Dosimetry in the therapy of metastatic differentiated thyroid cancer administering high 131I activity: the experience of Busto Arsizio Hospital (Italy).

AIM: The purpose of the present work was to evaluate the impact of 131I high activity therapy treatments of metastatic differentiated thyroid cancer (MDTC) in terms of feasibility, tolerance, efficacy, and the impact of dosimetry in order to optimize the process.

METHODS: Seventeen MDTC patients underwent 27 treatments with 131I, with activity ranging from 6.2 GBq to 24.1 GBq. Red marrow (RM) peritherapy dosimetry was based on the Standard Operating Procedure of European Association of Nuclear Medicine (EANM SOP), while metastases dosimetry on the guidelines of Italian Association of Physicists in Medicine and Italian Association of Nuclear Medicine. In 12 cases prospective dosimetry was performed too, with the purpose of evaluating the possibility of maximizing the therapeutic activity, complying the 2 Gy red marrow (RM) dose constraint. The absorbed dose to 45 lesions was evaluated. The severity of myelotoxic effects was monitored during the follow-up.

RESULTS: Treatments were generally well tolerated, also at the highest RM absorbed doses. RM absorbed doses ranged from 0.49 to 6.67 Gy, lesion doses from 1.1 Gy to 778 Gy. In case of repeated treatments on the same site, in 13 cases on a total of 15, an absorbed dose reduction was observed. RM prospective and peritherapeutic dosimetry differed somewhat: absorbed doses measured during therapy ranged from -7% to +40% with respect to provisional absorbed doses.

CONCLUSION: In our experience high activity treatments were well-tolerated. Prospective dosimetry needs further investigation to become sufficiently reliable in order to comply the 2 Gy constraint. Lesions became progressively less iodine-avid in case of repeated treatments, so the "first big-shoot" treatment with the highest safe activity seems to be desirable to obtain the maximum efficacy.