Optimization of organ dose calculation parameters in internal radionuclide treatment

In nuclear medicine, the treatments applied and the effectiveness of the treatments continue to increase day by day. The general aim in systemic treatments is that the intravenously administered radiopharmaceutical reaches the target tissue through the circulatory system and accumulates there. In addition to the target tissue, radiopharmaceuticals can actually cause unwanted organ irradiation in the areas of physiological uptake and excretion pathway. If this effect is in critical organs, this situation can be limiting or terminating the treatment. Accurate calculation of the radiation dose absorbed by organs and lesions are important in terms of not harming the patient while treating the disease effectively. The determination of cumulative activity depends on many variables, as it is done by quantitative methods and significant deviations may occur in the calculation results. While performing dosimetric calculations, the stability of the devices where the measurements are made should be high and the margin of error of imaging protocols should be kept at the lowest rate. In this study, device stability was evaluated for 10 months by taking consecutive measurements at fixed parameters in a gamma camera. A human-like phantom was produced to imitate the standard and overweight human body to be used in the calculation of the Standard Activity Conversion Factor (SACF) used in the determination of organ activities. SACF detected in air, human-like phantom and overweight human-like phantom with constant volume activity, respectively. Then, SACF was determined by diluting the same amount activity in different volumes. SACF results were compared and evaluated. Measurements were performed in different imaging parameters by using the NEMA PET phantom, which is used in lesion dosimetry calculations and provides an accurate determination of the lesion volume. The data obtained with different parameters were evaluated. A deviation of 2.88% was detected in consecutive measurements performed with constant parameters for 10 months. Standard and overweight human-like phantom results were found to be similar in images with single peak attenuation correction and scatter correction. When compare the vial/human-like phantom and the diluted source/human-like phantom, the measurements in the human-like phantom were 23% and 16% higher, respectively. The volumes obtained by using 40% threshold value in the data measured with 37 mm diameter bead using NEMA PET phantom are calculated smaller than the actual diameter. When the scanning time was reduced from 20 seconds to 10 seconds per projection, a 1% change in counting efficiency was detected. When the number of projections was reduced, the deviation in counting efficiency increased to 3.6%. As a result; in order to keep error rates in minimum, a human-like phantom should be used when determining the SACF.