Late cervical lymph node recurrence in differentiated thyroid carcinoma. An inherent problem of 131I beta dose...

In differentiated thyroid carcinoma, postoperative radioiodine therapy is an undisputed element of the integral therapeutic concept. Sometimes, however, and years after initial diagnosis and therapy, cervical lymph node metastases occur without any evidence of local relapse or distant metastases, indicating that radioiodine therapy has not achieved definite tumor cell sterilization. A Monte Carlo computer simulation of the spatial energy dose distribution of 131I in small functioning tumor manifestations was performed to explain this phenomenon. It appeared that only a small fraction of the total beta-energy dose can be deposited inside the tumor if the latter becomes smaller than 1 mm in diameter: for tumors with homogeneous 131I uptake and diameters of 1.0, 0.5, 0.2, 0.1, 0.05 and 0.02 mm, this value does not exceed 86, 73, 39, 16, 8 and 4%, respectively. These data suggest a potential therapeutic gap for small tumor manifestations not apparent at the time of initial diagnosis and could explain the occasional occurrence of late cervical lymph node metastases.