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Purpose: Carrying out dosimetric investigation of possibility to replace a traditional combined radiation therapy of cervical cancer by combinations only external irradiation, without change of total course dose and number of fractions. Material and Methods: Eleven patients with a diagnosis of cervical cancer (stages T2bNxM0 and T3NxM0) who received a course of combined radiotherapy (CRT) have been considered in this study. The combination of dose delivery techniques 3D-CRT + high dose rate brachytherapy (HDR) was used as a basic one. The following fractionation regimes for CRT were simulated: external beam RT (EBRT) of the first stage – total dose 50 Gy and fraction dose 2 Gy (25 fractions), the second stage – total dose 28 Gy and fraction dose 7 Gy (4 fractions). Total CRT course dose was 89.7 Gy EQD2. Dosimetric planning of EBRT using conventional radiography and 3D-CRT has been carried out using XIO dosimetry planning system. Dosimetric planning of first-stage EBRT and second-stage EBRT using the VMAT technique has been performed in the Monaco dosimetry planning system. HDR of the second stage has been planned using the HDRplus dosimetric planning system for the Multisource HDR unit with a 60Co source. Results: Coverage of the clinical volume of the tumor using HDR, on average, was equal to 95 % of the prescribed dose at 91.8 % of the volume, 110 % of the dose – 75.7 % of the volume. 60Co + VMAT results in the coverage level 95 % of the dose at 97.1 % of the volume and 110 % of the dose at 2.1 % of the volume. 3D-CRT + VMAT provide the coverage level of 95 % of the dose at 98 % of the volume and 110 % of the dose at 2.6 % of the volume. Using the combination VMAT + VMAT allows achieving the average coverage of the target at the level of 98 % of the dose at 97 % of the volume, 110 % of the dose at 8.8 % of the volume. The maximum dose per volume of the organs at risk equal to 2 cm3 did not exceed their tolerant levels both for the bladder and for the rectum. Conclusion: At present, there is a technical possibility to replace the second stage of CRT cervical cancer by EBRT using the VMAT technique. Implementation of the VMAT technique allows to increase the uniformity of irradiated volume coverage comparing with traditional HDR. While using VMAT technique the tolerant levels of organs at risk are not exceeded.

Purpose: Carrying out the analysis of the physical and radiobiological equivalence of dose distributions obtained during the planning of hypofractionated stereotactic radiation therapy of the prostate cancer and verification using a three-dimensional cylindrical dosimeter. Material and Methods: Based on the anatomical data of twelve patients diagnosed with prostate carcinoma, stage T2N0M0 with low risk, plans were developed for stereotactic radiation therapy with volumetric modulates arc therapy (VMAT). The dose per fraction was 7,25 Gy for 5 fractions (total dose 36,25 Gy) with a normal photon energy of 10 MV. The developed plans were verified using a three-dimensional cylindrical ArcCHECK phantom. During the verification process, the three-dimensional dose distribution in the phantom was measured, based on which the values of the three-dimensional gamma index and the dose–volume histogram within each contoured anatomical structures were calculated with 3DVH software. The gamma index value γ (3 %, 2 mm, GN) at a threshold equal to 20 % of the dose maximum of the plan and the percentage of coincidence of points at least 95 % was chosen as a criterion of physical convergence of the calculated and measured dose distribution according to the recommendations of AAPM TG-218. To analyze the radiobiological equivalence of the calculated and measured dose distribution, the local control probability (TCP) and normal tissue complication probability (NTCP) criteria were used based on the calculated and measured dose–volume histograms. Contours of the target (PTV) and the anterior wall of the rectum were used for the analysis. The approach based on the concept of equivalent uniform dose (EUD) by A. Niemierko was used to calculate the values of TCP/NTCP criteria. Results: The results of physical convergence of plans for all patients on the contour of the whole body were higher than 95 % for the criteria γ (3 %, 2 mm, GN). The convergence along the PTV contour is in the range (75.5–95.2)%. The TCP and NTCP values obtained from the measured dose-volume histograms were higher than the planned values for all patients. It was found that the accelerator delivered a slightly higher dose to the PTV and the anterior wall of the rectum than originally planned. Conclusion: The capabilities of modern dosimetric equipment allow us move to the verification of treatment plans based on the analysis of TCP / NTCP radiobiological equivalence, taking into account the individual characteristics of the patient and the capabilities of radiation therapy equipment.

Purpose: Investigation of the relative errors of absorbed dose measurement based on polymer films Gafchromic EBT3 for clinical electron and photon beams of medical accelerators. Material and methods: Polymer Gafchromic EBT3 films were calibrated using different radiation beams, namely photon and electron beams of Elekta Axesse medical accelerator with beam energy equal to 10 MV and 10 MeV, correspondingly, and electron beam of a betatron for intraoperative radiotherapy with beam energy equal to 6 MeV. The film pieces were irradiated by the uniform dose field in the dose range from 0.5 to 40 Gy. The dose value was controlled by cylindrical ionization chamber in the case of Elekta Axesse accelerator and by the Markus parallel chamber in the case of betatron. The irradiated films were scanned using Epson Perfection V750 Pro flatbed scanner in 16 bit RGB color mode with 150 dpi resolution. The red and green channels were used for further analysis. The central part of each film was used for calculation of average values of net optical density and its root-mean-square. As a result, the calibration curves, i.e. dependence on the reference absorbed dose measured by ionization chamber on the net optical density were constructed taking into account uncertainties of dose measurement and optical density measurement. Results: The relative uncertainty for the dose measurement lies within 7 % for low doses (less than 1 Gy) and within 4 % for higher doses. The green channel is less sensitive to the radiation, but its relative uncertainty values are in general 1–2 % lower than the ones for the red channel. The use of different calibration sources results in different calibration curves with difference up to ±6 % for the green channel. Conclusion: The polymer Gafchromic EBT3 films could be used for absorbed dose measurement for the doses not less than 0.5 Gy. For lower dose values the dose measurement uncertainty caused by statistical reasons amounts 15 %. For dose values of about 1 Gy and higher the dose measurement uncertainty amounts 5 % that allows to use the films for transverse and longitudinal prescription treatment dose distribution measurement with very high spatial resolution.

Purpose: Evaluation of the expected effectiveness of radiation therapy based on models of the local tumor control probability (Tumor Control Probability – TCP) for the head-neck cancer. Material and methods: The study used data from 11 patients with locally advanced head-neck cancer (larynx, oropharynx, and oral cavity). For each patient two dosimetric treatment plans have been prepared: SIB-VMAT (70 Gy per tumor, 50 Gy per lymph nodes, 25 fractions) and SEQ-VMAT (70 Gy per tumor, 50 Gy per lymph nodes, 35 fractions). The developed plans were analyzed using A. Niemierko's TCP model with parameters obtained by B. Maciejewski (TCD50 = 70.26 Gy with a 49-day total treatment time), taking into account the dose–volume histograms and the total treatment time. Results: The developed plans ensured a high level of coverage (98–98 %) of the Clinical treatment volume (CTV) in all but one patient. The average TCP SIB-VMAT is 99.9 % due to the very short total treatment time. The average TCP for SEQ-VMAT is 61.0%. For one patient, both SIB-VMAT and SEQ-VMAT showed zero expected efficacy due to 95–95 % CTV coverage. Conclusion: The use of TCP model allows analyzing personalized treatment plans for patients and developing adaptive treatment regimens with an increase in the total dose, dose per fraction, and a decrease in the total treatment time.