7. Dose Measurements
The strength of a radioactive source (in Bq) and the energy of the emission (in eV) can be measured. This is, however, not a dose measurement.
The radiation dose is the energy deposited in the irradiated compound. If the radiation hits a human being, the dose is defined as the energy deposited in the human body. The amount of energy deposited is almost always different from the amount of energy coming from the source. Deposited energy determines dose.
Counters observe particles or photons sequentially. In dose measurements, the concern is not with the individual particles or photons but with the total energy absorbed in the exposed materials (e.g., tissue). It is difficult to observe energy absorption in tissue. Two of the problems are:
An exposure to one roentgen (1.0 R) of x- or y-radiation results in a radiation dose to soft tissue of approximately 9.3 mGy. The precision can be no better since the roentgen unit is based on the radiation absorption in air, whereas doses to a biological system (soft tissue or bone) are based on the energy absorbed in that system.
The absorption increases with the electron density of the exposed material and is therefore larger in bone compared with soft tissue. Furthermore, the energy absorption increases with decreasing radiation energy. Since these properties are not the same for air, soft tissue and bone, the doses delivered by a 1 R exposure are different.
When the radiation strikes a body, the dose changes with depth (i.e., the distance the radiation traverses in the body). This is illustrated in Figure 4.2 for different types of x-rays as well as a beam of charged particles (C-12 ions).
Depth dose curves for soft tissue. The dose is measured from the surface of the skin. On the left are data for x- and γ-rays as indicated. To the right is shown the dose curve for high energy charged particles. In this example carbon atoms, with all 6 orbital electrons stripped away, were used. The energy of the carbon ions when they hit the soft tissue is 5,688 MeV.
In order to use radiation for cancer treatment it is important to have knowledge of these depth dose curves. As you can see from Figure 4.2, the region for maximum dose can be changed by changing the x-ray energy. For tumors positioned deeper than 6 cm into the tissue, x-rays with energy of more than 20 MeV should be used. The goal is to give a high killing dose to the tumor while minimizing the dose to the surrounding healthy tissue.
As you can see, the charged particle radiation has a striking depth dose relationship (Figure 4.2). The dose peaks at the end of the track (called the Bragg peak). The Bragg peak occurs at a depth which depends on the energy of the particles. This type of radiation has been used for cancer treatment. It requires large accelerators and it is, therefore, quite expensive. The depth dose curve shown in Figure 4.2 was obtained with a research accelerator at the Lawrence Berkeley Laboratory in the 1960s; it is no longer in operation.
The "dose rate" is usually given when describing the intensity of the radiation being absorbed by the target, i.e. the radiation dose per unit time. The total dose is then obtained by a simple multiplication of the dose rate by exposed time.
In order to measure the dose to soft tissue exposed to γ-radiation, the walls of the instrument as well as the compound in the sensitive volume (usually a gas) must have a composition that has absorption similar to soft tissue. The observed ion current is then proportional to the dose rate. The sensitive volume must be small when the dose rate is large and vice versa.
Thermoluminescence dosimeters (TLD) are frequently used for dose measurements. TLDs are well suited for different types of radiation as well as for large and small doses and dose rates. If LiF-crystals are used, x- and γ-rays yield a response which is proportional to the dose to soft tissue. If crystals with calcium are used, the response is proportional to the bone tissue dose.
Since the TLD crystals are small, they are well-suited for measurement of doses to patients. They can be placed on and in the body.