OBJECTIVE: To compare the probe detection method with the image quantification method when estimating ¹³¹I biokinetics and radiation doses to the red marrow and whole body in the treatment of thyroid cancer patients. MATERIALS AND METHODS: Fourteen patients with metastatic thyroid cancer, without metastatic bone involvement, were submitted to therapy planning in order to tailor the therapeutic amount of ¹³¹I to each individual. Whole-body scans and probe measurements were performed at 4, 24, 48, 72, and 96 h after ¹³¹I administration in order to estimate the effective half-life (T_eff) and residence time of ¹³¹I in the body. RESULTS: The mean values for T_eff and residence time, respectively, were 19 ± 9 h and 28 ± 12 h for probe detection, compared with 20 ± 13 h and 29 ± 18 h for image quantification. The average dose to the red marrow and whole body, respectively, was 0.061 ± 0.041 mGy/MBq and 0.073 ± 0.040 mGy/MBq for probe detection, compared with 0.066 ± 0.055 mGy/MBq and 0.078 ± 0.056 mGy/MBq for image quantification. Statistical analysis proved that there were no significant differences between the two methods for estimating the T_eff (p = 0.801), residence time (p = 0.801), dose to the red marrow (p = 0.708), and dose to the whole body (p = 0.811), even when we considered an optimized approach for calculating doses only at 4 h and 96 h after ¹³¹I administration (p > 0.914). CONCLUSION: There is full agreement as to the feasibility of using probe detection and image quantification when estimating ¹³¹I biokinetics and red-marrow/whole-body doses. However, because the probe detection method is inefficacious in identifying tumor sites and critical organs during radionuclide therapy and therefore liable to skew adjustment of the amount of ¹³¹I to be administered to patients under such therapy, it should be used with caution.

Keywords: Thyroid neoplasms; Iodine radioisotopes/therapeutic use; Radioisotopes/pharmacokinetics; Dosimetry; Radiotherapy.