Comparison of dose distributions for 6MV and 15MV energy for Total Body Irradiation (TBI).
Authors: Kowalik Anna Natalia , Koper Tomasz , Adamczyk Sebastian , Malicki Julian 
Published: March 2019

Introduction             Contemporary radiotherapy uses a number of highly specialized irradiation techniques dedicated to well-defined clinical diagnoses. Among these methods are techniques to irradiate the skin (TSEI), bone marrow (TMI) or the whole body of the patient (TBI). TBI has over the last century been used in the treatment of a variety of conditions, both benign and malignant. However, its importance has increased with the development of knowledge about the impact of ionizing radiation on the human body and the development of clinical dosimetry techniques. At present, however, this method is primarily used in the treatment of hyperplasia.

Aim             The general aim of the study is to compare dose distributions at selected points of the anthropomorphic phantom under full body radiation conditions for X: 6MV and 15MV radiation. Specific objectives are defined: comparison of percent depth and function of photon emission profiles: 6MV and 15MV measured with radiofrequency hydrophobic films; measurement of doses in selected cross sections of the anthropomorphic phantom.

Material and method

            A number of measuring devices and materials used in daily work by staff of the Medical Physics Department of the Greater Poland Cancer Centre were used to carry out the study part, but Alderson's anthropomorphic phantom and the radiochromic films in the form of point detectors were essential. In addition to each step of the research part, a special measuring system was prepared to reproduce the conditions prevailing during the TBI session as closely as possible. The research was carried out in three stages: Calibration of radiochromic films; PDD and OCR measurement for: 6MV and 15MV photon beam under TBI conditions; Measurement of dose distribution in selected anthropomorphic phantom's cross sections using radiochromic films in the form of point detectors.

 

Results             For the lateral field irradiated with 6MV photon beam, the maximum compliance (less than 2%) was obtained for the elbows at the entrance and in the center of the phantom; abdomen for the detector positioned in the center of the phantom, the lungs at the entrance and the arms in the middle, and the neck at the position of the film at the entrance. In the case of the lateral field X 15MV, the highest correspondence occurred for the points: the head and the PC in the position of the film in the center of the phantom and the entrance neck. In the case of AP/PA fields for 6MV energy, the highest compatibility was obtained for the mediastinum in all positions of the film. A small difference was also obtained for the points: head in the middle and at the output of the beam; as well as PC on the output. For AP/PA X 15MV fields, the highest dose compliance not exceeding 1% was obtained for the location of the neck - at the beam entrance, and the lung and mediastinum at the detector position at the center of the phantom.

Conclusions             On the basis of measurements of dose distribution at selected points of the patient's body for radiation X: 6MV and 15MV in the TBI procedure, the following conclusions can be made: Gafchromic EBT (radiochromic type film) can be successfully used for dosimetric measurements, among others. Due to their properties, such as the ability to cut from the sheet of film spot detectors of any shape and size, flexibility, low sensitivity to daylight, resistance to humidity, etc.; Their main drawback is the high cost of buying films and the long time required to prepare the detectors and then read the measured doses. Because of the low popularity of point-based EBTs in point dosing, further research is needed to improve their response to ionizing radiation. There is a noticeable increase in the difference between the dose calculated and measured as the distance between the position of individual detectors increases from the center point. The difference between the dose measured and planned in any of the cases examined does not exceed 9%. The measurements show that the method used is fast, accurate, and can be successfully used as a validation tool not only for the TBI procedure but also for other methods of cancer radiotherapy.