Quality Assurance of Small Animal Irradiation: Validation of a 3D-Printed Phantom for “Quasi In-Vivo” Dosimetry

Abstract

New therapeutic options in radiation oncology are often explored in preclinical in vivo studies using small animals. In this study we report on the use of a 3D-printed rat phantom for radiotherapy dosimetric “quasi-in vivo”quality assurance. In the context of a research project, we aimed to irradiate with a dose of 2Gy per fraction the half-brain of a rat model as homogeneously as possible and with maximal sparing of the contralateral half-brain. The dose was delivered by a 4 MV X-ray beam using a direct anterior-posterior, hemi-field beam, with a size of 3cm x 4cm. A 2cm thick 3D-printed bolus made of poly lactic acid (PLA) was used to avoid the build-up region of the beam in the target volume. Target alignment within the planned position was performed using online kilo-voltage on-board imaging. To verify proper dose delivery, considering the small size of the animal and the region to treat, a 3D 1:1 scale printed copy of the rat body was produced from the CT dataset of the rat to be used as a phantom for dose measurements. A plane section of the phantom perpendicular to the beam axis was created to insert thermoluminescent dosimeters (TLDs) and accommodate a radiochromic film. TLD measurements were made in field and out of field in the brain region, and just beneath the bolus, and were repeated 3 times per position. Kilo-voltage doses as well as linac daily output dose fluctuations were taken into account for dose corrections. The radiochromic film measurements were performed to analyse the dose profiles within the brain, from the cold to the hot region. Radiochromic film dosimetry showed a good concordance between the planned dose distribution and the measured dose, with the steep dose gradient at the edge of the hemi-field properly aligned to spare the contralateral half-brain. Mean TLDs percentage dose differences, (measurements minus planned/ planned), were 1.3% in-field and 0.9% beneath bolus. Out of field, dose measurements gave a mean of 0.05Gy for an expected dose of 0.04Gy. A 3D-printed 1:1 scale rat phantom is an effective and reliable tool for “quasi in-vivo” dosimetric quality assurance in the setting of small animal radiotherapy research.