28.02.2013 Views

autologous blood and marrow transplantation - Blog Science ...

autologous blood and marrow transplantation - Blog Science ...

autologous blood and marrow transplantation - Blog Science ...

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

Fisher 683<br />

bution to <strong>marrow</strong> dose is mostly from activity in circulating <strong>blood</strong>. The dose rate<br />

to <strong>marrow</strong> is greatest at early times postinjection, <strong>and</strong> the dose rate decreases as<br />

activity clears from the <strong>blood</strong>. Specific uptake <strong>and</strong> retention of activity in <strong>marrow</strong><br />

tissue may increase <strong>and</strong> prolong this irradiation. The amount of activity on bone<br />

surfaces <strong>and</strong> in other nearby organs, as well as the total-body activity over time,<br />

must also be determined so that the total contribution to dose from penetrating<br />

gamma radiation may be assessed. The dose assessment method usually depends<br />

on the amount <strong>and</strong> type of information that can be obtained to describe the time-<br />

activity curves for <strong>marrow</strong> <strong>and</strong> other dose-contributing organs.<br />

METHODS<br />

Three common methods are used to calculate <strong>marrow</strong> dose. The method of<br />

choice will depend on the purpose of the clinical study, the ability to obtain reliable<br />

measurement data, the relative need for data from highly invasive measurements,<br />

<strong>and</strong> the level of activity administered to patients.<br />

Dosimetry parameters<br />

The radiation dose to internal organs is a function of<br />

• the total activity administered to a patient,<br />

• the fraction of the administered activity taken up by each major source organ or<br />

tissue,<br />

• the retention of activity in organs over time, <strong>and</strong> the total number of radioactive<br />

decays,<br />

• the energies, emissions, <strong>and</strong> physical half-life of the radionuclide (<strong>and</strong> decay<br />

products, if any),<br />

• the spatial distribution of activity within tissues,<br />

• the size <strong>and</strong> weight of the patient,<br />

• the geometry <strong>and</strong> density of tissues absorbing radiation, <strong>and</strong><br />

• the cross-organ irradiation component for penetrating gamma radiation.<br />

Each of these elements enters into a dose calculation. Simplified methods <strong>and</strong><br />

computer software have been developed to facilitate the calculation.<br />

MIRD schema<br />

A formal system for internal dosimetry was developed by the Medical Internal<br />

Radiation Dose (MIRD) Committee of the Society of Nuclear Medicine. 4-5<br />

According to this schema, the absorbed dose to an organ or tissue such as red<br />

<strong>marrow</strong> is the sum of contributions from several different components. Radiations<br />

from internally deposited activity were classified as either penetrating (gamma

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!