The impact of patient table on size-specific dose estimate (SSDE)

The use of parameters water equivalent diameter (D _W ) and size-specific dose estimate (SSDE) are becoming increasingly established as a recognised method to relate patient dose from a CT examination to the dose indicator volume CT dose index (CTDI_VOL). However, the role of the attenuation due to the patient table in these estimations requires careful consideration and is the subject of this study. The aim of this study is to investigate the impact of a minimal part of the patient table when calculating the D _W and SSDE. We investigated 164 patients who had undergone CT examinations for the pelvis, abdomen, thorax and head. We subsequently calculated D _W and SSDE using two methods: one using a small circular region of interest (ROI) including a minimal part of the patient table and the other using a ROI fitted to the patient border alone. The results showed that the water equivalent diameter calculated with the table included in the ROI (D _W,t ) is greater, compared to that without the consideration of the patient table (D _W,nt ), by 1.5–6.2% depending on the anatomy being imaged. On the other hand, the SSDE calculated with inclusion of the patient table (SSDE_t) is smaller than otherwise (SSDE_nt) by 1.0–5.5% again depending on the anatomy being imaged. The effect of the patient table on D _W and SSDE in the thorax CT examination was statistically significant, but its effect on D _W and SSDE in the other examinations of head, pelvis and abdomen was relatively small and not statistically significant.     

Investigation of photon beam output factors for conformal radiation therapy--Monte Carlo simulations and measurements

The purpose of this study was to investigate beam output factors (OFs) for conformal radiation therapy and to compare the OFs measured with different detectors with those simulated with Monte Carlo methods. Four different detectors (diode, diamond, pinpoint and ionization chamber) were used to measure photon beam OFs in a water phantom at a depth of 10 cm with a source-surface distance (SSD) of 100 cm. Square fields with widths ranging from 1 cm to 15 cm were observed; the OF for the different field sizes was normalized to that measured at a 5 cm x 5 cm field size at a depth of 10 cm. The BEAM/EGS4 program was used to simulate the exact geometry of a 6 MV photon beam generated by the linear accelerator, and the DOSXYZ-code was implemented to calculate the OFs for all field sizes. Two resolutions (0.1 cm and 0.5 cm voxel size) were chosen here. In addition, to model the detector four kinds of material, water, air, graphite or silicon, were placed in the corresponding voxels. Profiles and depth dose distributions resulting from the simulation show good agreement with the measurements. Deviations of less than 2% can be observed. The OF measured with different detectors in water vary by more than 35% for 1 cm x 1 cm fields. This result can also be found for the simulated OF with different voxel sizes and materials. For field sizes of at least 2 cm x 2 cm the deviations between all measurements and simulations are below 3%. This demonstrates that very small fields have a bad effect on dosimetric accuracy and precision. Finally, Monte Carlo methods can be significant in determining the OF for small fields.