**8. 131I therapy of differentiated thyroid carcinoma**

Differentiated thyroid cancer (DTC) is the most common cancer of the endocrine system.

The first line therapy is represented by total or near-total thyroidectomy (with dissect of the sixth lymph nodes level and, if necessary, of the lateral-cervical lymph nodes of the same side respect to primary lesion).

After Total or near-Total Thyroidectomy (nTT) it is useful to ablate the thyroid remnant with 131-radioiodine therapy (RaIT). In fact, several Authors, such as Mazzaferri et al. (1997) demonstrated that the *prognosis quod vitam* and the survival curve of the DTC-patients significantly improve if RaIT follows TT or NTT.

In addition, the ablation of thyroid remnant (TRA) allows a better management of the follow-up of these patients.

In fact, in the patients treated with TT or NTT and TRA, the Thyroglobulin (hTg) serum levels should be undetectable. Thus, any enhancement of hTg serum (both under L-T4 suppressive therapy or after exogenous TSH stimulation -rhTSH-) can be considered as a relapse of disease.

RaIT can be carried out in hypothyroidism state (TSH>=30) or after rhTSH stimulation.

Post dose whole body scan and static images of the head, neck and thorax acquired 4-8 days after RIT allow to identify the thyroid remnant and metastases (loco-regional and/or distant).

For the RaIT of TRA, fixed activities are employed more frequently: 1110, 2220 or 3700 MBq. In the patients treated after rhTSH stimulation it is necessary to employ a medium-to-high activity of radioiodine (2220 and 3700 MBq, respectively), because in this condition the effective half-life of radioiodine in the thyroid remnant is shorter than in the state of hypothyroidism.

However, the TRA activity can be adjusted through a dosimetric approach (Lassmann, 2010), which requires a pre-therapeutic scintigraphy with 131I or 124I PET. The dose to the thyroid remnant can be calculated using 131I post-therapy whole body scan, too.

Both methods show advantages and disadvantages. The main disadvantage of the pretherapeutic scintigraphic method is correlated to the stunning or mass change effects that could be determined by a diagnostic activity of 131I.

On the other hand, the main disadvantage of the post-therapeutic scintigraphic method is correlated to technical difficulties such as the limitations deriving from the gamma-camera dead time.

The clinical evidence demonstrated that the dosimetry adjusted activities do not differ significantly from the fixed values. Thus, in the clinical practice, the dosimetric approach is not employed frequently. However, the dosimetric approach is useful for the treatment of loco-regional and/or distant metastases.

Dosimetry is particularly useful in patients with lung and/or bone metastases, where standard activities can lead to a radiation dose imparted to the lesions lower than the necessary. In such cases, it is necessary to acquire some whole scans starting from 6-7 hours after radio-iodine therapy. (Eschmann et al., 2002) (Sgouros et al., 2004).
