The AccuBoost system has been used to apply radiotherapy boost for breast cancer patients using round applicators. More conformal coverage at the chest wall can be obtained by reshaping the applicators into a D-shape with the flat edge positioned near the patient. Preceding clinical use, these new brachytherapy applicators require dosimetric characterization. Two D-type applicators were studied: one measuring 45 mm (D45) and the other 60 mm (D60) in anterior-posterior width, each having a 67% larger flat edge length. Radiation dose distributions were simulated using Monte Carlo methods (MCNP5), and compared to dose measurements in polystyrene (PS) using radiochromic film (GafChromic EBT) and air ionization chambers (Farmer and Markus). Simulations were performed in PS and ICRU 44 breast tissue. HDR 192Ir source dwell times were either constant or optimized for uniform skin dose. Unlike round AccuBoost applicators where simulations could take advantage of cylindrical symmetry, D-shaped applicators required a mesh-based tally (1 mm3 grid) to determine 3D dose distributions. Film and ion chamber dose response was traceable to 60Co calibrations, and validated through spot checks. RIT film scanning software (v.5.0) was used at 0.4 mm resolution, with films oriented parallel and perpendicular to the phantom surface. Ion chamber measurements of depth dose ranged from 0-8 cm. Monte Carlo results in breast tissue indicated similar central axis dose falloff to the round AccuBoost applicators. However, dose conformity changed as a function of azimuthal angle due to variable tungsten applicator collimation. For both applicators, scatter decreased as depth decreased with dose reductions 1 cm outside the lateral field edge of 50% and 90% at depths of 3 cm and at the skin, respectively. Similarly, dose uniformity improved as depth decreased, and was also due to scatter. At 3 cm, in-field dose uniformity was 14% and 17% for the D45 and D60, respectively, and 10% and 12% at the skin. The measured divergence angle (∼3°) matched the machined geometry. Dose determined using film was within 5% of Monte Carlo PS results inside the field for all depths studied, with larger variations observed outside the field. Ion chamber depth dose absolute measurements agreed with Monte Carlo PS results within ∼4% with improved agreement when normalized to a midline depth such as 3 cm. The D-shaped AccuBoost applicator dose distributions have been characterized in 3D using multiple techniques. Comparisons of dosimetry results amongst the techniques show good agreement. These results indicate the applicator designs are able to produce the desired dose distributions, and that prior recommendations on treatment planning margins should apply.