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

Abstract

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 ; and (b) dose to water in medium, D. The discussion about the preferable quantity to be reported is underway. The dose conversion factors, DCF, between dose to water in medium, D , and dose to medium in medium, D, 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 . In this case, DCF to convert from D to D or D 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 cm 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, Co,Ir and Cs, 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.

On the experimental validation of model-based dose calculation algorithms for192Ir HDR brachytherapy treatment planning

“…The denominator in the above equation was calculated using MC simulation in a previous work (Moutsatsos et al 2014) and found equal to 1.231 ± 0.56%. The numerator was obtained from MC simulations of this work that accounted for the geometry and physical properties of both the TLDs and the phantom in the experimental irradiation (see section 2.6) under the large cavity assumption (Branco et al 2016). k bq accounts for the difference in the relation between detector reading and dose to the detector in the experimental ( 192 Ir) and calibration (6 MV) irradiations.…”

“…Additional MC simulations were performed in order to support experimental dosimetry. Dose to TLD in PMMA required for the calculation of the TLD relative absorbed dose energy dependence according to equation (3) was estimated by scoring collision KERMA in PMMA using rectangular * FMESH4 tallies (Branco et al 2016) superimposed over the axial lattice slices including the TLDs. The resolution of each * FMESH4 tally coincided with the lattice resolution (1.37 × 1.37 × 1 mm 3 ).…”

Estudos dosimétricos dos efeitos da heterogeneidade dos tecidos em braquiterapia utilizando o método Monte Carlo

“…Em braquiterapia, tal conversão é um pouco mais complexa, já que dependendo da fonte utilizada a cavidade pode se comportar como pequena, média ou grande em relação ao alcance dos elétrons secundários 79 . Em geral, para fontes que emitem fótons com energia abaixo de 50 keV, como as avaliadas neste trabalho, o alcance dos elétrons é comparado às dimensões das células (alguns µm) 14 , de forma que a cavidade pode ser considerada grande e a dose absorvida proveniente da energia depositada pelas partículas carregadas dentro da d Poder de freamento (stopping-power): representa a perda média de energia por unidade de caminho em um determinado meio de densidade  , devido à colisões Coulombianas com elétrons atômicos.…”

“…As dimensões das células mamárias, por exemplo, são de aproximadamente 10 µm, o que faz com que tais cavidades sejam consideradas pequenas quando irradiadas com fontes de braquiterapia de energia alta, e grandes quando irradiadas com fontes de braquiterapia de energia baixa. Fatores como estes tornam a metodologia de conversão entre as doses o objeto de estudo para muitos pesquisadores 18,23,79,82 .…”