Dosimetry in MRT: Our recent experience.

Introduction: Personalized dosimetry is tending to become the "gold standard" in Molecular Radiotherapy (MRT). Setting up carefully all the procedures involved in the workflow is crucial for the final clinical result.

Aim: Individualized MRT dosimetry using a recently installed commercial system, comprising of a dual SPECT/CT camera, a treatment planning software (TPS) and a dose calibrator was implemented on patients undergoing ¹⁷⁷Lu-DOTATATE and ¹⁷⁷Lu-PSMA therapies. The clinical workflow implemented in our department is presented in detail. Measurement and calculation of the Calibration Factor (CF) to translate the count rate into activity concentration (quantitative data), and system's commissioning, was discussed.

Materials and methods: Calibration of the dose calibrator, the SPECT/CT system and the TPS, measured using the clinical acquisition protocol, were analyzed along with potential errors introduced by the procedure and means of future optimization. In addition, image acquisition parameters, image reconstruction and image registration were discussed. Anatomical contouring of the organs at risk (OARs) and functional contouring of the lesions, followed by the dose calculation of the aforementioned structures, with the use of different calculation algorithms, were presented, compared and evaluated.

Results and discussion: According to our experience, different fitting of each organ's activity curve, results in differences in the final calculated dose. Use of bi-exponential fitting seems to better approach physical and metabolic decay. Calculated absorbed doses for the OARs were found similar to those expected from literature. Finally, department's future work was discussed, including reproducible patient setup for image acquisition, high dose CT for finer contouring and comparison of the calculated doses with other TPSs.