[Calculation of Experimental Penumbral Width of X-ray Dose Distributions Using Multiple Detectors]

X-ray penumbral width depends on the size of the detector in the off-axis ratio of medical linear accelerators. The use of detectors with appropriate sizes according to the irradiation field is recommended. However, when measuring dose distributions, the shape of the off-axis ratio differs depending on detectors even if the dose distribution remains unchanged. This study aims to calculate the penumbral width using multiple detectors and obtain the independent penumbral width according to irradiation size.

Using the X-ray output from a medical linear accelerator, off-axis ratios in water were measured using nine types of detectors. Penumbral width was calculated from each measured off-axis ratio by linear approximation and determining the intercept for analysis. The experimental irradiation fields were square areas (1×1 cm² to 10×10 cm²). Penumbral widths were obtained from three types of detectors with different sensitive region widths of at least 1.2 mm.

The values estimated from the nine detectors were 2.51-4.07 and 2.93-4.70 mm for 6 and 10 MV X-rays, respectively. The penumbral width and variation due to detector size increased with the irradiation field. The results estimated from the three selected detectors varied within ±0.5 mm compared with those from the nine detectors and were generally consistent. The reliability of the results was evaluated by comparing with the results from past studies and Monte Carlo simulations.

Calculation of the penumbral width can be done regardless of size for various detectors. Thus, dose distributions can be compared for the linear accelerator at different facilities.