An indirect high iodine (131 I) effective dose used for thyroid ablation in patients with thyroid cancer. Is the method of measurement important?

Abstract

BackgroundRadiation effective dose to the red bone‐marrow, a critical organ in the therapy of differentiated thyroid carcinoma (DTC) with radioiodine‐131 (131I), cannot be measured directly. As radioiodine concentration is comparable in blood and most organs, and is believed to be similar in red marrow, the effective dose to the blood seems to be a good first‐order approximation of the radiation effective dose to the hematopoietic system and a better means to quantifying exposure from therapy compared to the total amount of activity administered.PurposeWe applied four formulas (Lassmann et al (standard) [2008], Eur J Nucl Med Molecul Imaging, 35:1405–1412), (Thomas et al. [1993], Nucl Med Biol, 20:157–162), (Sisson et al. [2003], J Nucl Med, 44:898–903; Ha¨nscheid et al. [2009], Endocr Relat Cancer, 16:1283–1289) and (Ha¨nscheid et al. [2006], J Nucl Med, 47:648–654) and compared between the estimated values of the effective dose that were obtained by three formulas and those obtained by the standard one.Materials and methodsTwenty‐seven patients, 22 women and 5 men, suffering from DTC were enrolled in this study. Whole‐body probe measurements and blood collections (2 mL whole‐blood samples) were conducted at 2, 6, 24, 48, 72–96 h after the administration of 131I to obtain time–activity curves. Whole‐body measurements were performed as conjugate view (anterior and posterior) counts by scintillation camera imaging.ResultsBy comparing the values of blood effective dose that were obtained by applying Thomas et al. [1993], Nucl Med Biol, 20:157–162; Sisson et al. [2003], J Nucl Med, 44:898–903 and Ha¨nscheid et al. [2009], Endocr Relat Cancer, 16:1283–1289, and Ha¨nscheid et al. [2006], J Nucl Med, 47:648–654, techniques, with those obtained by (Lassmann et al (standard technique) [2008], Eur J Nucl Med Molecul Imaging, 35:1405–1412), we found that these values are, respectively, 15.0%, 40.0%, and 41.0% more than those obtained by using the standard method. To our knowledge no papers have been published previously that compare between these dosimetric approaches.ConclusionHighly overestimated or highly underestimated results obtained by a certain method or technique, compared with those obtained by the standard method, are not desirable, they tend to exaggerate in applying radiation protection procedures, by increasing or decreasing, which, in both cases, become far from the realistic or recommended procedures. As an operating philosophy, the objective of radiation safety practices simply should not be to keep radiation doses within legal limits or maximum permissible doses (MPDs), but to keep them “as low as reasonably achievable” (ALARA concept). MPDs should not be considered as thresholds below which exposure to radiation is of no concern, they are not assumed to be totally risk free, and any reasonable technique for reducing radiation dose may have potential benefits in the long run.