Dosimetric correction factors for a lung aperture used in mouse irradiation experiments
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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 irradiator is a Philips RT250 orthovoltage unit. A 12.5 cm diameter cone and 250 kVp x-rays were used for irradiation. The actual mouse irradiation setup also involves a 1.0 cm air-gap, off-axis prescription point, and plastic bag. These dosimetric effects were accounted for as correction factors in the formalism for irradiation time calculation. The procedure for evaluating dose rate consists of: dose calibration for the open cone as per standard protocol (Shultz et al., 1983), (Mah et al., 2001) to determine the dose rate in air (D ̇ air ), output factor measurement of the shielded field ( OR A/C ), Inverse Square Law [SSD/(SSD+Gap)] 2 to account for the air-gap of 1 cm, Percent Depth Dose ( PDD ) comparison of the concentric shield versus the open cone, plastic bag attenuation effects ( TF ), off-axis effects ( OAR ) due to the fact that the mouse lungs are at a position off of the central axis of the x-ray beam, geometric effects ( Geometric_Factor ), which accounts for differences in scattering conditions between the solid water and mouse phantom and a lung tissue correction factor ( TC ) accounting for differences in PDD due to lung tissue replacing water. Dose enhancement at the surface caused by photoelectrons and Compton electrons from the shielding material was also studied. At depth of 0.9 cm there was a cumulative effect of (31 ± 4) % drop in dose for the experimental setup as compared to the standard 12.5 cm diameter cone. TF is ( 0.99 ± 0.01), [SSD/(SSD+Gap)] 2 factor is (0.93 ± 0.01) , OR A/C is (0.88 ± 0.01) . The OAR was found to be (0.94 ± 0.03) and Geometric_Factor is (0.93 ± 0.02) . Also, there was a drop in PDD due to the reduced field size of (4.5 ± 2.0)% at the mid-plane of the lung (0.9 cm) as compared to the 12.5 cm diameter cone, or a PDD factor of (0.89 ± 0.01) . The heterogeneity correction factor, TC = (1.02 ± 0.06) . An increased surface dose from electrons scattered off the shield was noticed from the surface dose profile. This extra surface dose was reduced by the addition of an air-gap and plastic bag, thereby providing skin sparing for small animal irradiation. More accurate dose delivery will be achieved when the correction factors using the formalism developed in this work are applied. By adding a thin attenuating material and air-gap, surface dose due to Compton electrons and photoelectrons can be reduced.