Room: Exhibit Hall | Forum 8
Purpose: To evaluate entrance skin dose (ESD) using a dose area product (DAP) meter calibrated based on the â?¶â?°Co absorbed dose to water for diagnostic x-ray beams.
Methods: Recently, we have developed new dosimetry to evaluate ESD for kilovoltage x-rays, using a Farmer ionization chamber calibrated with the â?¶â?°Co absorbed dose to water. The ESD evaluated with new dosimetry agreed within their uncertainty to that based on the existing air-kerma calibration. The DAP meter used for ESD measurements has been conventionally calibrated based on the air-kerma. In this study, a PTW 23344 parallel-plate ionization chamber was cross-measured by a PTW 30013 Farmer chamber with a â?¶â?°Co calibration factor, at a depth of 1 cm for a 20 Ã— 20 cmÂ² field in a water-equivalent phantom. Next, the DAP meter was directly cross-measured by the PTW 23344 chamber at the phantom surface for a 20 Ã— 20 cmÂ² field. In addition, the ESD was measured with the DAP meter for field sizes of 10 Ã— 10 to 30 Ã— 30 cmÂ². Finally, the ESD was compared ESDs estimated directly by parallel-plate chamber for nine kilovoltage x-ray beams with aluminum half value-layers (Al-HVLs) of 1.80 to 6.72 mm.
Results: The calibration coefficient for the parallel-plate chamber was almost independent of the beam quality. This is because the PTW 23344 chamber has small window area density. The averaged value was 0.0829 mGy/pC Â± 0.23%. The calibration coefficient of the DAP meter increased with increasing Al-HVL. The ESD measured by the DAP meter agreed within Â± 5% to that measured by the PTW 23344 chamber for field sizes of 10 Ã— 10 to 30 Ã— 30 cmÂ².
Conclusion: It made it possible to directly measure the ESD using the DAP meter cross-calibrated by a parallel-plate chamber at the phantom surface for diagnostic x-ray beams.