Caren van Roekel

32 Chapter 2 Thresholds for an acceptable average normal liver absorbed dose have yet to be established. 4.5. Pretreatment dosimetry Different methods for pretreatment activity calculations for the different types of microspheres have been proposed. The first method that was used was the so-called ‘empirical’ method, based on the tumor burden. The used activity ranged between 2 and 3 GBq. This method was abandoned because it was associated with unacceptable clinical toxicity (58). For resin microspheres, the Body Surface Area (BSA) method is mostly used. This method adjusts the prescribed activity for the patient’s BSA and the fractional tumor burden. The formula for BSA in square meter is: = √ ∗ √ ∗ ) + √ ∗ = 0.2024 ∗ ℎ ℎ ( ) 0,725 ∗ ℎ ( ) 0,425 ( ) = ( − 0,2) + ( + ) ( ) = ( ( ) ∗ ( )) 50 ( ) = ( ) ( ) ( ) ( ) ⁄ ( ) = ( ) ∗ ([ ∗ ( )] + ( )) 49,670 ∗ (1 − ℎ ) ( ) = ℎ ( ) ∗ 3780 ( ) The formula for the injected activity (IA) is: = √ ∗ √ ∗ ) + √ ∗ = 0.2024 ∗ ℎ ℎ ( ) 0,725 ∗ ℎ ( ) 0,425 ( ) = ( − 0,2) + ( + ) ( ) = ( ( ) ∗ ( )) 50 ( ) = ( ) ( ) ( ) ( ) ⁄ ( ) = ( ) ∗ ([ ∗ ( )] + ( )) 49,670 ∗ (1 − ℎ ) ( ) = ℎ ( ) ∗ 3780 ( ) (1, 2). The prescribed activity is reduced in case of lung shunting, depending on the percentage of lung shunting: 20% activity reduction with 10-15% LSF, 40% activity reduction with 15-20% LSF and no treatment with >20% LSF (52). The BSA method assumes a correlation between BSA and liver weight. However, this is not necessarily true and may result in an undertreatment of small patients with large livers, and an overtreatment of large patients with small livers. Another limitation of this method is that it does not take into account the degree of tumor uptake (1, 2). For glass microspheres, Salem et al. have developed the Medical Internal Radiation Dose (MIRD) method (59-61). With this method, the prescribed activity is determined by calculating the activity required to achieve a desired absorbed dose (between 80-150 Gy). The MIRD method assumes a homogenous intrahepatic microsphere distribution throughout the treated portion of the liver, and an assumed yield of 50 Gy per GBq 90 Y per kilogram of liver tissue. This method also does not take into account the degree of tumor uptake, nor does it account for fractional or total tumor burden. The formula for the IA is: