eye plaques is beneficial for verifying current dose calibration methods and for standardizing dose prescriptions to the sclera in plaque treatments. Work is currently underway exploring the use of commercial ionization chambers to transfer extrapolation chamber results to clinics. PP15 Presentation Time: 2:54 PM Real Time Modelling of The Dose Distribution Adapted to The Present Treatment Requirements (TG186) Philippe Nicolai, Dr 1 , Gabriele Birindelli, PhD 1 , Jean-Luc Feugeas, Dr 1 , Jonathan Page, PhD 1 , Bruno Dubroca, Dr 1 , Jerome Caron, Dr 2 , Guy Kantor, Pr 2 , Vladimir Tikhonchuk, Pr 1 . 1 university of Bordeaux, CELIA, Talence, France; 2 Institut Bergonie, Bordeaux, France. Purpose: The current approach for brachytherapy usually relies on superposition of single sources in a homogeneous liquid water phantom. This technique is fast and practical in the clinical context but not adapted to the present treatment requirements (TG186[a]) The influence of tissue heterogeneities, inter-seed attenuation and finite patient dimensions, have to be considered. A full Monte-Carlo (MC) code allows computing the Dose Distribution (DD), with a high precision, but is usually too time consuming to be routinely used in clinical context. This work proposes a new method for accurately calculating the DD within a time compatible with clinical requirements. Materials and Methods: The code solves the three dimensional linear Boltzmann transport equation. The model is based on a multi-group energy approach combined with a specific angular momentum closure, deduced from the principle of entropy minimization. The code has been recently applied to external radio-therapy and results suggest that the method is as accurate as Monte-Carlo code for the considered test cases [b]. The transport, attenuation and scattering are based on microscopic cross sections in the energy range from a few keV up to Mev’s. We study a specific application for I-125 prostate brachytherapy. Our code, called M 1 , calculates dose distribution in the tumor volume divided in 1mm 3 voxels without statistical uncertainties in a few seconds. It can take into account the tissue inhomogeneities, strong density gradients and different chemical compositions. The dose computations follow the TG-186 recommendations, with densities and tissue composition effects. Results: Density effects of adipose, bone and calcification are small, in the range of 2 %, in the DICOM case used in this study. On the contrary, the chemical composition radically changes the dose distribution : relatively to the maximum dose calculated in the homogeneous case,the pelvic bones receive þ 70%, the calcifications þ 400% and the adipose À20%. The inter-seed attenuation is small, a few percents, due to the long interseed distance (5mmand more) The gamma-test (1mm - 1%) between the Monte Carlo and M1 simulation passes for 99.88 % of points. Photon energies are relatively low in brachytherapy and the contribution of scattered photons to the total dose can be of the same order of magnitude as the primary dose, particularly in the presence of inhomogeneities. The proposed method provides a fast, accurate and integrated dose computation without any corrections or adjustments. In addition, the discretization of the space dimension is well suited to problems in radiation therapy where voxel-based geometries of patients are conducted from tomographic images. Conclusions: As recommended by the TG-186, the M1 model is a promising choice for the development of deterministic models able to accurately describe particle transport and dose deposition in heterogeneous media with a computational time reduced to a a few seconds This work was performed in the framework of the project POPRA supported by the Aquitain Regional Council and the European Fund for the Regional Development a-Beaulieu et al, TG186, Med Phys. 39, 6208 (2012) b-G. Birindelli et al, Phys. Med. 2017 https://doi.org/10. 1016/J.ejmp.2017.06.20 PP16 Presentation Time: 3:03 PM Dynamic Modulated Brachytherapy (DMBT) Balloon Applicator for Accelerated Partial Breast Irradiation William Y. Song, PhD. Radiation Oncology, Virginia Commonwealth University, Richmond, VA, USA. Purpose: A novel Dynamic Modulated Brachytherapy (DMBT) balloon applicator is described that is capable of generating highly conformal plans that are at-least-equivalent-but-often-superior to all of the modern day balloon-based applicators in market, for HDR based APBI. Materials and Methods: Figure 1a describes the design. The DMBT balloon has two channels: 1) a central channel used for real-time treatment monitoring (i.e., through in vivo dosimetry) and 2) the flexible outer channel that can bow out to any degree (similar to SAVI) and can rotate to any angular position, controlled through a precision stepper motor (e.g., Figure 1a-2 vs 1a-4). Since the outer channel’s degree of flex & its angular position are both under operator control, one can theoretically place the dwell position anywhere within the balloon’s volume. This translates to being able to generate at least equivalent plans but essentially superior compared with those balloons in market including MammoSite, Contura, SAVI, and BEST’s double balloon. Additional benefits include, but not limited to, 1) convenient single-channel connection to an afterloader, 2) thinner shaft design due to having only two catheters, 3) essentially guarantees the optimal angular positioning of lumens with respect to patient geometry every time (e.g., Figure 1b), etc. Since the accuracy of angular positioning of the outer channel becoming important with the DMBT balloon, this can be ensured through the implementation of an electromagnetic tracking (EMT) system. Essentially, at each angular positioning of the channel during treatment, an EMT system will swiftly map out the trajectory and compare it to the treatment plan before the 192Ir source runs out. To reduce treatment time, the EMT source mapping can replace the dummy run. Figure 1. The proposed DMBT balloon-based applicator. (a) 1-dual-channel balloon inflated, 2-an in-vivo dosimeter placed at the central channel & the outer channel ‘‘bow flexed’’ out to a custom degree per the patient-specific balloon volume, with an EMT source mapping out the 3D catheter geometry of the outer channel, 3-an 192Ir HDR source stepping through the planned dwell positions and times, and 4-the process repeats at another catheter angle. (b) An example DMBT balloon plan with 4 channel-angles created and posi- tioned optimally for the given geometry, allowing highly conformal plan to be generated, demonstrating the flexibility of the DMBT balloon system. (c) & (d) Two Contura multi-lumen balloon cases comparing the clinical plans with those of the DMBT balloon plans (with 18 channel-angles created). As can be seen, the DMBT balloon plans demonstrate superior dosimetric conformal- ity sparing the ribs and skin, respectively, while achieving equivalent target coverage to those of the clinical plans. S22 Abstracts / Brachytherapy 17 (2018) S15eS142