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。     

加速器运行误差对盆腔肿瘤容积旋转调强放疗计划剂量验证的影响

目的 研究加速器机架旋转角度、机器跳数(MU)、准直器到位和多叶准直器(MLC)叶片到位等误差对容积旋转调强放疗(VMAT)计划剂量验证γ通过率的影响。方法 选取已行VMAT的直肠癌和宫颈癌各10例,分别引入加速器各参数运行误差。通过比较引入误差计划与临床计划的剂量验证γ通过率,分析各参数误差对γ通过率的影响及其敏感性。结果 评价指标取3%/3mm、3%/2mm和2%/2mm时,引入机架旋转误差、机器跳数误差和准直器到位误差后的直肠癌和宫颈癌计划相比临床计划的剂量验证γ通过率变化均<7.0%,引入两侧MLC叶片反向、相向、同向运动误差后,每毫米误差导致绝对剂量验证γ通过率变化分别<19.13%、18.53%、0.19%,19.87%、20.01%、0.42%和23.11%、23.45%、0.65%。结论 执行VMAT计划时,相比机架旋转角度误差、机器跳数误差、准直器到位误差和MLC叶片同向偏移误差,MLC叶片反向或相向运动误差对绝对剂量验证γ通过率的影响更加明显,评价指标取3%/3mm、3%/2mm和2%/2mm时绝对剂量验证γ通过率受加速器各参数误差影响依次递增。执行特定患者剂量验证时,应适当使用评价指标并以绝对剂量验证γ通过率为评估计算和测量剂量分布一致性的参考指标。     

基于剂量重建分析摆位误差对鼻腔NK/T细胞淋巴瘤剂量学影响

目的 通过将摆位误差引入放疗计划系统进行剂量重建,分析放疗摆位误差对鼻腔NK/T细胞淋巴瘤剂量学影响。方法 选取 10例鼻腔NK/T细胞淋巴瘤患者,对每一患者CT图像及靶区设计非共面容积旋转调强计划,计划完成后通过改变治疗等中心点参数,将摆位误差引入放疗计划中再进行剂量计算重建剂量分布。结果 随系统摆位误差增加,靶区剂量逐渐下降,影响大小顺序为左右方向>头脚方向>前后方向。各方向平移摆位误差在-3~3mm以及旋转摆位误差在-3°~3°内,靶区剂量变化范围均<±3%。各方向摆位误差≤3mm时危及器官均在处方剂量附近,>3mm时眼晶体、脊髓、腮腺、视神经逐渐超出处方剂量范围;旋转摆位误差仅当≥3°时眼晶体超量,尤其注意左右方向较大的摆位误差对眼晶体、脊髓、腮腺的影响和前后方向较大的摆位误差对脊髓的影响。引入本单位实际摆位误差后对GTV、CTV受量影响很小,均<±2%;少数危及器官有超出处方剂量限值风险,尤其要注意晶体和视神经超量。结论 摆位误差将会导致鼻腔NK/T细胞淋巴瘤靶区剂量欠量以及危及器官超量,左右方向摆位误差影响尤为大,3mm和3°内单一方向摆位误差对靶区和危及器官影响有限,建议将单一方向摆位误差控制在3mm和3°内。引入本单位实际摆位误差对靶区受量影响很小,但少数危及器官有超出处方剂量限值风险,需增加对危及器官外扩区域评价。     

EPID在VMAT技术验收测试中的应用研究

目的 采用EPID实现直线加速器VMAT技术的验收研究。方法 采用Shaper及Eclipse TPS编辑并利用EPID对TrueBeam加速器VMAT技术中MLC到位精度、临床剂量等核心内容进行测试。测试例:TA:机架在0°、90°、270°时分别曝光0.5 cm×20.0 cm狭缝野;TB:不同剂量率及机架角速度组合时射野平坦度稳定性;TC:静态及旋转状态下MLC到位精度;TD:静态及旋转状态下MLC变速控制准确性;TE:加速器同时变剂量率及机架旋转速度的准确性;TF:临床病例。利用Matlab对测试结果进行分析。结果 0°、90°、270°测得狭缝叠加后半高宽与零度时半高宽差别均为0.39 mm。TB结果显示射野平坦度差别<0.5%。TC测试显示EPID测得的狭缝中心与TPS设定的中心最大偏差为0.45 mm。TD测试显示MLC位置实测值与TPS设定值差别最大为0.69 mm。TE结果显示各狭缝相互间差别最大为0.42 mm。临床病例以3%/3 mm标准评价时γ通过率最低为96.4%。结论 利用EPID能准确、便捷的完成VMAT技术验收,此方法为减轻物理师工作负担提供了一个良好选择。     

不同三维探测器对鼻咽癌VMAT计划MLC位置误差探测灵敏度的研究

目的 研究鼻咽癌容积旋转调强(VMAT)计划剂量验证中,Delta⁴和ArcCHECK两种三维探测器对多叶准直器(MLC)叶片位置误差检测的灵敏度。方法 选取10例鼻咽癌VMAT计划,对原始文件中每个MLC子野的叶片分别引入0.5~4.0 mm的位置误差,使子野整体扩大、缩小或偏向一侧平移,模拟VMAT治疗中MLC可能出现的位置误差。分别用Delta⁴和ArcCHECK进行验证测量,比较计划系统计算值与测量结果的γ通过率并行配对t检验。结果 当评价标准取3 mm/3%时,两种探测器所有患者的原计划验证绝对剂量γ通过率均>95%,Delta⁴和ArcCHECK可以检测出的MLC外扩、内收以及平移误差分别是1.5、1.0、2.0 mm和3.0、1.0、3.0 mm;而取2 mm/2%评价标准时,患者原计划验证绝对剂量γ通过率有较大幅度下降,此时Delta⁴和ArcCHECK能检出的MLC外扩、内收和平移误差分别是1.0、1.0、2.0 mm和1.5、0.5、2.0 mm。结论 Delta⁴和ArcCHECK鼻咽癌VMAT计划的剂量验证可以检测出不同类型和大小的MLC位置误差,但两者的检测灵敏性略有差异,而对<1.0 mm微小误差的检测都不够敏感,日常工作中仍需加强MLC的质量保证。     

基于ArcCHECK系统六维摆位误差校正方法精确性观察

目的 观察使用ArcCHECK系统验证全六维摆位误差校正方法的精度。方法 选取 2015年5—9月接受IMRT技术治疗的 14例鼻咽癌患者,使用CBCT获取初次治疗摆位误差。在ArcCHECK模体上模拟摆位误差,使用全六维校正方法对摆位误差进行校正。分别对正确摆位、校正误差前以及校正误差后3种情况进行计划验证。正确摆位与校正误差前后计划验证的DTA和γ通过率行配对t检验。结果 正确摆位,校正误差前及校正误差后验证的DTA、γ通过率分别为(96.76±1.57)%、(98.35±0.92)%,(59±21.42)%、(62.86±21.63)%及(91.41±4.82)%、(94.11± 4.33)%。正确摆位与校正误差前后验证的DTA通过率不同(P均<0.05),正确摆位与校正误差前后验证的γ通过率也不同(P均<0.05)。结论 全六维摆位误差校正方法应用于鼻咽癌IMRT患者的临床治疗是可行的,而且可以更好的修正误差,获得高精度的IMRT治疗剂量分布。     

脑转移瘤立体定向放疗分次间和分次内摆位误差及残余误差分析

目的 分析脑转移患者立体定向放疗ExacTrac X线图像,计算分次间和分次内摆位误差及残余误差,分析进行逐弧位置验证的必要性。方法 通过对过去2年在本中心采用头部立体定向放疗的脑转移瘤病例的回顾性分析,配准其数字重建图像和ExacTrac正交kV级验证图像,计算患者3个方向的平移误差和旋转误差。数据包含分次间摆位误差、分次内摆位误差和残余误差。结果 75例116个病灶进行了337次头部立体定向放疗。分次间、分次内平移摆位误差分别为左右方向x (0.93±0.86)、(0.15±0.59) mm,头脚方向y (1.83±1.27)、(0.25±0.73) mm,腹背方向z (0.96±0.80)、(0.14±0.56) mm;分次间、分次内旋转摆位误差分别为矢状面Rx (0.65°±0.62°)、(0.19°±0.40°),横断面Ry (0.97°±0.94°)、(0.13°±0.25°),冠状面Rz (0.92°±0.71°)、(0.10°±0.29°)。残余平移误差左右、头脚、腹背方向分别为(0.06±0.23)、(0.08±0.24)、(0.08±0.22) mm;残余旋转误差矢状面、横断面、冠状面分别为(0.12°±0.27°)、(0.09°±0.18°)、(0.06°±0.19°)。337次分次间摆位误差99.1%超过误差阈值(0.7 mm,0.7°)需要至少校正1次;1 006组分次内摆位误差33.6%在治疗床转到位验证无需误差校正,66.4%需要校正至少1次。结论 头部立体定向放疗患者要重视分次间摆位误差和分次内摆位误差,进行逐弧体位验证是非常必要的。     

基于轨迹日志的TrueBeam加速器多叶准直器性能测试研究

目的 寻找利用加速器轨迹日志评估多叶准直器(MLC)性能的解决方案并对TrueBeam加速器MLC评估。方法 所有测量在不同机架/小机头组合下各测5次。用动、静态MLC构造宽度1 mm的狭缝,评估加速器小野到位精度控制能力。由MLC重复运动评估其重复性。由MLC构造宽度1 cm的狭缝以不同速度由-7 cm匀速滑至7 cm处停止或立马匀速滑回,评估其匀速、变方向运动。由交叉运动评估其在复杂计划中的表现。结果 动静态狭缝野MLC到位准确度高。重复性得机架0°、非0°时MLC误差频谱分布一致,绝对值差0.0011 mm。机架0°、MLC速度由5 mm/s增至25 mm/s时,其均方误差(RMSE)由0.0150 mm增至0.0598 mm。机架非0°时,RMSE变化趋势一致,但绝对值稍大。MLC变方向运动引起的“超速”较其由静止启动时明显性低,速度在交叉前后无明显变化,速度在设定速度附近上下波动,且与机架角度无关。结论 利用轨迹日志评估加速器MLC性能的方法,能对TrueBeam加速器MLC进行详细评估,可用于MLC快速质控。     

加速器机架角度对多叶准直器叶片到位精度的影响

目的 测量加速器机架角度对多叶准直器叶片到位精度的影响.方法 用柯达X-omat-V胶片进行测量,选定合适叶片位置和机架角度,保证精确摆位,用RIT113胶片分析软件对曝光的胶片做分析.结果 在所有测量情况下,绝大部分叶片到位偏差都<0.5 mm,只有在机架角270°、叶片从左向右运动情况下有2对叶片到位偏差>1 mm.结论 机架角度即重力对多叶准直器叶片到位精度确有影响,精确摆位对测量工作至关重要.     

1.5T 磁共振加速器X线束剂量学特性测试

目的 由于磁场会改变次级电子运动轨迹,继而影响剂量场分布,磁共振加速器(MR-Linac) X线束剂量学特性与常规加速器有差别。本项目旨在测量和分析1.5T MR-Linac的X线束剂量学特性。方法 中国医学科学院肿瘤医院于2019年5月安装1台瑞典医科达公司Unity型1.5T MR-Linac,使用磁场兼容工具对其进行测量,测量项目包括表面剂量、最大剂量点深度、射线质、离轴比曲线(OAR)中心位置、对称性、半影宽度、不同机架角度的输出量变化。结果 不同射野面积的平均表面剂量为40.48%,平均最大剂量点深度为1.25cm。10cm×10cm射野面积下,x轴方向的OAR中心位置往x₂侧偏移1.47mm,对称性为101.33%,两侧半影宽度分别为6.86mm和7.14mm;y轴方向的OAR中心位置偏移0.3mm,对称性为100.85%,两侧半影宽度分别为5.92mm和5.95mm。不同机架角度下输出量最大偏差达1.50%。结论 与常规加速器不同,MR-Linac不同射野面积表面剂量数值趋于一致,最大剂量点深度上升。x轴方向的OAR中心位置往x₂侧偏移,造成对称性变差和半影不对称。不同机架角度下的输出量变化明显,需要修正。     

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.     

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.     

Abutment region dosimetry for serial tomotherapy.

PURPOSE: A commercial intensity modulated radiation therapy system (Corvus, NOMOS Corp.) is presently used in our clinic to generate optimized dose distributions delivered using a proprietary dynamic multileaf collimator (DMLC) (MIMiC) composed of 20 opposed leaf pairs. On our accelerator (Clinac 600C/D, Varian Associates, Inc.) each MIMiC leaf projects to either 1.00 x 0.84 or 1.00 x 1.70 cm2 (depending on the treatment plan and termed 1 cm or 2 cm mode, respectively). The MIMiC is used to deliver serial (axial) tomotherapy treatment plans, in which the beam is delivered to a nearly cylindrical volume as the DMLC is rotated about the patient. For longer targets, the patient is moved (indexed) between treatments a distance corresponding to the projected leaf width. The treatment relies on precise indexing and a method was developed to measure the precision of indexing devices. A treatment planning study of the dosimetric effects of incorrect patient indexing and concluded that a dose heterogeneity of 10% mm(-1) resulted. Because the results may be sensitive to the dose model accuracy, we conducted a measurement-based investigation of the consequences of incorrect indexing using our accelerator. Although the indexing provides an accurate field abutment along the isocenter, due to beam divergence, hot and cold spots will be produced below and above isocenter, respectively, when less than 300 degree arcs were used. A preliminary study recently determined that for a 290 degree rotation in 1 cm mode, 15% cold and 7% hot spots were delivered to 7 cm above and below isocenter, respectively. This study completes the earlier work by investigating the dose heterogeneity as a function of position relative to the axis of rotation, arc length, and leaf width. The influence of random daily patient positioning errors is also investigated. METHODS AND MATERIALS: Treatment plans were generated using 8.0 cm diameter cylindrical target volumes within a homogeneous rectilinear film phantom. The plans included both 1 and 2 cm mode, optimized for 300 degrees, 240 degrees, and 180 degrees gantry rotations. Coronal-oriented films were irradiated throughout the target volumes and scanned using a laser film digitizer. The central target irradiated in 1 cm mode was also used to investigate the effects of incorrect couch indexing. RESULTS: The dose error as a function of couch index error was 25% mm(-1), significantly greater than previously reported. The clinically provided indexing system yielded 0.10 mm indexing precision. The intrinsic dose distributions indicated that more heterogeneous dose distributions resulted from the use of smaller gantry angle ranges and larger leaf projections. Using 300 degrees gantry angle and 1 cm mode yielded 7% hot and 15% cold spots 7 cm below and above isocenter, respectively. When a 180 degree gantry angle was used, the values changed to 22% hot and 27% cold spots for the same locations. The heterogeneities for the 2 cm mode were 70% greater than the corresponding 1 cm values. CONCLUSIONS: While serial tomotherapy is used to deliver highly conformal dose distributions, significant dosimetric factors must be considered before treatment. The patient must be immobilized during treatment to avoid dose heterogeneities caused by incorrect indexing due to patient movement. Even under ideal conditions, beam divergence can cause significant abutment-region dose heterogeneities. The use of larger gantry angle ranges, smaller leaf widths, and appropriate locations of the gantry rotation axis can minimize these effects.     

瓦里安Halcyon加速器的安装验收测试

目的:通过安装验收测试(IPA)了解掌握瓦里安新型Halcyon加速器的构造、性能、验收测试和质控方法。方法:参考瓦里安提供的IPA手册,AAPM TG-142 C形臂加速器质控和TG-148 tomotherapy质控标准,测试并验收Halcyon的软件授权、安全联锁、机械精度、射束性能、成像系统等,并与传统True...     

自制“倾倒式”治疗床减小胸腹部肿瘤放疗摆位误差可行性与有效性

目的 CT模拟定位和放疗过程中均使用自制“倾倒式”治疗床进行摆位,探讨其减小摆位误差的可行性和有效性。方法 选取 2016年3-9月于肿瘤医院进行放疗的 22例胸腹部肿瘤患者,根据是否使用“倾倒式”治疗床随机分为2个组,每组 11例。试验组使用“倾倒式”治疗床实现患者由直立位转换至仰卧位,对照组采用常规的患者自主仰卧位。所有患者均在自主呼吸的状态下接受定位CT扫描,根据IGRT协作组的规范化建议进行图像配准。记录并分析CBCT扫描平移误差和旋转误差配准数据,根据“四参数模型”计算2个组摆位误差。结果 试验组x、y、z轴向平移误差和范围分别为(-0.012±0.128)、(0.272±0.123)、(0.089±0.105) cm和 0.29~0.70、0.23~0.70、0.14~0.53 cm,对照组的分别为(0.006±0.198)、(-0.108±0.396)、(-0.096±0.176) cm和 0.27~0.75、0.56~2.08、0.34~0.89 cm。结论 自制“倾倒式”治疗床可以提高胸腹部肿瘤放疗的摆位重复性,减少摆位误差,尤其是y轴向上的摆位误差。     

Quality assurance for dynamic tumor tracking

The purpose of this work was to develop a treatment plan verification routine for a linear accelerator dedicated to SBRT treatments with gimbal based dynamic tumor tracking using three commercially available phantoms.The accelerator system has two special features: It operates with a rotation of the ring shaped gantry instead of a couch rotation and target motion can be compensated for via a gimbal system (dynamic tumor tracking, DTT). DTT plans were each measured with the three different phantoms. Afterwards the measured dose distribution was compared with the calculated dose distribution via global Gamma Index analysis (3 mm / 3%, threshold: 10%).The global gamma pass rates were on average (93.5 ± 7.2) % for ArcCHECK, (98.0 ± 2.6) % for OCTAVIUS® 4D and (98.4 ± 4.2) % for MatriXX Evolution.All three systems could be used for quality assurance with ring rotations and DTT, however, each with limitations.     

A system for stereotactic radiosurgery with a linear accelerator

A small field irradiation technique to deliver high doses of single fraction photon radiation to small, precisely located volumes (0.5 to 8 cm3) within the brain has been developed. Our method uses a modified Brown-Roberts-Wells (BRS), CT-guided, stereotactic system and a 6 MV linear accelerator equipped with a special collimator (diameters of 12.5 mm to 30.0 mm projected to isocenter) located 23 cm from isocenter. Target localization via planar angiography has been added. Treatment consists of a series of arcing beams using both gantry and couch rotations. During treatment, the patient's head is immobilized independently of the radiotherapy couch and is precisely positioned without reference to room lasers or light field. A precise verification of alignment precedes each treatment. Extensive performance tests have shown that a target, localized by CT, can be irradiated with a positional accuracy of 2.4 mm in any direction with 95% confidence. If angiography is used for localization, the results are better. The dose 1.0 cm outside the target volume is less than 20% of the prescribed dose for a medium sized collimator.     

A non-contacting intraoperative electron cone apparatus

Aluminum tubes, 20 cm long, of various diameter and face angle have been used to collimate electron beams for intraoperative radiotherapy. The tubes placed in a body cavity are clamped to the treatment couch. A thin rod is fixed coaxially to a disc which is placed on the entrance face of the tube. A stepwise procedure involving couch translation, rotation, and gantry rotation is used to align the central axis of the beam with the axis of the cone. Further collimation is provided by an aperture mounted in an assembly which slides into the standard accessory holder of the machine and is separated by a 10 cm air gap from the tube in the patient. Careful adjustment of this aperture diameter results in a relatively uniform dose across the treated area. The use of 3 mm wall thickness aluminum tube provides for minimal leakage outside of the cone. A camera has been mounted to a plate that also mounts in the standard accessory holder and is used to document the area treated.     

前列腺癌放疗的摆位误差分析

目的 探讨前列腺癌仰卧位放疗时左右、头脚、前后方向的摆位误差及各方向的旋转误差。方法 收集2011年10月至2013年6月接受前列腺癌根治性放疗的患者25例,采用仰卧位体模固定,锥形束CT（CBCT）骨配准校位,分析左右、头脚、前后方向的平均摆位误差及各方向的平均旋转误差。结果 全放疗疗程中每例患者校位9次,共计225次。各方向的平均摆位误差：左右（0.19±0.18）cm,头脚（0.36±0.30）cm,前后（0.21±0.16）cm;其中左右方向摆位误差≥5mm占5.8%,头脚占24.3%,前后占8.0%。各方向旋转误差：轴位（1.07±1.03）°,头脚（0.82±0.66）°,水平（0.79±0.68）°。前5次与后4次摆位及旋转误差比较差异无统计学意义（P＞0.05）。结论前列腺癌仰卧位放疗时,头脚摆位误差最大,左右及前后摆位误差相当,旋转误差较小可忽略不计。     

圆的标准方程预防放射治疗机头与治疗床碰撞的应用

背景与目的：在放射治疗中,由于肿瘤靶区位置不同,需要配合治疗床左右或者上下移动。在移床距离较大的情况下,治疗床或患者与治疗机机头可能会发生碰撞。该研究旨在通过探讨治疗机机头的运动轨迹以及床运动的范围,避免因操作不慎导致治疗机机头与治疗床发生碰撞,减少此类事故的发生。方法：选择直线加速器Varian Trilogy,在治疗床空载、准直器零位的情况下,模拟共面照射,测量等中心点到机头表面中心的距离以及床的宽度和床的厚度。由于直线加速器Varian Trilogy的治疗床横截面为倒置的梯形,治疗床上表面最外缘一点距离等中心点最远,在发生碰撞的情况下,最先与治疗机头发生碰撞,所以取该点作为参考点。选取40组移床数据,计算参考点的坐标。参考点的横坐标为1/2床宽与床的左右位移之和。参考点的纵坐标为降床的距离。计算参考点横坐标与纵坐标的平方和,与旋转半径的平方比较,预判治疗床与治疗机机头是否发生碰撞,并进行实际验证。结果：纵坐标与横坐标平方和大于半径的平方时,治疗床与治疗机机头发生碰撞,与预判吻合。纵坐标与横坐标平方和等于半径的平方时,治疗床与治疗机机头发生0距离接触,处于临界点,与预判吻合。当纵坐标与横坐标平方和小于半径的平方时,治疗床与治疗机机头不会发生碰撞,与预判吻合。结论：通过计算测量,发现这种计算方法是可行的,可以有效避免治疗床和治疗机的机头碰撞。