Individualized dosimetry in the management of metastatic differentiated thyroid cancer

Aim. This paper analyzes the available data on the dosimetric approach and describes the use of dosimetry in the Division of Nuclear Medicine of the National Cancer Institute in Milan. Dosimetry is rarely performed when planning radio-iodine activity, although most of the available guidelines do mention this possibility, without giving any well defined indication. Aim of the present research was to validate the usefulness of dosimetry in the management of metastatic thyroid cancer. Benua (1962) set the limit of blood absorbed dose at 2 Gy to avoid hematological toxicity. Maxon (1983) determined at 80 Gy the dose to achieve complete destruction of a metastatic lesion. Dorn (2003) combined red marrow and lesion dosimetry showing that high activity administrations with less that 3 Gy to the red marrow are a safe and more effective with respect to fixed activities administrations. Lee (2008) reported 50% responses with high activity administrations based on blood dosimetry, in 47 patients which were unsuccessfully previously treated with fixed activities. Sgouros (2005) and Song (2006) introduced key parameters as Biological Effective Dose and Uniform Equivalent Dose in order to describe the effects of continuos low dose rate irradiation and non uniform activity uptake, typical of nuclear medicine treatments. Methods. Red marrow and lesion dosimetry (planar view) were performed during the treatment, without changing the fixed activity schema. Results. This experience demonstrate first of all, that dosimetry is feasible in the clinical routine, and that it can provide the clinician with important information, no matter its often quoted limited numerical accuracy. A total of 17/20 lesion doses below 80 Gy have been detected. Three/17 (doses between 40 and 80 Gy) disappeared in the follow-up scintigram. Two/17 were undetectable at computed tomography or nuclear magnetic resonance. These data suggest that repetition of treatment on a lesion drastically reduces its uptake, with a loss of therapeutic efficacy along the sequence of fixed activity administrations. Conclusion. The usefulness of dosimetry should not be assessed only on the basis of patient survival or therapeutic efficacy; the possibility to avoid useless treatments should also be considered. According to the authors, individualized dosimetry could improve the management of metastatic differentiated thyroid cancer. Even post-therapeutic dosimetry, as performed at this institution, has a significant impact on clinical decision-making. The question for the future is how to include dosimetry into the patient management framework.