[In vivo dosimetry study of semi-conductors EPD-20 in total body irradiation technique]

The objective of this work was the study of in vivo dosimetry performed in a series of 54 patients receiving total body irradiation (TBI) at the Salah-Azaiz Institute of Tunis since 2004. In vivo dosimetry measurements were compared to analytically calculated doses from monitor units delivered.

The irradiation was conducted by a linear accelerator (Clinac 1800, Varian, Palo Alto, USA) using nominal X-rays energies of 6 MV and 18 MV, depending on the thickness of the patient at the abdomen. The dose was measured by semi-conductors p-type EPD-20. These diodes were calibrated in advance with an ionization chamber "PTW Farmer" type of 0.6cm(3) and were placed on the surface of plexiglas phantom in the same TBI conditions. A study of dosimetric characteristics of semi-conductors EPD-20 was carried out as a function of beam direction and temperature. Afterwards, we conducted a comparative analysis of doses measured using these detectors during irradiation to those calculated retrospectively from monitor units delivered to each patient conditioned by TBI.

Experience showed that semi-conductors are sensitive to the angle of beam radiation (0-90 degrees ) and the temperature (22-40 degrees C). The maximum variation is respectively 5 and 7%, but in our irradiation conditions these correction factors are less than 1%. The analysis of the results of the in vivo dosimetry had shown that the ratio of the average measured doses and analytically calculated doses at the abdomen, mediastina, right lung and head are 1.005, 1.007, 1.0135 and 1.008 with a standard deviation "type A" respectively of 3.04, 2.37, 7.09 et 4.15%.

In vivo dosimetry by semi-conductors is in perfect agreement with dosimetry by calculation. However, in vivo dosimetry using semiconductors is the only technique that can reflect the dose actually received instantly by the patient during TBI given the many factors that calculation can not take into account: patient and organs motions and the heterogeneity of the targets.