Purpose: We investigated, for the first time to our best knowledge, the feasibility of image-guided radiation therapy (IGRT) and emission-guided RT (EGRT) by using a small animal radiation irradiator (SARI) platform integrated with quad-modal on-board positron emission tomography (PET), single-photon emission computed tomography (SPECT), spectral CT, and cone-beam computed tomography (CBCT) imaging in a Monte Carlo (MC) model.
Methods: A SARI platform was modeled by the GATE MC code. A partial-ring thallium bromide detector designed for PET imaging was axially coplanar with a cadmium zinc telluride flat detector panel designed for SPECT, spectral-CT, and CBCT imaging. Two exemplary treatment schemes of conventional image-guided RT with three fixed-angle treatment beams and EGRT with and without a 2-mm setup error to the target of a phantom were illustrated within the platform. To guide more accurate RT, the radiotherapy-related parameters of iodine contrast agent fractions and virtual non-contrast electron densities in the Kidney1 inserts, mixed with an iodine contrast agent at electron fractions of 0.02 and 0.03, were decomposed quantitatively in spectral CT using the Bayesian eigentissue decomposition method.
Results: A typical workflow of incorporating IGRT and EGRT into small animal RT was designed. In EGRT with and without the setup error set, GATE simulations resulted in 4339 and 5197 beamlet responses in an exemplary dose slice delivery, respectively. Compared to the conventional RT, EGRT scheme was validated to be more robust to setup errors while providing improved tumor targeting and a lesser dose burden on organs-at-risk. For the insert with an iodine contrast agent fraction of 0.02, the average relative errors in the underlying electron density and the iodine contrast agent fraction estimated were 0.95% and 0.64%, respectively.
Conclusion: The results demonstrated the successful implementation of image-guided RT and emission-guided RT within the SARI platform with quad-modal imaging capability.