A Focused Small Animal Irradiation Device Built on a Shepherd Cesium-137 Irradiator

Purpose: Focused radiation delivery to the tumor, part of the tumor, or other volumes of interest is often required in small animal radiation studies to isolate confounding factors from radiation to other areas, and to increase radiation dose to the target by sparing other organs. Studying the synergy between radiotherapy and immunotherapy is one example that requires such a focused radiation beam. However, such focused beam is not usually provided by many animal irradiators. The purpose of this study is to develop a device that delivers such a focused beam in a Shepherd Cesium-137 irradiator.Methods: We fabricated a Cerrobend based circular collimator placed behind a slit collimator to produce a focused radiation beam. The device also includes a platform to aid in animal immobilization. The slit collimator is provided by the vendor. The thickness of Cerrobend was 4 cm to ensure <5% of radiation transmission. Various sizes of circular collimators were designed for different experiment needs. Because the Cs-137 radiation source in the irradiator is a non-uniform line source, the location of the collimator is very important to generate a relatively uniform, symmetric cylindrical beam. Gafchromic EBT2 film was used for 3D dosimetry measurements. TLDs from the Radiological Physics Center were also used to verify the results.Results: The radiation dose for the focused beam was calibrated using the EBT2 film dosimetry and confirmed with the TLDs. Using AAPM task report 45, flatness and symmetry of the vertical and horizontal profiles for the circular beam were calculated. For the vertical profile, Flatness was 7.7%, Symmetry was 7.0%, the penumbra (80, 20) width in the inferior direction was 1.41 cm, and in the superior direction was 1.76 cm. On the other hand, the horizontal profile had Flatness of 5.6%, Symmetry of 6.3%, left penumbra width of 1.41 mm, and right penumbra width of 1.41 mm.An interesting phenomenon we noted was a huge surface peak in the first a few millimeters when we measured the percent depth dose using a stack of the EPT2 gafchromic films. This peak is approximate 2-fold higher than the relatively flat dose distribution in the depths > 2 mm. We speculate that this peak is due to the presence of contaminating secondary electrons. A 2-mm acrylic flat piece was used to block the secondary electrons in the animal experiments for uniform dose delivery to the target.Conclusions:Cs137 irradiators can be used to deliver small field radiation using custom made collimators. Filed doses using these types of add-ons need to be measured and carefully evaluated. An acrylic piece or any other materials with equivalent of 2 mm tissue thickness should be used in front of the target to block potential secondary electron contamination. Gafchromic EBT2 films can provide a reliable way to measure percent depth dose and dose profile properties.

SSG13-06

A Focused Small Animal Irradiation Device Built on a Shepherd Cesium-137 Irradiator