Absorbed dose coefficients for adult thyroid cancer patients undergoing radioiodine therapy

Abstract

Use of radioactive iodine (RAI) for thyroid cancer patients is accompanied by elevated risks of radiation-induced adverse effects due to significant radiation exposure of normal tissues or organs other than the thyroid. The health risk estimation for thyroid cancer patients should thus be preceded by estimating normal tissue doses. Although organ dose estimation for a large cohort often relies on absorbed dose coefficients (i.e., absorbed dose per unit activity administered, mGy/MBq) based on population models, no data are available for thyroid cancer patients. In the current study, we calculated absorbed dose coefficients specific for adult thyroid cancer patients undergoing RAI treatment after recombinant human TSH (rhTSH) administration or thyroid hormone withdrawal (THW). We first adjusted the transfer rates in the biokinetic model previously developed for THW patients for use in rhTSH patients. We then implemented the biokinetic models for thyroid cancer patients coupled with S values from the International Commission on Radiological Protection (ICRP) reference voxel phantoms to calculate absorbed dose coefficients. The biokinetic model for rhTSH patients predicted the extrathyroidal iodine decreasing noticeably faster than in the model for THW patients (calculated half-times of 12 and 15 h for rhTSH administration and THW, respectively). All dose coefficients for rhTSH patients were lower than those for THW patients with the ratio (rhTSH administration/THW) ranging from 0.60 to 0.95 (mean = 0.67). The ratio of the absorbed dose coefficients in the current study to the ICRP dose coefficients, which were derived from models for normal subjects, varied widely from 0.21 to 7.19, stressing the importance of using the dose coefficients for thyroid cancer patients. The results of this study will provide medical physicists and dosimetrists with scientific evidence to protect patients from excess exposure or to assess radiation-induced health risks caused by RAI treatment.

Absorbed dose coefficients for pediatric differentiated thyroid cancer patients undergoing radioiodine therapy

“…This degradation of thyroidal iodine levels in the pediatric models was obviously due to the faster renal clearance rate that rapidly reduces the iodine resource in blood for the thyroidal uptake. This impact of the renal clearance rate on the thyroidal uptake of iodine was also observed in the comparison between the THW and rhTSH adult models [8]; that is, the thyroidal iodine levels were significantly lower in the rhTSH model with a faster renal clearance rate than in the THW model.…”

“…Unfortunately, the International Commission on Radiological Protection (ICRP) has not provided absorbed dose coefficients for pediatric thyroid cancer patients. The absorbed dose coefficients in ICRP Publication 128 [7], designed for diagnostic nuclear medicine, as well as those for adult thyroid cancer patients in our previous work [8], may not be suitable for dosimetry in pediatric thyroid cancer patients. In light of the significant anatomical and physiological differences between pediatric and adult patients, absorbed dose coefficients should be calculated using anatomical and biokinetic models that can appropriately predict the radiation energy transfer and the iodine biokinetics in an immature body.…”

“…We adopted the biokinetic models suggested by Taprogge et al [9] and Kwon et al [8] as the baseline models for adult thyroid cancer patients undergoing THW and rhTSH administration, respectively. These models adjusted the transfer rates in the ICRP reference iodine biokinetic model (figure 1) [7] for use in adult thyroid cancer patients.…”

“…The age-specific secretion rates in the ICRP reference model were calculated using a biological half-life of thyroidal iodine at a given age, which is associated with dietary intake, hormone secretion, and thyroid stores of stable iodine [13]. However, the age-dependency of the biological half-life of iodine observed in the normal thyroid is not applicable to thyroid cancer patients The ICRP reference iodine biokinetic model structure [7] employed in the development of biokinetic models for adult thyroid cancer patients [8,9] Reproduced with permission from ICRP Publication 128 [7].…”

“…A total of 10 sets of transfer rates for pediatric patients undergoing THW (table 1) or rhTSH (table 2) administration were created in the current study, alongside corresponding values for adult patients [8,9] for comparison. Among the transfer rates considered age-dependent (highlighted in bold), Kidneys 2-to-Blood rate, Liver 2-to-Blood rate, Other 3-to-Blood rate, and Blood-to-Kidneys 3 rate (only in the THW model) were derived specific for pediatric thyroid cancer patients in the current study, while the remaining transfer rates were adopted from the ICRP reference models.…”