A national dosimetry audit of intraoperative radiotherapy

Objective:

National dosimetry audits are a fundamental part of quality assurance in radiotherapy, especially for new techniques. Intraoperative radiotherapy with a compact mobile kilovoltage X-ray source is a novel approach for the treatment of breast and other cancers. All seven current clinical sites in the UK were audited by a single visiting group and set of measurement equipment.

Methods:

Measurements of output, isotropy and depth doses were performed using an ion chamber in solid water, thermoluminescent dosemeters and radiochromic film, respectively.

Results:

The mean difference between measured and planned dose across all centres was −3.2±2.7%. Measured isotropy was within ±3% around the lateral plane of the X-ray source and +11±4% in the forward direction compared with the lateral plane. Measured depth doses were agreed within 5±2% of manufacturer-provided calibration values or a mean gamma index of 97% at a tolerance of 7%/0.5 mm.

Conclusion:

Agreement within measurement uncertainties was found for all three parameters except forward anisotropy, which is unlikely to be clinically significant. Steep dose gradients increase the sensitivity to small variations in positioning, but these tests are practical for use in interdepartmental audits and local baseline comparison.

Advances in knowledge:

The first UK interdepartmental audit of intraoperative radiotherapy builds confidence in the delivery of this treatment.Dosimetry audits have been a fundamental part of quality assurance (QA) in UK radiotherapy departments for over 20 years [1–6]. More recently, ongoing verification of established systems has been delegated to regional audit groups; however, new techniques such as intensity-modulated radiotherapy [7,8] or volumetric modulated arc therapy [9] have been assessed on a national level. This gives confidence in the accurate, safe and consistent delivery of complex treatments, regardless of location or prior experience.Intraoperative radiotherapy (IORT) has also been practised in the UK for nearly 20 years, using a compact mobile 50 kV X-ray device called PRS400 (Photoelectron Corporation, Lexington, MA) and, more recently, INTRABEAM® (PRS500; Carl Zeiss Surgical, Oberkochen, Germany). Initially, the system was used for intracranial stereotaxy [10] but found wider application in the delivery of a single fraction of radiation soon after surgical excision of breast tumours [11]. Most patients have been treated as part of the targeted intraoperative radiotherapy (TARGIT) international randomised controlled trial [12], although some other cases have also been successfully treated where external beam therapy was contraindicated [13]. Full calibration of the system is provided by the manufacturer, but recently, methods for independent dosimetry and quality assurance have been described [14,15]. The aim of this study was to audit all UK clinical centres in 2012 (

热释光剂量计核查非参考条件下放射治疗剂量学参数的研究

目的　用TLD核查在非参考条件下 ,光子束轴向和电子束轴向最大剂量点处 ,剂量随深度、距离、照射野和 4 5°楔形板等变化的临床剂量学参数研究。方法　在非参考条件下 ,TLD经6 0 Coγ射线束 ,6MV和 15MV高能X射线束 ,9MeV和 16MeV高能电子束测量并估算剂量。结果经TLD核查 ,对不同射线质在不同深度 ,用不同指型电离室测量 ,除了TLD在水中深度 2 0cm处应做支架减弱效应减弱校正外 ,其余临床剂量学参数核查结果表明 ,TLD测量结果与指型电离室测量的参考剂量结果相对误差值均在± 4 9% (IAEA允许误差± 5 % )以内 ;对高能电子线束 ,用平行板电离室测量 ,绝大多数TLD相对误差值超过± 5 %。结论　用TLD核查非参考条件下 ,放射治疗临床剂量学参数方便 ,准确可靠 ,经在医院做可行性验证 ,IAEA推荐的两种TLD照射支架 (一种用于光子束 ,另一种用于电子束 )具有可行性 ;对高能电子线束用平行板电离室测量 ,TLD验证方法待做进一步研究。     

河南省核查放射治疗参考与非参考条件下剂量学参数方法验证

目的 研究用热释光剂量计（TLD）方法核查放射治疗参考条件和非参考条件下剂量学参数的可靠性验证。方法 在参考条件和非参考条件下,用建立的TLD方法,核查10条6 MV光子线束剂量随照射野大小和45°楔形板等变化,4条9 MeV电子线束轴向最大剂量点处等剂量学参数,TLD估算结果与剂量仪测量结果进行对比。结果 6 MV光子线束TLD估算结果与指形电离室测量结果的平均相对偏差为4.7%,按照IAEA要求允许偏差不超过±7%;9 MeV电子线束TLD估算结果与平行板电离室测量结果平均相对偏差为2.4%,均未超过IAEA允许偏差要求（±5%）。结论 用TLD核查参考条件和非参考条件下放射治疗剂量学参数方法可靠,简单易行。     

术中放疗的吸收剂量测算方法的研究

目的 研究和比较水、固体水及有机玻璃3种模体的术中放疗吸收剂量的测算方法。方法 对于3种模体,使用固定在水模体中的电离室对加速器的电子速术中放疗限光筒进行吸收剂量的测算,首先选定参考限光筒对所有能量的电子束在源皮距SSD=100cm,水模内射束中心轴上特定深度,通过调整加速器使1cGy=1MU,然后使用术中放疗及参考限光筒在相同的辐照条件下进行测量,即测量术中放疗限光筒的输出因数,对于水模体,计算出各限光筒的吸收剂量cGy对应的加速器输出MU数值,并据此计算出固体水模体和有机玻璃模体的各限光筒吸收剂量cGy对应的加速器输出的MU数值,结果 相对于水模体,有机玻璃模体的测量误差为0.27%,固体水为0.45%。结论 对没有专用测量水箱和固体水的医院使用有机玻璃模体进行吸收剂量的测量不失为一种切实可行的方法。     

全中枢神经系统肿瘤放疗技术的剂量学研究

目的 以三维适形放疗技术为参照,探讨中枢神经系统肿瘤简单调强(sIMRT)放疗技术的剂量学特性。方法 选取5名已行全脑全脊髓放疗患者,为每位患者设计3D-CRT计划、3野和5野sIMRT计划。利用剂量分布和剂量体积直方图(DVH),评价不同照射技术的靶区和正常器官的照射剂量、靶区剂量均匀性(HI),通过总的机器跳数(MU)间接比较不同照射技术的治疗时间。结果 3D-CRT在射野衔接处只有处方剂量的70%。计划靶区后缘的正常组织接受的剂量达到处方剂量的140%。3野和5野sIMRT计划的靶区剂量均匀性分别为0.09±0.01和0.08±0.01,优于3D-CRT计划的0.18±0.02 (t=7.80、7.65,P<0.05);心脏V₁₀分别为(8.4±1.9)%和(8.4±2.0)%,低于3D-CRT计划的(36.0±6.0)%(t=13.3、13,0,P<0.05);甲状腺V₂₀分别为(12.4±1.5)%和(12.4±1.6)%,低于3D-CRT计划的(69.4±5.7)%(t=26.3、26.4,P<0.05);喉V₂₀分别为(17.2±1.2)%和(17.9±1.5)%,低于3D-CRT计划的(89.4±7.0)%(t=25.5、26.5,P<0.05);靶区后缘正常组织V₃₀分别为(4.4±1.4)%,(4.9±1.9)%,低于3D-CRT计划的(31.9±6.1)%(t=8.5、10.1,P<0.05);平均机器跳数(MU)分别为1100±106和1160±129,高于3D-CRT计划的640±78。结论 3野和5野sIMRT计划在剂量分布、危及器官(OAR)保护、靶区剂量均匀性等方面均好于3D-CRT计划。     

Carotid dosimetry for T1 glottic cancer radiotherapy

Objective:

Radiotherapy for T1 glottic cancer is commonly delivered using a lateral parallel opposed pair of megavoltage photon fields. There is increasing reported evidence of cerebrovascular events due to radiation-induced carotid stenosis. An alternative field arrangement is to use an anterior oblique technique. This study compares the carotid dosimetry between the two techniques and reviews the evidence for the risk of radiation-induced vascular events.

Methods:

The radiotherapy plans of 10 patients with T1 glottic cancer treated with an anterior oblique technique were examined for carotid dose. Alternative plans were then created using a parallel opposed pair of fields and the dose to the carotids compared. All patients received 50 Gy in 16 fractions treating once daily, for 5 days in a week.

Results:

The average of the mean dose to the carotids with the anterior oblique technique was 21 Gy compared with 37 Gy using the lateral parallel opposed pair arrangement (p < 0.0001).

Conclusion:

An anterior oblique field arrangement for the treatment of T1 glottic cancer results in a significantly lower radiation dose to the carotid arteries, which may be clinically important in terms of reducing the risk of cerebrovascular events in long-term survivors.

Advances in knowledge:

Although the anterior oblique technique for treating early glottic cancers is well described, and it is predictable that the dose received by the carotid arteries should be lower with this technique, to our knowledge this is the first study to quantify that reduction in dose with a series of patients.For other than clearly defined vocal cord lesions clear of the anterior commissure, radiotherapy remains the mainstay of curative treatment for T1 glottic cancer.^1,2 The radiotherapy target volume should be confined to the larynx without any attempt to encompass neck nodes. There is increasing reported evidence of late cerebrovascular events such as transient ischaemic attacks and ischaemic strokes due to radiation-induced injury of the carotid artery, e.g. accelerated arteriosclerosis. These vascular events can occur many years after radiotherapy such that their relationship to previous radiotherapy is obscured.Classically, a lateral parallel opposed pair of megavoltage photon fields is used for the treatment of early glottic cancer. An alternative approach is an anterior oblique technique, which we adopted following the introduction of a linear accelerator in our centre in 1955 (Figure 1). We aim to assess the extent of carotid exposure, to compare the carotid dosimetry between anterior oblique and lateral parallel opposed pair techniques and also to review the evidence for the increased risk of vascular events.Open in a separate windowFigure 1.Diagram showing the anterior oblique field arrangement.     

非参考条件下放射治疗剂量的TLD比对结果

目的在北京地区三级甲等医院中,用TLD完成非参考条件下放射治疗剂量比对。方法TLD系统用60Coγ线束校准。TLD读数经能量响应及TLD支架减弱等校正因子的修正。结果高能光子束DEAG/DUSER的平均比值是1.001;高能电子束DEAG/DUSER的平均比值是1.000。结论按照IAEA的要求,医院的TLD剂量比对扩展不确定度（k=2）不大于5%。比对结果表明,参加比对的这些医院的高能光子束的合格率是98%,高能电子束的合格率是100%。     

An application of film dosimetry for helium ion radiotherapy

Clinical dosimetry of complex dose distribution associated with charged particle radiotherapy requires a rapid and convenient technique with a spatial resolution of 3 mm or better. Radiographic film stacks have been evaluated for three-dimensional dosimetry of charged particle beams. Sheets of Cronex 4 film are placed between lucite spacers in a light-tight box and the beam is directed perpendicularly to the film surface. Optical densities on each sheet are digitized with a video digitizer and corrected for density-dose nonlinearity and LET effects. A three-dimensional matrix of the measured dose is constructed and compared with calculated values. Both transverse and depth dose data recorded by film have been compared with diode measurements and generally shown agreement within 5%. The present technique appears to be an effective method to generate experimentally measured dose distributions.     

Small-scale dosimetry: challenges and future directions

The increased specificity of targeting agents has resulted in an interest in the use of radionuclides that emit particulate radiation: alpha particles, beta particles and Auger electrons. The potential advantage of these radionuclides is the ability to deliver therapeutic doses to individual tumor cells while minimizing the dose to the surrounding normal tissues. However, the dosimetry of these radionuclides is challenging because the dose must be characterized on a scale that is comparable to the range of these emissions, ie, millimeters for beta particles, micrometers for alpha particles, and nanometers for Auger electrons to. In this review, each class of particulate emitter is discussed along with the associated dosimetric techniques unique to calculating dose on these scales. The limitations of these approaches and the factors that hinder the clinical use of small-scale dosimetry are also discussed.     

Use of a modified natural-rubber phantom for radiotherapy dosimetry measurements

The utility of a phantom material, based on SMR(L) [Standard Malaysian Rubber] grade natural rubber and a formulation used for the proprietary rubber phantom-material, Temex, has been examined for the 1-MeV photon-energy range using γ radiation from a ⁶⁰Co source. Measurement has also been performed with 60-keV photons using the radionuclide ²⁴¹Am. At photon-therapy energy levels the measured response, when compared with tabulated central-axis percentage depth doses for the defined measuring conditions, produces everywhere (in the range 1–19 cm depth) better than 2% deviation. The favourable measured response characteristics combined with the ease of processing and casting the phantom material provide the basis for useful radiotherapy machine calibration and anthropomorphic dosimetry measurements. The measured mass-attenuation coefficient, at 60 keV, of 0.204 cm² g^-1 (±3%) is in close agreement with tabulated values for water (0.2055 cm² g^-1).     

Design of a multiblock phantom for radiotherapy dosimetry applications.

This note describes the design of a multiblock semi-anatomic phantom, which lends itself to a variety of radiotherapy dosimetry applications, in particular, the audit of external beam treatment planning and delivery. The basic building blocks of the phantom were formed from a variety of tissue substitute materials and could be assembled in many ways to model different cross-sections through the body.     

Use of alanine for dosimetry intercomparisons among Italian radiotherapy centers.

A pilot program of postal dosimetry intercomparison among 16 Italian Radiotherapy Centers was performed using the ISS Alanine/EPR dosimetry as a transfer system. Dosimeters were irradiated at 10 Gy with high-energy photon beams, both in reference condition in a water phantom and in an anthropomorphic phantom during the simulated treatment of rectum cancer. Intercomparison design along with alanine performances analyzing the different contributions to the combined uncertainty in dose assessment are reported. Main results of the pilot intercomparison, terminated in 2002, are also presented.     

2005年放疗水平二级剂量标准的TLD国际比对

目的通过参加国际原子能机构（IAEA）和世界卫生组织（WHO）组织的二级标准剂量学实验室（SSDL）TLD国际比对，检查SSDL放疗水平剂量标准和国际标准的一致性。方法SSDL对IAEA邮寄的TLD进行照射，并计算出其吸收剂量，然后将TLD及其计算结果寄往IAEA剂量学实验室，IAEA对其进行评价，给出比对的偏差。结果本次^60Co比对的偏差为-1．4％，高能X射线比对的偏差为-1．8％。结论按照IAEA要求，该项比对的最大允许偏差为±3．5％，所以这次比对结果是合格的。     

IAEA剂量实用规范与我国“放疗剂量学规定”的比较研究

目的研究用于放射治疗剂量测定的国际原子能机构(IAEA)第277号技术报告《光子束和电子束吸收剂量的确定》国际实用规范和我国1984年国家计量局和卫生部颁布的基于ICRU23号技术报告的《关于肿瘤放射治疗剂量学的若干规定》之间的差异。方法用理论计算和实验测量2种方法比较了不同的电离室在测量^60Co γ射线和3种能量的高能X射线水中吸收剂量所得结果。结果大部分常用的电离室在不同的光子能量下使用两种方法所测的结果差异小于1％，只有1个电离室的结果大于1％，并小于1．5％。结论现在放射治疗对剂量有更高的精度要求，所以我们建议尽量采用IAEA国际实用规范。     

Practical guidelines for routine intensity-modulated radiotherapy verification: pre-treatment verification with portal dosimetry and treatment verification with in vivo dosimetry

The purpose of this work is to provide guidelines for the routine use of portal dosimetry and in vivo diode measurements to verify intensity-modulated radiotherapy (IMRT) treatments. To achieve tolerance levels that are sensitive enough to intercept problems, both the portal dosimetry and the in vivo procedure must be optimised. Portal dosimetry was improved by the introduction of an optimised two-dimensional (2D) profile correction, which also accounted for the effect of backscatter from the R-arm. The scaled score, indicating the fraction of points not meeting the desired gamma evaluation criteria within the field opening, was determined as the parameter of interest. Using gamma criteria of a 3% dose difference and 3 mm distance to agreement, a "scaled score" threshold value of 1.5% was chosen to indicate excessive tongue and groove and other problems. The pre-treatment portal dosimetry quality assurance (QA) does not encompass verification of the patient dose calculation or position, and so it is complemented by in vivo diode measurements. Diode positioning is crucial in IMRT, and so we describe a method for diode positioning at any suitable point. We achieved 95% of IMRT field measurements within ±5% and 99% within ±8%, with improved accuracy being achieved over time owing to better positioning. Although the careful preparation and setup of the diode measurements can be time-consuming, this is compensated for by the time efficiency of the optimised procedure. Both methods are now easily absorbed into the routine work of the department.     

One year of experience with alanine dosimetry in radiotherapy

Commercially available alanine dosimeters from different manufacturers were purchased for this study. The response of the detectors was evaluated with ⁶⁰Co gamma radiation in the dose range 0.2–200 Gy, using a small EPR spectrometer dedicated to dosimetry. The batch sensitivity, inter-specimen scattering and background signal for the different selection of dosimeters were evaluated. The usefulness of the alanine dosimetry system for clinical routine is illustrated by in vivo measurements during ¹⁹²Ir brachytherapy of cervix carcinoma.