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UNIVERSITY OF BUCHAREST
FACULTY OF PHYSICS Guest
2024-11-22 1:43 |
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Conference: Bucharest University Faculty of Physics 2003 Meeting
Section: Nuclear and Elementary Particles Physics
Title:
A comparative Study of the Lung Bio-Kinetic Model
from the Dosimetric and Epidemiologic Approach /
Studiul comparativ al modelului bio-cinetic al plamanului
din perspectivele dozimetrica si epidemiologica
Authors:
Vlad C. Tomulescu, Tatiana Angelescu, Mircea V. Rusu, Octavian G. Duliu
Affiliation:
Bucharest University, Faculty of Physics, Department of Atomic and Nuclear Physics
Universitatea Bucuresti, Facultatea de Fizica, Fizica Atomica si Nucleara
E-mail
Keywords:
Abstract:
A correct assignment of organs’ weighting factors is of tremendous importance for assessing the effective dose due to inhalation of Radon short lived decay daughters. If for the lung (i.e. the trachea, bronchi, bronchioles and alveoli) there is established a 0.12 organ weighting factor, the other tissues of the respiratory tract (including the naso-oro-pharynx) are not listed. Studies have shown that extra-thoracic airways - as the entrance gate for aerosols - are subject to the highest deposition of solid state Radon progeny, therefore to the highest dose equivalent. Consequently, should we apply the International Commission for Radiological Protection recommendation and assign to the naso-pharynx a tissue weighting factor of 0.025 as half of remainder tissues. However, in such case, the epidemiological studies have shown a lack of correlation to the low incidence of nose and mouth cancers due to Radon progeny inhalation. Therefore, the assumption of a 0.025 weighting factor for the extra-thoracic airways may lead to an overestimate of the effective dose. This work presents comparative graphs of effective doses as function of aerosol size, obtained when considering each separate situation: i) based on the dosimetric conservative recommendation, and ii) on the basis of epidemiological statistics. The ICRP based ModeLung® software for dose calculation following Radon progeny inhalation was used.
When considering the reference worker performing usual labor for a month (i.e. 70 working hours) in an environment where a 0,1kBq/m3 Radon concentration was established at a 0,5 equilibrium factor, and in a classical aerosol distribution, one can obtain the following values for the effective dose coefficient:
Yd = 36,88 mSv/WLM (dosimetric case with Wet = 0,025 case)
Ye = 7,03 mSv/WLM (epidemiologic case Wet = 0)
It can easily be noticed a 5 times lower value in the epidemiologic case. Though 0,025 is considered a high value for the extra-thoracic compartment weighting factor, there is yet no proposal for a lower non-negative value based on the cancer incidence. The current study is proposing a mean value for the dose conversion coefficient of 19 mSv/WLM, but a weigthing factor value for the extra-thoracic airways remains an opened problem.
TOPICS + KEYWORDS: Radon daughters, inhalation, respiratory tract, compartment model, naso-oro-pharynx, weighting factor, effective dose, dosimetric conservative model, epidemiological studies.
RO
Atribuirea unui factor de pondere pentru tesuturile aflate la risc in cazul unui model bio-cinetic este un fapt de o importanta majora. Acest factor de pondere da o masura a radiosensibilitatii unui tesut sau organ, relativ la radiosensibilitatea intregului organism. Studiul prezinta comparativ dozele efective datorate inhalarii de descendenti de viata scurta ai Radonului, evaluate cu ajutorul programului original de calcul “ModeLung”, bazat pe modelul compartimental bio-cinetic al sistemului respirator.
Comisia Internationala pentru Protectie Radiologica a stabilit pentru plaman factorul de pondere Wp = 0,12 corespunzator urmatoarelor tesuturi: trahee, bronhii, bronhiole si alveole pulmonare. Totusi, regiunea extratoracica (naso-oro-faringeala) reprezinta un compartiment aparte. In cazul inhalarii de aerosoli, aceeasta regiune a sistemului respirator este poarta de intrare in organism a materialului radioactiv, si implicit cea mai expusa iradierii.
Studiile au aratat (1) ca indiferent de conditiile de mediu (temperatura, umiditate, dimensiunea aerosolilor) si indiferent de subiect (categorie de virsta, rata de respiratie), doza echivalenta in regiunea naso-oro-faringeala este de pina la 10 ori mai mare decit doza echivalenta in oricare din regiunile anatomice toracale ale sistemului respirator. Acest fapt ne indreptateste sa alegem pentru regiunea extra-toracica un factor de pondere Wet = 0,025 adica jumatate din valoarea factorului de pondere stabilit prin conventie pentru “celelalte organe” a caror radiosensibilitate nu este bine precizata (2), (3). Aceste considerente au fost cele care au stat la baza elaborarii versiunii “dozimetrice” a programului de calcul “ModeLung”.
Pe de alta parte, studiile epidemiologice au aratat ca este dificil de facut o corelatie intre incidenta cancerelor de naso-oro-faringe si depunerea de descendenti ai Radonului in aceasta regiune. Bazandu-se pe statistici, epidemiologii (4), (5), (6) au afirmat ca un factor de pondere de 0,025 pentru naso-oro-faringe va aduce o supraevaluare a dozelor efective. In consecinta, in calculele efectuate cu versiunea “epidemiologica” a programului “ModeLung” se va introduce un factor de pondere Wet = 0.
In grafice se prezinta comparativ evolutia dozelor efective ca functie de dimensiunea aerosolului, pentru cele doua situatii discutate mai sus. Calculele de doza au fost efectuate cu ajutorul programului “ModeLung”, in urmatoarele ipoteze: muncitor de referinta cu respiratie normala pe nas, desfasurand o activitate uzuala cu rata de respiratie de 1,2 m3/h, timp de o luna (70 ore), intr-un mediu in care factorul de echilibru intre Radon si descendentii sai de viata scurta este F=0,5.
Prin compararea celor doua curbe reprezentand dozele efective corespunzatoare cazului “dozimetric” si respectiv celui “epidemiologic”, se poate observa o diferenta semnificativa intre nivelele de doza, in special in domeniul aerosolilor de dimensiuni mici si mari. In domeniul aerosolilor de dimensiune medie (0,01-0,1um) dozele bazate pe considerentele dozimetrice sunt aproape duble fata de cele calculate conform propunerilor epidemiologice.
In cele ce urmeaza se face propunere unei distributii medii dupa dimensiune a aerosolilor:
aerosoli neatasati (<0,001um): 0,5%
aerosoli atasati:
modul nucleatie (0,015-0,03um): 30% / modul acumulare (0,2-0,3um): 60% / modul grosier (3-5um): 9,5%
Astfel, s-au obtinut urmatoarele doze efective medii: DD = 0,498 mSv (pentru cazul dozimetric) si DE = 0,095 mSv (pentru cazul epidemiologic).
Tinand cont de faptul ca expunerea la descendentii Radonului intr-un mediu in care s-a stabilit o concentratie de 0,1 kBq/m3 Radon si un factor de echilibru F = 0,5 este 0,0135 WLM,
s-au calculat cele doua valori ale coeficientului de conversie al dozei:
Yd = 36,88 mSv/WLM (dozimetric) ; Ye = 7,03 mSv/WLM (epidemiologic)
Se poate concluziona ca prin alegerea unui factor de pondere pentru regiunea extratoracica (naso-oro-faringe) Wet = 0,025 bazat pe considerente de natura dozimetrica, dozele efective sunt supraevaluate de aproximativ 5 ori. Acesta poate fi considerat drept un caz conservativ, iar dozele reale nu vor depasi niciodata valorile calculate in acest mod.
In urma statisticilor epidemiologice nu s-a facut o propunere clara in privinta alegerii unui factor de pondere pentru naso-oro-faringe. Pornind de la un factor de pondere Wet = 0, introdus in calculele realizate cu modelul biocinetic al sistemului respirator, se pot obtine valorile minime ale dozelor efective primite de subiectii studiati.
In realitate, dozele efective reale se situeaza intre cele doua curbe prezentate in figura de mai sus, iar coeficientul de conversie al dozei realist se poate situa in jurul valorii: 20mSv/WLM. Gasirea unui factor de pondere pentru tesuturile extratoracice in cazul inhalarii de Radon si descendenti ai acestuia ramane o porblema deschisa.
(1)V.Tomulescu – “Internal Dose Assessment for Workers Inhaling Radon Daughters as function of Aerosol Size” – Journal of Preventive Medicine, 2002, Volume 10 (1): 55-64
(2)International Commission on Radiological Protection – Recommendation of the ICRP; Publication No.66, “Human Respiratory Tract Model for Radiological Protection”; Pergamon Press, Oxford and New York, 1994;
(3)Bailey M.R. – “The new ICRRP Model for the respiratory Tract”; Radiation Protection Dosimetry, Vol.53, Nos.1-4, pp107-114, 1994;
(4)International Agency for Research on Cancer – “IARC Monographs on the Evaluation of the Carcinogenic Risk of Chemicals to Humans”; IARC-WHO, Vol.38, Geneva, Switzerland, 1985;
(5)Proceedings of the 29th Hanford Symposium on Health and the Environment – “Indoor Radon and Lung Cancer: Reality or Myth?”; October 15-19, 1990, Washington, D.C.;
(6)National Research Council– “Health Risks of Radon and Other Internally Deposited Alpha-Emitters”; BEIR-IV, National Academy Press, Washington, D.C.
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