# Patient-Specific Dosimetry Evaluations in Theranostics Software for Internal Radiotherapy
## 1 Introduction  
Internal radiotherapy (IR) currently represents a growing field of nuclear medicine [1], which has rapidly evolved over the last few years. Radiabelled vectors administered in IR concentrate cytotoxic quantities of radioactivity selectively in targets, preserving simultaneously the adjacent healthy tissues. At the same time radiopharmaceutical dosimetry has been evolving [2], providing an increasingly accurate estimation of absorbed radiation doses to healthy organs and tumours. The work of [PERSON] et al. [3] shows an overview of the practice of dosimetry for IR in Europe, concluding that there is a large variation in the practice of IR across and within countries. However, the general increase in the application of IR and the implementation of the Council Directive 2013/59/Euratom and other position papers is likely leading to modifications in the practice of dosimetry in the next few years, to ensure the standardisation of dosimetry for IR [4, 5]. Temporal and spatial biodistributions of administered radiopharmaceuticals are assessed as accurately as possible from clinical quantitative imaging used in radiopharmaceutical dosimetry. Throughout the calculations a few challenges are met at some stage in the process (i.e., accurate image quantification, image registration, or time activity integral calculations) and specific uncertainties can be associated with the resulting absorbed doses [6; 7]. The robustness of each step of dosimetry achieves more accurate absorbed dose estimations. Existing software tools for dosimetry calculations differ significantly in design, functionality and degree of personalization and are typically grounded on a variety of models and assumptions, ranging from simple look up table (as in the case of ICRP 128) to computationally heavy Monte-Carlo codes for radiation transport calculations [8; 9]. Dosimetry can be performed at different scales, ranging from volumetric level (whole-body or organ based), to voxel size and lower to cellular size. Dosimetry software for calculation at organ level has been used for many years as a unique solution for dosimetry: OLINDA/EXM [10] version 1.0 code for personal computer was the first piece of software implementing a standardised dosimetry schema in internal dosimetry. More recently, several software packages have become available for dosimetric applications in IR, such as IDAC-Dose 2.1 [11] and MIRD-calc [2]. Aside from that, kernel convolution and Monte Carlo-based tools have been implemented in the last few years [12; 13]. In favour of the implementation of voxel level techniques, some studies indicate that deterministic biologic effects are mispredicted by the mean absorbed dose alone and may be significantly conditioned by not homogeneous dose distributions [14]. Currently most IR treatments consist of the administration of a fixed activity, such as in the case of [17] Lu] Lu-DOTATATE (Lutathera, Novartis) for the treatment of neuroendocrine tumours (4 \(\times\) 7.4 GBq injections separated by 8 weeks intervals). Though the administration of fixed activities is not compliant with the rules of a personalised treatment, in which the optimal activity is chosen individually for each patient, the verification of the delivered absorbed dose can be assessed after administration [12]. Even if performed as verification, dosimetry improves the knowledge of the relationship between absorbed dose and clinical effect, as a prediction of the therapy outcome [15]. Since dosimetry hold an important position in personalised medicine, the use of an advanced software is relevant, because it allows covering all of the necessary steps in one environment (i.e., image registration, volume of interest contouring, absorbed dose computation and therapy planning) and because it most likely represents a robust technique to carry out accurate dosimetry. In this paper Velocity Theranostics, a piece of commercial software, developed by Varian Medical System (Varian Medical Systems, Palo Alto, CA, USA) as advanced dosimetry software, is presented.