Abstract: An optimized method and system to compute the dose to a patient (2) given a captured integrated exit-transit image (5) of the radiation rays (4) traveling from the source of x-rays (1) through the patient (2) to the imaging device (3) to product the exit-transit image (5). Each radiation field image (5) is transformed (6,8,10,12) to multiple images (7,9,11,13) for each phantom thickness (26) that was measured with the imaging device (3) for a range of field sizes (21). Given the water equivalent path (22) through the patient for a ray (4) reaching a pixel (15, 16), the final pixel value (19) is interpolated from the images (9, 11) that bracket the water equivalent path through the patient (22). Adaptive feedback calibration measures are carried out effectively ameliorate the detrimental effects of off axis attenuation of pixel values.