Caren van Roekel

272 Chapter 10 In this formula Dose (in Gy) is defined as the planned dose in healthy liver tissue and mass (in kg) is the delineated volume (in ml) multiplied by the conversion factor for soft tissue (1.06 g/cm 3 ). The 90 Y conversion factor of 49.67 is based on previous work by Ho et al., stating that 1 µCurie in 1 gram soft tissue results in an absorbed dose of 183.78 cGy. In other words, 1 GBq of 90 Y in 1 kg soft tissue results in 49.67 Gy absorbed dose(81). For 166 Ho, the conversion factor in the formula for injected activity is 15.87 (82). The lung shunt fraction (LSF) is defined as (78, 83): = √ ∗ √ ∗ ) + √ ∗ = 0.2024 ∗ ℎ ℎ ( ) 0,725 ∗ ℎ ( ) 0,425 ( ) = ( − 0,2) + ( + ) ( ) = ( ( ) ∗ ( )) 50 ( ) = ( ) ( ) ( ) ( ) ⁄ ( ) = ( ) ∗ ([ ∗ ( )] + ( )) 49,670 ∗ (1 − ℎ ) ( ) = ℎ ( ) ∗ 3780 ( ) The main benefit of the partition method over single compartment models is the separation between tumor and non-tumor tissue in the calculation of their respective absorbed dose. The previously described formula allows the physician to describe a certain absorbed dose to the healthy tissue (Dose (Gy)). Subsequently, as an indication of expected mean tumor-absorbed dose, the physician can multiply the described healthy tissue dose with the T/N ratio. Thus, based on the maximum acceptable healthy liver tissue dose, a physician can assess whether or not an efficacy threshold (minimal tumor-absorbed dose) will be reached. Limitations of the partition model is its assumption of a homogenous distribution of microspheres within the compartments, it is more time- consuming than single compartment models and tumor and healthy liver tissue delineation can be quite challenging on anatomical imaging, especially in patients with ill-defined tumors (54, 79). The partition method does correct for the difference in tumor and non-tumor absorbed dose. Most methods use a (variant of a) ‘one size fits all’ approach. Unfortunately, this has led to failure of studies, for example the large SIRFLOX, FOXFIRE and FOXFIRE-Global studies. In these studies, the BSA method was used for activity calculation, but as stated before, this method does not differentiate between tumor and the healthy liver tissue activity distribution and often leads to under- or overdosing (84). As the treatment activity was reduced in case of lung shunting or on the basis of the tumor burden, under-dosing is likely what