Heterogeneous cytogenetic dose assessment of patients trated by radiotherapy.
Roy L., Grégoire E., Giraudet A.L., Voisin Pa., Buard V., Delbos, M., Voisin Ph., Bourhis J, European Radiation Research 2004, 25-28/08/2004, Budapest, Hongrie.
Objective : Biological dose estimate following exposure to ionizing radiation is based on the yield of dicentrics observed in the lymphocytes of patients. This technique is especially precise if the exposure is recent and homogeneous. But in most of accidental overexposure cases, irradiation is heterogeneous leading to an under estimation of the dose received by the fraction of irradiated lymphocytes. Therefore some mathematical models have been developed to evaluate dose heterogeneity (QdR, Dolphin). They have been successfully validated by in vitro studies. The objective of this study is to test these models on an in vivo study. Methods: blood samples were collected from patients undergoing fractioned radiotherapy for cervical cancer before starting the treatment, after the first 2 Gy fraction, after the 12 Gy fraction, at the end of their treatment and 6 months after. Six patients were treated with a large field radiotherapy whereas two others had a reduced one. Dicentrics were observed on these samples. Results: An increase in the whole body dose is observed after the 2 Gy fraction with an average biological dose of 0.21 Gy for the large field patients and 0.10 for the reduced irradiation field. At the end of the treatment, a whole body dose of 3-4 Gy is measured. Using the mathematical models Dolphin and QdR, the dose delivered to the irradiated fraction of lymphocytes is between 5 and 6 Gy for the large field radiotherapy whereas it is only 2 Gy for the two other patients. The percentage of irradiated lymphocytes is estimated to 80 % for the large field irradiation and only 15 % in the other configuration. Conclusion: the use of mathematical models allows the detection of dose heterogeneity. A strong difference was observed between the two types of irradiation as dose heterogeneity is detected after the 12 Gy fraction for large field exposed patients whereas it is only detected at the end of the treatment for the other kind of exposure. This is explained by a reduction of the irradiated lymphocytes percentage when the treatment is applied to a reduced volume of the neck.