An experimental irradiation setup was designed to deliver a conformal field of thoracic irradiation to mice. The objective is to provide accurate dosimetric evaluation for the experimental setup, which involves a pie cage device holding up to 10 mice with concentric Cerrobend® shields to collimate the beam. The setup uses 250 kVp X rays, and it also involves an air gap, off-axis prescription point and plastic bag containing anesthetic isoflurane gas. The dose rate in cGy/min was determined as follows: absolute dose calibration for the open cone, measurements of output factor and percentage depth dose for the narrow ring-shaped lung aperture, measurements of bag attenuation, and evaluation of other factors specific to the treatment geometry. Dose enhancement at the skin surface caused by electron contamination from shielding material was also studied. The results showed an overall 25 ± 4% drop at lung mid-plane relative to the standard irradiation setup with the open cone. The increased surface dose from scattered electrons was reduced by addition of the air gap and plastic bag. In conclusion, more accurate dose delivery is achieved when correction factors specific to the animal irradiation setup are applied. Care should be taken when experiments with shields in direct contact with animal skin are involved.