Blood and bone marrow dosimetry for thyroid cancer patients prepared with rhTSH injection.

Purpose: Radioiodine (¹³¹I) dosimetry is used to maximise tumour dose while reducing the chances of toxicity. High thyroid-stimulating-hormone (TSH) levels are required for ¹³¹I treatment, achieved through hormone withdrawal or intramuscular injection of recombinant human TSH (rhTSH). Both approaches have shown equivalent results, with the rhTSH approach reported to reduce morbidity and avoid hypothyroidism. There are established differences in ¹³¹I biokinetics using each method. This clinical cohort study investigated if pretherapy iodine biokinetics as measured using a dosimetry protocol without a dose of rhTSH are predictive of post therapy biokinetics in patients prepared with rhTSH injection.

Methods: Thirteen patients with differentiated thyroid cancer (DTC) were recruited. An adaptation of the European Association of Nuclear Medicine (EANM) dosimetry protocol was conducted at St James's Hospital, Ireland. The maximum tolerable activity (MTA) was calculated using the EANM, Association of Physics in Medicine (AIFM) and Traino models, after administering ¹³¹I, and subsequent whole-body (WB) dose-rate measurements and blood-sampling were carried out. The MTA estimated from pre-therapeutic (PT) ¹³¹I tracer administration (6.07 ± 2.46 MBq) was compared to during therapy (DT) administration (3.88 ± 0.16 GBq).

Results: The PT WB residence-time overestimated the DT with a difference of - 7.72 ± 8.13% (p = 0.007), while no significant difference is reported between the blood residence-time (1.13 ± 6.49%, p = 0.559). The EANM model reported the lowest difference of 1.73 ± 4.83% (p = 0.241) in MTA.

Conclusion: This study validated the feasibility of using dosimetry in euthyroid patients to predict therapeutic ¹³¹I biokinetics in DTC patients prepared with rhTSH.