Dose from a Photon Source Inside the Body

2 Dose from a Photon Source Inside the Body 

Since photons have, essentially, an infinite range, the previous calculation for charged particles does not apply. A source of photons located anywhere in the body will deliver some dose to all the other parts of that body. The calculation of the dose rate proceeds as follows. 

Consider an internal organ containing a uniform concentration of a radioisotope emitting a gamma with energy E at the rate of 5y[y/(s m3)] inside the volume V, called the source volume (Fig. 16.4). The dose rate received by 

pulling out of the integral the quantities that are constant in space, Eq. 16.21 becomes 

One common case utilizing the concept of the geometry factor is the calculation of the dose rate in an organ from a radioisotope deposited in that organ. For example, what is the dose rate to the thyroid from radioactive iodine given to a patient If the organ is further assumed to be spherical, Eq. 16.23 takes the simple form 

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