1. A method for dynamic tracking of soft tissue targets comprising: registering a first ultrasound (US) image having a soft tissue target of a patient with a first reference image of a first modality having the soft tissue target of the patient; determining a first displacement of the soft tissue target based on the registering of the first US image with the first reference image; registering a second US image of the soft tissue target with the first US image of the soft tissue target, wherein the second US image is obtained subsequent to the first US image; determining a second displacement of the soft tissue target based on the registration of the second US image with the first US image; adjusting a position of a radiation source of radiation treatment delivery system using the second displacement; storing the second US image to establish a treatment delivery reference position of the soft tissue target for registration with one or more subsequent US images; and registering the one or more subsequent US images with the treatment delivery reference position to determine a third displacement of the soft tissue target with respect to the treatment delivery reference position of the soft tissue target, wherein the one or more subsequent US images are obtained at a first rate, and wherein determining the third displacement further comprises: tracking a second motion of one or more markers externally disposed on the patient by sensing the one or more markers at a second rate with respect to a fixed coordinate system, wherein the second rate is higher than the first rate; and calculating a predicted position of the soft tissue target as a function of a time using a correlation model based on tracking the second motion of the one or more markers, the correlation model correlates the second motion of the one or more markers to the predicted position of the soft tissue target; and readjusting the position of the radiation source using the third displacement.