Thyroid cancer dosimetry using clearance fitting

Since 1962, Memorial Sloan Kettering Cancer Center has used an individually optimized dosimetry method for patients with thyroid carcinoma undergoing radioiodine therapy. This traditional dosimetry method involves a determina tion of the maximum tolerated activity or the activity that will deliver 2 Gy to the blood (A,,), and the corresponding ablative lesion dose (D-lesion ) However, the traditional calculations of A,,, and Dlesion were based on e mpirical assumptions. The objective of this work was to develop a dosimetry method that eliminates these assumptions by incorporating patient kinetics and that is not restricted to I-131 as a tracer and therapeutic agent. Met hods: Patient kinetics were incorporated into the dosimetry algorithm by fi tting parameters to patient clearance measurements. The radioiodines I-123, I-124, I-125, and I-131 were accommodated as tracers and therapeutic agent s by incorporating their physical half lives and by precalculating photon-a bsorbed fractions for these radionuclides for several thousand patient geom etries using Monte Carlo simulations. Results: A(max) and D-lesion have bee n calculated using the traditional and new method for a group of patients, and errors associated with each of the above assumptions were examined. Ass uming that the initial blood activity is distributed instantaneously in 5 L was found to introduce an error in A(max) of up to 30%, whereas assuming p hysical decay beyond the last data point introduced an error of up to 50%. Conclusion: individualized fitting of clearance data is a practical method to accurately account for inter-patient kinetics variations. The substituti on of standard kinetics beyond measured data might lead to substantial erro rs in estimating A(max) and D-lesion. in addition, gamma camera images, rat her than neck probe readings, should be used to determine lesion uptakes fo r thyroid cancer patients.