Author: Thomas Gee, Sofia Michopoulou, Amit Nautiyal 👨🔬
Affiliation: University Hospital Southampton 🌍
Purpose: Dosimetry software that is accessible to departments offers new opportunities to improve patient-specific dosimetry. Prior to clinical decision-making, it is essential to validate dosimetry software used in patient-specific absorbed dose estimation. This study aimed to calculate time-integrated activity (TIA) and absorbed dose using NEMA IQ phantom and Monte Carlo simulation as a gold standard to compare the recently installed MIMSurePlanMRT dosimetry software.
Methods: We filled the three different-sized spheres (11, 26 and 113ml) mimicking tumour lesions in NEMA IQ Phantom with a known amount of 177Lu concentration (1.99 MBq/ml) and acquired SPECT/CT images at multi-time points (2,24 and 120h) using 177Lu PSMA patient imaging protocol. Data was reconstructed using xSPECT and CT-based segmentation was performed using a 3D analysis tool on Symbia software for image quantification. The time-integrated activity was calculated using mono-exponential fitting on MATLAB. The voxel S-values (VSV) for 177Lu were calculated using Monte Carlo simulations on Geant4. TIA in a source voxel was convoluted with VSV for a target voxel to determine the absorbed dose in each sphere mimicking tumour lesions.
The TIA (MBq-h) and absorbed dose (mGy/MBq) results obtained with the above Monte Carlo method were considered the gold standard, and they were compared with the results obtained from MIMSurePlanMRT software using the VSV method.
Results: The time-integrated activity estimated using MIMSurePlanMRT for all tumour lesions is within 5% of the standard method. The absorbed dose results produced by MIMSurePlanMRT were within 10% of the standard method.
Conclusion: The MIM SurePlan MRT agreed well with the golden standard method for patient treatment with 177Lu labelled radiopharmaceuticals, providing a practical approach to dosimetry software validation. However, tissue density might influence the dose estimation in patients for organs and tumour lesions using the VSV; this could be assessed in further validation studies with bone and lung inserts.