Purpose: To directly measure the entrance surface dose (ESD) of a phantom using a plane-parallel ionization chamber for interventional radiology (IVR) procedures.
Methods: A PTW 23344 plane-parallel chamber was calibrated in terms of the dose at a 1-cm depth determined using a PTW 30013 Farmer-type chamber. The ESD for six IVR devices of three manufacturers in three institutions was measured using the PTW 23344 chamber. The front surface of the PTW23344 was set at an IVR reference point on the examination tabletop located 15 cm below the isocenter. The irradiation conditions of the IVR procedures were determined according to the automatic exposure control for the abdomen and head in the digital acquisition mode and fluoroscopy mode. The ESD is presented as a function of a half-value layer (HVL (mm Al)) for the given quality index (QI) values, which are defined as the ratio of the effective energy to the maximum energy of photons. The relationship between the ESD and incident air kerma, Kₐᵢᵣ,ᵢ, was also analyzed. Kₐᵢᵣ,ᵢ was calculated using x-ray fluence spectra for the IVR irradiation conditions created by the SpekCalc program.
Results: The calibration coefficient of the PTW 23344 chamber was mostly constant for x-ray beam qualities with tube voltages of 60–140 kVp and HVLs of 2.24–9.22 mm Al. The ESD increased with an increasing tube voltage for x-ray beams with the same HVL, and even for the same QI. In addition, the ESD achieved a good linear relationship with Kₐᵢᵣ,ᵢ, regardless of the x-ray beam quality.
Conclusion: To reduce the ESD, reducing the tube voltage and using the x-ray beam quality with the QI close to unity was found to be effective. It is also possible to estimate the ESD using Kₐᵢᵣ,ᵢ based on the irradiation conditions for IVR procedures.
Funding Support, Disclosures, and Conflict of Interest: This study was funded by a research grant (2019) from the Japanese Society of Radiological Technology (JSRT), and was also partially supported by JSPS KAKENHI of the Ministry of Education, Culture, Sports, Science, and Technology through Grant No. JP19K08203. The authors have no conflicts of interest to disclose.