Verification of the dose delivered to a patient presents a difficult challenge. As mentioned previously, predicted dose can be calculated with mathematical models that take into account a number of patient and radiation-beam variables. Techniques exist for measuring radiation dose actually delivered to a point on the body surface or in the body cavity, but are impractical for daily use. These include diodes and thermoluminescence dosimeters (TLDs) (33). TLDs are crystalline solids that trap electrons in an excited state after exposure to ionizing radiation. Heating of TLDs following exposure to radiation, results in a luminescence that can be translated to a radiation dose.
Small implantable dosimetry devices are in development and undergoing early clinical trials (34). These devices measure the radiation delivered to a substrate, and relay this information to an electromagnetic sensor placed outside the patient. Much like fiducial markers, they must be surgically implanted prior to treatment, and may migrate during the course of therapy, confounding measurements if they are placed near an area of rapid fall-off in dose. Nonetheless, this technology may help elucidate the relationship between predicted and actual dose delivered for a given patient through a course of therapy, although it will be limited to a few dose points.
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