3-D dose distribution for organ dose measurement in CT thoracic exams using Gafchromic™ XR-QA2 films

The patient dose in radiodiagnostic is currently monitored through an ionizing radiation exposure index named Volume Computed Tomography Dose Index (CTDI_vol) and the Dose-Length Product (DLP), displayed by CT scanner and measurable through a 100 mm long pencil ionization chamber, inserted in a homogeneous cylindrical PMMA phantom. The phantom is 14 cm long and has a diameter of 16 (32) cm that represents the adult head (body). One of the main flaws of such a method is that it does not return any information on the dose distribution to organs, and the average absorbed dose value, due to the material and dimensions of both the phantom and the ionization chamber, might be either underestimated or overestimated depending upon the patient size. With the aim to obtain complementary information, this work presents a method to estimate organ dose in the thoracic area of an anthropomorphic woman phantom in CT, by employing Gafchromic^TM XR-QA2 and computational 3-D reconstruction methods. Lungs, heart, and spinal cord have been chosen as a dosimetry case study. XR-QA2 films have been placed between the phantom slabs in the thoracic area and scanned with a multi-slice CT scanner. 2-D and 3-D absorbed dose distributions for each organ have been analyzed, by means of a custom code in Matlab®. Critical aspects in the dose distributions have been found for the spinal-cord case (serial organ) where the dose distribution is non normal and the maximum dose value is ∼ 30 mGy while a maximum dose value between 26 and 28 mGy has been measured elsewhere.