A method for detecting a tumor, the method comprising: determining, by a processing circuit, in each of a plurality of raw density arrays, a respective median value within a region of the respective raw density array, each of the plurality of raw density arrays being a three dimensional array having a plurality of array elements, each of the raw density arrays being associated with a respective point in time, each element of each of the raw density arrays representing a density of a portion of a patient at the respective point in time; forming, by the processing circuit, a plurality of offset density arrays, each corresponding to a respective one of the raw density arrays, the forming of the offset density arrays comprising subtracting, from each of the raw density arrays, the respective median value; forming, by the processing circuit, a first difference array, the forming of the first difference array comprising subtracting, from a first offset density array, a second offset density array, the second offset density array being associated with a later point in time than the first offset density array; forming, by the processing circuit, a second difference array, the forming of the second difference array comprising subtracting, from a third offset density array, a fourth offset density array, the fourth offset density array being associated with a later point in time than the third offset density array; and forming, by the processing circuit, a discriminator array, the forming of the discriminator array comprising: adding the first difference array and the second difference array; and replacing with zero any value that is less than zero, in each of the first difference array and the second difference array. determining, by a processing circuit, in each of a plurality of raw density arrays, a respective median value within a region of the respective raw density array, each of the plurality of raw density arrays being a three dimensional array having a plurality of array elements, each of the raw density arrays being associated with a respective point in time, each element of each of the raw density arrays representing a density of a portion of a patient at the respective point in time; forming, by the processing circuit, a plurality of offset density arrays, each corresponding to a respective one of the raw density arrays, the forming of the offset density arrays comprising subtracting, from each of the raw density arrays, the respective median value; forming, by the processing circuit, a first difference array, the forming of the first difference array comprising subtracting, from a first offset density array, a second offset density array, the second offset density array being associated with a later point in time than the first offset density array; forming, by the processing circuit, a second difference array, the forming of the second difference array comprising subtracting, from a third offset density array, a fourth offset density array, the fourth offset density array being associated with a later point in time than the third offset density array; and forming, by the processing circuit, a discriminator array, the forming of the discriminator array comprising: adding the first difference array and the second difference array; and replacing with zero any value that is less than zero, in each of the first difference array and the second difference array. discriminator array comprising: adding the first difference array and the second difference array; and replacing with zero any value that is less than zero, in each of the first difference array and the second difference array.