Thyroid cancer dosimetry using clearance fitting

Area of Science:

• Nuclear medicine
• Medical physics
• Oncology

Background:

• Memorial Sloan Kettering Cancer Center has utilized an individualized dosimetry method for thyroid carcinoma radioiodine therapy since 1962.
• Traditional dosimetry relies on empirical assumptions for calculating maximum tolerated activity (A(max)) and ablative lesion dose (D(lesion)).

Purpose of the Study:

• To develop an improved dosimetry method that eliminates empirical assumptions by incorporating patient-specific kinetics.
• To create a method not limited to 131I, accommodating various radioiodine isotopes.

Main Methods:

• Patient kinetics were integrated into the dosimetry algorithm by fitting parameters to clearance measurements.
• Monte Carlo simulations were used to pre-calculate photon-absorbed fractions for multiple radioiodines (123I, 124I, 125I, 131I) across diverse patient geometries.
• Incorporated physical half-lives of different radioiodine isotopes.

Main Results:

• Calculations using traditional and new methods revealed significant errors in A(max) due to assumptions.
• Assuming instantaneous blood distribution introduced up to 30% error in A(max).
• Assuming physical decay beyond the last data point introduced up to 50% error in A(max).

Conclusions:

• Individualized fitting of clearance data accurately accounts for inter-patient kinetic variations.
• Using standard kinetics beyond measured data can lead to substantial errors in estimating A(max) and D(lesion).
• Gamma camera imaging is recommended over neck probe readings for determining lesion uptakes in thyroid cancer patients.