Threshold estimation in single photon emission computed tomography and planar imaging for clinical radioimmunotherapy

Thresholding is the most widely used organ or tumor segmentation technique used in single photon emission computed tomography (SPECT) and planar imaging for monoclonal antibodies. Selecting the optimal threshold requires a priori knowledge (volumes from CT or magnetic resonance) for the size and contrast level of the organ in question. Failure to select an optimal threshold leads to overestimation or underestimation of the volume and, subsequently, the organ-absorbed dose value in radio-immunotherapy. To investigate this threshold selection problem, we performed a phantom experiment using six lucite spheres ranging from 1 to 117 ml and filled with a uniform activity of 1 microCi/ml Tc-99m. These spheres were placed at the center and off-center locations of a Jasczsak phantom and scanned with a three-headed gamma camera in SPECT and planar modes. Target-nontarget (T:NT) ratios were changed by adding the appropriate activity to the background. A threshold search algorithm with an interpolative background correction was applied to sphere images. This algorithm selects a threshold that minimizes the difference between the true and measured volumes (SPECT) or areas (planar). It was found that for spheres equal to or larger than 20 ml [diameter (D) > 38 mm] and T:NT ratios higher than 5:1, mean thresholds at 42% for SPECT and 38% for planar imaging yielded minimum image segmentation errors, which is in agreement with current literature. However, for small T:NT ratios (< 5:1), the threshold values as high as 71% for SPECT and 85% for planar imaging were substantially different than those fixed thresholds for large spheres (D > 38 mm). Hence, the use of fixed thresholds in low contrasts and with tumor and organ sizes of clinical interest (25 < or = D < or = 50 mm) may result in limited volume estimation accuracy. Therefore, we have provided the investigator a method to obtain the threshold values in which the proper threshold can be selected based on the organ and tumor size and image contrast. By measuring and calibrating the proper threshold value derived through machine-specific phantom measurements, a more accurate volume and activity quantitation can be performed. This, in turn, will provide tumor-absorbed dose optimization and greater accuracy in the measurement of potentially subacute, toxic absorbed doses to normal organs for patients undergoing radioimmunotherapy.