Variations in skin dose using 6MV or 18MV x-ray beams

This research aimed to quantitatively evaluate the differences in percentage dose of maximum for 6MV and 18MV x-ray beams within the first 1 cm of interactions. Thus provide quantitative information regarding the basal, dermal and subcutaneous dose differences achievable with these two types of high-energy x-ray beams. Percentage dose of maximum build up curves are measured for most clinical field sizes using 6MV and 18MV x-ray beams. Calculations are performed to produce quantitative results highlighting the percentage dose of maximum differences delivered to various depths within the skin and subcutaneous tissue region by these two beams. Results have shown that basal cell layer doses are not significantly different for 6MV and 18MV x-ray beams. At depths beyond the surface and basal cell layer there is a measurable and significant difference in delivered dose. This variation increases to 20% of maximum and 22% of maximum at 1 mm and 1 cm depths respectively. The percentage variations are larger for smaller field sizes where the photon in phantom component of the delivered dose is the most significant contributor to dose. By producing graphs or tables of % dose differences in the build up region we can provide quantitative information to the oncologist for consideration (if skin and subcutaneous tissue doses are of importance) during the beam energy selection process for treatment.     

Measurement of TVLs in lead for 4, 6 and 10 MV bremsstrahlung x-ray beams at scattering angles between 30 degrees and 135 degrees

Measurements have been made of the transmission factors through lead for scattered radiation produced by 4, 6, and 10 MV bremsstrahlung x-ray beams incident on a polystyrene phantom; these measurements cover the range of scattering angles between 30 degrees and 135 degrees. The results show that the tenth value layer (TVL) for scattered radiation decreases sharply with increasing scattering angle and increases slightly with increasing beam energy, although at large scattering angles there is little energy dependence. TVLs range from 3.5 cm to 0.3 cm for 4 MV at scattering angles between 30 degrees and 135 degrees, from 3.8 cm to 0.6 cm for 6 MV, and from 4.2 cm to 0.7 cm for 10 MV, respectively, at scattering angles between 30 degrees and 120 degrees. Monte Carlo calculations, performed at 4 MV to simulate the transmission of scattered radiation, are in good agreement with the experimental measurements.     

Variations in 6MV x-ray radiotherapy build-up dose with treatment distance

Dose in the build up region for high energy x-rays produced by a medical linear accelerator is affected by the x-ray source to patient surface distance (SSD). The use of isocentric treatments whereby the tumour is positions 100 cm from the source means that depending of the depth of the tumour and the size of the patient, the SSD can vary from distances of 80 cm to 100 cm. To achieve larger field sizes, the SSD can also be extended out to 120 cm at times. Results have shown that open fields are not significantly affected by SSD changes with deviations in percentage dose being less than 4% of maximum dose for SSD's from 80 cm to 120 cm SSD. With the introduction of beam modifying devices such as Perspex blocking trays, the effects are significant with a deviation of up to 22% measured at 6MV energy with a 6 mm Perspex tray for SSD's from 80 cm to 120 cm. These variations are largest at the skin surface and reduce with depth. The use of a multi leaf collimator for blocking removes extra skin dose caused by the Perspex block trays with decreasing SSD.     

Extrapolated surface dose measurements using a NdFeB magnetic deflector for 6 MV x-ray beams

Extrapolated surface dose measurements have been performed using radiographic film to measure 2-Dimensional maps of skin and surface dose with and without a magnetic deflector device aimed at reducing surface dose. Experiments are also performed using an Attix parallel plate ionisation chamber for comparison to radiographic film extrapolation surface dose analysis. Extrapolated percentage surface dose assessments from radiographic film at the central axis of a 6 MV x-ray beam with magnetic deflector for field size 10 x 10 cm2, 15 x 15 cm2 and 20 x 20 cm2 are 9 +/- 3%, 13 +/- 3% and 16 +/- 3%, these compared to 14 +/- 3%, 19 +/- 3%, and 27 +/- 3% for open fields, respectively. Results from Attix chamber for the same field size are 12 +/- 1%, 15 +/- 1% and 18 +/- 1%, these compared to 16 +/- 1%, 21 +/- 1% and 27 +/- 1% for open fields, respectively. Results are also shown for profiles measured in-plane and cross-plane to the magnetic deflector and compared to open field data. Results have shown that the surface dose is reduced at all sites within the treatment field with larger reductions seen on one side of the field due to the sweeping nature of the designed magnetic field. Radiographic film extrapolation provides an advanced surface dose assessment and has matched well with Attix chamber results. Film measurement allows for easy 2 dimensional dose assessments.     

Depth dose-equivalent and effective energies of photoneutrons generated by 6-18 MV X-ray beams for radiotherapy

Photoneutron production was investigated on Siemens KD 2 and Varian Clinac accelerators operating in the 6-18 MV range. Neutron dose equivalent rates were measured on the surface of a water phantom at the isocenter of the accelerators and also inside the phantom at depths of 1, 5, and 10 cm and off-axis distances of 0, 20, and 50 cm. Superheated drop detectors based on dichlorofluoromethane and etched-track detectors with boronated converters were employed in this study. The energy response of these detectors permits a direct measurement of dose equivalent without prior knowledge of the neutron energy spectra. Dose equivalent rates were assessed using the Q(L) relationship from ICRP publication 60, as well as using earlier data from ICRP publication 21. This permitted both a comparison with previously published data and an assessment of the impact of the recent ICRP recommendations--which were found to increase the dose equivalent levels by about 30%. In addition, the depth corresponding to 50% of maximum dose equivalent, dH50, was determined along the central axis of the beams and at 50 cm off-axis. Monte Carlo neutron transport calculations were performed to determine the depth-dose equivalent distributions in a phantom irradiated with monoenergetic neutrons. Effective energies of the photoneutron spectra were then estimated by comparing our measured dH50 values to those calculated for monoenergetic neutrons. It was found that the effective photoneutron energy is 1.8-2.1 MeV within the 10-18 MV x-ray beams, and it is 0.5-0.8 MeV for photoneutrons transmitted through the accelerator head. Data from this work cover most of the x-ray beam energies in clinical use and permit an assessment of integral dose values as well as specific organ doses to a radiotherapy patient.     

Measurement of the leakage radiation from linear accelerators in the backward direction for 4, 6, 10, 15, and 18 MV x-ray energies

The x-ray leakage from the housing of a therapy x-ray source is regulated to be <0.1% of the useful beam exposure at 1 m from the source. It is to be expected that the machine leakage in the backward direction would be less because the gantry and stand contain significant amounts of additional metal to attenuate the x rays. A reduction in head leakage in this direction will have a direct effect on the thickness of the shielding wall behind the linear accelerator. However, no reports have been published to date on measurements in this area. The x-ray leakage in the backward direction has been measured from linacs having energies of 4, 6, 10, 15, and 18 MV using a 100 cm ionization chamber and Al2O3 dosimeters. The leakage was measured at nine different positions over the rear wall using a 3 x 3 matrix with a 1-m separation between adjacent horizontal and vertical points with either the leftmost or rightmost column aligned with the target and isocenter. In general, the leakage is less than the canonical value, but the exact value depends on energy, gantry angle, and measurement position. There is significantly greater attenuation directly behind the gantry stand for all energies. Leakage at 10 MV for some positions exceeded 0.1%. Additionally, neutron leakage measurements were made for 10, 15, and 18 MV x-ray beams using track-etch detectors. The average neutron leakage was less than 0.1% except for 18 MV, where neutron leakage was more than 0.1% of the useful beam at some positions.     

Superposition dose calculation in lung for 10MV photons

Currently available radiotherapy treatment planning systems employ scatter function models such as ETAR and Batho dSAR for dose calculation. Errors using these models for high energy photon irradiation occur in and beyond lung tissue for small fields. For larger fields, central axis dose is correctly predicted but penumbral broadening in lung is underestimated. The major source of error is the assumption that lateral electronic equilibrium is always established. A superposition algorithm has been developed for 10MV photons which calculates the dose by convolving the TERMA (Total Energy Released per unit MAss by primary photons) with a dose spread array formed using the EGS4 Monte Carlo code. TERMA and dose spread arrays are both generated using a 10 component photon energy spectrum. Dose in inhomogeneous media is calculated using dose spread arrays generated for different density media and by scaling dose spread arrays according to density variations. This method ensures that electronic disequilibrium is modelled in situations where it exists. Superposition results in a lung phantom for a 5 x 5 cm field agree with EGS4 Monte Carlo results to within 2% for p = 0.20 gcm-3 and p = 0.30 gcm-3 lung. Profiles generated by superposition for a 10 x 10 cm field at mid-lung and compared with film measurements show that penumbral broadening in low density material is also correctly predicted.     

Risks posed by neutron contamination in high energy radiotherapy

High energy X-rays penetrate tissue deeply, depositing most of their energy beyond the skin and shallow tissues. X-rays with energies above 8 MeV may interact to produce neutrons, to which the patient is then exposed. The overall number of neutrons produced is relatively low, because X-rays may interact in a variety ways and reactions producing neutrons are generally less likely. The biological damage inflicted by neutron radiation depends on the energy of the neutrons, and neutrons with energy around 1 MeV may be up to 20 times more damaging than X-rays. Treatment planning systems (TPS) do not...

     

On beam quality and flatness of radiotherapy megavoltage photon beams

Ratio of percentage depth dose (PDD) at two depths, PDD at a depth of 10 cm (PDD₁₀), and beam flatness are monitored regularly for radiotherapy beams for quality assurance. The purpose of this study is to understand the effects of changes in one of these parameters on the other. Is it possible to monitor only the beam flatness and not PDD? The investigation has two components. Naturally occurring i.e., unintended changes in PDD ratio and in-plane flatness for 6 and 10 MV photon beams for one particular Siemens Artiste Linac are monitored for a period of about 4 years. Secondly, deliberate changes in the beam parameters are induced by changing the bending magnet current (BMI). Relationships between various beam parameters for unintended changes as well as deliberate changes are characterized. Long term unintentional changes of PDD ratio are found to have no systematic trend. The flatness in the inplane direction for 6 and 10 MV beams show slow increase of 0.43 and 0.75 % respectively in about 4 years while the changes in the PDD ratio show no such trend. Over 10 % changes in BMI are required to induce changes in the beam quality indices at 2 % level. PDD ratio for the 10 MV beam is found to be less sensitive, while the depth of maximum dose, d_max, is more sensitive to the changes in BMI compared to the 6 MV beam. Tolerances are more stringent for PDD₁₀ than PDD ratio for the 10 MV beam. PDD ratio, PDD₁₀, and flatness must be monitored independently. Furthermore, off axis ratio alone cannot be used to monitor flatness. The effect of beam quality change in the absolute dose is clinically insignificant.     

某医院15MV医用电子加速器机房的屏蔽防护计算

目的 介绍医用电子加速器机房设计、辐射屏蔽厚度计算和天空反散估算。方法 依据相关计算公式,采用顺序替代法计算屏蔽墙体设计厚度,以NCRP151估算原则估算天空反散射剂量。结果 顺序替代法计算屏蔽层厚度既简便又省时,所获得数值准确,天空反散射剂量估算结果的影响可以忽略不计。结论 采取本方法计算的屏蔽厚度作为机房辐射屏蔽厚度值建造,能达到既确保辐射安全又经济的目的。     

某医院6MV医用电子直线加速器机房的屏蔽设计

目的 使某直线加速器机房防护达到标准要求。方法 根据放射防护基本原则及相关法规标准。结果 机房屏蔽设计各主要结果符合标准要求。结论 该机房防护设计达到预期目的。     

Validation of image registration and fusion of MV CBCT and planning CT for radiotherapy treatment planning

In areas like adaptive therapy, multi-phase radiotherapy, and single fraction palliative treatment or in the treatment of patients with metal implants where megavoltage(MV) CT could be considered as a treatment planning modality, the reduced contrast in the MV CT images could lead to limited accuracy in localization of the structures. This would affect the precision of the treatment. In this study, as an extension our previous work on bespoke MV cone beam CT (MV CBCT), we propose to register the MV CBCT with kilovoltage (kV) CT for treatment planning. The MV CBCT images registered with kV CT would be effective for treatment planning as it would account for the inadequate soft tissue information in the MV CBCT and would allow comparison of changes in patient dimensions and assist in localization of the structures. The intensity based registration algorithm of the BrainSCAN therapy planning software was used for image registration of the MV CBCT and kV CT images. The accuracy of the registration was validated using qualitative and quantitative measures. The effect of image quality on the level of agreement between the contouring done on both the MV CBCT and kV CT was assessed by comparing the volumes of six structures delineated. To assess the level of agreement between the plans after the registration, two independent plans were generated on the MV CBCT and the planning CT using the posterior fossa of the skull as the target. The dose volume histograms and conformity indices of the plans were compared. The results of this study show that treatment planning with MV CBCT images would be effective, using additional anatomical structure information derived from registering the MV CBCT image with a standard kVCT.     

Tenth value layers for 60Co gamma rays and for 4, 6, 10, 15, and 18 MV x rays in concrete for beams of cone angles between 0 degrees and 14 degrees calculated by Monte Carlo simulation

The calculation of shielding barrier thicknesses for radiation therapy facilities according to the NCRP formalism is based on the use of broad beams (that is, the maximum possible field sizes). However, in practice, treatment fields used in radiation therapy are, on average, less than half the maximum size. Indeed, many contemporary treatment techniques call for reduced field sizes to reduce co-morbidity and the risk of second cancers. Therefore, published tenth value layers (TVLs) for shielding materials do not apply to these very small fields. There is, hence, a need to determine the TVLs for various beam modalities as a function of field size. The attenuation of (60)Co gamma rays and photons of 4, 6, 10, 15, and 18 MV bremsstrahlung x ray beams by concrete has been studied using the Monte Carlo technique (MCNP version 4C2) for beams of half-opening angles of 0 degrees , 3 degrees , 6 degrees , 9 degrees , 12 degrees , and 14 degrees . The distance between the x-ray source and the distal surface of the shielding wall was fixed at 600 cm, a distance that is typical for modern radiation therapy rooms. The maximum concrete thickness varied between 76.5 cm and 151.5 cm for (60)Co and 18 MV x rays, respectively. Detectors were placed at 630 cm, 700 cm, and 800 cm from the source. TVLs have been determined down to the third TVL. Energy spectra for 4, 6, 10, 15, and 18 MV x rays for 10 x 10 cm(2) and 40 x 40 cm(2) field sizes were used to generate depth dose curves in water that were compared with experimentally measured values.     

Characterization of a Fricke dosimeter at high energy photon and electron beams used in radiotherapy

The dosimetric features of the Fricke dosimeter in clinical linear accelerator beams are considered. Experimental data were obtained using various nominal energies 6 and 18 MV, 12 and 15 MeV, including the ⁶⁰Co γ-ray beam. The calibration of the dosimeters was performed using the ionization chamber as a reference dosimeter. Some general characteristics of Fricke dosimeter such as energy dependence, optical density (OD)-dose relationship, reproducibility, accuracy, dose rate dependence were analyzed. The Fricke solution shows linearity in OD-dose relationship, energy independence and a good reproducibility over the energy range investigated. The Fricke dosimeter was found to be suitable for carrying out absorbed dose to water measurements in the calibration of high energy electron and photon beams.     

Measurement of scatter factors for 4, 6, 10, and 23 MV x rays at scattering angles between 30 degrees and 135 degrees

NCRP Report No. 49, published in 1976, describes how to calculate the shielding for the medical use of x rays and gamma rays for energies up to 10 MV, including primary, scattered, and leakage radiation. However, in that report, data for scattered radiation for linear accelerators exist only for 6 MV, and leakage radiation is assumed, incorrectly, to be equivalent to primary radiation. Since the publication of that report, linear accelerators with energies up to 25 MV have been widely used in the radiation therapy community. Thus, there is a need to measure additional data for all energies in the range 4-25 MV. In this study, measurements were made of the "a" factor for 4, 6, 10, and 23 MV x rays at scattering angles between 30 degrees and 135 degrees. The results show that the 6 and 10 MV "a" factor data are consistent with published data, and the 23 MV data are also consistent with recently published data at 18 and 25 MV. The data show that, in general, the "a" factor decreases with energy; the exception is that 23 MV data show a sharp increase at low scattering angles, much greater than at other energies.     

四维CT技术在放疗中的应用

介绍了四维CT技术在国内肿瘤放疗中的应用现状,阐述了四维cT图像采集的软件参数设置方法和具体操作步骤,以及使用四维CT图像勾画肿瘤靶区的应用方法,以期提高诊断的准确率,更好地为临床服务。     

Choice of optimal centering points of radiation beams in planning radiotherapy in malignant tumors]

A programmed complex has been developed to determine the optimum radiation protocols which include the optimum positions of intersection centers of central axes of radiation beams, the optimum directions of radiation, the optimum time of exposure. Analyzing the optimum radiation protocols has shown that variations in the positions of intersection centers of radiation beams play a crucial role in the targeted formation of the optimum dose fields. To choose the optimum radiation protocols for central lung cancer is a challenge whose solution depends on the initial position of intersection centers of radiation beams, which give rise to iterative solution of emergency problems. An effective radiation protocol has been chosen among its varieties by making a multicriteria assessment of gross characteristics of dose fields in the lung tissue.     

高能电子线TLD剂量质量核查

目的：基于国家专项经费的支持,开展江苏、河南和湖北3省常用临床治疗条件高能电子线水中吸收剂量的核查工作。方法：采用平行板电离室和指型电离室完成高能电子线水中吸收剂量的测量,剂量核查使用的热释光剂量计（TLD）在电子线下照射。TLD系统使用60Coγ射线校准。结果：DEAG/DUSER的平均比值为1.012,最大值为1.056,最小值为0.961。结论：按照国际原子能机构（IAEA）的要求,TLD扩展不确定度（k=2）≤5%。核查结果表明,参加此次核查的医院高能电子线合格率为94.2%。     

15 MV医用电子加速器机房迷路内、外墙体厚度确定

目的确定医用加速器机房迷路内、外墙最佳厚度。方法由迷路内取两个不同剂量限值计算出两种内、外墙厚度,比较不同厚度对漏、散射线的防护效果。结果迷路内取限值P=0.01cGy/wk确定迷路内墙厚度,迷路外墙是计算值再加一个半值层。结论用本方法确定迷路内、外墙厚度,能达到最佳防护要求。