The System Of Microdosimetry Estimation In Nuclear Medicine

Radionuclide was widely used for diagnostic and therapeutical application innuclear medicine. It provided a new approach for medical research, and wasimportant for understanding the mechanism of pathogeny and pharmacology, aswell as the nature of life.As the same with other medical method, the risk and curative effect should beevaluated. Absorbed dose is the basic parameter for quantifying the radiation,evaluating the risk and predicting the outcome of therapy in nuclear medicine andradiology.The cell radiation damage is the hypostasis for radiation effect of body. Hence,the absorbed dose in individual cell and corresponding radiation biological effecthas been becoming more and more interested in diagnostic and therapeuticalapplication of radionuclide used in nuclear medicine.To theoretical research of absorbed dose in cellular or Subcellular targetdeposited by radionuclide used in nuclear medicine, we firstly observed themicrodistribution of radionuclide using microautoradiography and frozensectioning technology. The result that the distribution of radionuclide isnonuniform at cellular level suggested us to establish a set of microdosimetryestimation system in nuclear medicine by using Visual Basic program language.This system improved the MIRD microdosimetric scheme and had capability tofleetly calculate cellular S factor in different conditions when radionuclidedistributed uniformly, as well as nonuniformly. On the other hand, the specificenergy distribution in target can also be calculated based on Monte Carlosingle-even simulation and benefited our understanding of stochastic nature ofradiation.We calculated cellular S factor and specific energy distribution for differentradionuclide in different source-target configuration and various cell geometricalconditions. After comparing the influence of several factors, we find that:The researched fill mediums have little influence on microdosimetry. It isconvenient to use the cell modal filled with unit density water, and the results canreflect the absorbed dose deposited in tissue or cell well.Secondly, using detailed radiation spectra of radionuclide is important formicrodosimetry. The mean energy of beta emitters' spectra cannot approximate the continuous beta spectra well. And the dosimetric contribution of the large number of auger electron released from O-shell or N-shell transition cannot be ignored in microdosimetry.Next, the radiation type of radionuclide can also significantly influence the microdosimetry in target. The self-absorbed dose contribution of alpha-emitter with high linear energy transfer is much higher than auger electron emitter and beta emitter. However, the relative short range of alpha particle and auger electron limits the energy deposited in surrounding cell. It indicates that alpha or auger electron emitters are suitable for radioimmunotherapy of dispersive and small tumor cell. It can deposit high dose in target, giving rise to high cell kill effect with little effect to normal cell near the tumor. On the other hand, beta emitters are useful for internal radiotherapy to solid tumor with lager dimension. Although the linear energy transfer is relative low, the long range of beta ray ensures the considerable dose deposited in interior of large solid tumor.In addition, the change of the energy loss per unit length along the track in target has little influence in microdosimetry. The distribution of radionuclide in source region influences the target dose mainly by changing the average chord traveled by particle in target. The target cell dimension will also change the average chord traveled by particle in target, meanwhile, significantly change the volume and mass of the target.For radionuclide which contain radioactive daughter in decay chain, it's more reasonable to use cell dose conversion factor to describe the dose deposited in target cell. Generally, in system where radionuclide is expurgated rapidly, such as blood circulation system, the likelihood that daughters generated within the circulation will contribute significantly to the dose at the site of parent decay is considerably less than if the daughters were generated within the interstitium. However, in the tumor interstitium, longer cutoff times may be appropriate and the dose contribution of radioactive daughter in decay chain should be involved.After calculating the specific energy distribution in target based on Monte Carlo single-event simulation, we find that, because of the high stochastic nature of radiation, the amount of energy deposited vary greatly from event to event, giving rise to a wide frequency distribution. The specific energy, which is less than the mean of specific energy deposited in target per single event, appears with more probability. This suggests the actual dose in target is much likely less than the average value by theoretical computation. So, to deliver a certain amount of dose to target, we can increase the cellular radionuclide concentration appropriately. Compared with the results reported by literatures, we find the results of our microdosimetry estimation system are exact. At the same time, the system is convenient, friendly and useful for microdosimetry estimation.