Quantitative ultrasound (QUS) imaging is a model-based approach aimed at lesion detection and classification. In this study, the RF backscattered signals from rat fibroadenomas were fit to various mathematical models to yield effective scatterer diameter (ESD) estimates which are tied to tissue microstructure. The goal of these experiments was to understand potential sources of scattering in live tissue across a wide frequency range and how results from different models compare to one another. The ESD was computed from the RF backscattered signals from Sprague Dawley rats with fibroadenoma tumors ranging in size from 1 to 6 cm in diameter. The tumors were scanned using three single-element transducers with center frequencies of 3.5, 7.5 and 13 MHz with a collective -10-dB frequency bandwidth of 1.4 to 18 MHz. Theoretical models of scattering, i.e., form factors, were used to estimate the average ESD of each tumor. Glass bead (Faran), fluid-filled sphere and spherical Gaussian form factors were used, allowing for a comparison between different models. Sixteen histologically confirmed fibroadenomas were included in the analysis. The ESD values were highest for the spherical Gaussian form factor and were 250 μm ± 63 μm, 115 μm ± 38 μm and 52 μm ± 25 μm for 3.5 MHz, 7.5 MHz, and 13 MHz respectively. A trend of decreased ESD with increased frequency was observed for all three form factors, which fits with theory that the scattering at higher frequencies is due to smaller scatterers. The range of values obtained across the form factors and frequencies resulted in ESDs in the typical range of the acini of the rat fibroadenomas, which range in cross sections from 10s to 100s of micrometers. This work was supported by NIH Grant R01CA111289.