Organ Doses to Adult Patients for Chest CT

Purpose: The goal of this study was to estimate organ doses for chest CT examinations using volume computed tomography dose index (CTDIvol) data as well as accounting for patient weight.

Methods: A CT dosimetry spreadsheet (ImPACT CT patient dosimetry calculator) was used to compute organ doses for a 70 kg patient undergoing chest CT examinations, as well as volume computed tomography dose index (CTDIvol) in a body CT dosimetry phantom at the same CT technique factors. Ratios of organ dose to CTDIvoI (f(organ)) were generated as a function of anatomical location in the chest for the breasts, lungs, stomach, red bone marrow, liver, thyroid, liver, and thymus. Values of f(organ) were obtained for x-ray tube voltages ranging from 80 to 140 kV for 1, 4, 16, and 64 slice CT scanners from two vendors. For constant CT techniques, we computed ratios of dose in water phantoms of differing diameter. By modeling patients of different weights as equivalent water cylinders of different diameters, we generated factors that permit the estimation of the organ doses in patients weighing between 50 and 100 kg who undergo chest CT examinations relative to the corresponding organ doses received by a 70 kg adult.

Results: For a 32 cm long CT scan encompassing the complete lungs, values of f(organ) ranged from 1.7 (thymus) to 0.3 (stomach). Organs that are directly in the x-ray beam, and are completely irradiated, generally had f(organ), values well above 1 (i.e., breast, lung, heart, and thymus). Organs that are not completely irradiated in a total chest CT scan generally had f(organ) values that are less than 1 (e.g., red bone marrow, liver, and stomach). Increasing the x-ray tube voltage from 80 to 140 kV resulted in modest increases in f(organ) for the heart (9%) and thymus (8%), but resulted in larger increases for the breast (19%) and red bone marrow (21%). Adult patient chests have been modeled by water cylinders with diameters between approximately 20 cm for a 50 kg patient and approximately 28 cm for a 100 kg patient. At constant x-ray techniques, a 50 kg patient is expected to have doses that are approximately 18% higher than those in a 70 kg adult, whereas a 100 kg patient will have doses that are apparoximately 18% lower.

Conclusions: We describe a practical method to use CTDI data provided by commercial CT scanners to obtain patient and examination specific estimates of organ dose for chest CT examinations.