Room: AAPM ePoster Library
Purpose: The IAEA and the AAPM have encouraged the research of using depth-dose (dd) as beam quality index (BQI) for low-energy x-ray beams, a task that imposes significant challenges. We have studied the uncertainties arising when using dd as BQI in the beam calibration of an electronic surface brachytherapy (eBT) system of 69.5 kVp. A novel methodology easily applied to other eBT systems is proposed.
Methods: Several Monte Carlo (MC) simulations of the Esteya® eBT system (Elekta, Veenendaal, The Netherlands) were performed (PENELOPE-2014) to obtain dd’s, backscatter factors (Bw), (µen/?)water,air, and HVL’s, considering the manufacturing tolerances (MT). Additionally, the uncertainties analysis included both MC cross-sections and dd experimental uncertainties (sdd). The latter was estimated by comparing the dd measurements made by different groups in different facilities and with different units of the parallel-plate ionization chambers T34013 (PTW, Freiburg, Germany). We tested the dd (normalized at 0.3 cm) at several depths, dd(z (cm)), evaluating their sensitivity to the sdd using the partial derivative method.
Results: The sdd found in this work (sdd(z)) was 2% for dd(1) and dd(2), and 5% for dd(5). The effect of the MT and sdd(z) on the (µen/?)water,air, and Bw is within 0.1%, giving rise to combined uncertainties of about 0.4%. The use of dd(2) and dd(5) kept the uncertainties in the HVL determination under 10%. Using this approach, the added uncertainty to the calibration budget is = 0.1% (k = 2).
Conclusion: The use of depth-dose adds a negligible uncertainty to the beam calibration of an eBT x-ray beam of 70 kVp, a method that can be applied to other eBT systems. dd(2) and dd(5) appear as a good candidate for BQI due to their low sensitivity to sdd. The uncertainty of using dd(z) to obtain the HVL's is comparable to the experimental data published elsewhere.