瓦里安加速器6 MV X线小野数据测量与计算比较

目的 通过比较不同电离室测量及蒙特卡罗计算结果,研究不同方法对小野数据测量的适用性。方法 用不同电离室测量及蒙特卡罗计算瓦里安6 MV X线在0.5 cm×0.5 cm~10 cm×10 cm射野的总散射因子和百分深度剂量,对测量和计算结果进行比较和分析。结果 当射野≥3.5 cm×3.5 cm时各电离室测量及蒙特卡罗计算结果间差异较小,当射野≤3.0 cm×3.0 cm时不同结果间差异明显。CC04、CC13电离室适合于≥2.0 cm×2.0 cm射野的总散射因子和百分深度剂量测量。结论 CC04、CC13电离室测量和蒙特卡罗计算方法能确定≥2.0 cm×2.0 cm射野的总散射因子和百分深度剂量,更小射野测量和计算数据需慎重评估。     

基于菊花链射野输出因子测量方法

目的 针对响应随射野面积变化的探测器,应用基于菊花链(Daisy-Chaining)的射野输出因子测量方法,提高测量结果的准确性。方法 分别使用IBA CC13电离室、IBA CC01电离室、IBA Razor半导体探测器、IBA EFD半导体探测器和Gafchromic EBT3胶片测量Varian Edge加速器 6 MV X线的射野输出因子。结果 同Razor和CC13衔接的菊花链测量结果相比,常规测量方法使用CC13测量小野时结果偏小,在射野1 cm×1 cm时偏差达到16.71%。使用CC01测量大野时结果偏大,在射野40 cm×40 cm时偏差达到8.39%。使用Razor测量大野时结果偏大,在射野40 cm×40 cm时偏差达到9.40%。EFD的测量结果与Razor结果接近,在射野40 cm×40 cm时偏差为9.14%。使用胶片测量1 cm×1 cm以上的射野时,与菊花链测量结果接近,偏差在1.60%以内,在射野1 cm×1 cm时偏差则达到3.13%。选择射野3 cm×3 cm或4 cm×4 cm作为中间野的菊花链测量结果一致,最大偏差0.29%。结论 对于响应随射野面积变化的探测器可通过菊花链的方法来扩大测量范围,提高测量结果的准确性。     

二维电离室矩阵实时验证VMAT剂量价值研究

目的 探讨利用二维电离室矩阵进行VMAT患者透射剂量实时验证的临床价值。方法 将二维电离室矩阵面板粘贴固定在加速器EPID探测面板上,源到EPID探测面板距离为140 cm。电离室矩阵面板上加8 mm的RW3固体水以提高信躁比。选取食管癌、前列腺癌、肝癌患者计划,在圆柱形Cheese模体上照射测量5次,研究患者计划在模体中剂量验证的可行性与准确性。患者每次治疗时进行实时测量,第1次测量结果作为参考剂量,利用γ分析比较分次间剂量误差。结果采用3%3 mm标准,Cheese模体VMAT计划的γ通过率为98%左右,食管癌、前列腺癌和肝癌患者实时照射γ通过率分别约为92%、92%和94%。整个治疗过程中各分次的γ通过率都在90%以上。     

穿透型参考电离室在立体定向放疗的相对剂量学中的应用

目的 探讨Stealth穿透型参考电离室在立体定向放疗的相对剂量学中的实际应用价值。方法 在IBABlue Phantom水箱中使用2种方法对于配备Brianlab圆锥形限光筒的VarianNovalis 6 MV光子射线进行相对剂量测定。第1种方法无Stealth穿透型参考电离室,单独使用SFD半导体电离室配合“Step by Step”逐点测量;第2种方法有Stealth穿透型参考电离室,使用SFD半导体电离室配合“Continuous”连续测量。测量内容包括直径4、15 mm限光筒下中心轴百分深度剂量(PDD)曲线和离轴剂量比(OAR)曲线并记录每次测量所需时间。结果 对于配备直径4 mm及15 mm的X刀限光筒的6 MV光子射线,逐点测量和连续测量所得的射野中心轴PDD曲线及10 cm深度处OAR曲线吻合度较好,差异在1%以内。连续测量PDD曲线及OAR曲线所需时间大幅缩短,较逐点测量平均缩短为13.1%和20.7%。结论 Stealth穿透型参考电离室在立体定向放疗相对剂量学中的应用可以在保证剂量结果准确性前提下大幅提高测量效率。     

基于IAEA 483号报告的小野射野输出因子测量及修正方法

目的 IAEA 483号报告阐述了最新的小野剂量学方法,本研究应用报告中的射野输出因子测量及修正方法,提高不同探测器小野输出因子测量结果的准确性和一致性。方法 分别使用IBA公司的 CC13电离室、 CC01电离室、PFD半导体探测器、EFD半导体探测器和Razor半导体探测器测量Varian Edge加速器6 MVX射线射野面积从0.6 cm×0.6 cm到10 cm×10 cm的射野输出因子,使用射野输出修正因子对测量结果进行修正。结果 与修正后数据相比,由于电离室主要受到体积平均效应和注量扰动的影响,造成测量结果偏低,在0.6 cm×0.6 cm时偏低4.70%;有屏蔽半导体主要受到注量扰动的影响,造成测量结果偏高,在0.6 cm×0.6 cm时偏高4.80%;无屏蔽半导体主要受到能量响应和注量扰动的影响,造成射野>0.8 cm×0.8 cm时测量结果偏低,在1.5 cm×1.5 cm时偏低2.10%,射野<0.8 cm×0.8 cm时测量结果偏高,在0.6 cm×0.6 cm时偏高1.10%。修正前不同类型探测器测的测量结果差异较大,平均标准差为0.016 6。经过修正后各探测器之间的差异明显减小,平均标准差为0.006 6。结论 对于电离室、半导体等探测器,在测量小野射野输出因子时可以通过射野输出修正因子进行修正,从而提高测量结果的准确性和一致性。     

Hi-ART螺旋断层治疗机剂量学参数的测量

目的 探讨Hi-ART螺旋断层治疗机照射野剂量学参数测量的内容和方法.方法 用断层治疗机专门配置的微型扫描水箱在治疗条件下测量了6 MV X线的百分深度剂量和射野离轴比,并与常规Primus加速器6 MV X线进行比较.根据AAPM TG51号报告用Tomotrometer剂量仪和A1SL电离室在源皮距85 cm、照射野40 cm×5 cm、1.5 cm深度条件下对断层治疗机进行输出剂量刻度,并对剂量线性和重复性进行测量分析.输出剂量率随机架角的变化分别用0.6 cm3电离室和Unidos剂量仪在直径为3 cm有机玻璃体模中测量和用治疗机自身的MVCT探测器测量.设置不同的照射范围,在固体水组织等效材料中对多叶准直器照射野输出因子进行测量.结果 Hi-ART断层治疗机6 MV X线百分深度剂量的最大剂量点在1.0 cm左右.Hi-ART断层治疗机和Primus 6 MV X线在源皮距85 cm、深度10 cm处的百分深度剂量分别为59.6%和64.7%.单个照射野内剂量分布是不均匀的,在人体左右方向剂量分布呈锥形,在人体头脚方向剂量分布和照射野的宽度有关,40 cm×5 cm照射野的输出剂量率为848.38 cGy/min.剂量仪的读数R和照射时间t的关系为R=-0.017+0.256t,线性相关系数为0.999.重复测量的输出剂量率的最大偏差为1.6%,标准偏差＜0.5%;输出剂量率随机架角度变化的最大偏差为1.1%,标准偏差＜0.5%.多叶准直器相邻叶片对单个叶片照射野的剂量贡献比较大,继续增加叶片数目输出因子基本保持不变.结论 Hi-ART断层治疗机的输出剂量率高,照射野剂量分布不均匀.独特的设计和剂量学特性使其剂量计算模型和调强实现方式更加简单、高效.     

SIEMENS 6MV X刀剂量测量

用P型半导体探测器测量X刀4mm~41．2mm射野的输出因子Scp、体模散射因子Sp、百分深度剂量PDD和离轴比曲线OAR；用0．1cc电离室测量射野的输出因子、体模散射因子和组织最大剂量比（TMR）。通过两者的比较及与已发表文献比较，对结果给予评价。结果显示：用小型半导体探测器测量X刀，可获得准确的结果；测量百分深度剂量获得TMR比用电离室直接测量TMR更快捷更简便；0．1cc电离室测量射野输出因子，电离室直径要小于射野直径的一半，否则将引起较大误差。     

Elekta直线加速器全碳纤维六维治疗床床板对后斜野放疗剂量的影响

[目的]探讨Elekta Synergy直线加速器全碳纤维六维治疗床床板对后斜野放疗剂量的影响。[方法]采用等中心技术测量,深度6cm,射野10cm×10cm,能量6MV和15MV,每个射野照射剂量100MU,机架角间隔5°设一个野,设两组射野,一组射野机架角为270°至0°,另一组射野机架角为90°至180°,两组射野为等中心对穿野;准直器和治疗床角度为0°。将标准固体水模中轴与治疗床中心纵轴重合置于治疗床上,用PTW剂量仪0.6CCFarmer电离室比对测量,计算出全碳纤维六维治疗床主床板、延长板及其衔接处对后斜野放疗剂量的衰减。[结果]6MV能量,全碳纤维六维治疗床主床板对剂量的衰减在1.7%～6.0%;延长板对剂量的衰减在1.8%～6.1%;延长板和主床板衔接处对剂量的衰减在8.4%～25.6%。15MV能量,全碳纤维六维治疗床主床板对剂量的衰减在1.1%～4.8%;延长板对剂量的衰减在1.1%～4.7%;延长板和主床板衔接处对剂量的衰减在6.0%～19.8%。[结论]全碳纤维六维治疗床主床板及其体部延长板对剂量的衰减接近,进行常规二维或三维适形治疗时后斜野应做剂量修正;因延长板和主床板衔接处都为床板的边框且有固定螺栓,故衰减比较大,在治疗摆位时应避免射野穿过衔接处。同一能量由于机架角度的改变穿过床板的厚度不同,不同厚度床板衰减因子也不同,随床板厚度的增加衰减因子减小;不同能量同一位置床板衰减也不同,能量增加衰减变小。计划系统计算模型没有加入治疗床板的修正,后斜野由于床板对剂量的衰减,一方面会导致靶区欠剂量,另一方面会增加皮肤剂量,应引起临床治疗的注意。     

直线加速器小照野物理数据的测量和比较

背景与目的：对于精确的肿瘤放射治疗特别是立体定向和调强放射治疗,为了建立可靠的治疗计划系统剂量计算模型,提供准确的小照射物理数据尤其重要。本研究通过测量不同能量下小照野的物理数据,分析和比较不同方法和不同电离室之间相应的测量误差。方法：在直线加速器4、6、8MV光子线下,采用0．65、0．13、0．01cm^3的三种指形电离室,在30cm×30cm×30cm的固体水体模中测量了1cm×1cm～10cm×10cm照射野的总散射因子（total scatter factor,Scp）、准直器散射因子（collimator scatter factor,Sc）和组织最大剂量比（tissue-maximum ratio,TMR）等物理数据。对相应的测量结果进行了分析和比较。结果：照射野〉3cm×3cm时,不同电离室的Scp和Sc测量结果偏差在0．8％以内：3cm×3cm以下的照射野的测量结果差别较大（最大64％）;在4、6、8MV光子线1cm×1cm和2cm×2cm照射野的Sc测量中。0．13cm^3电离室拉长源皮距（〉150cm）比标准源皮距处（100cm）的测量结果分别大25．4％、6．9％、24．6％和1．4％、1．4％、2．2％;两种电离室0．01cm^3和0．13cm^3拉长距离测量的Sc对≥2cm×2cm照射野没有明显的偏差．对1cm×1cm照射野0．13cm^3比0．01cm^3测量值小0．2％、8．5％、3．4％。在1cm×1cm照射野的TMR测量中,0．01cm,和0．13cm^3电离室在15cm以下区域的测量偏差较大,约为4％左右。对于2cm×2cm及以上照射野TMR的测量结果偏差较小（〈1％）。〉3cm×3cm的照射野中,TMR测量的结果与百分深度剂量（percentage depth dose,PDD）转换得到的TMR数据在深度15cm之前一致性较好。15cm深度之后有明显的偏差（〉2％）。结论：测量小照射野物理数据时。由于侧向电子散射不够,需要谨慎选择测量探头。不同的测量探头对小照野物理数据的准确性可能存在较大的影响。当侧向电子平衡不能建立时,测量?     

ArcCHECK半导体探头特性及在容积调强弧形治疗剂量验证应用研究

目的 研究ArcCHECK半导体探头特性及在容积调强弧形治疗(VMAT)剂量验证应用。
方法 用PTWRW3型固体水模体对ArcCHECK探头测量固有敏感性、稳定性、剂量响应、剂量率响应、每脉冲剂量响应、射野大小依赖性、角度依赖性,并与PTW31010型0.125 cm³指形电离室测量值或VMAT计划系统计算值比较。随机选取211例已验证过的VMAT计划,分析计划与测量剂量分布的γ通过率差异,两两比较采用配对t检验。
结果 除角度依赖性外ArcCHECK探头其余特性均符合临床验证要求,当射束从探头底部入射时探头响应最小(180°时约为－3.9%),射束从两侧入射时响应最大(255°时约为 7.7%)。113例鼻咽癌、48例宫颈癌和50例直肠癌VMAT计划3 mm3%的γ平均通过率分别为93.5%、95.7%和97.5%,两两比较t=－12.69~－4.88,P均<0.01。
结论ArcCHECK半导体探头进行VMAT剂量验证前需精心校正,计划复杂度是影响VMAT计划验证通过率主要因素。     

二维半导体探测器阵列的方向性响应和在调强放射治疗合成剂量验证中的应用研究

Background and Objective:The planning dose distribution of intensity-modulated radiation therapy(IMRT) has to be verified before clinical implementation.The commonly used verification method is to measure the beam fluency at 0 degree(0°) gantry angle with a 2-dimensional(2D) detector array,but not the composite dose distribution of the real delivery in the planned gantry angles.This study was to investigate the angular dependence of a 2D diode array(2D array) and the feasibility of using it to verify the co...     

A reduction in the AAPM TG-36 reported peripheral dose distributions with tertiary multileaf collimation. American Association of Physicists in Medicine Task Group 36.

PURPOSE: The American Association of Physicists in Medicine Task Group 36 (AAPM TG-36) data can be used to estimate peripheral dose (PD) distributions outside the primary radiation field. However, the report data apply to linear accelerators not equipped with tertiary multileaf collimators (MLCs). Peripheral dose distributions consist of internal scatter, collimation scatter, transmission through collimation, head leakage, and room scatter. Tertiary MLCs may significantly reduce the PD due to a reduction in collimation scatter, transmission through collimation, and head leakage. Measurements were performed on a multimodality linear accelerator, equipped with a tertiary MLC, to determine PD distributions as a function of energy, field size, distance from the primary radiation field edge, MLC position, and collimator orientation. METHODS AND MATERIALS: Measurements were made using an ionization chamber embedded in a 20 x 40 x 120-cm3 water-equivalent plastic phantom with the secondary collimator and MLC settings of 10 x 10, 15 x 15, 20 x 20, 25 x 25 cm2, and with the MLC fully retracted. Data were taken along the longitudinal axis of the machine for 6 and 18 MV photons. Peripheral dose distributions were evaluated with the collimator set to 180 and 90 degrees. Rotation of the collimator allowed measurements parallel and orthogonal to the direction of motion of the MLC. RESULTS: For both photon energies, peripheral doses measured on a MLC machine were lower than the TG-36 data. When the collimator is rotated by 90 degrees, placing the lower jaws and the MLC leaves along the plane of interest, PD was reduced by as much as a factor of three compared with PDs measured with the MLC fully retracted and the collimator rotated to 180 degrees. PDs measured with the MLC fully retracted and collimator rotated to 180 degrees were comparable to the TG-36 data. Measured PDs were lower when the MLC was used to shape the field than when the MLC was fully retracted. CONCLUSION: A strategic orientation of the collimator with a tertiary MLC can reduce PD distributions by more than a factor of two. This decrease significantly lessens or eliminates the need for external lead shielding to reduce the critical organ dose. This method can be used even when Lipowitz metal blocking (such as for mantle fields) is used, with the MLC leaves oriented along the longitudinal plane.     

Experimental determination of peripheral photon dose components for different IMRT techniques and linear accelerators

PurposeTo quantify the relative peripheral photon doses (PD) to healthy tissues outside the treated region for different IMRT technologies and linac head designs.Material and MethodsMeasurements were performed on an Elekta linac for various energies (6 MV, 10 MV, 25 MV) at different depths at a distance of 29 cm off-axis (vertical measurements) and different distances from the field edge at constant depth of 10 cm (horizontal measurements). These measurements were compared with results obtained on a Siemens linac at 6 MV and 15 MV [26]. TLD-700 detectors were used to quantify the PDs relative to the dose in the volume exposed with the primary beam. Intensity modulated (IM)-beams with identical fluence patterns were generated with a segmental multileaf (sMLC) technique and with lead-containing cerrobend compensators (MCP96). PD values of IM beams were compared with open beam values. All measurement results of the two different linacs, the different IM methods and the different energies were normalized to the same mean dose.ResultsPD values were distinctly higher near the surface (0.5–20 mm) than at larger depth and showed the same trend for all photon beam energies. In comparison with the open field, the photon dose component of PD for IM beams delivered with a segmental MLC technique were increased by a factor varying from 1.2 to 1.8, depending on photon energy and depth. This ratio was around 2 for compensator based IMRT. Depending on depth and distance from the field edge the PD on the Siemens machine was about 30% to 50% higher than on the Elekta machine for the same nominal photon energy.ConclusionThe treatment head design of a linac has a large impact on PD in IMRT as well as for open beams. PD can be minimized by proper selection of treatment delivery method and photon beam energy.     

Peripheral dose from a dual energy linear accelerator equipped with tertiary multileaf collimators and enhanced dynamic wedge

Peripheral dose (PD) or the dose outside the geometrical boundaries of the radiation field is of clinical importance when anatomical structures with low dose tolerances might be involved(1). It is the aim of this study is to estimate the PD on linear accelerators on different wedge systems without multileaf collimator (MLC). Measurements were performed on a dual energy linear accelerator equipped with tertiary MLC and enhanced dynamic wedge (EDW). Measurements were made using an ionization chamber embedded in a Radiation Field Analyser (RFA-300) with the secondary collimator and MLC setting of 5x5, 10x10, 15x15, and 20x20 cm2, and with the MLC fully retracted. The effects of SSD on PD were measured at three SSDs of 90, 100, and 110 cm for the irradiation fields of 5x5, 10x10, 15x15, and 20x20 cm2 and the effects of the three different wedges (Upper wedge, Lower Wedge and Enhanced Dynamic Wedge) on PD were measured for 45° wedges with field size of 15x15 cm2. Data were taken from 3 cm to 24 cm away from the field edge. Results show that due to tertiary MLC, PD can be reduced by means of a factor of two to three at certain distance from the edge of the field compared with TG-36 data. In between the wedges, the PD was less for the EDW when compared with the upper and lower physical wedges. We conclude that the reduction in PD is significant in reducing or eliminating the need for external peripheral shielding to reduce the dose on affected critical organs. Keywords: peripheral dose, multileaf collimator, enhanced dynamic wedge, linac, MLC.     

Flattening filter free beams in SBRT and IMRT: dosimetric assessment of peripheral doses

Purpose

Recently, there has been a growing interest in operating medical linear accelerators without a flattening filter. Due to reduced scatter, leaf transmission and radiation head leakage a reduction of out-of-field dose is expected for flattening filter free beams. The aim of the present study was to determine the impact of unflattened beams on peripheral dose for advanced treatment techniques with a large number of MUs.

Material and methods

An Elekta Precise linac was modified to provide 6 and 10 MV photon beams without a flattening filter. Basic beam data were collected and implemented into the TPS Oncentra Masterplan (Nucletron). Leakage radiation, which predominantly contributes to peripheral dose at larger distances from the field edge, was measured using a Farmer type ionisation chamber. SBRT (lung) and IMRT (prostate, head&neck) treatment plans were generated for 6 and 10 MV for both flattened and unflattened beams. All treatment plans were delivered to the relevant anatomic region of an anthropomorphic phantom which was extended by a solid water slab phantom. Dosimetric measurements were performed with TLD-700 rods, radiochromic films and a Farmer type ionisation chamber. The detectors were placed within the slab phantom and positioned along the isocentric longitudinal axis.

Results

Using unflattened beams results in a reduction of treatment head leakage by 52% for 6 and 65% for 10 MV. Thus, peripheral doses were in general smaller for treatment plans calculated with unflattened beams. At about 20 cm distance from the field edge the dose was on average reduced by 23 and 31% for the 6 and 10 MV SBRT plans. For the IMRT plans (10 MV) the average reduction was 16% for the prostate and 18% for the head&neck case, respectively. For all examined cases, the relative deviation between peripheral doses of flattened and unflattened beams was found to increase with increasing distance from the field.

Conclusions

Removing the flattening filter lead to reduced peripheral doses for advanced treatment techniques. The relative difference between peripheral doses of flattened and unflattened beams was more pronounced when the nominal beam energy was increased. Patients may benefit by decreased exposure of normal tissue to scattered dose outside the field.     

Phantom and in-vivo measurements of dose exposure by image-guided radiotherapy (IGRT): MV portal images vs. kV portal images vs. cone-beam CT.

PURPOSE: Positioning verification is usually performed with treatment beam (MV) portal images (PI) using an electronic portal imaging device (EPID). A new alternative is the use of a low energy photon source (kV) and an additional EPID mounted to the accelerator gantry. This system may be used for PI or--with rotating gantry--as cone-beam CT (CBCT). The dose delivered to the patient by different imaging processes was measured. METHODS AND MATERIALS: A total of 15 in-vivo dose measurements were done in five patients receiving prostate IMRT. For anterior-posterior (AP) and lateral PI with MV and kV photons measurement points were inside the rectum and at the patient's skin. Dose for CBCT was measured in the rectum. Additional measurements for CBCT were done in a cylindrical CT-dose-index (CTDI) phantom to determine peripheral, central and weighted CTDI. RESULTS: The dose for AP MV PI was 57.8 mGy at the surface and 33.9 mGy in the rectum, for lateral MV PI 69.4 mGy and 31.7 mGy, respectively (5 MU/exposure). The dose for AP kV PI was 0.8 mGy at the surface and 0.2 mGy in the rectum, for lateral PI 1.1 mGy and 0. 1 mGy, respectively. For a CBCT the rectal dose was 17.2 mGy. The peripheral CTDI was 23.6 mGy and the center dose was 10.2 mGy, resulting in a weighted CTDI of 19.1 mGy in the phantom and an estimated surface dose of < or =28 mGy. CONCLUSIONS: Even taking into account an RBE (Relative Biological Effectiveness) of 2 for kV vs. MV radiation, for kV PI the delivered dose is lower and image quality is better than for MV PI. CBCT provides a 3D-image dataset and dose exposure for one scan is lower than for two MV PI, thus rendering frequent volume imaging during a fractionated course of radiotherapy possible.     

头颈部兆伏级锥形束CT成像周边剂量测量与评价

目的 测量兆伏级锥形束CT (MVCBCT)成像在头颈部图像引导放疗的周边剂量(PD),评价PD与MVCBCT成像机器跳数(MU)间关系,探讨MVCBCT扫描射野对PD的影响。方法将0.65 cm³电离室插放在特设固体水体模中,使用MVCBCT图像采集预案对体模头颈部扫描,测量射野外体模中心垂直平面在不同深度和距射野边缘不同距离的PD。使用相同电离室在体模中参考点上审视MVCBCT成像的MU与PD的线性关系,并在射野外体模横断面上测量PD均匀性。     

A mobile shield to reduce scatter radiation to the contralateral breast during radiotherapy for breast cancer: preclinical results

Purpose: To design a practical breast shield and to investigate its efficacy in reducing scattered radiation to the contralateral breast of patients undergoing radiation therapy for breast cancer.

Methods and Materials: We constructed a mobile shield consisting of (a) a mobile base and a counterweight; (b) a vertical column adjustable in height and a diagonal arm adjustable in angle; (c) a curved, 2.5-cm thick lead sheet with a 1-cm thick polystyrene liner for blocking scattered radiation; and (d) diode detectors to verify that the edge of the lead sheet is not in the useful beam in addition to the use of the field light. Measurements were performed with thermoluminescent dosimeters on 10 patients without the shield and on an anthropomorphic phantom with a pair of wax breasts with and without the shield. All of the patients were treated with 6-MV photons (Varian 6/100). The scattered radiation from the medial and lateral fields was measured separately.

Results: The contribution of the medial field to the total scattered dose was 70% to 75%, whether a medial wedge was used or not. However, without a medial wedge, the scattered dose was reduced by nearly 33% at 3 to 9 cm away from the medial border. In the anthropomorphic phantom study with wax breast, the mobile shield reduced the medial field contribution to the total scatter dose to less than the contribution from the lateral field without a shield. With a prescribed dose of 50 Gy and a medial wedge, the median scatter dose to the contralateral breast from 6 patients was 5.3 Gy; without a medial wedge, it was 3.8 Gy from 4 patients at 6 cm from the medial border. In the phantom study, with the shield the total dose to the contralateral breast was 1.0 Gy at 6 cm from the medial border with a same prescribed dose.

Conclusion: The mobile shield reduced the scatter dose to the contralateral breast from the linear accelerator (Varian 6/100, 6-MV photons) by a factor of 3 to 4. The shield greatly reduced the scattered dose in the wax phantom. Equivalent reductions in patients may be clinically significant by reducing the risk of radiation-induced breast cancer in the contralateral breast of woman undergoing radiation therapy for breast cancer. The shield is safe and easy to adjust to each patient.