Background:  Cell viability is an important factor in radiation therapy and thus is a method to quantify the effect of the therapy. Materials and Methods: The viability of human hepatoma (HepG2) cells exposed to radiation was evaluated by both the MTT and Trypan blue assays. The cells were seeded on 96 well-plates at a density of 1 x 10⁴ cells/well, incubated overnight, and irradiated with 1-100 Gy. Results: The cell viability was decreased in a dose- and time- dependent manner when using the Trypan blue assay, but no significant changes in the response to dose could be detected using the MTT assay. It indicated that the MTT assay was not efficient at a cell density of 1 x 10⁴ cells/well on 96 well-plates to determine cell viability. Subsequently, the relationship between cell viability and lower cell density (1 x 10³, 3 x 10³, and 5 x 10³ cells/well) was investigated. A cell density of 1 x 10³ was found to be the most effective when using the MTT assay. Results show that the cell density is most important when using the MTT assay in 96 well-plates to follow in radiation effects. Furthermore, the radiation-induced cell viability dependent on cell density was confirmed by using the traditional Clonogenic assay. Conclusion: Our results suggest that the MTT and Trypan blue assays are rapid methods to detect radiation-induced cell viability of HepG2 cells in about 3 days as compared with 14 days of assay time in the Clonogenic assay. To obtain accurate cell viability measures using both rapid assays, an incubation time of at least 3 days is needed after irradiation.

Background: Dose calculation algorithms play a very important role in predicting the explicit dose distribution. We evaluated the percent depth dose (PDD), lateral depth dose profile, and surface dose volume histogram in inhomogeneous media using calculation algorithms and inhomogeneity correction methods. Materials and Methods: The homogeneous and inhomogeneous virtual slab phantoms used in this study were manufactured in the radiation treatment planning system to represent the air, lung, and bone density with planned radiation treatment of 6 MV photons, a field size of 10 × 10 cm², and a source-to-surface distance of 100 cm. Results: The PDD of air density slab for the Acuros XB (AXB) algorithm was differed by an average of 20% in comparison with other algorithms. Rebuild up occurred in the region below the air density slab (10–10.6 cm) for the AXB algorithm. The lateral dose profiles for the air density slab showed relatively large differences (over 30%) in the field. There were large differences (20.0%–26.1%) at the second homogeneous–inhomogeneous junction (depth of 10 cm) in the field for all calculation methods. The surface dose volume histogram for the pencil beam algorithm showed a response that was approximately 4% lower than that for the AXB algorithm. Conclusion: The dose calculation uncertainties were shown to change at the interface between different densities and in varied densities using the dose calculation methods. In particular, the AXB algorithm showed large differences in and out of the field in inhomogeneous media.

Background: This study was conducted to assess the accuracy of dose calculation near the air-phantom interface of a heterogeneous phantom for Acuros XB (AXB) and Anisotropic Analytical Algorithm (AAA) algorithm of a           6-MV flattening-filter-free beam, compared with film measurements. Materials and Methods: A phantom included air gap was specially manufactured for this study. In order to evaluate the dose near air gap-phantom interface, Eclipse treatment planning system equipped both AXB and AAA was used for the dose calculations. Measurements in this region were performed with radiochromic film. The central-axis dose (CAD) and off-axis dose (OAD) between calculations and measurements were analyzed for various field sizes and air gaps. The root-mean-square-error (RMSE) was used to evaluate the difference between the calculated and measured OAD. In order to quantify agreement between the calculated and measured dose distributions, the gamma analysis was performed with the 2%/2 mm and 3%/3 mm criteria. Results: For all fields traveling through 1 and 3 cm air gap, the maximum difference in the calculated CAD was -5.3% for AXB and 214.8% for AAA, compared to the measured CAD. For the RMSE between the calculated and measured OAD, the calculated OAD using AXB showed interval in the RMSE (from 4.4 to 12.7) while using AAA indicated broad (from 7.7 to 101.0). In addition, the gamma passing rates showed that AXB was higher agreement than AAA. Conclusion: This study demonstrated that AXB was more accurate in heterogeneous media near air-phantom interface than AAA when comparing the measured data.