## 6 Final Conclusions and Further Work  
The number of distributions in volume could be unknown in a real case and the determination of this number is possible by the application of two strategies. The first strategy assumes exhaustive search with independent processing of one, two, and three expected components of mixture. The results are based on the achieved error, and the minimal value selects the set of parameters as well as the number of components. The second strategy assumes single optimization process with dimensionality switching so that the number of components (dimensions) is variable--components are added or removed dynamically.  
The improvement of computation time is still important, e.g., with the use graphics processing unit (GPU) or multiple computers, but the achieved results offer the possibility of investigating real parathyroid glands, a matter that could be investigated in further works.  
The described method allows calculating a tracer activity in small lesions. It applies, mainly, to secondary hyperparathyroidism because the volume of lesions is lower compared to primary hyperhyroidism. Parathyroid adenomas or hyperplasias, especially in secondary hyperparathyroidismogism [8] could be assessed more precisely. There are examples of using the same approach but on different organs in the literature [57; 58; 59]. There have also been first attempts at quantitative calculation of parathyroid adenoma in SPECT/CT [60]. An alternate method (NM Quantification Q.Metrix for SPECT/CT Package [61]) has been shown to validate the quantitative assessment of preoperative parathyroid SPECT/CT scintigraphy results in a new approach with standard uptake value (SUV). The technique is described as a promising tool to improve diagnostic accuracy of \({}^{99m}\)Tc-Sestamibi in localizing/lateralizing the parathyroid adenoma. Incidentally, it can be quite interesting to compare both methods in future.  
The concept of precise or personalized medicine (PM) is currently a subject of considerable debate. As it is stated in the editorial commentary of the European Journal of Nuclear Medicine and Molecular Imaging, \"the concept of PM, prevention and treatment strategies taking into account individual variability, might be strictly related to molecular imaging and nuclear medicine, through understanding the molecular basis of disease and defining molecular changes or markers associated with disease progression, response to treatment, and relapse\" [62]. This statement, though limited to the diagnostic approach, closely corresponds with the approach presented here. Development of PM would be hindered without SPECT quantification as is it is a routine approach for PET/CT [63]. We hope that this paper adequately addresses the challenges of PM.  
M.H.L. (study conception and design, principal investigator and project manager, and drafting of the manuscript); H.P.-B. (acquisition of data from SPECT/CT gamma cameras, quality control, and study protocols); K.S. (database design and statistical analysis); J.I. (acquisition and processing of planar and SPECT/CT parathyroid scintigraphy data); M.L. (pathological examination of removed specimens); M.C. (pathological examination of removed specimens); M.O. (performed surgical removal of parathyroid pathology); B.B. (final approval of the paper); D.O.-M. (study conception and design, drafting of the manuscript, testing software, and statistical analysis); P.M. (development of the mathematical model, writing software, and final approval of the paper); All authors read and approved the final manuscript.  
This study was supported by grant from budget resources for science from 2010 to 2015 as a research project No. N N402 463339 (Poland).  
The authors declare no conflicts of interest.