Objective: No satisfactory methods exist for postsurgical remnant mass estimation required for dosimetric calculations in differentiated thyroid cancer patients undergoing I-131 remnant ablation. We estimated the remnant mass by Tc-99m pertechnetate single-photon emission computed tomography to calculate the radiation dose delivered.
Materials and methods: Thyroid phantoms of known volumes (1.1-6 cm3) were fabricated, filled with different concentrations of Tc-99m pertechnetate activity, and single-photon emission computed tomography acquired on a dual head camera and processed with Hanning filter. Area of the organ on coronal slices was calculated at different thresholds and the sum was multiplied by slice thickness to get the volume. A threshold of 50% was found to distinguish the organ from the background activity and give the most accurate results. Remnant mass was calculated by this method in 50 patients of differentiated thyroid cancer after thyroid surgery. Effective half-life (EHL) of I-131 and 24 h radioactive iodine uptake (RAIU) were calculated by repeated neck count measurements for 4 days. After administering I-131 for therapy, 39 patients were followed up at 6 months and the effects of various parameters in achieving complete ablation were analyzed.
Results: Mean values of remnant mass, RAIU, and EHL were 6.6+/-4.3 g, 8.8+/-6.1%, and 4.5+/-0.9 days, respectively. Type of surgery, remnant mass, and RAIU were the most important predictors of complete ablation (P<0.05). Radiation-absorbed dose, EHL, and the administered activity had no significant effect on complete ablation.
Conclusion: Remnant thyroid mass estimation by Tc-99m pertechnetate single-photon emission computed tomography is a simple technique and allows overcoming the dependence on structural imaging techniques or making empirical assumptions during dosimetry.