As part of dosimetric verification for IMRT intensity modulated radiation therapy, we examined the selection of a dosimeter in accordance with the purpose of physical measurement and the process of data analysis. Because of the high dose conformation in the target volume and minimum dose in the organs at risk(OAR) in IMRT, dosimetric verification is essential. Because the performance of dosimetric verification in a patient is not allowed, a physical phantom and dosimeter must be used. Dose verification using a physical phantom, from which the beam data optimized for a patient slated for IMRT are transferred, may cause latent error as a result of change in the depth of each beam toward an isocenter. This effect may change the dose distribution and prescription dose. The basic methods of dosimetric verification with physical measurement are point dosimetry, when the reference dose is given at a point by planning software, and volumetric dosimetry, when planning software gives the dose as a volumetric configuration. While the most accurate dosimetry is done using a calibrated ionization chamber, IMRT requires volumetric dosimetry using some kind of portal film or a polymer gel dosimeter, because of the need for dosimetric verification for an irregular dose distribution in IMRT. The importance of indirect dosimetry using these methods is to provide calibration as a dosimeter, absolute dose, and preservation of calibration. In our study, the verification of dose distribution for IMRT using portal film and a RANDO phantom could be performed with an error of less than 2% in all cases. The measurement error for the central dose using a JARP-type ionization chamber and MixDP was less than 3% in all cases except for the case with the maximum error. At the moment, IMRT requires a great deal of effort in the processes of planning, dosimetric verification, and isocenter checking in every fraction to maintain high accuracy. Although the need for a large amount of effort in the service of maintaining accuracy may be reasonable, it could be enough to inhibit the spread of IMRT. It is hoped that an easy method of dosimetric verification that still maintains a high level of accuracy will develop as a result of this great effort.
The number of dose monitor units in IMRT QA plans differs form the actual conditions of IMRT in dosimetry for verification of dynamic multileaf-IMRT(DMLC-IMRT). We measured the accuracy of the position of the dynamic multileaf collimator and the dose profile for various numbers of dose monitor units, and verified the accuracy of dosimetry in an IMRT QA plan. The accuracy of the position of the dynamic multileaf collimator was measured by using the software of the external irradiation device, and the dose profile was measured by using the semiconductor profiler. Deviation in the position of the dynamic multileaf collimator increased as the number of dose monitor units decreased. When deviation in the position of the dynamic multileaf collimator was large and the gap width of the multileaf collimator was narrow, the change in dose profile was large. Therefore, verification of IMRT QA plans requires a phantom and measurement device close to the actual conditions of IMRT.
Since the year 2000, our hospital has been equipped with an intensity modulated radiation therapy (IMRT) facility. Before IMRT is administered, the absorbed dose is measured by the ionization chamber to provide verification for the IMRT procedure. In utilizing the current point dose evaluation, large discrepancies have been experienced when the measured dose is compared with the calculated dose. This discrepancy is due to the lack of uniformity in IMRT irradiation in comparison with that of the present method of dose distribution. In order to reduce the margin of error, the average dose of the ionization chamber calculated on a dose-volume histogram was compared with the measured dose. As a result, the margin of error was minimized to <2% in uniform areas and <4% in non-uniform areas.
Because intensity-modulated radiation therapy (IMRT) is complicated by many small, irregular, and off-center fields, dosimetry quality assurance(QA)is extremely important. QA is performed with verifications of both dose distributions and some arbitrary point doses. In most institutes, verifications are carried out in comparison with dose values generated from radiation treatment planning systems (RTPs) and actually measured doses. However, the estimation of arbitrary point doses without RTPs should be feasible in order to perform IMRT delivery more safely and accurately in terms of the clinical aspect. In this paper, we propose a new algorithm for calculating output factors at the center point of the collimations in an IMRT field with step and shoot delivery machines in which the lower jaws were replaced with multileaf collimators(MLC). We assumed that output is independently affected by collimator scatter and total scatter according to the position of the upper jaws and each of the MLC leaves (lower jaws). Then, the two scatter factors are accurately measured when changing their position. Thus, the output factor for an irregular field could be calculated with the new algorithm. We adopted this technique for some irregular fields and actual IMRT fields for head-and-neck cancer and found that the differences between calculated and measured output values were both small and acceptable. This study suggests that our methods and this algorithm are useful for dosimetry quality assurance.
A method for quantitatively checking the position to be used for stereotactic radiosurgery was devised using a digital reconstruction radiogram (DRR) and electric portal images (EPI). On each image, the radiation field and head contour were extracted by binary-coded processing. The center of gravity position vector of the extracted image was calculated, and the vector from the head contour to the radiation field was obtained. By comparing the difference in DRR and EPI, an index was made of positional error. The error in the center of gravity position coordinate in changing 40〜80 of 256 image tones in the binary operation was 0.5mm or less. The effect on the center of gravity position vector caused by the image distortion of EPII was within 0.5mm in the 200×200mm^2 region. Statistical processing was carried out on this index value, and a 95% confidence interval was estimated. The index value of the Z component of the lateral image became -1.43±0.66, and it shifted to the negative side. Error was evaluated by the verification method devised for the target point position. Results indicate the usefulness of the verification method using center of gravity for the target point position.
The MM5O is a racetrack microtron that can emit photon beams or electron beams up to 50 MeV. The MM22 using the scanning beam method and the MM22 using a flattening filter method both to flatten the emission field and a water phantom with particular function measurable of PDD etc. in an accelerator using the scanning beam method to make up the PDD curve of photon beams from the linear accelerator. The Clinac21EX was thus employed. The maximum depth of beam flux was shallow, the gradient of the flux decrement large, the surface dose large, and the estimated nominal energy low to the same nominal energy. From these findings, it can be said that thorough comprehension of the characteristics of beam flux properties for these units is necessary when photon beams are to be used.
This study aimed to decrease the radiation dose to the disease-free testis in postoperative irradiation for seminoma patients. We consider the factors influencing the peripheral dose(PD)of 10MV X-ray radiotherapy to be the distance between the caudal edge of the irradiation field and the measuring point, the size of the therapeutic irradiation field, the thickness of the lead shield laid above and lateral to the disease-free testis, and the thickness of the lateral absorber. Materials and methods: We measured the scattering radiation dose coming from the accelerator head and that due to irradiation volume. We measured these doses using a testicular phantom as the non-diseased testis. Results: Scattering radiation from the accelerator head mainly contributes to PD, whereas the larger the size of the irradiation field the more the scattering radiation from the irradiation volume contributed to PD. PD changed more at the surface of the phantom than at its center. PD at the testicular phantom could be reduced to less than 1 % of the therapeutic dose when it was situated more than 5cm distant from the caudal limit of the irradiation field, the lead shield above the testicular phantom was 7.5cm thick, and the lateral lead shield was 2mm thick. Conclusions: PD is influenced by many factors. It is necessary to clarify the change in PD at the testicular phantom, and it is important to limit the caudal edge of the irradiation field and to lay the lead shield for the attenuation of radiation on the disease-free testis.
In the case of total skin electron therapy without the beam guide, the electron beam is scattered just outside the gantry exit, dose uniformity in the field is broken, and dose is spread outside the light field. The aims of this study were to measure the mean energy of the off-axis incident electron beam without the beam guide and to establish a reference for the clinical situation. For the measurement, a 4 MeV electron beam was selected among several energies from the linear accelerator. A scintillating fiber beam energy monitor measured the mean energy of the incident electron beam. This energy monitor is a small, light-weight piece of equipment composed of a wedge absorber, scintillation fiber, and photodiode. We found the relationship between electron energy and the indicated value of the energy monitor by means of the estimation of correction factors for five different kinds of electron energy. The preferable linear correlation of 0.997 of the coefficient of determination (R^2) was obtained. From the results of measurement at each point, those variations due to the off-axial distance were about 5% within the measured area. It was assumed that the energy did not change rapidly beyond the light field. Clinically, this amount of variation in energy may not cause any problem.
Although one-dimensional densitometry has been widely performed for linac dosimetry, it is a time-consuming, labor-intensive procedure. We studied a new densitometry system (DD-System)that consists of a personal computer and common flatbed-type scanner, with regard to its usefulness for daily QC checks in comparison with conventional densitometry. The spatial resolution of the DD-System is very high, and its accuracy of measurements of field size and flatness are equivalent to those of a conventional system at an optical density <2.0. Although the performance of the image scanner limits the maximum optical density of our system, this system can acquire high-resolution two-dimensional dose distributions quickly and easily. In conclusion, the DD-System is very useful for reducing the amount of time and effort required in daily QC activities. We consider this system applicable to the analysis of complicated dose distributions in this era of IMRT.
The wide radiation field for mediastinal dose distribution should be inhomogeneous with the usual simple opposed beam irradiation. The purpose of this study was to improve the dose distribution of the mediastinum using a conventional planning system with a dose-volume histogram(DVH)and the field-in-field technique. Three-dimensional (3D)dose distribution is obtained in bilateral opposed-field irradiation. An overdose area obtained from the 3D dose distribution is defined and reprojected into the irradiation field. A new reduced field is created by removing the reprojected overdose area. A 3D dose distribution is again obtained and compared with the results from first one. Procedures were repeated until each of the target volumes was within ±5 % of the prescribed dose and the irradiation volume within 107% or less of the prescribed dose. From the DVH analysis, our field-within-a-field technique resulted in a more uniform dose distribution within the conventional planning. The field-within-a-field technique involves many parameters, and an inverse planning algorithm is suitable for computation. However, with our method, the forward planning system is adequate for planning, at least in a relatively straightforward planning system such as bilateral opposed fields therapy.
In the measurement of 4 MV and 10 MV X-ray collimator scatter factors (S_c), the method of using an acrylic mini-phantom showed no significant differences between cases in which the chamber axis was either parallel or perpendicular to the beam axis. Chamber readings with an aluminum or acrylic build-up cap were not reflected by contaminant electrons when the chamber axis was parallel to the beam axis. On the basis of the data on 4 MV and 10 MV X-ray collimator S_c measured using an acrylic mini-phantom, we examined three methods of obtaining square fields equivalent to rectangular fields, and reached the following conclusions: (1)The A/P method was not accurate because it did not take into account the structure of the radiation head. (2)Regarding the geometrical weight factor(k) used in the field-mapping method, more accurate k values were obtained when using the geometrical places of the flattening filter(or the second source, taken from the concept of extra-focal radiation), the upper and lower collimators, and the chamber, rather than when using the geometrical places of the source, the upper and lower collimators, and the chamber. (3)The most accurate k values could, in general, be obtained when determined on the basis of measured S_c data.