When using inhomogeneity corrections on treatment plans, explain how the dose is effected by the presence of bone and air. Which way do the isodose lines shift? Why? How accurately do the algorithms display these isodose lines?
When comparing the Isodose distribution of heterogeneous corrections to a plan to a homogenous plan the dose distribution can vary greatly and cause and inaccurate patient dose calculation. Homogeneous plans assume that the entire body is equal to the density of water, which we know is untrue. When a radiation beam enters the body, it interacts with various densities causing a difference in penetration and scatter of the beam.1 Water or tissue has a density of approximately 1.0g/cm3 whereas bone is about 1.8g/cm3 and air is about 0.25g/cm3. Radiation reacts differently with each of these densities due to their differences in electron densities.1 When radiation interacts with bone it will be attenuated more than when compared to tissues of the body. Whereas when the radiation beam interacts with air, such as the lungs, it is able to penetrate deeper in the body because there is nothing there to slow it down.
Treatment planning systems are able to use the electron densities of the various densities in the body and obtain their attenuation coefficient that are measured by CT at therapeutic energies.2 Using this information the TPS is able to create accurate isodose lines as they pass through various densities within the body.
When comparing the Isodose distribution of heterogeneous corrections to a plan to a homogenous plan the dose distribution can vary greatly and cause and inaccurate patient dose calculation. Homogeneous plans assume that the entire body is equal to the density of water, which we know is untrue. When a radiation beam enters the body, it interacts with various densities causing a difference in penetration and scatter of the beam.1 Water or tissue has a density of approximately 1.0g/cm3 whereas bone is about 1.8g/cm3 and air is about 0.25g/cm3. Radiation reacts differently with each of these densities due to their differences in electron densities.1 When radiation interacts with bone it will be attenuated more than when compared to tissues of the body. Whereas when the radiation beam interacts with air, such as the lungs, it is able to penetrate deeper in the body because there is nothing there to slow it down.
Treatment planning systems are able to use the electron densities of the various densities in the body and obtain their attenuation coefficient that are measured by CT at therapeutic energies.2 Using this information the TPS is able to create accurate isodose lines as they pass through various densities within the body.
- Bentel G. Dose determination for external beams: Radiation Therapy Planning. 2nd ed. New York, NY: McGraw-Hill; 1996.
- Khan FM, Gibbons JP. Khan’s The Physics of Radiation Therapy. 5th Philadelphia, PA: Lippincott Williams & Wilkins; 2014.