CT模拟定位机扫描条件对图像质量的影响

目的:分析大孔径CT模拟定位机不同扫描条件对CT图像的影响,为模拟定位工作中扫描协议的选择提供参考。方法:利用飞利浦大孔径CT模拟定位机扫描Catphan504模体,分析管电压、管电流、层厚、过滤器、扫描方式、扫描视野、摆放位置等扫描条件对CT图像的影响。结果:CT值参数受管电压及过滤器影响明显,140 kV管电压CT值最大偏差为69.6 HU;Sharp Edge过滤器最大偏差为215.3 HU。图像均匀性受过滤器Detail、Y-Sharp、Y-Detail影响明显。空间分辨率参数受矩阵、过滤器、摆放位置参数影响明显。对比度分辨率参数受管电流及过滤器影响明显。结论:管电压、管电流、过滤器、矩阵、摆放位置对CT图像的影响不可忽略,临床工作中可根据实际需要选择合适的扫描条件,在保证CT值准确的同时提高图像质量。     

CT模拟定位机的扫描参数对放疗计划系统剂量计算的影响

目的:探讨模拟定位CT的扫描参数对放射治疗计划系统剂量计算的影响。方法:在不同扫描条件下,将可拆分的带有人体组织等效密度插件的参考模体(CIRS-062)在同一台CT模拟定位机上进行扫描,比较得到的各密度插件CT值;将同一IMRT治疗计划移植到CT图像上,在相同条件下进行剂量计算,测量和分析模体内剂量变化。结果:不同扫描条件下同一模块的CT值有改变,CT值受管电压影响较大;扫描条件对剂量分布有一定影响,其中低剂量区域影响较高剂量区域大。结论:CT模拟定位机的扫描条件对放疗计划系统剂量计算有一定影响,CT模拟定位机的质量保证与质量控制也是放射治疗过程中的一个重要环节。     

放疗计划CT值的校准检测及其影响因素分析

背景与目的:放射治疗计划系统(treatmentplanningsystem,TPS)依赖CT扫描得到的CT值建立CT-密度转换曲线,并根据转换所得的组织密度(或电子密度)进行组织不均匀性剂量校正计算,从而得出放射治疗计划的剂量分布。同一组织在不同的扫描条件下得到的CT值可能产生相当大的差异,从而导致剂量计算的误差。本研究利用等效模体在不同条件下进行CT扫描,测量各因素对CT值影响的大小及分析各影响的相对严重程度。方法:用含有已知密度的不同组织替代材料组成的模体在不同的CT扫描条件下进行CT值测量,评价扫描参数、患者体形和CT床面等因素对CT值的影响。实验条件分为3组:(1)以不同的扫描电压、层厚及扫描方式(逐层/螺旋扫描)对相同模体进行CT扫描;(2)改变各不同密度的组织替代材料在模体中的位置和排列顺序,固定CT参数进行图像扫描;(3)扫描参数不变,分别将相同模体置于床面和悬空进行比较扫描和CT值测量。结果:扫描电压和床面分别对高密度组织和低密度组织的CT值影响较大,前者对皮质骨模型的CT值改变可达150Hu(12.7%),后者对水模的CT值改变达26.34%。模体几何位置对CT值的影响可以忽略(<3%)。结论:不同的CT扫描参数和定位条件可能使某一组织重建影像的CT值发生改变,从而造成TPS剂量计算误差。尤其以扫描电压和定位床面的散射对CT值的影响为甚。已知密度的不均匀体模可以用于放射治疗计划CT扫描的质量保证检测和CT值的校准。各放疗中心应根据实际测量和校准结果制定统一的CT定位扫描规范,对每一患者采用一致的扫描参数和定位条件,以保证治疗计划剂量计算的精度。     

大孔径CT模拟定位机扫描条件对CT值影响分析

目的:分析大孔径CT模拟定位机扫描条件对CT值的影响,为模拟定位工作中不同扫描条件的选择提供参考。方法:采用飞利浦大孔径CT模拟定位机扫描CIRS-062M电子密度模体并建立CT值-相对电子密度曲线。分析管电流、管电压、层厚、层间距、分辨率、准直器、旋转时间、重建模型、过滤器、几何位置多种扫描条件对CT值的影响。结果:管电压变化对CT值影响最为显著,90 kV、140 kV管电压CT值平均偏差66.1 HU、-24.0 HU,差异主要集中在高密度区域。过滤器YA、YB对CT值影响明显,CT值平均偏差-40.7 HU、-39.9 HU。不同扫描位置对CT值有一定影响。结论:管电压、部分过滤器、扫描位置对CT值的影响不可忽略,临床工作中需规范上述参数的使用,以免造成剂量计算偏差。     

不同CT扫描条件模拟定位对放射治疗计划的影响

目的:利用不均匀等效模体,探讨不同CT扫描条件对CT值及照射跳数(MU)的影响.方法:采用大小2种CT孔径(80和70 cm)、2种模体几何摆放顺序及2种扫描电压(120和140 KV),测量不同组合,比较各自的CT值,建立相应的CT-电子密度(ED)转换曲线,选取盆腔、胸部、头颈部各10例患者的CT图像模拟适形计划(CRT)和调强计划(IMRT),分析照射MU数值的偏差.结果:对于小孔径CT,无论扫描电压、模体几何位置如何变化,其MU数值相差均≤0.1％；大孔径CT扫描电压改变对MU值无影响,模体几何摆放顺序的改变有影响,但＜0.3％；CRT计划和IMRT计划各自偏差值相同.结论:对于精确放疗计划系统,CT扫描条件改变和测量模体位置改变均可引起照射MU数值误差.若使用大孔径CT进行模拟定位,需根据其特性建立合理的电子密度曲线,并应用在计划制定中.     

锥形束CT重建影像CT值空间均匀性分析

目的 锥形束CT对均匀模体进行扫描后重建图像的CT值在空间分布是否均匀.方法 使用SynergyTM的锥形束CT对IBA调强验证均匀固体水模体进行扫描,将结果通过放疗网络传至治疗计划系统.测量重建图像的CT值在三维空间的分布,并与扇形束CT扫描重建图像的测量结果进行对比,从而得到锥形束CT重建影像CT值空间均匀性.结果 该模体在扇形束cT扫描重建图像中CT值分布均匀,波动范围在±50内.相同扫描条件下锥形束CT重建图像的CT值在水平方向具有一定的对称性,在垂直方向和头脚方向均不具有对称性.不同kV值扫描重建图像的CT值有一定差别,不同滤过时重建影像的CT值有明显差别.结论 锥形束CT扫描重建的CT值在空间分布不均匀.     

瓦里安23EX加速器附加锥形束CT图像的CT值线性分析

目的 分析瓦里安23EX加速器附加千伏级X线锥形束CT (CBCT)在不同扫描条件下模体图像在不同位置处的CT值线性变化。方法 应用安装在直线加速器上的CBCT系统在标准头(体)部扫描条件下重复扫描不同位置Catphan504模体,将结果传至计划系统及Matlab 7.0,测量不同位置处不同密度插件的CT值线性情况。经与传统扇形束CT重建后图像CT值线性结果进行比较,了解CBCT图像CT值线性空间分布。结果 模体CBCT图像在标准头(体)部扫描条件下,在横断面、矢状面、冠状面及偏中心位置处CT值线性均有良好表现,其线性拟合因子R²值均>0.953。Bowtie滤过器虽然使得被测量物质的CT值不同,但并不改变CBCT图像CT值的线性。结论 瓦里安23EX加速器附加CBCT图像CT值线性良好,如对CT值做进一步校正,使CBCT图像用于治疗计划系统的剂量计算将成为可能。     

对HT系统MVCT扫描图像CT值及噪声影响因素的研究

目的 探讨HT的MVCT扫描剂量率波动和不同扫描精度选择对图像CT值及噪声的影响。方法 MVCT在不同剂量率和扫描层厚下扫描带各种密度棒的水模体,对应各密度棒的CT值得出各自条件下的IVDT曲线进行对比。MVCT在不同剂量率和扫描层厚下扫描带有均匀密度棒的水模体,测量不同扫描条件下均匀水模体中的CT图像噪声。对结果进行方差及相关性分析。结果不同扫描剂量率下对CT值影响显著(P=0.000),各密度棒对应CT值的变化大小与密度呈正相关(R²=0.846),既高密度物质的CT值受的影响较大。不同剂量率对噪声也有显著影响,扫描剂量率越低图像噪声越大(P=0.000)。2、4、6 mm扫描层厚对CT值(P=1.000)及图像噪声(P=0.667)无显著影响。结论 MVCT剂量率改变会引起CT值及图像噪声改变,在临床中应定期对MVCT剂量率及IVDT曲线稳定性进行监测,以保证ART计划中MVCT图像剂量计算的准确。     

头颈肿瘤立体定向分次照射靶区定位的误差分析

背景与目的:明确靶区定位的精确度是立体定向分次照射质量保证的基本要求。本文主要分析头颈肿瘤立体定向分次照射(fractionatedstereotacticradiotherapy,FSRT)中机械等中心、CT定位、治疗摆位以及CT图像误差等可能引起的靶区定位误差。方法:使用立体定向治疗计划系统、靶点模拟器、头部定位框架检查各个治疗阶段靶区定位的误差。设置任意5个参考点,使用靶点模拟器检查CT定位误差;选取7个不同机器臂架/治疗床角度,定期用胶片检验使用的PhilipsSL-18直线加速器等中心误差大小;用验证片检查治疗摆位误差;对自制模体行CT扫描,分析CT图像伪影可能引起的图像误差。结果:CT定位误差约为(1.5±0.4)mm;在检查的不同机器臂架/治疗床角度中机械等中心最大误差为(1.0±0.6)mm;患者摆位的距离误差为(1.0±0.3)mm;整个治疗过程中靶区定位误差约为(2.1±0.8)mm。结论:立体定向分次照射中需要综合考虑各个阶段中可能对治疗靶区定位产生的影响,误差分析结果可用来确定治疗的计划靶区。     

CT扫描/重建参数对三维治疗计划系统影像的影响

[目的]研究CT模拟定位中,CT扫描/重建参数对三维治疗计划重建的三维假体的几何精确度的影响.[方法]在西门子CT模拟机（Somatom plus 4）上扫描自制模体,扫描所得图像登记到ADAC三维治疗计划系统重建成三维假体,测量假体的相关坐标数据并与模体的实际数据相比较;对Catphan 412模体扫描并测量各组图像的实际层厚,讨论实际层厚的变化对计划系统中登记影像的几何精度影响.[结果]（1）CT扫描所采用的不同扫描/重建参数对三维计划系统中重建的三维假体的左右及上下方向的几何精度影响不大,但对重建假体的前后方向（即模体扫描的步进方向）的几何精度有一定的影响.（2）CT扫描所采用的螺距及重建模式会对层厚敏感度曲线（SSP）半高宽值产生影响,该变化对重建假体的前后方向几何精度同样有一定的影响.[结论]重建CT图像的前后方向的几何误差是随着扫描层厚增加而增加,主要是由于CT扫描的部分容积效应影响.单纯增加螺距或使用360度线性内插（Wide）重建模式,都会引起CT图像实际层厚的增加,引起更大的容积效应影响.同时部分容积效应也会导致三维治疗计划系统中数字重建影像（DRR）分辨率的降低.     

VMAT模式下MLC叶片运动速度对到位误差影响

目的 探讨RapidArc模式下MLC叶片运动速度对叶片到位误差的影响,完善RapidArc QA方案,验证RapidArc可靠性。方法 参考PicketFenceStatic_M120.dcm、PicketFenceRA_M120.dcm文件,设计模拟相邻叶片不同速度的Tilt测试,分析EPID图像得出叶片到位误差。结果 静态机架和RapidArc模式下,Tilt测试中gap11~gap50的位置误差都逐渐增大。机架270°时gap41误差最大,为-0.55 mm。RapidArc模式下gap46误差最大,为-0.67 mm。所有模式下gap宽度偏差均≤15%。对比4个固定机架角度的宽度偏差图形几乎相似,每个条纹相同gap处宽度偏差呈现一致趋势。与静态机架模式下gap宽度偏差百分比相比,RapidArc模式下的波动更大。结论 随MLC叶片速度增加,到位误差随之增大。叶片速度不同并未对gap宽度造成明显影响,不同叶片速度形成的gap宽度偏差无规律。4个固定机架角度下gap宽度偏差图像相似,说明gap宽度偏差与叶片本身有关,与机架所处角度几乎无关。RapidArc模式比固定机架模式对gap宽度影响更大。     

Treatment planning for stereotactic radiosurgery of intra-cranial lesions

Stereotactic radiosurgery of intra-cranial lesions is a treatment modality where a well defined target volume receives a high radiation dose in a single treatment. Our technique delivers this dose using a set of non-coplanar arcs and small circular collimators. We use a standard linear accelerator in our treatments, and the adjustable treatment parameters are: isocenter location, gantry arc rotation interval, couch angle, collimator field size, and dose. The treatment planning phase of the treatment determines these parameters such that the target volume is sufficiently irradiated, and dose to surrounding healthy tissue and critical, dose-limiting structures is minimized. The attachment of a BRW localizing frame to the patient's cranium combined with CT imaging (and optionally MRI or angiography) provides the required accuracy for localizing individual structures in the treatment volume. The treatment is fundamentally 3-dimensional and requires a volumetric assessment of the treatment plan. The selection of treatment arcs relies primarily on geometric constraints and the beam's eye view concept to avoid irradiating critical structures. The assessment of a treatment plan involves isodose distributions throughout the volume and integral dose-volume histograms. We present the essential concepts of our treatment planning approach, and illustrate these in three clinical cases.     

A new irradiation unit constructed of self-moving gantry-CT and linac

PURPOSE: To improve reproducibility in stereotactic irradiation (STI) without using noninvasive immobilization devices or body frames, we have developed an integrated computed tomography (CT)-linac irradiation system connecting CT scanner and linac via a common treatment couch. METHODS AND MATERIALS: This system consists of a linac, a CT scanner, and a common treatment couch. The linac and the CT gantry are positioned on opposite ends of the couch so that, by rotating the treatment couch, linac radiotherapy or CT scanning can be performed. The rotational axis of the linac gantry is coaxial with that of the CT gantry, and the position of the linac isocenter on the couch matches the origin of the coordinate system for CT scanning when the couch is rotated 180 degrees toward the CT side. Instead of the couch moving into the gantry, as in conventional CT, in this case the table is fixed and scanning is accomplished by moving the gantry. We evaluated the rotational accuracy of the common couch and the scan-position accuracy of the self-moving gantry CT. RESULTS: The positional accuracy of the common couch was 0.20, 0.18, and 0.39 mm in the lateral, longitudinal, and vertical directions, respectively. The scan-position accuracy of the CT gantry was less than 0.4 mm in the lateral, longitudinal, and vertical directions. CONCLUSION: This irradiation system has a high accuracy and is useful for noninvasive STI and for verification of the position of a target in three-dimensional conformal radiotherapy.     

Dosimetric impacts of gantry angle misalignment on prostate cancer treatment using helical tomotherapy

During helical tomotherapy, gantry angle accuracy is one of the vital geometric factors that assure accurate dose delivery to the target and organs at risk adjacent to it. The purpose of this study is to investigate the dosimetric impact of gantry angle misalignment on the target volume and critical organs during helical tomotherapy treatment. Five prostate cases were chosen to calculate the effects of gantry angle deviations on both patient-specific delivery quality assurance (DQA) and helical tomotherapy treatment plans. For DQA plans, the cheese phantom was rotated for up to +/-5 degrees from the preset position to simulate the gantry angle deviations during tomotherapy. Point doses at 5 mm below the isocenter and the dose distribution for each gantry angle were measured and reconstructed, respectively. For helical tomotherapy treatment plans, the same gantry misalignment effect was simulated by adjusting the automatic roll correction for up to +/-5 degrees using Planned Adaptive software. Variations of dose volume histograms (DVHs) and isodose lines were evaluated for both target and critical organs. There was no significant difference found, however, among the point dose measurements for gantry rotation up to +/-5 degrees in DQA plans. Shifts of isodose lines could be observed for gantry rotations larger than +/-27 degrees. Dosimetric discrepancies (less than 2%) were also found among DVHs of the PTV in the cases when gantry angle misalignment was larger than +/-2 degrees. However, for DVHs of either bladder or rectum under different gantry rotations, no significant differences were detected when gantry angle errors were up to +/-5 degrees. In summary, point dose measurements alone cannot reveal the dosimetric deviation due to gantry angle misalignment in DQA plans. For a 5 degrees gantry deviation, the dose to PTV increased by 0.5% comparing to the planned dose. The influence on organs at risk, i.e., rectum and bladder, is also negligible. Further studies are needed on the dosimetric impacts of gantry angle deviations for other treatment sites.     

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.     

Assessment of patient-independent intrinsic error for a noninvasive frame for fractionated stereotactic radiotherapy

The purpose of our study was to examine the extent of patient-independent intrinsic error associated with multiple, repeat remounting of the Laitinen Stereoadapter. The Laitinen frame was repeatedly mounted on a solid water phantom and imaged using computed tomography (CT). The phantom contained five targets located in the center, anterior, right, left, and posterior orientations. The images were processed, fused, and analyzed on the Pinnacle 3-D treatment planning system. The coordinate values (in the x, y, and z directions) for each target were determined for each mounting, and an absolute mean deviation was calculated for 11 repetitions. The mean deviation in the x, y, and z direction for the central and right target, and in the x and y direction for the posterior and anterior target was less than 2.0 mm. However, the mean error in the z direction of the anterior and posterior targets was 1.79 +/- 1.02 mm and 2.20 +/- 1.32 mm, respectively. Rotational misalignment during repeat frame fixation contributed to the observed deviations and in particular affected the antero-posterior plane. With the exception of two occasions where an obvious mounting error occurred, a significant portion of error from remounting the Laitinen Stereoadapter is associated with the operator and the imaging process. The observation of an angular displacement around the axis through the earplugs suggests that a certain degree of rotational misalignment in daily remounting is possible. Targets in the antero-posterior plane are most susceptible to localization error as a consequence of rotational misalignment. In summary, the overall error is within the limits of current imaging technology but not within submillimeter accuracy. Clinical application should take these errors into consideration when designing field margins.     

Stereotactic radiotherapy of extracranial targets: CT-simulation and accuracy of treatment in the stereotactic body frame.

BACKGROUND AND PURPOSE: Evaluation of set-up accuracy and analysis of target reproducibility in the stereotactic body frame (SBF), designed by Blomgren and Lax from Karolinska Hospital, Stockholm. Different types of targets were analyzed for the risk of target deviation. The correlation of target deviation to bony structures was analyzed to evaluate the value of bones as reference structures for isocenter verification. MATERIALS AND METHODS: Thirty patients with 32 targets were treated in the SBF for primary or metastatic peripheral lung cancer, liver metastases, abdominal and pelvic tumor recurrences or bone metastases. Set-up accuracy and target mobility were evaluated by CT-simulation and port films. The contours of the target at isocenter level, bony structures and body outline were compared by matching the CT-slices for treatment planning and simulation using the stereotactic coordinates of the SBF as external reference system. The matching procedure was performed by using a 3D treatment planning program. RESULTS: Set-up accuracy represented by bony structures revealed standard deviations (SD) of 3.5 mm in longitudinal, 2.2 mm in anterior-posterior and 3.9 mm in lateral directions. Target reproducibility showed a SD of 4.4 mm in longitudinal, 3.4 mm ap and 3.3 mm in lateral direction prior to correction. Correlation of target deviation to bones ranged from 33% (soft tissue targets) to 100% (bones). CONCLUSION: A security margin of 5 mm for PTV definition is sufficient, if CT simulation is performed prior to each treatment to correct larger target deviations or set-up errors. Isocenter verification relative to bony structures is only safe for bony targets but not for soft tissue targets.     

Radiation dose and risk to the lens of the eye during CT examinations of the brain

Cataracts are the leading cause of blindness and visual disability worldwide. Of the known contributing factors to this condition, ionising radiation is considered the primary concern in a radiological context given the particular radiosensitivity of the lens of the eye. In light of the substantially increased application of computed tomography in brain imaging, an investigation of the relevent literature is warranted to assess thresholds, lens radiation doses and dose reduction techniques in respect to the cataractogenic risk of such examinations. The value and very existence of a lens dose threshold is debatable given different considerations of radiation dose, latency, opacity classifications and historical sample populations, though ICRP guidelines suggest a threshold of 0.5 Gy. Documented CT‐specific radiation doses to the eye following scans of the brain are highly variable between studies (2–130 mGy), primarily owing to discrepancies in scanning technique. These findings, when coupled with the relative ambiguity of known threshold values, present difficulties in assessing the overall risk of cataracts following serial CT examinations to the head. In the absence of definitive risk evaluations, a cautionary approach is advised. The implementation of gantry tilt along the supraorbital margin is recommended as standard practice on account of its highly effective radiation dose reduction outcomes. Organ‐based tube modulation and reductions in tube current may also be considered beneficial. Bismuth eye shielding is only advised where gantry tilting is unachievable, and in such cases, ensure careful adherence to appropriate shield placement and infection control measures.     

机载CBCT扫描协议对图像质量的影响分析

目的:分析不同扫描协议对CBCT图像的质量影响,为临床扫描协议使用提供参考。方法:使用瓦里安VitalBeam加速器机载CBCT扫描Catphan 604模体,分析扫描协议中管电压、管电流、脉冲时间、机架速率、帧速率、滤线器对图像的影响。结果:头部扫描协议下图像HU值受管电压、帧速率影响明显,体部扫描协议图像HU值受管电压、管电流影响明显。管电压、管电流增加可改善图像均匀性同时降低图像噪声。高对比度分辨力受参数变化影响不明显,增加管电压、管电流或降低机架速度可明显提高图像低对比度分辨力。结论:管电压、管电流、帧速率等参数对CBCT图像影响不可忽略,应根据临床需要设置合理的扫描协议。     

Variation in the isocentre of a Philips Linear Accelerator (SL–20) used for stereotactic radiosurgery/stereotactic radiotherapy

Stereotactic irradiation, either in the form of stereotactic radiosurgery (SRS) or stereotactic radiotherapy (SRT) of brain lesions requires high precision and submillimetre accuracy in the isocentre, the main determinants being gantry and couch rotations. It is thus necessary to evaluate the isocentre variation due to gantry and couch rotations in the particular setup for SRS/SRT. This paper describes variation in the isocentre of a Philips (now Elekta) SL-20 linear accelerator modified for adapting a couch-mounted radiosurgery system. By considering the isocentre as defined by a mechanical index as the standard, the variations in the isocentre of the linear accelerator were independently measured for the gantry and for couch rotations. The variation in the isocentre for gantry rotation was found to be between 0.1 mm and 0.9 mm, conforming to the submillimetre accuracy required for SRS/SRT. However, the isocentre variation due to couch rotation varied considerably, possibly because the couch is of the RAM type. The isocentre variation due to couch rotation is rectified by microadjusting the couch mount at the time of treatment using a laser target localizing frame. It is our conclusion that a modified linear accelerator can be used for performing SRS/SRT after careful and separate evaluation of the isocentre stability due to gantry and couch rotations.