Methodology of Defining of the Radiation Therapy Components for Various Methods of Patients’ Treating Using Medical Linear Accelerators and Gamma-Therapeutic Devices

One of the main factors affecting the effectiveness of radiation therapy is the constancy of the patient’s position on the treatment table created by immobilization devices of various designs and held throughout the entire irradiation procedure, which guarantees the accuracy of the delivery of the prescribed dose distribution. The purpose of the work was to establish the numerical values of the dominant components of a radiation therapy session for each of the irradiation techniques most commonly used in clinical practice of the radiation therapy. To determine the numerical values of the components of the radiation therapy session, the authors have measured each component for some clinical cases of patients’ irradiation placed. The patients had been diagnosed with the following malignant tumours: prostate cancer, breast cancer, lung cancer, head and neck tumours. More than 2000 individual measurements have been carried out with the help of such medical linear accelerators as "Clinac", "Unique", "Truebeam", and the gamma-therapeutic apparatus named "Theratron". The numerical values of the time spent on 3 groups of parameters of an irradiation session were established: the mechanical parameters of the radiation therapy equipment, the functional characteristics of the irradiation systems and the parameters that directly depend on the personnel involved in an irradiation procedure. According to the measurement results, the flow diagram for the procedures of verifying a patient’s position on the therapeutic table (2 different techniques), preceding their irradiation and the radiation therapy procedures themselves was proposed. It has been shown that a number of session components can run in parallel to each other thus optimizing the time spent by a patient in the treatment room. Using the obtained values of the time spent on the radiation session parameters it is possible to actualize the mathematical model that will allow the medical physicist to determine in advance the duration of the irradiation session at the stage of treatment planning and choose a radiation therapy technique taking into account the individual parameters of the irradiation session in each particular clinical case.

Ключевые слова: сеанс облучения, линейный ускоритель, лучевая терапия, временные характеристики. DOI: 10.21122/2220-9506-2020-11-4-289-297 Introduction For receiving radiation therapy (RT) oncological patients' radiation safety, it is necessary to ensure their constancy on the worktable by using fixing devices of a standard design throughout the entire irradiation procedure [1]. In 1976, the International Commission on Radiological Systems (ICRU) presented a recommendation according to which the uncertainty in the delivery of the absorbed dose to the target on any therapeutic apparatus should not exceed 5 % [2]. One of the key factors on which the effectiveness of irradiation depends is the time parameters (time spent by a patient in the treatment room), which directly depend on their position, and, consequently, the accuracy of the delivery of an individual three-dimensional dose distribution [3].
In [4], the authors established the temporal characteristics of the irradiation session components, which have a dominant effect on its duration, and proposed an algorithm that allows to set the duration of cancer patients treatment, depending on the use of various methods of radiation therapy and tumour localization.
The process of modern remote RT includes pre-radiation preparation of the patient, treatment planning, verification of the treatment plan and irradiation with a radiotherapy apparatus.
During the radiation planning stage, the medical physicist creates several treatment plans. The plan that best meets the target coverage criteria and exerts the least dose to nearby organs includes more radiation fields, which significantly increases the duration of a patient's treatment session. The session time is also influenced by the choice of a radiation technique. An increased radiation time can surge the errors in the internal position of the target of radiation due to the biological activity of organs [5]. It should be noted that the treatment selection approach described above does not take into account the timing of the RT session.
The aim of this work was to establish methods for the dominant numerical components of a radiation therapy session for each of the methods of radiation used in the radiation therapy. On the basis of the presented experimental data, the program is presented that determines the duration of the radiation session at the stage of pre-radiation preparation and chooses a radiation therapy technique taking into account the individual parameters of the radiation session in each specific clinical case.

Research results
To determine the temporal characteristics of the components of the RT session, the authors experimentally established the values of the dominant components of the RT session for each of the most commonly used RT in clinical practice. The measurements for clinical cases of irradiation of patients with localizations of malignant neoplasms: prostate cancer, breast cancer, lung cancer, head and neck tumours were taken [6]. For each patient from the specified sample, dosimetry plans were calculated additionally for the rest of RT techniques used in the clinical practice of radiotherapy departments around the world for these localizations [7]: 3D CRT -threedimensional conformal radiation therapy, IMRTintensity modulated radiation therapy, VMATsector radiation therapy with volumetric intensity modulation or Gating -radiation therapy with respiratory control. A sample of 100 patients was carried out for each site. The measurements were taken for RT sessions carried out using medical linear accelerators of the following models: "Clinac", "Unique", "Truebeam", as well as the gamma-therapy device named "Theratron".
The authors have divided all components of the RT session into 3 groups: mechanical parameters, functional parameters, and parameters that directly depend on the personnel involved in the irradiation procedure.
Mechanical parameters include those ones, the duration of which is not constant and is associated with the components selected at the factory for the implementation of individual manipulations. They include rotation of the therapy table, the tripod of the radiotherapy apparatus and the collimator. The dependences of the rotation time of the therapy table, tripod and collimator on the setting of the required angles are presented in Figures 1-3, respectively.
The following operations were included in the group of functional parameters: loading the irradiation plan on an operator's computer, checking the irradiation plan for errors in data transmission, initializing of the radiotherapy device with the irradiation parameters relevant to the first treatment field, initializing of the accelerator with the irradiation parameters relevant to the second and subsequent medicinal fields. The duration of these procedures depends on the volume of transmitted and processed information.   The parameters that directly depend on the personnel involved in the irradiation procedure include these operations, the performance of which is entrusted with a person, and rely on the individual characteristics of an individual specialist. These include placing the patient and centering in the prescribed for the irradiation position, entering the irradiation parameters, setting the required mechanical parameters of the accelerator under visual control from the treatment room, installing / Devices and Methods of Measurements 2020, vol. 11, no. 4, pp. 289-297 E.V. Titovich et al. Приборы и методы измерений 2020 removing dose modulating devices, switching on the irradiation, and verifying the patient's position [8].
The table shows the measurements of mechanical procedures during the RT session, depending on the radiotherapy device.  , vol. 11, no. 4, pp. 289-297 E.V. Titovich et al. Приборы и методы измерений 2020   The duration of the RT session directly depends on the number of treatment fields of irradiation [9]. Figure 4 shows the dependences of the radiation time for the considered localizations on the number of irradiation fields for various RT techniques.

Flow diagram of a typical radiation therapy session
The RT session on modern radiation devices begins with the verification of the patient's position according to the planned one [10]. Today, the RT uses two types of the patient position verification: kV and CBCT. During kV-verification two orthogonal images are taken and then used to compare the patient's position before the RT session in two projections. During CBCT, a volumetric computed tomography is built and comparison is made over the entire volume of tissues covered by the scanned area. Flow diagram of a typical radiation therapy session is shown in Figure 5. Devices andMethods of Measurements 2020, vol. 11, no. 4, pp. 289-297 E.V. Titovich et al. Приборы и методы измерений 2020. -Т. 11, № 4. -С. 289-297 E.V. Titovich et al. Figure 5 -Flow diagram of a typical radiation therapy session As seen in Figure 5, some of the RT session procedures run in parallel, which should be considered while defining its time at the stage of the patient dosimetric planning. Such procedures comprise a patient positioning and loading of the verification field or CBCT parameters, setting the collimator and beam arm angle values.

Conclusion
Experimental measurements of the irradiation session time parameters were taken for clinical cases of irradiation of patients with localizations of malignant neoplasms: prostate cancer, breast cancer, lung cancer, head and neck tumours (more than 2000 individual measurements for three different medical linear accelerators: "Clinac", "Unique", "Truebeam", as well as the gamma-therapeutic apparatus "Theratron".
The numerical values of the time spent for 3 groups of parameters of the irradiation session have been established: mechanical parameters of the radiation therapy apparatus, functional characteristics of the systems for the implementation of irradiation and parameters that directly depend on the personnel involved in the irradiation procedure; time dependences for mechanical parameters are constructed.
The flow diagram is proposed for the procedures of verifying the patient's position on the therapeutic table (two different techniques): the patient's previous radiation and the radiation therapy procedures themselves, which shows that a number of session components can run in parallel to each other, hence the time spent by the patient in the treatment room can be optimized.
Using the obtained values of the time spent on the implementation of various irradiation session components, it is possible to imbed the mathematical model that will determine in a preliminary manner the duration of the irradiation session at the stage of pre-irradiation preparation and choose the radiation therapy technique bearing in mind the individual parameters of the irradiation session in each specific clinical case.