Individualized dosimetry in children and young adults with differentiated thyroid cancer undergoing iodine-131...

Objectives The amount of Iodine-131 to treat young patients with differentiated thyroid cancer (DTC) has not been established so far. The purpose of this study was to perform and compare blood dosimetry by "Hanscheid's approach"and lesion dosimetry by "Maxon's approach". Methods Seventy-one DTC patients ≤21 years were given diagnostic activity of 74 MBq 131I followed by whole-body scan (WBS) at 2 h (pre-void), 24 h, 48 h, and ≥72 h. Pre-therapy blood and lesion dosimetry were conducted to determine the absorbed doses to blood and lesions and to predict the therapeutic activity. The administered activities were varied from 1.11-5.55 GBq of 131I depending on disease extent. Post therapy dosimetries were again performed by acquiring WBS data at 24 h, 48 h, and ≥72 h. Results In blood dosimetry, the difference between predicted therapy activity (PTA) and actual therapeutic activity (ATA) was statistically significant in remnant and lung lesions but insignificant in nodal metastases (p=0.287). In lesion dosimetry, the difference between PTA and ATA was statistically significant for lung metastasis patients; however, not significant in remnant (p=0.163) and nodal metastases (p=0.054). The difference between predicted and observed absorbed dose was insignificant in blood dosimetry whereas, significant in lesion dosimetry. Conclusions The PTA based on 0.3 Gy recommendations of Hanscheid et al. may be adequate for patients with remnant or nodal metastases but inadequate for lung metastases. Lesion dosimetry demonstrated that there is scope to decrease the 131I empiric ATA for remnant and nodal metastases; at the same time, there is scope to increase in lung metastasis patients.