RADIATION DOSIMETRY INTRODUCTION NEW MODALITIES

Transcription

1 RADIATION DOSIMETRY M. Ragheb 1/17/2006 INTRODUCTION Radiation dosimetry depends on the aumulated knowledge in nulear siene in general and in nulear and radio hemistry in partiular. The latter is onerned with the study of the inner workings of radioative atomi nulei and their interation with inert and biologial matter. Earlier on, nulear and radio hemistry was involved with the understanding of the behavior of fission and fusion produts, and the protetion against radiation in the design of nulear weapons. This evolved into researh in the servie of the nulear power industry, maintaining the nulear stokpile and the appliations of radiotherapy and nulear mediine. NEW MODALITIES Fasinating new diretions are taking shape for the future. New moleules are being tagged with radioative isotopes to deliver deadly radiation straight to aner ells that may have metastasized or spread throughout the body. Conventional radiation beam therapy works well in the ase of well loalized aner, but he use of Syntheti High affinity Ligands (SHALs) and monolonal antibodies, promises the reahing of the Holy Grail of aner treatment in the form of a magi or silver bullet diretly delivered to the sites of tumors. The same ompounds targeting the tumors an also be used with imaging tehniques to identify the loations of tumors in the body. New detetors are being designed to use the radioisotope tagged moleules to reveal the loations of aner ells. New mass spetrosopy tehniques are observing how isotopes interat with human ells. Radiation transport methodologies suh as Monte Carlo methods are being linked to nulear data libraries in for planning radiation beam therapy. Programs in safeguards, nonproliferation and homeland seurity are evolving to meet the demands of a hanging world. Sientists are alled to detet minute amounts of radiation and to manage the onsequenes of radiologial events. In this regard, radiologial detetors large and small, permanently installed and hand held are being developed. New tehnologies for deteting gamma radiation and neutron emissions are under development. Another promising area of endeavor involves the measurement of the ways the human body responds to small doses of radiation. With a better understanding of the effets of radiation on living tissue, medial personnel will be able to measure the dose reeived and intervene before the treated individuals beome sik and die. The objetive is to treat the human body as a walking dosimeter. If the onept of radiation preonditioning or even the ontroversial radiation hormesis idea are understood or

2 beome realities, thousands of lives ould be saved, in what is the prime national and international health issue of aner prevention and treatment. SCOPE OF RADIATION DOSIMETRY In radiation dosimetry we are onerned with the transport and interation of ionizing radiation in matter. We start with a radiation soure S, and onsider a material M interposed between it and a detetor D. The radiation from the soure interats with the interposing material before arriving at the detetor. The material M ould be inert. In that ase it is treated as a shield, and the objetive is to design it in suh a way so as to shape the radiation field before its arrival at the detetor D. The material ould be biologial in nature, and the task beomes the assessment of the biologial effets in the intervening material and how the radiation would affet the detetor D, whih ould be a lesion or a tumor. Attenuation and absorption are important in shielding and in radiotherapy and radiation proessing where the aim is to deliver a preise amount of dose of radiation to a sample or tumor. Radiation an be in the form eletromagneti radiation as x rays or gamma rays. It ould also be in the form of neutral partiles suh as neutrons, or in the form of harged partiles suh as eletrons, protons or alpha partiles. Their effets and penetration in various materials, inluding the human body and its different onstituents must be onsidered. The damaging or benefiial potential of radiation of ionizing radiation is what is designated as dosimetry. The biologial effet of radiation is related to the energy deposited by ionization in a mass of tissue, or the absorbed dose. This is modified by several fators inluding: 1. The mirosopi spatial distribution of the ionizations, 2. The onentration of oxygen, desribed by the oxygen enhanement ratio, OER, 3. The rate of energy deposition, 4. The type of radiation, 5. The response of the biologial material to the ionization reated in it. Neutrons damage in metals is related to the displaements per atom (dpa) and vaanies reated by neutron ollisions with the nulei. It also reates transmutations by nulear reations leading to ativation of the materials. Hydrogen and helium nulei an be generated in the lattie, leading to volumetri deformation and swelling. Embrittlement of steel orrelates with the neutron fluene with neutron energies above 1 MeV, whih is a major onsideration in designing future breeder or fusion reators. In experimental and theoretial investigations, the energy spetrum of the partiles is obtained and then onvoluted with an energy dependent response funtion to get estimates of different reation rates. Thus the dosimetry task involves both the methods of alulating the spetrum and of alulating the response funtion or dose. DOSIMETRY PROBLEM Several steps are involved in a dosimetry problem:

3 1. Soure speifiation, inluding its strength, energy spetrum, angular distribution, spatial distribution and time distribution. 2. Speifiation of geometry of the soure, intervening medium and detetor. This inludes material ompositions and densities. Simplified geometrial onfigurations are adequate for most appliations. In detailed studies detailed models or phantoms are used to aount for heterogeneity of the human body or the designed shield. Phantoms are artifiial bodies, approximately the size, shape and density of a human body, used for alibrating ounters, or for numerial omputations. The experimental phantoms are designed so that the radioisotopes they ontain have a similar distribution as the isotopes expeted in the real body. Figures 1-3 show different phantom onfigurations. Fig. 1: Human phantom used to study I 131 in the thyroid gland. 3. Data mine for the ross setions for the different interations from a data warehouse. These ould inlude attenuation oeffiients, seondary radiation prodution, ativation ross setions, response funtions, and multigroup or point ross setions. The data that is obtained may need to be redued to a form suitable for the partile transport alulations.

4 4. Speify the detetors loations energy and angular responses aording to the purpose of the alulation. Response funtions for the alulation of the dose at different loations that are of interest are obtained. Point, surfae or volume detetors an be used. 5. Choose a alulation methodology analytial, numerial or statistial and an assoiated omputer program to be used in solving the problem based on its omplexity and the auray desired in the result. A new omputer program an be written for the onsidered problem, or an existing one modified to suit the purpose at hand. The transport of partile is arried out from the soure through the intervening materials and at eah of the detetors. Figure 1 is an experimental phantom used to study the radiation spetrum from the thyroid gland area in the nek. The radiation field is determined by a sodium iodide (NaI) sintillation detetor from I 131 in the thyroid used to treat thyroid nodules or Graves syndrome. In the alulation geometrial model of Fig. 3, the human trunk is modeled by an elliptial ylinder: 2 2 x y + 1, 0 z 70, (1) the head is modeled as; 2 2 x y + 1, 70 z 94, 7 10 (2) and the legs are modeled as trunated elliptial one; x y z , 80 z < (3)

5 Fig. 2: Anterior view of the prinipal organs in the head and trunk of human phantom used in the alulation of dose to an organ from radionulides deposited in another organ.

6 Fig. 3: Adult human phantom geometrial model. 6. Comparison of alulated response to the limiting riteria of the problem. This inludes maximum allowable doses or regulatory riteria. If neessary modify thre geometries, the design, substitute other materials, and iterate until a satisfatory response is obtained. 7. Estimation of ost and other onstraints may have to be taken into onsideration. The proedure or the shield may have to be optimized. Interplay between the safety of the proedure or shield design and its ost may require modifiation of the surrounding strutures or their shapes. In aerospae and naval appliations, spae and weight beome paramount riteria. 8. Performane verifiation must be undertaken of the dose within the body or the within the shield. Atual measurements may be needed, or the measurements are made in a mokup or a phantom representing the body NEUMANN SERIES SOLUTION OF THE TRANSPORT EQUATION

7 Using matrix notation and onsidering the transport operator T and ollision operator C as matrix operators the partile transport equation an be written in terms of the ingoing ollision density ψ as: ψ = S + Hψ, ψ Hψ = S, [ I H] ψ = S (4) where: I is a unit matrix operator, H=TC is the transport operator, S = TS is the one transported soure, S is the physial soure. Premultiplying by the inverse in Eqn. 4 we get: Using the expansion: 1 1 [ ].[ ] ψ = [ ]. I H I H I H S I (5) I. ψ =. S I H 1 1 x = < x x..., x 1, (6) Eqn. 5 yields the Neumann series expansion for the ingoing ollision density: ψ = [ I H H... ]. S = S + TC S + TC S + TC < 2 ( ) ( )..., ρ( ) 1. (7) where the spetral radius of (H = TC) is less than unity. RESPONSE FUNCTIONS The auray of the dose determination depends on the problem at hand. In radiation therapy an auray of 10 perent or better is needed to ahieve the required therapeuti response. This is neessary to avoid damaging the intervening tissue between soure and detetor. In shield alulation a relaxed requirement on the auray of 30 perent may be adequate. The dose or other response funtion is normally omputed as the integral quantity: where: D is the dose, D = Σr( r, E) ψ ( r, E) drde (4)

8 ψ ( r, E) is the ollision density desribing the radiation field, Σ ( r, E) is the spaially and energy dependent response funtion. r Computational methods benefit today from the availability of a multitude of platforms from desktops to superomputers making radiation transport in realisti three dimensional geometries possible. DISCUSSION Doses of radiation to workers and the publi are regulated by state and federal agenies and it beomes a matter of professionalism and ethis for the radiation pratitioner to abide by these laws and rules. The reommendations and guidelines of national and international professional groups suh as the National Counil on Radiation Protetion and Measusements (NCRP) and the International Commission on Radiologial Protetion (ICRP) must be taken into aount. Protetion of one self, oworkers and the publi is a matter of publi servie and requires the highest level of professionalism and ethis. EXERCISE 1. Obtain the Neumann series solution of the Transport Equation in terms of the outgoing ollision density as: χ = S + ( CT) χ Compare the solution to that for the ingoing ollision density: REFERENCES ψ = S + TCψ, S = TS. 1. Tomas Diaz de la Rubia, Calling All Nulear Sientists, Siene and Tehnology Review, National Nulear Seurity Administration, Lawrene Livermore National Laboratory, July/August, A. Edward Profio, Radiation Shielding and Dosimetry, Wiley-Intersiene, W. S. Snyder, H. L. Fisher, Jr., M. R. Ford, and G. G. Warner, Estimates of absorbed dose frations for monoenergeti photon soures uniformly distributed in various organs of a heterogeneous phantom, Soiety of Nulear Mediine, Medial Internal Radiation Dose (MIRD) Pamphlet 5, J. Nul. Med. Supple. 3, August John H. Woodburn and Frederik W. Lengemann, Whole Body Counters, Division of Tehnial Information, USAEC, 1967.