# Patient-Specific Dosimetry Evaluations in Theranostics Software for Internal Radiotherapy
## 2 Materials and Methods  
**OLINDA 2.0**  
OLINDA/EXM version 2.0 software (OLINDA 2.0) implements the RADAR method for internal dose assessment [16]. It is used to estimate mean absorbed doses to organs or lesions, through the use of tabulated S-values and computational phantoms (e.g., adult male and adult female for organs' estimates) or specific models (e.g., sphere model for lesions' estimates), with a low level of personalization. OLINDA 2.0 uses a series of voxel-based, realistic human computational phantoms developed by RADAR committee of SNMMI (Society of Nuclear Medicine and Molecular Imaging), which are based on 2007 ICRP (International Commission on Radiological Protection) recommendations. Compared to the previous versions of OLINDA/EXM, OLINDA 2.0 implements the most recent biokinetic models. The concepts on which OLINDA 2.0 works are identical to the MIRD method for internal dosimetry. However, MIRD and RADAR methods use different terminologies [16]. **Velocity Theranostics**  
Velocity Theranostics is a piece of research software available on the Velocity 4.0 workstation produced by Varian Medical Systems (Palo Alto, USA) capable of estimating and reporting doses in selected structures at organ-level (in terms of mean absorbed dose) or at voxel-level (e.g., dose distributions in organs). The Linear Boltzman equation solver performs a voxel based calculation, and the mean absorbed dose is also reported. Velocity Theranostics provides all the necessary steps to perform internal dosimetry, from clinical images to absorbed dose calculations. The workflow of the Theranostics calculations includes the following steps, which are also shown in Figure 1:  
1. **Volume delineation:** Volume of interest (VOI) delineation of Organ at Risk (OAR) and/or lesions is performed using the Velocity contouring tool. This tool enables the user to choose among different options, such as manual contouring, adaptive semi-automatic and threshold-based. 2. **Image registration:** Sequential SPECT/CT images are automatically registered with a deformable algorithm based on CT data. 3. **Activity quantification:** The counts-to-activity calibration factor (CF) is entered by the user. It can be extrapolated by acquiring a homogeneous phantom filled with a radioactive solution with the SPECT clinical protocol. 4. **CT Calibration**: The accuracy of the calibration of CT numbers in relation to electron density is a relevant factor for absorbed dose calculations in a not homogeneous medium. It is possible to verify and customize the conversion of Hounsfield units (HU) into material composition and mass density using a tissue characterization phantom. 5. **Partial Volume Effect (PVE) correction**: This can be applied by entering an equation which allows to estimate the recovery coefficients for VOI statistics, that are specific for the SPECT scanner in use. 6. **Time-integrated activities (TIAs):** The algorithm uses a fitting technique at voxel level, which estimates a fitting function (sums of exponentials) for each voxel, This process automatically selects the best model from a predefined list of possibilities, using the Akaike Information Criterion (AIC) [17]. The algorithm provides an error metrics statistics section based on the calculation of the symmetric mean absolute percentage error (SMAPE), which allows measuring the accuracy in terms of percentage (or relative) errors. 7. **Absorbed dose calculated with Acuros MRT algorithm:** This deterministic solver is customized for internal therapies. It includes the solution of the linear Boltzmann transport equation (LBTE) for photons and the linear Boltzmann-Fokker-Planck transport equation (LBFFTE) for electrons and it takes into account that photon and electron energies for internal therapy applications are significantly lower than the energies used in external beam radiation therapy applications. The performances of the LBTE solver for internal therapies with \({}^{177}\)Lu were considered and described in-depth by [PERSON] et al. [18]. The algorithm may perform voxel dosimetry.