomb scattering. The dependence of the maximum acceptable dose grid size on the orientation of the beam with respect to the dose grid was also investigated. The maximum acceptable dose grid size depends on the gradient of dose profile and in turn the range of proton beam. In the case that only the phantom scattering was considered and that the beam was aligned with the dose grid, grid sizes from 0.4 to 6.8 mm were required for proton beams with ranges from 2 to 30 cm for 2% error limit at the Bragg peak point. A near linear relation between the maximum acceptable grid size and beam range was observed. For this analysis model, the resolution requirement was not significantly related to the orientation of the beam with respect to the grid.     

Creating a spread-out Bragg peak in proton beams

The model of Bortfeld and Schlegel (1996 Phys. Med. Biol. 41 1331-9) for determining the weights of proton beams required to create a spread-out Bragg peak (SOBP) gives a significantly tilted SOBP. However, by arbitrarily varying its parameter p, which relates the range of protons to their energy, we have been able to create satisfactory SOBPs. MCNPX Monte Carlo calculations have been carried out to determine p, demonstrating the success of this modification. Optimal values of p are tabulated for various combinations of maximum beam energy E(0) (50, 100, 150, 200 and 250 MeV) and SOBP width χ (15%, 20%, 25%, 30%, 35% and 40%), as well as for a correction factor needed to calculate the SOBP dose. An example shows the application of these results to analyzing the dose deposited by deuterons and alpha particles in broad proton beams.     

Depth ionization curves for an unmodulated proton beam measured with different ionization chambers

Differences in depth dose curves for a 78 MeV unmodulated proton beam were measured with four commercially available ionization chambers. Measurements were performed both in water and in a commercially available solid water phantom. A depth scaling factor (Cpl) was determined from the ratio of depths distal to the Bragg peak where the dose is reduced to 80% of the maximum dose in water and in the solid water phantom. This scaling factor provides good agreement between the ionization curves at all depths in water and in this solid water phantom. There is no significant difference in the value of the depth scaling factor between the ratios (R80wat/R80med) and (R50wat/R50med), or (R100wat/R100med) for 78 MeV unmodulated proton beams. The depth scaling factor for this commercially available solid water phantom is 1.023. An effective point of measurement for a cylindrical ionization chamber was found to be slightly greater than the 50% of the cavity radius proposed by the AAPM-TG25 dosimetry protocol for electron beams and amounts to 62.5% of the cavity radius of cylindrical ionization chambers. The ion collection efficiency, Pion, and the polarity correction factor, Ppol, for all the ionization chambers studied are within 1% and 0.4% of unity, respectively. Absolute doses measured with a parallel plate ionization chamber in water and in the solid water phantom show that the doses measured in the solid water phantom are 1.4% +/- 0.5% lower than in water. The dose rate dependent response of the beam line monitor chamber was also investigated. Agreement between all the chambers was within 1.5% at the dose rates studied but the results showed that all four ionization chambers are less dose rate dependent than the monitor chamber.     

Parameterization of multiple Bragg curves for scanning proton beams using simultaneous fitting of multiple curves

Although Bortfeld's analytical formula is useful for describing Bragg curves, measured data can deviate from the values predicted by the model. Thus, we sought to determine the parameters of a closed analytical expression of multiple Bragg curves for scanning proton pencil beams using a simultaneous optimization algorithm and to determine the minimum number of energies that need to be measured in treatment planning so that complete Bragg curves required by the treatment planning system (TPS) can be accurately predicted. We modified Bortfeld's original analytical expression of Bragg curves to accurately describe the dose deposition resulting from secondary particles. The parameters of the modified analytical expression were expressed as the parabolic cylinder function of the ranges of the proton pencil beams in water. Thirty-nine discrete Bragg curves were measured in our center using a PTW Bragg Peak chamber during acceptance and commission of the scanning beam proton delivery system. The coefficients of parabolic function were fitted by applying a simultaneous optimization algorithm to seven measured curves. The required Bragg curves for 45 energies in the TPS were calculated using our parameterized analytical expression. Finally, the 10 cm width of spread-out Bragg peaks (SOBPs) of beams with maximum energies of 221.8 and 121.2 MeV were then calculated in the TPS and compared with measured data. Compared with Bortfeld's original formula, our modified formula improved fitting of the measured depth dose curves at depths around three-quarters of the maximum range and in the beam entrance region. The parabolic function described the relationship between the parameters of the analytic expression of different energies. The predicted Bragg curves based on the parameters fitted using the seven measured curves accurately described the Bragg curves of proton pencil beams of 45 energies configured in our TPS. When we used the calculated Bragg curves as the input to TPS, the standard deviations of the measured and calculated data points along the 10 cm SOBPs created with proton pencil beams with maximum energies of 221.8 and 121.2 MeV were 1.19% and 1.18%, respectively, using curves predicted by the algorithm generated from the seven measured curves. Our method would be a valuable tool to analyze measured Bragg curves without the need for time-consuming measurements and correctly describe multiple Bragg curves using a closed analytical expression.     

Dosimetric considerations of d(15) + Be and p(26) + Be neutron beams from an isocentric cyclotron facility

To select the optimum therapeutic neutron beam available from a CS30 medical cyclotron (manufactured by the Cyclotron Corporation, Berkeley, California), central axis depth dose data and output dose rates were compared for the bombardment of beryllium with either the proton or deuteron beams available from the machine. The effect on these parameters of filtering the beams with either pure polyethylene, polyethylene loaded with 5% boron, or polyethylene loaded with 10% lithium was studied. A 4-cm, 10% lithiated filter used with a 26-MeV proton beam was selected for therapeutic use. Buildup curves, beam profiles at several transverse planes for different field sizes, and comparison of beam profiles with 60Co are given.     

Intensity and energy modulated radiotherapy with proton beams: variables affecting optimal prostate plan

Inverse planning for intensity- and energy-modulated radiotherapy (IEMRT) with proton beams involves the selection of (i) the relative importance factors to control the relative importance of the target and sensitive structures, (ii) an appropriate energy resolution to achieve an acceptable depth modulation, (iii) an appropriate beamlet width to modulate the beam laterally, and (iv) a sufficient number of beams and their orientations. In this article we investigate the influence of these variables on the optimized dose distribution of a simulated prostate cancer IEMRT treatment. Good dose conformation for this prostate case was achieved using a constellation of I factors for the target, rectum, bladder, and normal tissues of 500, 50, 15, and 1, respectively. It was found that for an active beam delivery system, the energy resolution should be selected on the basis of the incident beams' energy spread (sigmaE) and the appropriate energy resolution varied from 1 MeV at sigmaE = 0.0 to 5 MeV at sigmaE= 2.0 MeV. For a passive beam delivery system the value of the appropriate depth resolution for inverse planning may not be critical as long as the value chosen is at least equal to one-half the FWHM of the primary beam Bragg peak. Results indicate that the dose grid element dimension should be equal to or no less than 70% of the beamlet width. For this prostate case, we found that a maximum of three to four beam ports is required since there was no significant advantage to using a larger number of beams. However for a small number (< or = 4) of beams the selection of beam orientations, while having only a minor effect on target coverage, strongly influenced the sensitive structure sparing and normal tissue integral dose.     

Physical aspects of dynamic stereotactic radiosurgery with very small photon beams (1.5 and 3 mm in diameter)

Stereotactic radiosurgery is often used for treating functional disorders. For some of these disorders, the size of the target can be on the order of a millimeter and the radiation dose required for treatment on the order of 80 Gy. The very small radiation field and high prescribed dose present a difficult challenge in beam calibration, dose distribution calculation, and dose delivery. In this work the dose distribution for dynamic stereotactic radiosurgery, carried out with 1.5 and 3 mm circular fields, was studied. A 10 MV beam from a Clinac-18 linac (Varian, Palo Alto, CA) was used as the radiation source. The BEAM/EGS4 Monte Carlo code was used to model the treatment head of the machine along with the small-field collimators. The models were validated with the EGSnrc code, first through a calculation of percent depth doses (PDD) and dose profiles in a water phantom for the two small stationary circular beams and then through a comparison of the calculated with measured PDD and profile data. The three-dimensional (3-D) dose distributions for the dynamic rotation with the two small radiosurgical fields were calculated in a spherical water phantom using a modified version of the fast XVMC Monte Carlo code and the validated models of the machine. The dose distributions in a horizontal plane at the isocenter of the linac were measured with low-speed radiographic film. The maximum sizes of the Monte Carlo-calculated 50% isodose surfaces in this horizontal plane were 2.3 mm for the 1.5 mm diameter beam and 3.8 mm for the 3 mm diameter beam. The maximum discrepancies between the 50% isodose surface on the film and the 50% Monte Carlo-calculated isodose surfaces were 0.3 mm for both the 1.5 and 3 mm beams. In addition, the displacement of the delivered dose distributions with respect to the laser-defined isocenter of the machine was studied. The results showed that dynamic radiosurgery with very small beams has a potential for clinical use.     

Performance of a fluorescent screen and CCD camera as a two-dimensional dosimetry system for dynamic treatment techniques

A two-dimensionally position sensitive dosimetry system has been tested for different dosimetric applications in a radiation therapy facility with a scanning proton beam. The system consists of a scintillating (fluorescent) screen, mounted at the beam-exit side of a phantom and it is observed by a charge coupled device (CCD) camera. The observed light distribution at the screen is equivalent to the two-dimensional (2D)-dose distribution at the screen position. It has been found that the dosimetric properties of the system, measured in a scanning proton beam, are equal to those measured in a proton beam broadened by a scattering system. Measurements of the transversal dose distribution of a single pencil beam are consistent with dose measurements as well as with dose calculations in clinically relevant fields made with multiple pencil beams. Measurements of inhomogeneous dose distributions have shown to be of sufficient accuracy to be suitable for the verification of dose calculation algorithms. The good sensitivity and sub-mm spatial resolution of the system allows for the detection of deviations of a few percent in dose from the expected (intended or calculated) dose distribution. Its dosimetric properties and the immediate availability of the data make this device a useful tool in the quality control of scanning proton beams.