Reply: THYROPET Study: Is It Biology or Technology That Is the Issue?

REPLY: Thank you for the opportunity to respond to the letter to the editor by Pattison et al. regarding our article presenting the results of the THYROPET study (1). That study showed that after preparation with recombinant human thyroid-stimulating hormone (rh-TSH), ¹²⁴I PET/CT could not predict the outcome of the posttherapy ¹³¹I scan after thyroid hormone withdrawal (THW) in patients with suspected recurrence of differentiated thyroid cancer. We discussed several factors potentially causing these disappointing results; both technical issues regarding ¹²⁴I PET/CT acquisition and biologic matters were put forward.

Pattison et al. argue that the biologic explanations are most probably causing the false-negative ¹²⁴I PET/CT scans. We fully agree that the difference in the method of preparation on the ¹²⁴I PET/CT (after rhTSH injections) and the ¹³¹I therapy with posttherapy scanning (after THW) is a likely cause of the ¹²⁴I PET/CT being false-negative. Data from ours and other studies support this hypothesis (1–3). Confirmation through prospective studies with intrapatient comparisons of rhTSH- and THW-stimulated ¹²⁴I PET/CT is warranted.

Notably, the one important downside of ¹²⁴I PET/CT scanning after THW as a diagnostic tool is the reduced quality of life during the period of THW (4).

Whether technical issues can be ruled out as a factor, as argued by Pattison et al., is in our opinion not definite. The sensitivity of ¹²⁴I PET/CT is shown to be confined by specific scanner characteristics (e.g., no time-of-flight or point-spread function) (5). New scan protocols and new-generation scanners will—to a greater or lesser degree—improve the sensitivity of ¹²⁴I PET/CT. Similarly, higher administered doses of ¹²⁴I (e.g., 222 MBq as applied by Ho et al. (6)) might improve lesion detection. Currently, it is not known whether the lesions detected by the improved sensitivity of ¹²⁴I PET/CT can be treated with a curative administered dose of ¹³¹I without exceeding safety limits for bone marrow. Future studies are needed to elucidate this.

In conclusion, we agree with Pattison et al. that the method of preparation on the ¹²⁴I PET/CT is probably the most important factor causing false-negative ¹²⁴I PET/CT findings. However, it is too early to state that it is the only factor.

• © 2017 by the Society of Nuclear Medicine and Molecular Imaging.