The increasing use of computed tomography (CT) in diagnostic imaging is associated with a relevant increase in patient dose and requires CT dose optimization. Anthropomorphic phantoms and mathematical patient models have been developed to improve the dosimetry in diagnostic imaging. Nevertheless, the doses calculated in these models and the ones individual patients can receive may differ considerably. In particular, the assessment of organ doses is problematic when organs and tissues receive only a partial exposure. A typical example for this situation is the exposure of the liver within a thoracic CT. To evaluate the impact of the field boundary and the liver volume on the individual organ dose, 50 CT scans from 25 male and 25 female patients between the ages of 27 to 87 were analyzed in this study with the volumetric tools of a treatment planning system for radiotherapy. The relative volume of the liver within a thoracic CT was assessed and compared to results from dosimetry methods using standardized patient models. The differences between an individual dose and the results from standardized patients are considerable. The fraction of the liver volume within a thoracic CT with a standard lower boundary extends from 48–92%, resulting in a possible dose difference of up to a factor of 1.7. Results from mathematical phantoms can underestimate the liver dose by more than a factor of 2.6. From the determined data, correction factors for the dosimetry of the liver using standard programs can be derived.