Special Dosimetry Research Articles

Application of the Ion Chamber Array in Magnetic Resonance Accelerator QA

The magnetic resonance accelerator (MR-Linac) is gradually widely used due to high-quality soft tissue contrast and real-time tracking. However, the special dosimetry characteristics and wide field sizes of MR-Linac increase the QA difficulty with conventional measurement method. The purpose of this study was to confirm an ion chamber array could be used for measuring the beam quality, the profiles, as well as the positioning accuracy of all MLC leaves efficiently, by comparing results with the conventional method. To propose a new QA approach for solving the common problem in data acquisition caused by the wide fields of MR-Linac. The research was based on a MR-Linac fixed with 1.5T MR and 7MeV energy photon beam. The conventional QA method adopted the MR water tank with a gantry angle of 0°and an SSD of 133.5 cm, both microdiamond and ionization chamber detector were used to acquire the dose profiles (PDD, inline, crossline and diagonal). Field sizes 1 × 1 cm2, 2 × 2 cm2, 3 × 3 cm2, 5 × 5 cm2, 10 × 10 cm2, 15 × 15 cm2, 22 × 22 cm2, 40 × 22 cm2,57 × 22 cm2 were measured with depth 13mm, 50mm, 100mm for vertical beam. As for the wide fields (larger than 15 × 15 cm2), two profiles of x axis (one from left to right, the other from right to left) needed to be gathered and then stitched into one final profile. A boot phantom with an ionization chamber detector was used for measuring beam quality. We defined the profiles measured by conventional method as the baseline. An ion chamber array was adopted to acquire TPR, PDD, profiles and MLC positioning, comparing to the conventional method. The center of ion chamber array was placed to the isocenter of MR-Linac, the array could move to the right and left offset positions through engaging the pin into correct hole of QA platform, such 'once positioning and twice movements' operation could finish within 3 minutes. The central detector of the ion chamber array was used for measuring beam quality. TPRs for different depths were acquired by stacking solid water on the ion chamber array. As for the profiles, we could get the final profile by 'once positioning and twice movements' efficiently. As for the positioning accuracy of MLC leaves, firstly the central leaf pair was put on y = 0 to measure 'open profile' under the open field. Then we moved the MLC leaves to different positions to get the n profile (n for different leaf positions). The ratio of n profile to open profile could show the positioning accuracy of MLC. We adopted 2D gamma (1mm / 2%) to compare the profiles between the ion chamber array and the conventional method, the results were within 98%. The beam quality consistency of ion chamber array comparing to the wedge tank was within 1% according to daily measurement. The ion chamber array could reflect the MLC positioning differences, the sensitivity was 0.5 mm. The ion chamber array showed a convenient QA method both for the dosimetry and for the MLC positioning accuracy which did reduce the overall measurement time, it was recommended for daily and monthly QA for MR-Linac.

Preliminary assessment of dose distribution on the spatial micro level for internal exposure of alveolar epithelium of rats by 56Mn

Special dosimetry study of experimental rats exposure by sprayed 56Mn powder was conducted during experiments in order to study internal irradiation effects. All experiments were performed in Kurchatov’s reactor complex «Baikal-1» (Kurchatov city, East-Kazakhstan region) after neutron activation of stable Mn powder. This study was performed by group of scientists from Japan, Kazakhstan, and Russian Federation. The results of estimated doses in lungs alveolar epithelium of rats are shown in this paper. Absorbed dose on the «surface» of epitelium is equal to 160 Gy and absorbed dose in the «bottom» of epithelium for minimalthickness of epithelium cells is 8.9 Gy and for maximal thickness of epithelium cells equal to 0.4 Gy.

Preliminary assessment of dose distribution on the spatial micro level for internal exposure of alveolar epithelium of rats by 56Mn

Special dosimetry study of experimental rats exposure by sprayed 56Mn powder was conducted during experiments in order to study internal irradiation effects. All experiments were performed in Kurchatov’s reactor complex «Baikal-1» (Kurchatov city, East-Kazakhstan region) after neutron activation of stable Mn powder. This study was performed by group of scientists from Japan, Kazakhstan, and Russian Federation. The results of estimated doses in lungs alveolar epithelium of rats are shown in this paper. Absorbed dose on the «surface» of epitelium is equal to 160 Gy and absorbed dose in the «bottom» of epithelium for minimal thickness of epithelium cells is 8.9 Gy and for maximal thickness of epithelium cells equal to 0.4 Gy.

Hendee's Radiation Therapy Physics, 4th ed. TPawlicki, DJScanderbeg & GStarkschall. New Jersey: Wiley‐Blackwell, 2016. Hardcover 352. pp. Price: $159.95. ISBN: 978‐0‐470‐37651‐5.

This is the 4th edition of a book on the physics of radiation therapy that covers basic principles and new technologies in the field and how these apply to the clinical practice. It is a well-referenced and easy to read text. The text is designed as an alternative or supplement to other well-established books in the field of radiotherapy physics. In addition, the authors have made an extra effort to provide a concise summary of the objectives for every chapter. They even set aside a chapter on patient safety and quality improvement. This article is protected by copyright. All rights reserved.

Dosimetric response of radiochromic films to protons of low energies in the Bragg peak region

One of the major advantages of proton or ion beams, applied in cancer treatment, is their excellent depth-dose profile exhibiting a low dose in the entrance channel and a distinct dose maximum (Bragg peak) near the end of range in tissue. In the region of the Bragg peak, where the protons or ions are almost stopped, experimental studies with low-energy particle beams and thin biological samples may contribute valuable information on the biological effectiveness in the stopping region. Such experiments, however, require beam optimization and special dosimetry techniques for determining the absolute dose and dose homogeneity for very thin biological samples. At the National Centre of Accelerators in Seville, one of the beam lines at the 3 MV Tandem Accelerator was equipped with a scattering device, a special parallel-plate ionization chamber with very thin electrode foils and target holders for cell cultures. In this work, we present the calibration in absolute dose of EBT3 films [Gafchromic radiotherapy films, http://www.ashland.com/products/gafchromic-radiotherapy-films] for proton energies in the region of the Bragg peak, where the linear energy transfer increases and becomes more significant for radiobiology studies, as well as the response of the EBT3 films for different proton energy values. To irradiate the films in the Bragg peak region, the energy of the beam was degraded passively, by interposing Mylar foils of variable thickness to place the Bragg peak inside the active layer of the film. The results obtained for the beam degraded in Mylar foils are compared with the dose calculated by means of the measurement of the beam fluence with an ionization chamber and the energy loss predicted by srim2008 code.

WE‐E‐213A‐04: Practical Aspects of Electron Beam Dosimetry

Electron beam dosimetry can be a deceptively simple endeavor. On one hand, the process of measuring and using unrestricted electron cone factors is remarkably straight forward. However, making measurements in restricted fields especially at extended treatment distances can prove to be quite a challenge, especially if the field is extremely narrow. Ionization chamber correction coefficients for cylindrical and plane‐parallel chambers can depend on many factors and ensuring that the correct factors are applied for measurements at depth in water or solid phantoms can be confusing.This talk will cover proper measurement set up and technique, problems and pitfalls associated with detector selection for various electron measurement situations, chamber response characteristics that should be known and determined before use for electron dosimetry, and efficient techniques for performing routine commissioning measurements and special dosimetry for electron beams. The influence of TG‐51 calibration protocol on routine clinical measurements will also be covered along with some clinical techniques where electron beam measurements are particularly challenging and interesting.Learning Objectives:1. Ionization chamber response characteristics in electron beams2. Selection of detectors for measurements in electron beams3. Use of ionization chambers and other detectors in electron beams measurements4. Special measurement situations: restricted field measurements, measurements at extended distances, measurements in non‐water phantoms5. Clinical situations involving special electron beam measurements

Effectiveness of syringe shieldings using radionuclides in radiation synovectomy

The radiation exposure in radiation synovectomy was investigated for technician and therapist using Erbium-169, Rhenium-186 and Yttrium-90 with and without syringe shieldings. Dose rates were measured in relation to the distance of the syringe containing the radionuclide. Measurements were repeated using syringe shieldings which consist of plastic surrounded by a lead layer. The most relevant radiation exposure arises from Yttrium-90. Using syringe shieldings radiation exposure can be reduced by a factor of thousand. This kind of radiological protection is completely sufficient for the therapist. Concerning the technician preparing the radiopharmaceutics, the limit of the official German dosimetry service (500 mSv) might be exceeded if no special radiological protection is established. Thus, special dosimetry is recommended.

Validation of neutron propagation calculations using the DORT and DOTSYN codes and the special dosimetry benchmark experiment at the French St. Laurent reactor

The St. Laurent B1 unit is a 900 MWe PWR that was loaded with MOX fuel at the beginning of its 8th cycle. In order to provide the means of validating the calculation of neutron propagation in a power reactor, the French national electric utility EDF installed numerous neutron dosimeters in special surveillance capsules and in three ex-vessel chains in the reactor pit. These dosimeters were irradiated for one cycle and then removed, providing measured reaction rates at locations both inside and outside the reactor vessel. This paper describes the analysis performed by Framatome using the DORT + DOTSYN 3-D synthesis modules to obtain calculated dosimeter responses at each instrument location. Several 2-D and 1-D models were developed, paying particular attention to the geometrical representation of the reactor: the variable mesh capability of the DORT code was used and a special variable mesh generator was written for it, along with a graphical interface for visual checks. A 100 energy group transport cross-section library derived from the master ENDF/B-IV and ENDF/B-VI libraries was used. The results show very good agreement between the calculated and measured fast neutron reaction rates at the in-vessel locations, with an average calculated (C) to measured (M) ratio, CM, of 0.94 and a small dispersion in the CM distribution. The CM values at the ex-vessel dosimeter locations averaged 1.12, which indicates that the transport cross-section library slightly underestimates the attenuation of the fast neutrons passing through the reactor vessel. These results are comparable to those from a similar analysis carried out independently based on the same St. Laurent dosimetry benchmark, using a Monte Carlo code which is reported elsewhere.

The patient equivalence of the RANDO phantom for cobalt gamma rays.

The RANDO2 “average man” or “average woman” phantoms are especially designed for use in connection with radiation dosimetry studies. The construction of the phantom, which has been discussed in detail in the literature (1, 2), consists of a special thermosetting synthetic rubber which is molded around a natural human skeleton. Lungs are simulated with a special foam reported to have a density of 0.32. During the past three years a RANDO “average man” phantom has been in use at Walter Reed General Hospital in the development of a number of special dosimetry technics using 60Co gamma rays and 2 MVp x rays. In all of these studies the exposure in the phantom was measured with film or miniature ionization chambers. In the application of the measurements to treatment planning we have assumed that the phantom was “patient equivalent,” i.e., that an exposure at a depth and in any particular anatomical site in the phantom would accurately predict the exposure under similar conditions in the patient. This assumpti...

Dosimetry for fast-neutron irradiations of cultured cells and intact animals. II. Comparison between activation and ionization methods.

SummaryIn addition to the use of tissue-equivalent ionization chambers for neutron dosimetry, sulphur activation detectors were employed for special dosimetry purposes. A comparison of the two dosimetry methods was carried out for neutrons produced by the D-D and the D-T reactions. For 15 mev neutron irradiations a good agreement between both methods was obtained. However, for 3 mev neutron irradiations, it was found that the dose calculated on the basis of the sulphur activation had to be multiplied by a factor 1·65 to obtain the correct value corresponding to the tissue-equivalent ionization chamber. The large resonances in the cross-section curve for the (n, p) reaction in sulphur for the neutron energy region between 2·5 and 4 mev may be responsible for the discrepancy in the doses measured with these independent methods.

The physical aspects of the utilization of the tripartite rigid cobalt 60 applicator for the treatment of carcinoma of the cervix.

The basic physical considerations in the use of the cobalt 60 applicator for treatment of carcinoma of the cervix described in the preceding paper include: (1) A provision of special dosimetry tables so that the gamma-roentgen contribution from each capsule of the applicator to any point in the pelvis may be summated. (2) The projection of the applicator and its modifications on a three-axial geometric system, predicating certain fixed relationships between the applicator capsules and pelvic points which are to be representative of the desired radiation to the pelvis. (3) The determination of the required radioactivity in each capsule so that the most desirable dosage to the various points may be obtained. (4) The selection of the total capsular strength of each capsule so that the radiation time will be optimal and frequent manipulation of the contents of the capsule may be avoided. (5) A method of adjustment of the cobalt 60 intensity within the capsules so that the desired total strengths can be obtain...