Experimental evaluation of midline dose calculation methods in In vivo dosimetry using anatomic thorax phantom

Accuracy in the delivered dose to the patients plays an important and vital role in radiotherapy treatments. This happens because of steep gradient of dose-local control and dose-normal tissue complication probability curves. Dosimetric uncertainty of 3%-4% (one standard deviation) in the delivered dose to the target volume as an acceptable level has been recommended(1-3). In many centers, in vivo dosimetry using thermoluminance dosimeters (TLD) and diodes are used routinely to verify the actual delivered dose to the patients(1, 4, 5). Recently, application of portal imaging devices as a tool for in vivo dosimetry has been evaluated(6-8). In some radiotherapy centers entrance doses are measured and used for evaluation of source to patient distance (SSD), dose rate, and treatment time(9-11). However, all errors in the delivered dose to the patients could not be revealed using entrance dose. So, in some occasions exit dose measurements are performed to provide the required data for delivered dose evaluations(5,9,10). Exit dose measurements are useful for dose verification of dose calculation methods in treatment planning process. Finally, the combination of entrance and exit doses results could differentiate the origin of the observed errors in patient dosimetry. It is desirable to compare the measured dose to the prescribed dose to the center of target volume. In order to estimate the dose received to the center of target volume, measured entrance and exit doses can be used(4,5,9,12). There are a few methods for determination of dose at the dose specified point from the measured entrance and exit dose values(13-17). Using these methods, the midline dose and the dose of points situated on the central axis could be determined from the entrance and exit doses, measured on the patients during treatment. In our study the arithmetic and geometric mean methods for the midline dose Background: In vivo dosimetry is a method for estimation of overall error in the delivered dose to the patients at the end of radiotherapy process. In this research, two methods for target dose calculation were evaluated on midline and central axis of photon beams in in vivo dosimetry of thorax fields. Materials and Methods: Entrance and exit doses for anterior and lateral fields of thorax were measured in thorax phantom using diode dosimeter. Also, the doses of some points on midline and central axis were measured in thorax phantom using ionization chamber. The dose at these points was calculated using entrance and exit doses by geometric and arithmetic mean methods. The calculated doses were compared with measured doses. Results: In all cases, arithmetic mean method showed errors from %8.8 to 19% for points on midline and central axis in comparison to measurements. The range of errors for geometric method was from %1.5 to %8 depending on distance from midline. Conclusion: The results showed that doses of points on midline and central axis can be calculated with acceptable accuracy from entrance and exit doses using geometric mean in thorax fields. Iran. J. Radiat. Res., 2007; 5 (2): 91-95