Radiation therapy dose calculation methods, systems and computer program products, for use with a treatment delivery device for treating a patient and utilizing a scattered radiation detector in making independent dose calculations. The scattered radiation detector is configured to acquire measurement information during patient treatment, which is used to determine an estimate of the output of the treatment delivery device. Other information is acquired including patient imaging data, gantry angle information and collimator position information utilized during patient treatment. The acquired information is utilized along with the treatment delivery device output estimate to determine dose delivered to the patient.
