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J Nucl Med. 2008; 49 (Supplement 1):48P
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Radiopharmaceutical Chemistry: Dosimetry/Radiobiology

Clinical Dosimetry

Building up the electron dose to target cells in a complex environment

Paolo Zanotti-Fregonara1, Christophe Champion2, Jean-Luc Moretti3 and Elif Hindié3

1 DSV/I2BM/SHFJ, CEA, Orsay, France; 2 Laboratoire de Physique, Institut de Physique, Metz, France; 3 Nuclear Medicine, St Louis Hospital, Paris, France

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Objectives: We used our Monte Carlo code "CELLDOSE" to assess the dose to target cells from electron emissions in a complex environment. 131I in the thyroid was used as a model.

Methods: Thyroid follicles were represented by spherical units of 170µm diameter composed of a lumen of 150µm diameter, and a peripheral layer of thyroid cells of 10µm thickness. The distance separating two neighbouring follicles is 4µm. 131I is homogeneously distributed in the lumen and absent from cells. Electron dose distribution in an isolated follicle was first assessed. Then, the simulation continued while progressively adding the contribution of neighbouring follicles.

Results: Electron dose gradient around a point source showed that 131I electrons can contribute to energy deposit up to 2090 µm. We thus studied all contributions to the central follicle up to a distance encompassing 12 orders of neighbourhood (15624 follicles). Even when all neighbours were taken into account, the dose in the central follicle was heterogeneous. For an average dose to tissue of 1Gy, the dose in the colloid was 1.168 Gy, the dose received by thyroid cells was 0.982 Gy, and the dose to the interfollicular tissue 0.895 Gy. 17.3% of the dose of thyroid cells came from the colloidal matter of their own follicle, while the remaining 82.7% came from surrounding follicles. It is shown that when different follicles contain different concentrations of 131I, the impact in terms of cell dose heterogeneity can be important.

Conclusions: Using 131I in the thyroid as a model, we showed how to map electron dose deposit and build-up the dose to target cells in a complex multi-source environment. This approach can be of considerable interest for comparing different radiopharmaceuticals as therapy agents.





This Article
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Right arrow Articles by Zanotti-Fregonara, P.
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Right arrow Articles by Zanotti-Fregonara, P.
Right arrow Articles by Hindié, E.