(Sunday, 7/29/2018) 3:00 PM - 6:00 PM
Room: Exhibit Hall
Purpose: This study was performed to analyze the effect of material type and density on dose calculation reporting to dose-to-medium D(m) and dose-to-water D(w). The effect on dose calculation with and without assigning material type in calculation algorithm was also assessed.
Methods: The calculations were performed in a water phantom (30x30x30 cm³) with 6xFFF and 10xFFF beams. A cylindrical volume of 5.7cm diameter and 6.0cm length was contoured, with centre at 9.0cm from the phantom surface. To assess the dosimetric effects against material density, the Hounsfield Units (HU), were varied from -1000 to +1500. The analysis was performed with and without assigning the material type to the cylindrical volume. The absolute dose was normalized to the mid-phantom (d=15cm). All the dosimetric effects were assessed at the centre of the cylinder.
Results: D(w) vs D(m) differences in dose calculation follow approximately a similar trend for both energies. The calculated dose difference is expressed as Δ(%) = (D(w) ̶ D(m)) / D(m))x100. In the low HU region (-1000 to -965) Δ is 14% for 6xFFF and 12% for 10xFFF. At -960 HU there is a sharp drop in Δ to about -1.0% and -0.4% for 6xFFF and 10xFFF respectively and then approaches -3.2% at -400 to -100 HU. As expected, there is no difference in calculated doses at HU equal zero. For positive HUs, the Δ follows a trend similar to the material density and reaches a maximum of 13%. When material type is not assigned, D(m) is same in the overlapping density range for skeletal muscle, cartilage and bone. Assigning material type also affects the D(m), but to a lesser extent (±3%).
Conclusion: There is a considerable dose difference between D(w) and D(m) calculations, and it increases with material density. Further, dose differences were observed when assigning material type in AcurosXB.
Keywords
Not Applicable / None Entered.
Taxonomy
TH- External beam- photons: dose computation engines- deterministic
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