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 a beam enters the body it may go through many different tissues, such as bone, lung, muscle, and fat, that will affect both the depth of penetration and the scatter of the beam.[1] This is dependent on the volume and density of the inhomogeneity and the beam energy. Lung, which is greatly filled with air, is less dense compared to bone. The density of lung falls somewhere between 0.25 to 1.0 g/cm3, depending on how much air is in the lung, and the density of bone is typically 1.8 g/cm3, but can vary depending on whether it is compact or spongy bone. Since lung has a relatively low density that is similar to air it will attenuate the beam less which causes the isodose line to shift forward or away from the patient’s skin. Bone has higher density which attenuates the beam more and causes the isodose line to move back or towards the patient’s skin. When calculating to a point just beyond an air to tissue interface for higher energy beams, the primary dose will be higher because of the area of low density the beam passed through prior to this point, but there may be some loss of electronic equilibrium, similar to what happens in the buildup region when the beam first enters the patient. For bone to tissue interfaces for higher energy beams, the dose is increased at the interface due to increased electron fluence in the bone caused by pair production interactions.[2]
References
References
- Bentel G. Dose determination for external beams: Radiation Therapy Planning. 2nd ed. New York, NY: McGraw-Hill; 1996.
- Khan FM. The Physics of Radiation Therapy.5th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2014