Abstract
ObjectiveTo 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 MethodsFourteen 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 (Teff) and residence time of 131I in the body.ResultsThe mean values for Teff 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 Teff (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).ConclusionThere 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.
Highlights
The management of patients with differentiated thyroid cancer involves radioiodine therapy to ablate remnants of thyroid tissues after surgical resection of the gland or in the treatment of metastases[1]
Several of the dosimetric methods used for adjusting the amount of 131I to be administered in therapy are based on delivering a maximum radiation dose of 2–3 Gy to red-marrow tissues, while abiding by the rules for radioiodine-avid lung metastases[2,4]
Fourteen patients with metastatic differentiated thyroid cancer were submitted to a dosimetric protocol in order to tailor the amount of 131I to be administered in individual therapy
Summary
The management of patients with differentiated thyroid cancer involves radioiodine therapy to ablate remnants of thyroid tissues after surgical resection of the gland or in the treatment of metastases[1]. A large amount of 131I is generally administered to patients during the treatment of metastatic diseases Under these circumstances, it would be more appropriate to tailor the amount according to individual needs and to the radiotoxic effect on healthy organs, such as the red marrow, lungs, kidneys, and salivary glands[2,3]. Several of the dosimetric methods used for adjusting the amount of 131I to be administered in therapy are based on delivering a maximum radiation dose of 2–3 Gy to red-marrow tissues, while abiding by the rules for radioiodine-avid lung metastases[2,4]. When determining the radiation dose per unit of 131I activity (mGy/MBq) to be received by the red marrow and whole body, sequential measurements of the circulating levels of 131I are generally required. The circulating level of 131I is either estimated through invasive procedures, such as the collection and analysis of blood samples, or inferred from whole-body radiation measurements with a radiation detection probe or by image quantification[5,6]
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