Room: Exhibit Hall
Purpose: Total body irradiation (TBI) at extended SSD has long been used in our institution primarily as part of a preparatory cytoreductive conditioning regimen prior to bone marrow transplantation. TBI absolute and relative dosimetry is associated with a number of problems that are not normally found at shorter SSDs and with smaller field sizes. There is no well-defined standard calibration condition for absolute dosimetry in TBI setup. In this study we aim at investigating the dosimetric accuracy of our TBI in-vivo system.
Methods: TBI was done using the bilateral technique with the patient seated on a couch in a semifetal position. An acrylic beam spoiler was placed between the patient and the radiation source. Head and neck compensators as well as body compensators were placed remotely. In this study, we have analyzed the in-vivo results for 20 TBI patients. Measurements were done using OSLDs at 6 different body sites. We have studied detector responses by conducting ion chamber and OSLD measurements in phantoms at both standard and TBI setups.
Results: Our preliminary results showed a difference in the order of 5.5% between the OSLD measured values as compared to those expected based on ion chamber measurements. The mean ratio of OSLD measured dose to the expected dose was in the range of 0.92-0.98 based on a detailed analysis of all studied cases. Large uncertainties were associated with dose measurements for the TBI setup and its correlation to the TG51 standard condition. Detector response should be corrected for the differences in the stopping power ratio for the detectors and should take into account electron contamination resulting from the beam spoiler.
Conclusion: In-vivo measurement uncertainties can be minimized by formulating corrections for detector response under the TBI treatment conditions.