SU-F-T-679: Monte Carlo Simulation On Surface Dose in Preclinical Irradiation Using Monoenergetic Photon Beams.

PURPOSE: This study investigated the surface dose variation in preclinical irradiation using small animal, when monoenergetic photon beams with energy range from 50 keV to 1.25 MeV were used.

METHODS: Inhomogeneous, homogeneous and bone-tissue homogeneous mouse phantom based on the same CT image set were used. The homogeneous and bone-tissue homogeneous phantom were created with the relative electron density of all and only bone voxels of the mouse overridden to one, respectively. Monte Carlo simulation based on the EGSnrc-based code was used to calculate the surface dose, when the phantoms were irradiated by a 360° photon arc with energies ranging from 50 keV to 1.25 MeV. The mean surface doses of the three phantoms were calculated. In addition, the surface doses from partial arcs, 45°-315°, 125°-225°, 45°-125° and 225°-315° covering the anterior, posterior, right lateral and left lateral region of the mouse were determined using different photon beam energies.

RESULTS: When the prescribed dose at the isocenter of the mouse was 2 Gy, the maximum mean surface doses, found at the 50-keV photon beams, were 0.358 Gy, 0.363 Gy and 0.350 Gy for the inhomogeneous, homogeneous and bone-tissue homogeneous mouse phantom, respectively. The mean surface dose of the mouse was found decreasing with an increase of the photon beam energy. For surface dose in different orientations, the lateral regions of the mouse were receiving lower dose than the anterior and posterior regions. This may be due to the increase of beam attenuation along the horizontal (left-right) axis than the vertical (anterior-posterior) in the mouse.

CONCLUSION: It is concluded that consideration of phantom inhomogeneity in the dose calculation resulted in a lower mean surface dose of the mouse. The mean surface dose also decreased with an increase of photon beam energy in the kilovoltage range.