Spatial Resolution and Frequency Characteristics for Dose Modulation on the TrueBeam Medical Accelerator

Purpose: To study the spatial resolution achievable by dose modulation in a water phantom using a multi-leaf collimator, jaws and their combination. To estimate the power spectrum density of the useful signal (dose distribution) and statistical noise, evaluate the frequency interval containing the useful signal. Material and methods: Using the Gafchromic EBT2 radiochromic film, nested squares dose patterns formed in a water-equivalent phantom by 6 and 15 MV photon beams of the TrueBeam medical accelerator, jaws, a multi-leaf collimator, and a combination of these devices were measured. Dose response to step function (ESF) data was extracted from the penumbra and the linear photon source dose response function (LSF) was calculated. To move to frequency domain, fast Fourier transform was performed over the obtained datasets, as well as over individual LSF peaks, and then power spectra densities were calculated. The Nyquist frequency associated with data sampling was 1.42 mm-1, the Hann window was used to minimize leakage effect. Results: The shape of the obtained LSF peaks was approximated by a sum of two Gaussian distributions with the same center positions but different widths. The LSF peak width at half maximum (FWHM) was 1.7-3.9 mm depending on the modulation device. No significant difference was observed in the peak widths at energies of 6 and 15 MV. In most cases, the width of the peak along the X-axis was wider than along the Y-axis. The power spectrum of the useful signal had a maximum near zero frequency, a 50 % level was near 0.09 mm-1 and its high frequency limit was about 0.4 mm-1. Above this value, only the spectrum of statistical noise was recorded, uniformly distributed over frequency. Conclusion: The obtained values of the LSF peak width in the range 1.7-3.9 mm characterize the ability of dose modulation by the considered devices or their combination, which can be significant for treatment of small targets (less than 3-4 cm), where these limits of spatial resolution can be reached. The obtained relationships in frequency domain can be used for optimal removal of statistical noise using Wiener filters from profiles or two-dimensional dose distributions.

Modern Approaches for the Objective Assessment of Minimal Erythema Dose

At present, the assessment of the degree of ultraviolet (UV) exposure is based on the calculation of the minimal erythema dose (MED). The concept of MED is inextricably linked to the process of acute UV skin damage, which is accompanied by damage to the structures of the epidermis and dermis, dermal vasodilation, and it manifests clinically in erythema formation. The knowing of the MED value is necessary to choose the starting dose for UV-phototherapy of various dermatoses and to evaluate individual photosensitivity. Meanwhile, the determination of MED in clinical practice is most often performed visually by naked eye. The traditional assessment of MED is a subjective, inaccurate, poorly reproducible and not quantifiable method. In addition, preliminary UV irradiation is necessary for the estimation of MED and waiting for the results takes 24 hours. Inaccurate determination of MED may lead to the wrong dosing of UV-exposure, which leads to various complications. Today, there are a number of non-invasive methods that allow making the calculation of the MED more objective and accurate. However, these techniques have a number of limitations: the difficulty in mastering the methods, the lack of standardization of the data recording process and the evaluation of results. The majority of methods are based on the assessment of the subjective parameters of UV erythema, but not its objective pathophysiological criteria. In addition, available technologies still do not allow to estimate the MED in the first hours after UV exposure (on-site) or predict MED without UV exposure.

Hardware and Software Complex for Detection and Visualization of Subsurface Blood Vessels During Brain Tumors Resection

Purpose: The purpose of this work is to prove the possibility of subsurface blood vessels detection during endoscopic resection of brain tumors using the method of endoscopy in red and near infrared light. Material and methods: This work was accomplished with an experimental setup, simulating the geometry of endoscopic resection of brain tumor. The setup realizes the backlight of operational field with light from diagnostic window of electromagnetic spectrum (650 1000 nm) and takes photos of operational field. After that special algorithm increases the contrast of the photos and detect subsurface blood vessels. The pieces of bovine brain have served as brain samples. And thin-walled transparent plastic tube with an internal diameter 1 mm filled with bovine blood has served as a blood vessel. The tube was placed into brain samples on different depths. Results: During the experiments the series of photos of bovine brain with artificial blood vessels located on different depths was received. For every photo contrast was increased and blood vessel was recognized. Conclusion: The series of experiments has showed the possibility to detect the blood vessels with outer diameter 1 mm in the depth of 2 mm and 3 mm in brain tissues using the method of endoscopy in red and near infrared light. The depth of 3 mm is enough for preliminary detection of blood vessel during the endoscopic resection of brain tumor.

Dictionary of Abbreviations in Medical Radiology, Medical Physics and Radiation Safety

Based on the analysis of numerous literary sources and the authors' many years of their own experience in medical physics and medical radiology, a dictionary of abbreviations (abbreviations) that are most often used in scientific publications, guidelines, regulatory documents in these areas of high-tech medicine has been developed. The dictionary contains abbreviations in English, which are usually not deciphered in English-language publications, as well as abbreviations in Russian with the corresponding English abbreviations, if only they are available in scientific and educational literature. The dictionary is intended both for use in professional education, including postgraduate education, and for medical physicists, radiation diagnosticians and radiation oncologists working in radiological medical organizations.

Dosimetric Audit for Clinical Linac X-Rays Calibration

The CDT guidelines on external dosimetry audit of linear accelerators x-ray beams absolute calibration is described. The guidelines specify audits organization, conduction and results analysis for 4-20 MeV photon beams, including measurement methodology, equipment and staff specification. The guidelines are designed for medical physicists, dosimetrists and management of external beam radiotherapy departments.

Differences of Ring Applicator Reconstruction Methods in Brachytherapy

Purpose: Analysis and comparison of existing reconstruction methods of the ring applicator, as well as the development of reconstruction method that is closest to the actual source path in dosimetric planning using brachytherapy. Material and methods: Evaluation of reconstruction methods was carried out on the basis of CT images, which were used for dosimetric planning of patient treatment. The obtained data on the positioning of the radiation source using X-ray images were transferred to CT images using matrix transformation methods. A method of reconstruction along the source path is proposed, in which the minimum discrepancies in the location of the source positions in comparison with the coordinates of the actual source positions were obtained. For comparison, the reconstruction of the applicator was performed by three different methods: the method using the manufacturer’s applicator libraries, along the source path, and the method proposed above. Results: The results were assessed using a dose-volume histogram. Dose characteristics were selected based on the recommended ICRU Report 89. To assess the presence of the effect of the reconstruction method on the dose characteristics, the Friedman criterion was used. The differences between the doses for different reconstruction methods turned out to be significant (the critical value of the significance level was considered p=0.05). The maximum deviations of the median values of the parameters of the dose-volume histogram were 7.8 % for the “lib” method, 8.6 % for the tracking method, and 7.9 % for the track_new method. We calculated the median values of the deviations of the source positions obtained using the previously described reconstruction methods from the actual stopping positions of the source found experimentally. Conclusion: The use of the proposed reconstruction method track_new increases the accuracy of determining the positions of the stop of the radiation source in the applicator and, as a consequence, the accuracy of dose delivery on the brachytherapy apparatus in comparison with the manual tracking method. The methods used in this study can be used when putting the applicators into clinical use as part of a radiotherapy quality assurance program. The obtained results will be useful to medical physicists if it is necessary to choose a technique for carrying out the reconstruction of the applicator in accordance with the available capabilities and imaging methods and, as a result, will increase the accuracy of dosimetric planning and dose delivery on brachytherapy afterloaders.

The Global Quality Assurance of Radiation Therapy Clinical Trials Harmonization Group Recommendations for Сontouring of OARs

QUANTEC report summarized the data of the dose constrains for critical structures in radiotherapy. The authors of this report determined that one of the obstacles to obtaining meaningful data on tolerant doses was inconsistency in the nomenclature of names. And inconsistencies in the guidelines for contouring the organs at risk increase the variability in contouring. Eliminating these inconsistencies increases the speed and safety of the workflow within each individual healthcare facility and improves the accuracy and reliability of the data underlying the dose limits that are developed. The standardization of terminology facilitates the integration of dosimetry data, the creation of various templates and scripts to automate the creation of a prescription for the RT course for similar cases, and the automation of reports. It helps to train the systems supporting artificial intelligence. AAPM report TG-263 resolved the nomenclature problem, however, different anatomical boundaries of different organs were still hidden under the same names in various large research centers. In 2020 The Global Quality Assurance of Radiation Therapy Clinical Trials Harmonization Group published an article. It summarized the many years of work by the scientific radiotherapy community to standardize the approach to delineating the OARs. This article provides an overview of the article key points and provides links to outline guides. A special attention it was paid to clarifying the anatomical boundaries of such OARs: heart, femoral heads, skin, individual structures of the gastrointestinal tract and substructures of the eye.

Mathematical Modeling of Uniform and Nonuniform Malignian and Normal Tissues Irradiation. Mathematical Analysis of the Tumour Grid Irradiation

Purpose: On base created mathematical model (MM) modified Veybull distribution, intended for NTCP calculation, depending on irradiated volume V and uniform irradiation dose D, to develop the MM, which allows to realize transition from nonuniform dose in tissue to the equivalent, uniform identical dose. To research the hypothesis, under what condition lumpy sharing dose in system tumors+normal tissues will be more efficient, than uniform dose. Material and Methods: Lumpy sharing dose are described as a differential histogram dose-volume (DDVH). The MM for calculation transition lumpy distribution of the absence of the beam complication probability (ACPr) in tissue is designed. It was used for MM conclusion, which allows to calculate transition values of the Adequate Dose (AD) of the uniform irradiation tissues, which use brings about ACPr in tissues. Results: On base of the suggestions and proved affirmation MMs are received, which allow for lumpy distribution of local NTCP values, presented as a DHDV, to calculate transition values ACPr, as well as select from it MM for reduction of the lumpy sharing dose. On base created MM question was explored, in what case tumors and normal tissue through a grid irradiation will be more effective than uniform irradiation. Conclusion: The transition from lumpy sharing doses in tissues to equivalent uniform dose present the significant interest for the RT planning efficiency. Their study is necessary, where the radiation of biosubjects is influenced.

Extracranial Dose and the Risk of Radiation-Induced Malignancy after Intracranial Stereotactic Radiosurgery: is it Time to Establish a Therapeutic Reference Level?

Background. To measure extracranial doses from Gamma Knife Perfexion (GKP) intracranial stereotactic radiosurgery (SRS) and model the risk of malignancy after SRS for different treatment platforms. Methods. Doses were measured for 20 patients undergoing SRS on a GKP at distances of 18, 43 and 75 cm from the target, corresponding to the approximate positions of the thyroid, breast and gonads respectively. The National Cancer Institute (NCI) RadRAT calculator was used to estimate excess lifetime cancer risk from this exposure. Five different age groups covering childhood and younger adults were modelled for both sexes. Results. Extracranial doses delivered during SRS with the GKP were a median 0.04 %, 0.008 % and 0.002 % of prescription dose at 18 cm, 43 cm and 70 cm from the isocentre respectively. Comparison with the literature revealed that the extracranial dose was lowest from GKP, then linacs equipped with micro-multileaf collimators (mMLC), then linacs equipped with circular collimators (cones), and highest from Cyberknife (CK). Estimated lifetime risks of radiation-induced malignancy in the body for patients treated with SRS aged 5–45 years were 0.03 0.88 %, 0.36–11 %, 0.61–18 % and 2.2–39 % for GKP, mMLC, cones and CK respectively. Conclusions. We have compared typical extracranial doses from different platforms and quantified the lifetime risk of radiation-induced malignancy. The risk varies with platform. This should be taken into account when treating children and young adults with SRS. The concept of a therapeutic reference level (TRL), similar to the diagnostic reference level (DRL) established in radiology, is proposed.