ESTRO 2020 Abstract Book

S1090 ESTRO 2020

from a CT to density curve. Tissue elemental compositions were assigned according to TG-186. Dose was computed and reported as dose to water in water, Dw,w, dose to medium in medium, Dm,m, as well as Dw,m, where radiation transport is carried out in the tissue, while dose is scored and reported to a water cavity embedded in the tissue. Dose volume histogram (DVH) parameters evaluated in this study were: Dose Homogeneity Index (DHI), CTV D100%, PTV V200%, 100% and 90% and D0.1cc for ipsilateral lung, chest wall and skin. Results The DHI varied with 0.72, 0.71, 0.70 and 0.68 for MC calculated Dm,m, Dw,m, Dw,w and the dose calculated in Oncentra, respectively. The differences between Dm,m and Dw,m were less than 1% for all DVH parameters except for the PTV V200%, where Dm,m was lower than Dw,m by 3.83%. When compared to Dm,m, Dw,w overestimated the D0.1cc to the lung, chest wall and skin by 4.94%, 2.57% and 3.90% respectively, the CTV D100% by 4.42% and the PTV V100% by 2.23%. The PTV V200%, was overestimated by 8.93%. Comparison between Dm,m and Oncentra revealed overestimation in Oncentra by 17.2% for the CTV D100%, 23.71% for the PTV V200%, 6.09% for the PTV V100%, 2.13% for the PTV V90% and 4.46% for the D0.1cc of the skin while the D0.1cc for the lung and the chest wall were 1.94% and 2.99% lower. However, every calculation method yielded a PTV coverage acceptable by the NSABP protocol B-39 i.e. 90% of the PTV volume was covered by 90% of the prescription dose.

Conclusion There is an important difference in prescribed and delivered dose when using simplified TG-43 based clinical dose calculation algorithms compared to MC calculated Dm,m in breast HDR brachytherapy. Differences less than 1% between the DVH parameters of Dm,m and Dw,m suggest that Dw,m is a good surrogate to Dm,m for reporting of dose. None of the patients in this study had a recurrence, suggesting that even if the delivered dose to the PTV is lower than the prescribed dose, the treatment of breast cancer with HDR brachytherapy is efficient, however, there is room for dose escalation to improve the outcomes further. OC-1036 Intensity Modulated Ir-192 Brachytherapy Using 3D Printed Shielded Applicators L. Skinner 1 , T. Niedermayr 1 , J. Perl 2 , N. Prionas 3 , F. Benjamin 1 , E. Kidd 1 1 Stanford University, Radiation Oncology, Stanford, USA ; 2 Stanford Unvisersity, Stanford Linear Accelerator Lab., Stanford, USA ; 3 University of California- San Fransisco, Radiation Oncology, San Fransisco, USA Purpose or Objective Gynecologic (Gyn) cancers treated with brachytherapy are often asymmetric, yet current Ir-192 brachytherapy techniques provide only limited radial modulation of the dose. The shielded Gyn brachytherapy applicators investigated here solve this by providing the ability to modulate between highly directional and radially symmetric dose distributions at a given location. Material and Methods T o find applicator designs that can modulate between full full dose and less than of the 50% dose, on a scale comparable to the urethra, a 2D calculation algorithm was developed to rapidly narrow down the search space. Two shielding design types were then further investigated using the TOPAS Monte Carlo package and the Varian Brachyvision AXB dose calculation algorithms. 3D printing techniques using ISO10993 biocompatible plastics combined with high density materials that are 95% tungsten powder by weight (approximate density 9g/cc), were tested for the manufacture of applicator parts. Results 3D printable designs were modeled in both Monte Carlo, and Varian Accuros calculation algorithms. The two designs (6-channel star and 8-channel rounded) allow for reduction of the dose by over 50% at 5 mm from the applicator surface in desired regions. Achievable dose gradients circumferentially around the shielded applicator were 5 times steeper than multi-channel cylinders without