Patient-specific anatomical models for radioiodine dosimetry in treatment of hyperthyroidism: is it necessary?

Purpose: To investigate the necessity of patient-specific dosimetry calculations using individualized models for hyperthyroid patients treated with radioactive iodine (RAI). This treatment modality was considered to be safe and effective; however, a recent publication indicated associations between greater organ-absorbed doses of RAI and risk of cancer death.

Methods: Ten patient-specific models which ranged in size were used (from 152.5 to 184 cm in height and from 44 to 88 kg in mass). The time-integrated activity coefficients (TIAC) were evaluated from the 2017 Leggett's model assuming 24 h radioactive iodine uptakes (RAIU) of 30, 50, 70, and 90% and two intake routes for normal uptake (ingestion and injection). A set of ¹³¹ I S factors (mGy MBq^-1 h^-1 ) from the patient-specific phantoms including 12 source regions were provided in this study. These S factors were used together with the new TIACs to present dose coefficients.

Results: The MC-based patient-specific S factors were compared with the ICRP standard data and the variation ranges (%) of (-65, +210) and (-57, +193) were reported for self and cross S factors, respectively. However, for self S factors, those intervals were reduced to (-8.3, +4.6) when mass correction was applied. Moreover, variations on organ dose coefficients were evaluated and the thyroid contributions were also assessed for 24 h RAIU of 30, 50, 70, and 90%. Considering that the thyroid contribution to adjacent normal organs is high and the variations on cross dose coefficients are also considerable, variations (%) on normal organ doses were estimated to be up to (-63, +132), with a planned thyroid absorbed dose of 150 Gy.

Conclusion: Given the large variations on organ doses, the standard data are not an appropriate substitute for patient-specific data. Particularly, when accurate patient-specific dose estimation is a serious concern in RAI treatment (RAIT) for nuclear medicine practitioners. However, acquiring computed tomography (CT) images for patient-specific modeling will impose additional radiation dose to patients. It was concluded that CT imaging limited to the region from skull base to mid thorax (i.e., for organs with RAIT doses of >~50 mGy with a dose of 150 Gy prescribed to the thyroid) may be suggested and is clinically relevant because the normal organ dose increments are not greater than 10%.