QUALITY CONTROL OF LINEAR ACCELERATOR OF NOUAKCHOTT.

Ould Mohamed Yeslem Ahmed El Mouna 1 , Choukri Abdelmajid 1 , Hakam Oum Keltoum 1 and Semghouli Slimane 2 . 1. Univesity of Ibn Tofail, Kenitra (Morocco). 2. Higher Institute of Nursing Professions and Health Techniques, Agadir, Morocco. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History

At the National Center of Oncology of Nouakchott, the external radiotherapy treatments are performed using a linear accelerator particles of two energies 6 MV and 18 MV. This particle accelerator is originally set to deliver a dose under reference conditions. It is essential that the measured data on the particle accelerator be consistent with the data calculated by the Treatment Planning System (TPS). The purpose of this work is to perform a general quality control of this accelerator, and to compare the results obtained by two ionization chambers (PTW 0.125 cm 3 and PTW 0.6 cm 3 ). This control is based on the measure of percentage depth dose (PDD) and beam profiles (BP) and the comparison of these measurements with the same results performed by TPS calculations for different field sizes. In general, the results measured are in good concordance with the results calculated by TPS. Regarding comparison between the two ionizations chambers, we notice that the light difference between the percentage depth doses measured by the two ionizations is related to the energy and to the field size of the incident photons beam. In order to check the quality of the accelerator and related equipment, we have performed measurements (percentage depth dose and beam profiles) in water phantom by an ionization chamber for energies of 6 MV and 18 MV for different four field sizes.

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To compare the measured results we have calculated the same parameters by Treatment Planning Systems (TPS), in order to compare measurements with calculation following the (IAEA) recommendations (N. Villani, 2010; Charles W. Coffey,1980; C. McKerracher, 1985) The (TPS) is a treatment planning software allowing to predict, according to a given ballistics, an established medical prescription, a chosen energy, an anatomical configuration, the dose at all points of the space (G. Krithivas, 1985; Haluk Yucel, 2016; Sang Hyoun Choi, 2016; Miljenko Markovic, 2014).
To perform a comparison between the two chambers (PTW 0.125 cm 3 and PTW 0.6 cm 3 ), we have measured the PDD by these two ionizations, for different field sizes and for the two energies 6 MV and 18 MV.

Materials and Methods:-
Measurements of percentage depth dose and dose (beam) profile were carried out using a water phantom, connected to a PC. The system is controlled for the acquisition of the dosimetric data by MEPHYSTO mc² software. The dosimetric measurements were realized using an ionization chamber associated with an electrometer and the chamber used for acquisition can move in three directions (K.A. Johansson, 1986    Looking at the previous figures, we notice the following; for PDD, we find that the PDD measurements agree with TPS calculations, for the two energies.
For the BP, there are some light differences between the curves; these differences exist in the tails of the curves and in the maximum of doses. The difference is greater for the energy of 18 MV than for the energy of 6 MV but this difference is still acceptable.

Comparison of results obtained by 0.125 cm 3 and 0.6 cm 3 ionization chambers
We have undertaken measurements of PDD by the two chambers for 6MV and 18MV for different treatments field dimensions. Figure 5

Discussion:-
According to the Figures 5 and 6, there is a difference between the percentage depth doses measured by the two detectors; this difference is mainly and clearly observed in the first zone (the buil-up zone). The largest difference calculated as a between results given by the two ionization chambers varies between about 10 % and 22%.
The relative deviations between the measurements obtained by the two ionizations chambers are given for the four field sizes in the following These results show that the difference between the curves of the percentage depth doses measured by the two detectors is bigger if the field size is lower, and it is higher when the energy is low

Conclusion:-
In this general quality control work, we have compared the PDD and BP measurements obtained by ionization chamber (PTW 0.125 cm 3 ) with the results calculated by TPS.
The results of PDD and BP measured by the ionization chamber are comparable for all chosen treatment field dimensions to those calculated by TPS.
This comparison show that the quality of linear accelerator at Nouakchott Center of Radiotherapy is good and assure the conditions of different medical interventions in this Center.
Regarding comparison between the two ionizations chambers, we notice that the difference between the percentage depth doses measured by the two ionizations is related to the energy and to the field size of the incident photons beam.