Bernhard Schmidt PhD, Presenter: Employee, Siemens AG

Thomas G. Flohr PhD, Abstract Co-Author: Employee, Siemens AG

Christoph Suess PhD, Abstract Co-Author: Employee, Siemens AG

Rosenmarie Banckwitz, Abstract Co-Author: Employee, Siemens AG

Recent publications suggest that for CT scans, the values for effective dose determined from thermoluminescent dosimeters (TLDs) using anthropomorphic Alderson-Rando phantoms differ significantly from published values determined by Monte Carlo-based simulations on mathematical anthropomorphic phantoms like the MIRD5 phantom. Further on, similar differences were observed between the results from TLD measurements and effective dose values determined by techniques using conversion factors k to calculate the effective dose (Eeff = DLP x k). The goal of our study is the assessment of the impact of the different used anthropomorphical phantoms on estimated patient dose.

For the organ dose measurements an Alderson-Rando phantom was equipped for each measurement with 131 TDLs. Dose values were obtained for standard chest (30cm) and abdomen (17cm) protocols. To simulate dose, a hardware accelerated voxel-based Monte Carlo tool was used which allowed the consideration of CT system specific parameters (e.g. x-ray filters). Simulations were performed on the one hand on voxel data sets derived from CT scans of the Alderson phantom, which also allowed the comparison between TLD measurements and corresponding simulations on the same phantom. On the other hand, the voxel data from established gender specific anthropomorphic phantoms MIRD5 (ADAM and EVA) were used for the simulations. For the scan parameters used for the measurements, simulations of dose distribution were performed for both phantoms.

For the Alderson there was for both, chest and abdomen, a good agreement between simulated and measured organ doses (differences < 10%). For the effective dose there was an excellent agreement between measured and simulated data. The comparison of simulated dose values of the Alderson and Adam/Eva phantoms shows, that significantly higher values (up to 35%) were obtained for Alderson compared to Eva and Adam. One reason might be the differences in cross-section of both established phantoms, e.g. for the abdomen of Alderson 27x21cm² and of Adam 40x20 cm².

Depending on the anatomical area, significant differences in organ dose and dose distribution were observed between the Alderson Rando and the MIRD (Adam/EVA) phantom.

Patient size and cross section has a significant impact on patient dose. The simple approach ‘Eeff = DLP x k’ needs corrections to account for patient size.

Schmidt, B, Flohr, T, Suess, C, Banckwitz, R, Comparison of CT Effective Dose Values and Dose Distributions Estimated by Different Established Anthropomorphical Phantoms.  Radiological Society of North America 2010 Scientific Assembly and Annual Meeting, November 28 - December 3, 2010 ,Chicago IL. http://archive.rsna.org/2010/9014211.html