ESTRO 2023 - Abstract Book

S226

Saturday 13 May

ESTRO 2023

source for brachytherapy applications and investigate its shielding properties combined with a novel rectal applicator developed for intensity modulated brachytherapy. Materials and Methods A 75Se source (Figure 1a) was designed with its active core (0.65 mm diameter, 7 mm length, 3.7 g/cm3 packed density) encapsulated in a titanium (4.5 g/cm3, 0.90 mm outer diameter, 0.25 mm wall thickness) capsule. The length of the active core was chosen such that it can contain 23 Ci of 75Se which gives a dose rate equivalent of 10 Ci 192Ir. The AAPM TG-43U1 brachytherapy dosimetry parameters were calculated for the 75Se using RapidBrachyMCTPS, which is a Monte Carlo based treatment planning system. RapidBrachyMCTPS, was further used to acquire dose distributions in a water phantom. Four different scenarios were simulated where a novel rectal applicator was combined with 3 types of rotating tungsten shields. These shields were developed for intensity modulated brachytherapy and include two rigid shields and a flexible shield. A no shield scenario was also simulated. The results were used to calculate the transmission factors (TF) for the different shield models. The results were compared with simulations performed with a 192Ir source. Results The radial dose function and 2D anisotropy function were calculated and plotted in comparison with 192Ir as presented in Figure 1b and 1c. The air kerma strength per unit activity and dose rate constant were calculated for 75Se to be 4.751 +/- 0.005 x 10^-8 U/Bq and 1.116 +/- 0.001 cm^-2 respectively and for 192Ir to be 9.79 +/- 0.01 x 10^-8 U/Bq and 1.110 +/- 0.001 cm^-2 respectively. Dose distributions in a water phantom were calculated and the dose colour maps for all scenarios can be seen in Figure 2a-e. For the rigid shields, 75Se had TF values of 2.7 +/- 0.5 % and 2.3 +/- 0.7 %, and 192Ir had TF values of 15.15 +/- 0.05 % and 13.2 +/- 0.2 %. For the flexible shield, 75Se and 192Ir had TF values of 16.0 +/- 0.5 % and 31.4 +/- 0.1 % respectively. This displayed that 75Se had 5.61 and 5.74 times better attenuation than 192Ir with the rigid shields and 1.96 times better with the flexible shield.

Conclusion The designed 75Se source was superior with regards to attenuation through tungsten shields due to its lower energy while still being able to produce an equivalent dose rate to 192Ir. These results allow for justification of further analysis of this source for use in conventional brachytherapy and intensity modulated brachytherapy as it is expected to deliver the same absorbed dose to the tumours as 192Ir with similar treatment times while reducing the dose to surrounding organs at risk. OC-0291 Improving HDR brachytherapy needle visibility during US imaging via vibration-induced color-Doppler J. Dupere 1 , E. Brost 1 , M. Urban 2 , M. Hainy 3 , C. Deufel 1 1 Mayo Clinic, Radiation Oncology, Rochester, USA; 2 Mayo Clinic, Radiology, Rochester, USA; 3 Mayo Clinic, Biomechanical Development, Rochester, USA Purpose or Objective Ultrasound-guided brachytherapy requires conspicuous applicators for proper applicator placement and accurate digital reconstruction. The conspicuity of brachytherapy applicators during ultrasound (US) prostate brachytherapy may be reduced by the presence of artifacts and tissue interfaces, unintentionally resulting in geometrically inaccurate treatment plans. The US visibility of HDR brachytherapy needles may be improved by including color-Doppler (CD) alongside standard B-mode imaging. This work describes the development of, and initial results for, a prototype vibrational device that produces a strong and reliable mechanical CD signal that consistently enhances applicator conspicuity. Materials and Methods