Model for age-dependent decline in dopamine transporter
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LETTER TO THE EDITOR
Model for age‑dependent decline in dopamine transporter Masashi Kameyama1,2 · Yumi Umeda‑Kameyama3 · Sumito Ogawa3 Received: 20 June 2019 / Accepted: 11 July 2019 © The Japanese Society of Nuclear Medicine 2019
We read with great interest the article by Roberts et al. [1] published in the Annals of Nuclear Medicine. The proposed thesis that dopamine transporter (DaT) does not decrease with age in older people would affect many nuclear medicine physicians who diagnose older patients. We feel that the thesis is reasonable; however, we also feel that the data presented is inconclusive. Linear regression is an acceptable tool for analysis; however, its limitation should be recognized. A model to explain the non-linear relationship would be preferable. A large coefficient of determination ( R2 ) would provide a convincing argument. However, merely finite number of equations can be evaluated. The best fit equation cannot be determined in principle without a theoretical model. In mathematics, there are numerous functions that are arbitrarily close to a given function within a certain range. A bi-linear relationship can be sometimes merged into an exponential relationship. A semi-log relationship between erythrocyte creatine and mean erythrocyte age based on a model [2] might be preferable to the previously proposed bi-linear relationship. A poisson process model of random elimination of DaT would provide a basis for an exponential relationship, such as a radionuclide. An exponential relationship assures a positive value for SBR.
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s12149-019-01388-z) contains supplementary material, which is available to authorized users. * Masashi Kameyama kame‑[email protected] 1
Department of Diagnostic Radiology, Tokyo Metropolitan Geriatric Hospital and Institute of Gerontology, 35‑2 Sakae‑cho, Itabashi‑ku, Tokyo 173‑0015, Japan
2
Division of Nuclear Medicine, Department of Radiology, School of Medicine, Keio University, 35, Shinanomachi, Shinjuku‑ku, Tokyo 160‑8582, Japan
3
Department of Geriatric Medicine, School of Medicine, the University of Tokyo, 7‑3‑1 Hongo, Bunkyo‑ku, Tokyo, Japan
In view of the above, we are afraid that the data do not seem to support conclusively the model presented. First of all, limiting the analysis to the old people may cause breakage effect (Supplementary Information). Because of this effect, the observed slope in the derived function may be different from the entire population. Second, certainly striatal volume loss causes underestimation of SBR by Southampton method. However, usually, cerebral cortices show more severe atrophy than striata. Cortical atrophy causes underestimation of background and hence overestimation of SBR. The Japanese database [3] did not apply the correction for atrophy. Therefore, we do not think that the rate of decline with age in the Japanese database was exaggerated. We hope that, in future studies, an exponential relationship based on random
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