Evaluation of Tissue Heterogeneity Effect on the Received Dose of Orthovoltage X-Rays in Rat's Cervical Spinal Cord Region

The most important physical quantity of irradiation to predict and evaluate the response of biological samples to irradiation is absorbed dose. Recently, according to ICRU(International Commission on Radiation Units and Measurements) reports, determination of absorbed dose with 3.5% accuracy is recommended. Dose distribution is usually estimated in homogeneous mass by unit density, while irradiated volume consists of different layers such as fat, bones, etc that cause changes in dose distribution. Absorbed dose determination with maximum accuracy is essential for crucial tissues when they are located in body cavities, encircled with heterogeneities(doubted charged particle equilibrium) and when the low quality beams are used in radiobiological studies. The goal of this study is to determine absorbed dose in such situations — rat’s spinal cord. In order to investigate the effect of heterogeneity on the received dose of orthovoltage x-rays in the cervical spinal cord region of rat, a homogeneous phantom of polyethylene, a frozen sample of the animal using liquid nitrogen, and a jig for the reproducibility of the sample were designed and made. The used dosimeter in the project was rod shaped TLD(Thermduminscence Dosimetry) chips, TLD-100. The x-ray generating device was a radiotherapeutic x-ray in potential of 200 Kv(HVL: Half Value Layer=1.5mmCu). The reader device used was Hardshaw TLD reader model 3500 with the heating accuracy of ±1˚C. We took advantage of various dosimetry protocols including American association of physical medicine(AAPM) report TG-61, 2001, dosimetry protocol in radiobiology by Zoteliefe et al, 2001, NSC-report 3, TR-IAEA, 277 protocol and also reports by ICRU-23, 24, 30, 48. Statistical calculations using SPSS for running t-test on the resulting doses within the recommended limit of 3.5%(P<0.001) showed no significant difference between the two groups. Thus, in usual fields in radiobiological studies and within the spectrum of orthovoltage energies used for rats and smaller animals, normal accuracy in calculations could be accepted. However, in the regions where there is no equilibrium in the charged particle and where microdosimetry is concerned the results are not to be crucially applied.