ESTRO 2023 - Abstract Book

S403

Sunday 14 May 2023

ESTRO 2023

Poster Discussion: Physics brachytherapy

PD-0495 Improved heterogeneity handling for the Advanced Collapse-cone Algorithm F. Alpsten 1 , B. van Veelen 2 , C. Valdes-Cortez 3 , F. Berumen Murillo 4 , A. Ahnesjö 5 , Å. Carlsson Tedgren 6 1 Karolinska University Hospital , Medical Radiation Physics and Nuclear Medicine, Stockholm, Sweden; 2 Elekta Brachytherapy, Physics and Advanced Development, Veenendaal, The Netherlands; 3 Hospital Regional de Antofagasta, Nuclear Medicine, Antofagasta, Chile; 4 Université Laval, Département de physique, de génie physique et d'optique, Quebec, Canada; 5 Uppsala University, Department of Immunology, Genetics and Pathology, Uppsala, Sweden; 6 Karolinska University Hospital, Medical Radiation Physics and Nuclear Medicine, Stockholm, Sweden Purpose or Objective The Advanced Collapsed-cone Engine (ACE) is a brachytherapy model-based dose calculation algorithm (MBDCA) developed for dose computations in heterogeneous media. However, it has been shown that ACE underestimates the dose to cortical bone, likely caused by shortcomings in the handling of the multiple scatter dose. The aim of this work was to investigate dose depositions with respect to scatter order in different media, and to test a correction to ACE in which the multiple scatter (msc) dose component has been modeled in more detail with respect to the balance between local energy absorption and further transport. Materials and Methods We calculated dose distributions with the current version of ACE, an updated version of ACE (ACEcorr), and with the TG43 formalism. To benchmark the results, we used the Monte Carlo (MC) package TOPAS 3.7 to simulate the generic Ir-192 source recommended by TG186 for MBDCA testing. The source was positioned in the center of a large, cubic water phantom of side 30.1 cm. The phantom contained a boxed-shaped cortical bone heterogeneity of dimensions 2.1x2.1x3.1 cm3, placed with its center 6.0 cm from the source. The total dose was separated into dose from the photon transport generations primary, first scattered (1sc) and msc that all were scored separately with MC, ACE, and ACEcorr. Deviations between ACE and ACEcorr versus MC were analyzed based on the ratio of the dose difference to the total dose, Results The mean dose difference ratio in the cortical bone heterogeneity for ACE was -13±2%, while for the primary, 1sc, and msc dose components it was -0.3±0.4%, 0±1%, and -12±2%, respectively. These results show that the deviation of ACE from the MC system comes from the msc dose calculation. For the corrected algorithm ACEcorr, the mean deviation was reduced to -2±2% for the msc dose. Comparing the calculation times between ACE and ACEcorr showed no significant difference. Conclusion We have shown that the dose underestimation to cortical bone by the MBDCA ACE is a consequence of oversimplified multiple scatter transport in non-water media. The results for ACEcorr showed that an improved handling of the balance between local absorption and further transport for multiple scattered photons yields satisfying results for cortical bone without increasing the calculation time. Supportive Material , where gen indicates which scatter generation(s) is being tested and where the superscript ACE stands for either ACE or ACEcorr.

Figure 1: A) Color maps of the dose difference ratio to the total dose for ACE (top) and ACEcorr (bottom) versus the MC dose. The left panels show results for the total dose, while the right panels show results for the multiple scatter dose. B) Dose profiles through y=0, normalized to the dose as calculated with the TG-43 formalism (DTG43) for MC (gray solid lines), ACE (gray dash-dotted lines), and ACEcorr (black dotted lines) for the total dose (upper set of graphs) and multiple scatter dose (the lower set of graphs).