Influence of heterogeneities in the dosimetric calculation of gynecological HDR brachytherapy. A Monte Carlo study

M. HERVIAS; M. SANTIBÁÑEZ; F. MALANO; C. DE LA BARRA; M. VALENTE

Zacatecas

Congreso; ISSSD 2021 - Int. Symposium of Solid State Dosimetry; 2021

Univ. Zacatecas & Soc. Mex de Dosim.

Cervical cancer is the fifth most prevalent malignancy worldwide and represents a major public health problem [1-2]. Although new treatment techniques have been introduced in recent decades, gynecological brachytherapy remains to be an important treatment modality [3-4]. Traditionally, clinical dose calculations in brachytherapy is carried out using the TG-43 formalism [5], which describes the absorbed-dose deposition produced by a radiation source in an infinite water medium, which is why most of the Treatment Planning Systems (TPS) currently model the patient considering all volume elements (voxels) made of water, thus approximating the real patient anatomy without considering heterogeneities. On the other hand, during the last decade, several Model Based Calculation Algorithms for brachytherapy (MBCDA's) have been introduced. These models consider the influence of heterogeneities, the composition of applicators and the patient's anatomy [6]. In the case of HDR gynecologic brachytherapy, it has been found that the use of MBDCAs produces an average difference of less than 5% with respect to the TG-43 formalism used in TPS [7]. However, it has been reported that there may be important uncertainties due to not considering the specific atomic composition of the tissue and the applicator [8] and due to the dose distribution produced from discrete step movement of the source [9].In this work, the dosimetric impact in the uterus and other organs at risk due to heterogeneities commonly found in the pelvis was studied by means of Monte Carlo simulations carried out with the PENELOPE 2014 code [10-11]. The simulation scenario consisted of a 192Ir HDR brachytherapy source placed in an anthropomorphic pelvis phantom specially designed for this study. The 192Ir source was simulated according to the GAMMAMED PLUS model (Varian Medical Systems, Palo Alto, CA.). Eight positions of the source in the tandem separated by 1 cm and two positions in the ovoids with the same separation were simulated independently. The dose obtained in the organs at risk were compared with results obtained in simulations carried out in a homogeneous water phantom. The dose distributions in both cases were compared using the gamma index method. The effect produced by different sizes of rectum on the dose distribution was also studied.It was found that the dose calculation in the homogeneous phantom was 1.47 ± 0.01 times bigger as compared with the anthropomorphic phantom. Conversely, the dose in the femoral heads was 0.56 ± 0.02 times lower with respect to the anthropomorphic phantom. For the uterus and the bladder, the differences were remarkably lower, finding that they were less than 5% in both cases when compared with the anthropomorphic phantom. Besides, it was found that a rectum with a 40% reduced volume produces an increase in the dose of the rectal wall of 1.34 ± 0.01 times and a bigger increase in the rectum enclosed volume of 1.53 ± 0.04 times.It is concluded that, even though in most cases the calculations in homogeneous water phantom yields similar results to those obtained in an anthropomorphic phantom, attention should be paid when large heterogeneities are present in the anatomy.Keywords: gynecological HDR brachytherapy, heterogeneities, Monte Carlo, anthropomorphic phantom[1] J. Ferlay et al., International Journal of Cancer, vol. 144, no. 8. 2019.[2] W. Small et al., Cancer, vol. 123, no. Jul. 13, 2017.[3] C. H. Holschneider et al., Brachytherapy, vol. 18, no. 2, Mar. 2019.[4] K. Tanderup, P. J. Eifel, C. M. Yashar, R. Pötter, and P. W. Grigsby, Int. J. Radiat. Oncol., Vol. 88, no. 3, Mar. 2014.[5] M. J. Rivard et al., Med. Phys., Vol. 31, no. 3, Feb. 2004.[6] L. Beaulieu et al., Med. Phys., Vol. 39, no. 10, Sep. 2012.[7] D. Jacob, M. Lamberto, L. DeSouza Lawrence, and F. Moutti, Brachytherapy, vol. 16, no. 3, May 2017.[8] S. A. Enger, J. Vijande, and M. J. Rivard, Semin. Radiat. Oncol., Vol. 30, no. 1, Jan. 2020.[9] B. Farhood and M. Ghorbani, Radiol. Phys. Technol., Vol. 10, no. 4, Dec. 2017.[10] J. Baró, J. Sempau, J. M. Fernández-Varea, and F. Salvat, Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms, vol. 100, no. 1, May 1995.[11] F. Salvat, Ann. Nucl. Energy, vol. 82, Aug. 2015.