WE‐A‐BRB‐06: 3D In‐Patient Dose Reconstruction from the PET‐CT Imaging of Y‐90 Microspheres for Metastatic Cancer to the Liver

Purpose: The development of radioembolization with SIR‐Spheres represents a significant advance in the treatment of patients with metastatic disease to the liver. This technique has evolved to use a formula for dose calculation that relies on body surface area as the main determinant of dose. However, what is prescribed is not dose, but rather activity. It has been traditionally thought that the tracer 90Y is a pure electron emitter. However, the decay of 90Y has also a minor ß+ branch to the first excited state of 90Zr, with 34 ppm branching ratio. While the positron emission is rare, it can be observed in a PET scan. Therefore the main objective of this work is to develop a new method for 3D dose reconstruction based on the PET imaging of a patient injected with SIR‐Spheres. Methods: Using Fluka Monte Carlo code, the voxel dose kernel (VDK) was calculated for 90Y source of size equal to that of the PET scanner voxel size. Subsequently, the convolution of the VDK with measured PET data was performed to recover the absorbed dose. The absolute dose calibration was done by taking the ratio of the measured positron activity to the known activity due to electrons Results: The developed model was used to reconstruct the dose distribution for two patients treated with 90Y microspheres. The 3D dose calculation was subsequently superimposed on the patient's CT information to provide a complete description of the absorbed dose to the liver, tumor and adjacent structures. Conclusions: The proposed method offers significant improvement in characterization of the dose deposited by SIR‐Spheres with the hope that this may help answer some of the clinical questions concerning the influence of the dose distribution on the response rate, progression‐free or overall survival.