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SYMPOSIUM PAPER For reprint orders, please contact: [email protected]
Selective internal radiation therapy dosimetry
Andrew Kennedy*
Dosimetry is the calculation of radiation energy absorbed by tissue (measured in Grays [Gy]) as a result of exposure to indirect and direct ionizing radiation. The strength of a radioactive source is called its activity, which is defined as the rate at which the isotope decays over time. The concentration of radioactivity, or the relationship between the mass of the radioactive material and the activity, is called the specific activity. Specific activity is expressed as the number of curies or becquerels (Bq) per unit mass or volume. The higher the specific activity of a material, the smaller the physical size that the source is likely to be. For selective internal radiation therapy (SIRT), a predefined activity (measured in GBq) of yttrium-90 (Y-90) is prescribed and delivered to a prespecified volume of liver (either whole, lobe or segment). However, there is no direct way to measure the absorbed energy (or dose) delivered by the Y-90 microspheres to the tumor and non-tumor tissue during the SIRT procedure, except using techniques such as Y-90 PET–computed tomography. Consequently, a direct correlation between the absorbed dose and outcomes (i.e., radiological response and toxicity) cannot be made at most centers. For safety purposes, the biological effect of the radiation (expressed as a summation of the effects of the radiation on the tissue parts) is calculated in Sieverts (Sv). Currently, most centers calculate the percentage activity of Y-90 that will be absorbed into the lung, tumor(s) and normal liver parenchyma based on the intensity of the technetium-99m-labeled macroaggregated albumin cloud on SPECT during the pretreatment simulation. From this image, it is possible to estimate the isodoses within and around the tumor (Figure 1) [1] .
KEYWORDS
• dosimetry • radioembolization-
induced liver disease • selective internal radiation therapy
Radioembolization-induced liver disease The goal of SIRT is to achieve uniform coverage by Y-90 in the tumor tissue while minimizing its effects on nontarget tissue. Seminal work in hepatic radiotherapy by Lawrence and colleagues has shown that complication rates do not occur unless the threshold of liver damage exceeds the functional reserve [2] . This concept (also known as the parallel architect model) means that small portions of the liver can tolerate irradiation well above 35 Gy without significant complications, as long as sufficient normal liver is spared from high-dose radiation exposure. While the clinical features of radiation-induced liver disease (RILD) caused by external beam radiotherapy are well recognized, radioembolization-induced liver disease (REILD) has clinical features much more akin to the effects of veno-occlusive disease caused by chemoradiation (Table 1) [3,4] . Risk factors associated with REILD, as identified by Sangro and colleagues of the Pamplona group, include activity >0.8 GBq/l delivered to the target liver volume, prior chemotherapy, cirrhosis, small liver volume (1.2 mg/dl [3,5] . The Pamplona group also found that the severity and frequency of REILD can be significantly reduced (from 13.3% to 2.2%; p 3 mg/dl) Present Present 4–8 weeks Normal Possible Present
SYMPOSIUM PAPER For reprint orders, please contact: [email protected]
Selective internal radiation therapy dosimetry
Andrew Kennedy*
Dosimetry is the calculation of radiation energy absorbed by tissue (measured in Grays [Gy]) as a result of exposure to indirect and direct ionizing radiation. The strength of a radioactive source is called its activity, which is defined as the rate at which the isotope decays over time. The concentration of radioactivity, or the relationship between the mass of the radioactive material and the activity, is called the specific activity. Specific activity is expressed as the number of curies or becquerels (Bq) per unit mass or volume. The higher the specific activity of a material, the smaller the physical size that the source is likely to be. For selective internal radiation therapy (SIRT), a predefined activity (measured in GBq) of yttrium-90 (Y-90) is prescribed and delivered to a prespecified volume of liver (either whole, lobe or segment). However, there is no direct way to measure the absorbed energy (or dose) delivered by the Y-90 microspheres to the tumor and non-tumor tissue during the SIRT procedure, except using techniques such as Y-90 PET–computed tomography. Consequently, a direct correlation between the absorbed dose and outcomes (i.e., radiological response and toxicity) cannot be made at most centers. For safety purposes, the biological effect of the radiation (expressed as a summation of the effects of the radiation on the tissue parts) is calculated in Sieverts (Sv). Currently, most centers calculate the percentage activity of Y-90 that will be absorbed into the lung, tumor(s) and normal liver parenchyma based on the intensity of the technetium-99m-labeled macroaggregated albumin cloud on SPECT during the pretreatment simulation. From this image, it is possible to estimate the isodoses within and around the tumor (Figure 1) [1] .
KEYWORDS
• dosimetry • radioembolization-
induced liver disease • selective internal radiation therapy
Radioembolization-induced liver disease The goal of SIRT is to achieve uniform coverage by Y-90 in the tumor tissue while minimizing its effects on nontarget tissue. Seminal work in hepatic radiotherapy by Lawrence and colleagues has shown that complication rates do not occur unless the threshold of liver damage exceeds the functional reserve [2] . This concept (also known as the parallel architect model) means that small portions of the liver can tolerate irradiation well above 35 Gy without significant complications, as long as sufficient normal liver is spared from high-dose radiation exposure. While the clinical features of radiation-induced liver disease (RILD) caused by external beam radiotherapy are well recognized, radioembolization-induced liver disease (REILD) has clinical features much more akin to the effects of veno-occlusive disease caused by chemoradiation (Table 1) [3,4] . Risk factors associated with REILD, as identified by Sangro and colleagues of the Pamplona group, include activity >0.8 GBq/l delivered to the target liver volume, prior chemotherapy, cirrhosis, small liver volume (1.2 mg/dl [3,5] . The Pamplona group also found that the severity and frequency of REILD can be significantly reduced (from 13.3% to 2.2%; p 3 mg/dl) Present Present 4–8 weeks Normal Possible Present
