**8.3 Patient radiation protection**

Before finishing let´s have few words about the protection of patients. In patient protection we have to had in mind not only the physical half life because sometimes biological half life is longer and radiation protection must to be concern to the excretion system of patient so Nuclear Medicine define an effective half life to take into account the two contributions to patient doses coming from physical and biological behavior of radioisotope. Biological half live is the time living organism needs to excrete any radioisotope trace of radioactive material. It depends no only of physical time but on effectiveness of excretion system of the body. So effective life is :

$$\frac{1}{T\_{1/2}^{eff}} = \frac{1}{T\_{1/2}^{phy}} + \frac{1}{T\_{1/2}^{bio}}$$


Table 4. Physical and Biological half lives

Radioisotopes like P and Sr like to stay in bone, so as far as they can be used to treat bone lesions the maximize its doses due to the combination of physical and biological exposure. Phosphorous decay faster so Sr is better to treat bone metastasis due to it relatively long physical and biological half life. For diagnostic studies Tc has proven to be the best due to its short physical and biological lives, so it is excrete from the body just after images has be taken.
