Monte Carlo studies on water and LiF cavity properties for dose-reporting quantities when using x-ray and brachytherapy sources

Model-based dose calculation algorithms (MBDCAs) are the current tools to estimate dose in brachytherapy, which takes into account heterogeneous medium, therefore, departing from water-based formalism (TG-43). One aspect associated to MBCDA is the choice of dose specification medium since it offers two possibilities to report dose: (a) dose to medium in medium, D _m,m; and (b) dose to water in medium, D _w,m. The discussion about the preferable quantity to be reported is underway. The dose conversion factors, DCF, between dose to water in medium, D _w,m, and dose to medium in medium, D _m,m, is based on cavity theory and can be obtained using different approaches. When experimental dose verification is desired using, for example, thermoluminescent LiF dosimeters, as in in vivo dose measurements, a third quantity is obtained, which is the dose to LiF in medium, D _LiF,m. In this case, DCF to convert from D _LiF,m to D _w,m or D _m,m is necessary. The objective of this study is to estimate DCFs using different approaches, present in the literature, quantifying the differences between them. Also, dose in water and LiF cavities in different tissue media and respective conversion factors to be able to convert LiF-based dose measured values into dose in water or tissue were obtained. Simple cylindrical phantoms composed by different tissue equivalent materials (bone, lung, water and adipose) are modelled. The phantoms contain a radiation source and a cavity with 0.002 69 cm³ in size, which is a typical volume of a disc type LiF dosimeter. Three x-rays qualities with average energies ranging from 47 to 250 keV, and three brachytherapy sources, ⁶⁰Co, ¹⁹²Ir and ¹³⁷Cs, are considered. Different cavity theory approaches for DCF calculations and different cavity/medium combinations have been considered in this study. DCF values for water/bone and LiF/bone cases have strong dependence with energy increasing as the photon energy increases. DCF values also increase with energy for LiF/lung and water/lung cases but, they are much less dependent of energy. For LiF/adipose, water/adipose and LiF/water cases, the DCF values are also dependent of photon energy but, decreases as the energy increases. Maximum difference of 12% has been found compared to values in literature. (paper)