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Purpose: To investigate the dose distributions in water cylinders simulating patients undergoing Interventional
Radiological examinations.
Method: The irradiation geometry consisted of an x-ray source, dose-area-product chamber, and image intensifier as
currently used in Interventional Radiology. Water cylinders of diameters ranging between 17 and 30 cm were used to
simulate patients weighing between 20 and 90 kg. X-ray spectra data with peak x-ray tube voltages ranging from 60 to
120 kV were generated using XCOMP3R. Radiation dose distributions inside the water cylinder (Dw) were obtained
using MCNP5. The depth dose distribution along the x-ray beam central axis was normalized to free-in-air air kerma
(AK) that is incident on the phantom. Scattered radiation within the water cylinders but outside the directly irradiated
region was normalized to the dose at the edge of the radiation field. The total absorbed energy to the directly irradiated
volume (Ep) and indirectly irradiated volume (Es) were also determined and investigated as a function of x-ray tube
voltage and phantom size.
Results: At 80 kV, the average Dw/AK near the x-ray entrance point was 1.3. The ratio of Dw near the entrance point to
Dw near the exit point increased from ~ 26 for the 17 cm water cylinder to ~ 290 for the 30 cm water cylinder. At 80 kV,
the relative dose for a 17 cm water cylinder fell to 0.1% at 49 cm away from the central ray of the x-ray beam. For a 30
cm water cylinder, the relative dose fell to 0.1% at 53 cm away from the central ray of the x-ray beam. At a fixed x-ray
tube voltage of 80 kV, increasing the water cylinder diameter from 17 to 30 cm increased the Es/(Ep+Es) ratio by about
50%. At a fixed water cylinder diameter of 24 cm, increasing the tube voltage from 60 kV to 120 kV increased the
Es/(Ep+Es) ratio by about 12%. The absorbed energy from scattered radiation was between 20-30% of the total energy
absorbed by the water cylinder, and was affected more by patient size than x-ray beam energy.
Conclusion: MCNP offers a powerful tool to study the absorption and transmission of x-ray energy in phantoms that can
be designed to represent patients undergoing Interventional Radiological procedures. This ability will permit a
systematic investigation of the relationship between patient dose and diagnostic image quality, and thereby keep patient
doses As Low As Reasonably Achievable (ALARA).
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