Dose distribution in 42 MV roentgen irradiation of cervical carcinoma.

The dose distribution obtained with two different techniques for external irradiation of cervical carcinoma is described. The dose to the central area is somewhat higher with the multiple beam technique compared with the newly introduced shielding block technique. The difference between the calculated CRE values for the two techniques is small. When the shielding block is not placed over the area corresponding to the position of the applicators from the previous intracavitary treatment the considerable difference in absorbed dose between the two techniques does not correspond to the difference in the calculated CRE values. A comparison of the relative distributions laterally, in total dose and CRE, shows that for central volumes the relative CRE is much higher than the relative dose, when the normalization is made at the pelvic wall.     

Absorbed dose to organs in the head and neck from bitewing radiography.

Dose measurements were carried out to estimate the absorbed dose to organs at risk from bitewing radiography under exposure conditions similar to those found in general dental practice in The Netherlands. The measurements were carried out in the head and neck of a Rando phantom using TLD-100 ribbons. Three different X-ray machines were used simulating 10 different exposure conditions to determine the effects of beam energy, beam size and focus-skin distance on the absorbed dose. The absorbed dose in the primary beam, close to the focal spot, decreased as the beam energy increased, while behind the film plane it increased with the beam energy. Outside the primary beam, at short distances from the skin surface, the absorbed dose decreased as the energy increased, while behind the film plane and at greater distances from the primary beam the absorbed dose increased as the beam energy increased. The absorbed dose was significantly lower for smaller beam sizes and for larger focus-skin distances, independent of beam energy.     

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。     

Dosimetric characteristics of unflattened 6 MV photon beams of a clinical linear accelerator: a Monte Carlo study.

The purpose of this study is to evaluate the dosimetric properties of a flattening filter free 6 MV photon beam. The 6 MV photon beam of a Varian Clinac 21 EX linac was modeled using the MCNP4C Monte Carlo (MC) code. Dosimetric features including central axis absorbed doses, beam profiles and photon energy spectra were calculated for flattened and unflattened 6 MV photon beams. A substantial increase in the dose rate was seen for the unflattened beam, which was decreased with field size and depth. The penumbra width was decreased less than 0.2 mm (about 5%) and a 25% decrease in out-of-field dose was observed for the unflattened beam. The photon energy spectra were softer for the unflattened beam and the mean energies of spectra were higher for smaller field size. Our study showed that increase in the dose rate and lower out-of-field dose could be considered as practical advantages for unflattened 6 MV beams.     

Pantak Therapax SXT 150: performance assessment and dose determination using IAEA TRS-398 protocol

The performance assessment and beam characteristics of the Therapax SXT 150 unit, which encompass both low and medium-energy beams, were evaluated. Dose determination was carried out by implementing the International Atomic Energy Agency (IAEA) TRS-398 protocol and measuring all the dosimetric parameters in order to have a solid, consistent and reliable data set for the unit. Mechanical movements, interlocks and applicator characteristics agreed with specifications. The timer exhibited good accuracy and linearity. The output was very stable, with good repeatability, long-term reproducibility and no dependence on tube head orientation. The measured dosimetric parameters included beam first and second half-value layers (HVLs), absorbed dose rate to water under reference conditions, central axis depth dose distributions, output factors and beam profiles. Measured first HVLs agreed with comparable published data, but the homogeneity coefficients were low in comparison with typical values found in the literature. The timer error was significant for all filters and should be taken into consideration for the absorbed dose rate determination under reference conditions as well as for the calculation of treatment times. Percentage depth-dose (PDD) measurements are strongly recommended for each filter-applicator combination. The output factor definition of the IAEA TRS-398 protocol for medium-energy X-ray qualities involves the use of data that is difficult to measure. Beam profiles had small penumbras and good symmetry and flatness except for the lowest energy beam, for which a heel effect was observed.     

Electron dose calculations: a comparison of two commercial treatment planning computers.

The accuracy of electron dose calculations performed by two commercially available treatment planning systems, Varian Cadplan and MDS Nordion Helax-TMS, were assessed. Three tests designed to reproduce clinical treatments likely to result in dose nonuniformity have been carried out. The tests examined oblique incidence of the electron beam; incidence on a surface containing a step shape; and incidence on a phantom containing a small air cavity. Dose calculations performed by the planning systems were compared with thermoluminescence dosimetry (TLD) measurements in a WTe electron solid water phantom. A Varian 2100C linear accelerator was used. In most situations, the discrepancy between calculated and measured dose was within the tolerance specified by the ICRU; however, some exceptions were noted. Helax-TMS produced errors of 5 mm in the position of the 10% isodose line in the penumbra of the obliquely incident beam. Both Cadplan and Helax-TMS overestimated the surface dose adjacent to a step in the beam entry surface by approximately 15%. An overestimation of 10% in dose was calculated by both systems downstream of the small air cavity. Discrepancies between the measured and calculated monitor units lay within the uncertainty limits of the measurements. In conclusion, calculations of absorbed dose from electron beams performed by Varian Cadplan and MDS Nordion Helax-TMS result in significant errors at shallow depths near surface irregularities and downstream of small air cavities.     

用能量沉积核函数方法计算~(60)Co照射野的吸收剂量

目的　介绍了用能量沉积核函数方法计算60 Co照射野吸收剂量的方法。方法　能量沉积核函数方法将吸收剂量的贡献分为 3部分 :原射线、单次散射和多次散射。它使用基本的剂量学数据 ,如射野中心轴百分深度剂量、离轴比和准直系统散射输出因子等 ,这些数据在Fyc 5 0H治疗机上用方形照射野测量得到。再用能量沉积核函数计算吸收剂量。并讨论了散射线对吸收剂量的影响。结果　从测量数据得到了原射线和散射线的能量沉积核函数 ,并利用能量沉积核函数计算60 Co照射野的主要剂量学参数 ,计算值和测量值是一致的 ;不规则照射野的吸收剂量及其分布的计算结果也和测量结果符合得很好。结论　能量沉积核函数方法适用于较精确地计算60 Co不规则照射野的吸收剂量。     

Detection size dependence of field factor for narrow beam]

An absorbed dose in the narrow beam is calculated based on the depth dose distribution and a field factor. The field factor has to be measured with especially high accuracy because it is highly variable. The field factor was calculated when detection size was changed, by using Monte Carlo simulation, which had no energy dependency or geometrical error. Then the relation between field factor and detection size in the narrow beam was investigated. An absorbed dose in peak depth and reference depth according to detection size was calculated for each field size. Detection size dependency was recognized in the case of measuring a field factor, because the absorbed dose tended to decrease as detection size increased. The absorbed dose in the narrow beam has to be calculated within a change of +/- 2%. The change in peak depth according to detection size was eliminated, and then the relation between an absorbed dose at the ideal detection size of 0 mm phi by extrapolation and detection size which has a deference of 2% from it, were acquired. In addition, the maximum usable detection size was estimated in the case of measuring the field factor.     

Dosimetric assessment of nonperfectly abutted fields using asymmetric collimators.

The abutment of adjacent fields has been facilitated through the use of asymmetric collimators. Conceptually, the abutment yields a perfectly uniform dose distribution across the junction, provided the asymmetric jaw is set precisely at the beam central axis. However, the asymmetric jaw has an associated tolerance, which can cause the abutment to be misaligned. This study examined the dose distribution at the junction of nonperfectly abutted fields. The abutment of fields was carried out using an asymmetric collimation of 5 x 10 cm, with an asymmetric jaw positioned at the beam central axis. A film was initially exposed using this field with the collimator set at 90 degrees. The collimator was then rotated 180 degrees and the same film was exposed for the second time to create the field abutment. Positioning the asymmetric jaw with respect to the beam central axis set the amount of gap and overlap between the abutted fields. The dose distribution was measured for asymmetric jaw positioning of -2, -1, 0, + 1, and +2 mm from the beam central axis. In addition, the dose distribution was also computed mathematically by summing the 2 dose profiles with defined gap or overlap. A field mismatch of +/-1 mm would result in a dose nonuniformity of 17%, and a +/-2 mm mismatch would produce a 35% dose nonuniformity.     

Breast compression and radiation dose in two different mammographic oblique projections: 45 and 60 degrees.

INTRODUCTION: Standard mammography includes two views, craniocaudal and medio-lateral oblique. Depending on patient's body constitution, central beam angle in mediolateral oblique projection may vary, with 45 degrees being suitable for the majority of patients in routine daily practice. With continuous improvement in X-ray technology and radiographers' training, the risk of radiation induced cancerogenesis is considerably reduced and acceptable when compared to benefit. However, the risk still exists, being cumulative and directly related to absorbed glandular dose. There is no minimal dose of radiation which is absolutely harmless, and every effort to reduce the dose is welcome. In this retrospective study two different angles (45 vs. 60 degrees) of mediolateral oblique view were compared according to radiation dose and efficacy of breast compression. PATIENTS AND METHODS: In 52 women, additional 60 degrees oblique films were done after craniocaudal and mediolateral oblique 45 degrees-films, with the same kVp and positioning technique. Breast thickness, time-current products (mA s) and absorbed doses were compared between 45 degrees- and 60 degrees-films. Subgroups of women with large, small, prominent and pendulous breasts were analyzed separately, following the same methodology as for the whole group. RESULTS: mA s were 11.5% lower and compression 7% better with an angle of 60 degrees than with 45 degrees. In the subgroup of women with small breasts, mA s values were 13% lower and compression 9% better with 60 degrees than with 45 degrees, while in the subgroup with large breasts, mA s were 9% lower and compression 5% better. In the subgroup of patients with pendulous breasts, mA s values were 12% lower and compression 10% better with 60 degrees than with 45 degrees, while in the subgroup with prominent breasts, mA s values were 4% lower and compression 3% better. Absorbed glandular dose was estimated to be approximately 20% lower when an oblique mammogram was done with 60 degrees instead of 45 degrees. The compression with 15 kp was well tolerated by the majority of patients. DISCUSSION AND CONCLUSION: Mammograms of excellent quality should be done with as low a radiation dose as possible. Adequate breast compression is fundamental in mammography due to immobilization of the breast, shortening of the exposure times, reduction of motion and geometric blur and prevention of overpenetration by means of equalizing breast thickness. As the absorbed glandular dose cannot be accurately measured, it is convenient to estimate the dose approximately, on the basis of its linear proportionality with exposure dose. With constant technical properties of X-ray machines, exposure dose is determined only by mA s. Hence, the absorbed glandular dose in our study was influenced only by changes of mA s and breast thickness. As the absorbed dose reduction is proportional to the product of the reduction of mA s and thickness, we estimated that absorbed dose was 7-22% lower if 60 degrees is applied instead of 45 degrees. Breast compression and mA s were more favourable in women with pendulous breasts, possibly because of elongation of the glandular disc in the lateroascending direction, with its longer axis directed more perpendicularly. Fibroglandular tissue in the 60 degrees-view is thus projected onto a larger film area, with less effect of superimposition. In conclusion, because of lower mA s values and better compression, which finally result in a 25% lower absorbed dose, we recommend the oblique view be done with an angle of 60 degrees, especially for small and pendulous breasts.     

Monte Carlo study on a flattening filter-free 18-MV photon beam of a medical linear accelerator

PURPOSE: Several studies on the dosimetric properties of unflattened photon beams have shown some advantages for radiotherapy. In this study, the effect of removing the flattening filter from an 18-MV photon beam was investigated using the Monte Carlo method. MATERIALS AND METHODS: The 18-MV photon beam of an Elekta SL25 linear accelerator was simulated using the MCNP4C Monte Carlo (MC) code. Beam dosimetric features, including central axis absorbed doses, beam profiles, and photon energy spectra, were calculated for flattened and unflattened 18-MV photon beams. RESULTS: A 4.24-fold increase in the dose rate was seen for the unflattened beam with a field size of 10 x 10 cm(2). A decrease in the out-of-field dose up to 30% was seen for the unflattened beam. For the unflattened beam, photon energy spectra were softer, and the mean energies of the spectra were higher for a smaller field size. CONCLUSION: Our study showed that the increase in dose rate and lower out-of-field dose can be possible advantages for an unflattened 18-MV beam.     

Polarity effects of ionization chambers used in tbi dosimetry due to cable irradiation.

This paper presents the results of an investigation on polarity effects in total-body irradiation (TBI) dosimetry. Thimble (NE2571, 0.6 cc) and plane-parallel (Markus NE2534 0.055 cc) chambers were investigated in a 30 x 30 x 30-cm3 acrylic phantom in TBI conditions (6-MV x-rays). The thimble chamber was positioned at the midline and at the entrance and exit Dmax (1.5 cm from the phantom surface) levels. The Markus chamber, which is generally used for skin dose estimations, was positioned at various depths from the entrance surface of the phantom (from 0- to 2-cm depth). The polarity factor (Ppol) was defined as (Q+ + Q-)/2Q-, where Q+ and Q- were the collected charges at positive and negative bias voltage, respectively. The variations of Ppol with many parameters (absorbed dose, dose rate, the presence or absence of a 1-cm acrylic spoiler, irradiated cable length) were investigated. Results show that Ppol is quite small (within 1.002 for on-axis measurements and 1.005 for off-axis measurements) for the NE2571 chamber when the beam spoiler is placed. Ppol was significantly higher without the beam spoiler (within 1.008 for on-axis measurements, up to 1.02 for off-axis measurements). Concerning the Markus chamber, for on-axis skin dose measurements, Ppol was found to be less than unity (around 0.988) or more than unity (around 1.0035), respectively, with and without the beam spoiler. Possible "directional effects" of the currents generated in the cable were investigated for both chambers and found to be insignificant. This shows that the application of Ppol correction has to be considered a reliable procedure in minimizing these effects. When the beam spoiler is placed, the cable has to be drawn to minimize the portion of cable just outside the beam; if this is not the case, Ppol may significantly vary (for the NE2571 chamber values up to 1.0035 were found for on-axis measurements).     

用能量沉积核函数方法计算60Co照射野的吸收剂量

目的 介绍了用能量沉积核函数方法计算＾６０Ｃｏ照射野吸收剂理的方法。方法 能量沉积核函数方法将吸收剂量的贡献分为３部分：原射线、单次散射和多次散射。它使用基本的剂量学数据,如射野中心轴百分深度剂量、离轴比和淮直系统散射输出因子等,这些数据在Ｆｙｃ５０Ｈ治疗机上用方形照射野测量得到,再用能量沉积核函数计算吸收剂量,并讨论了散射线对吸收剂量的影响。结果 从测量数据得到了原射线和散射线的能量沉积核函数,并利用能量沉积核函数计算＾６０Ｃｏ照射野的主要剂量学参数。计算值和测量值是一致的;不规则照射野的吸收剂量及其分布的计算结果也和测量结果符合得很好。结论 能量沉积核函数方法适用于较精确地计算＾６０Ｃｏ不规则照射野的吸收剂量。     

Basic concepts of CORVUS dose model.

Basic concepts of the dose model utilized in the CORVUS treatment planning system are reviewed. Following the Peacock delivery tool (MIMiC) by NOMOS Corporation, CORVUS "delivers" radiation to a patient by means of narrow x-ray beams (pencil beams), which are subject to lateral electronic disequilibrium. Dose data for such beams are difficult to obtain experimentally. Therefore, the CORVUS dose model uses analytically calculated (rather than experimentally measured) narrow-beam dose data. The model is based on the idea that physical parameters necessary to calculate absorbed dose in narrow x-ray beams can be derived from measured broad-beam dose data. Calculation of central-axis and off-axis absorbed dose in narrow beams as well as a method of generating beam profiles are described.     

Absorbed doses and energy imparted from tomography for dental implant installation. Spiral tomography using the Scanora technique compared with hypocycloidal tomography.

Tomography is sometimes needed to obtain information on the amount of bone available in the maxilla and in the posterior parts of the mandible prior to implant surgery. Both conventional and computed tomography can be employed. Recently a new imaging device, the Scanora, has been introduced which can be used for spiral tomography. The aim of this study was to compare absorbed doses and energy imparted from this new unit with those from conventional hypocycloidal tomography using the Philips Universal Polytome. A multi-film cassette with five pairs of calcium tungstate screens was used in the latter while a single film technique was used with the Scanora. The absorbed dose measurements were made on an anthropomorphic phantom. Most absorbed doses were found to be below 0.2 mGy except those to the major salivary glands. The absorbed doses with the Scanora were higher than with the Polytome. The beam direction and shorter focus-object distance in the Scanora influenced the absorbed dose distribution. The energy imparted was found to be low for both techniques, 1.8-1.9 mJ with the Scanora for both jaws, and for hypocycloidal tomography 0.78 mJ in the maxilla and 1.3 mJ in the mandible.     

Empirical equations for the representation of depth dose data for computerized treatment planning.

Equations of the form (see article) have been used to represent the variation of central axis percentage depth dose P or tissue-air ratio (TAR) with depth d below the dose maximum. The equations were originally developed for the representation of cobalt 60 depth dose data but have also been fitted to the central axis depth dose data published in the British Journal of Radiology Supplement 11, for radiations ranging in energy from 1-5 mm Cu HVT to 8 MV. Values of the constants Q and M for standard field sizes are presented together with an estimate of the goodness of fit in each case. Two different approaches have been used in determining the dose at points other than those on the central axis. In the simpler method, used for rotation techniques, the off-axis ratio (OAR) is calculated from the equation. (see article) where x is the off-axis distance, w the field width at the depth and k1 and k2 are constants. In the more accurate method, used for fixed field techniques, different equations are used within the main beam, within the geometrical penumbra and outside the beam.     

Dose calculation for very small irregular electron beam fields. 1. Dose calculation for the central beam using a simple field zone method

Very small electron beams show considerable reduction of the dose output factor, the therapeutically relevant range, the practical range and the therapeutically relevant field size, compared to broad beams. The first three effects were measured on the central beam axis for different quadratic field sizes and are presented in the first part of this publication. A simple field zone method for calculation of irregular shaped electron beams was developed from the different depth dose curves. The method allows the determination of the maximum dose and the 80% range with sufficient accuracy for practical purposes. The examination of the reduction of the therapeutically relevant field size and the consequences following thereof with respect to the adequate electron therapy of a small irregular target volume will be published in a second part.     

Characteristics of the new THOR epithermal neutron beam for BNCT.

A characterization of the new Tsing Hua open-pool reactor (THOR) epithermal neutron beam designed for boron neutron capture therapy (BNCT) has been performed. The facility is currently under construction and expected in completion in March 2004. The designed epithermal neutron flux for 1 MW power is 1.7x10(9)n cm(-2)s(-1) in air at the beam exit, accompanied by photon and fast neutron absorbed dose rates of 0.21 and 0.47 mGys(-1), respectively. With (10)B concentrations in normal tissue and tumor of 11.4 and 40 ppm, the calculated advantage depth dose rate to the modified Snyder head phantom is 0.53RBE-Gymin(-1) at the advantage depth of 85 mm, giving an advantage ratio of 4.8. The dose patterns determined by the NCTPlan treatment planning system using the new THOR beam for a patient treated in the Harvard-MIT clinical trial were compared with results of the MITR-II M67 beam. The present study confirms the suitability of the new THOR beam for possible BNCT clinical trials.     

Techniques for measurement of dose width product in panoramic dental radiography

Dose width product (DWP) is the quantity recommended for assessment of patient dose for panoramic dental radiography. It is the product of the absorbed dose in air in the X-ray beam integrated over an exposure cycle and the width of the beam, both measured at the receiving slit. A robust method for measuring the DWP is required in order to facilitate optimization of practices and enable comparison of dose levels at different centres. In this study, three techniques for measuring the DWP have been evaluated through comparison of results from 20 orthopantomographic units. These used a small in-beam semiconductor detector and X-ray film, a pencil ionization chamber and an array of thermoluminescent dosemeters (TLDs). The mean results obtained with the three techniques agreed within +/-6%. The technique employing a pencil ionization chamber of the type used for dose assessment of CT scanners is the simplest and most reliable method. The in-beam detector and film method has larger errors both from positioning the radiation detector and from measurement of X-ray beam width, which should be the full width at half maximum obtained from a scan of the film optical density. The TLD array method was accurate, but more time consuming to carry out. The mean DWP for the units studied was 65 mGy mm and the mean dose-area product was 89 mGy cm2. The DWP for 30% of the units tested exceeded the diagnostic reference dose of 65 mGy mm, recommended by the National Radiological Protection Board.