Scientists' Contributions

Bliznakov V., * Ya. Kirova-Pancheva, **A. Bayrakova*
* National Center of Radiobiology and Radiation Protection, Sofia
** University Hospital "Queen Yoanna", Medical University, Sofia

Lung and bronchus cancer constitute a substantial part of the oncological pathology, especially with men [1]. As it is well known, the origin and development of oncological diseases is a process in which factors of different nature participate: natural components from the surrounding environment, harms originating from the way of life and vocation, heredity etc. The estimation of the role of the different cancerogenic factors in the complex ethiopathogenesis of cancer formations is a prerequisite for the development of effective preventive programs.

The ionising radiation is only one of the lung and bronchus cancer risk factors known to us so far [2,3]. Here is meant the population's exposure to both man-made and natural sources of ionising radiation. Background radiation, a substantial part of which is attributed to radon inhalation, is deemed to account for nearly 2/3 of the total exposure of the population in a number of developed countries, Bulgaria including [4,5]. The objective of this analysis is formulated with a view to this aspect, namely:
to determine the attributable risk (AR) for lung and bronchus cancer for the Bulgarian population caused by radon inhalation.

Materials and Methods:

The analysis is based on data about:

1. the mean individual lung doses from radon inhalation for the Bulgarian population - 0.34 mSv/a [5], and
2. the lung and bronchus cancer diseases (ICD-9 Code 162) for 1966 according to the National Cancer Register: number of new cases by age at diagnosis for male and female [1].

AR (in %) is determined from the ratio between the estimated excess cases of lung and bronchus cancer and all cases of cancer of the same locality, recorded among the exposed population within a year.

To estimate the excess cancer cases, excess relative risk estimates at 1 Sv for cancers of the trachea, bronchus, and lung by sex and age at exposure were used according to Thompson et al. [6].

The collective doses were estimated as the product of the mean yearly organ dose for the lung for radon inhalation, and the sum of the person-years for each age group.

Results and comments

The results from the performed AR estimations are shown on Table 1. According to them, radon inhalation may be responsible for a relatively small part of the cases of lung and bronchus cancer among the Bulgarian population. In absolute figures this makes 41 of the newly diagnosed 2756 men, respectively 19 of the newly diagnosed 466 women. Differences depending on sex are shown: greater AR from radon inhalation with women compared to AR with men. The risk factor is manifested only after a relatively long interval of exposure (more than 40 years).

According to the BEIR VI committee [3], 10% of lung cancer deaths occurring each year in the U.S. (or 3000 to 32000 death cases) are attributable to indoor radon. About one third of these deaths could be prevented if radon levels were reduced so that no home exceeded 148Bq/m³ in air (the action level recommended by the U.S. Environmental Protection Agency)) [7].

On interpretation of the estimates for AR from radon inhalation, the conditions of the different estimation procedures should be kept in mind, namely:

One and the same mean organ dose is used, regardless of the age and the different duration of stay in the open and in premises with different radon concentration. One single example: by data of the National Center of Radiobiology and Radiation Protection [8], the radon concentration in premises-contained air is estimated to be from 3 to 250Bq/m³.

Risk coefficients based on the results from epidemiological study of those survived the atom bombing of Hiroshima and Nagasaki are applied, i.e. a cohort exposed under rather different conditions (as to the dose's amount and distribution with time, type of radiation etc.). This involves, above all, extrapolations of the high dose effects for the lower dose range. The uncertainty, characteristic for the concept of the linear non-threshold dose-effect dependence accompanies the estimations of the BEIR VI Committee [3] in which WLM-related-risk coefficients are used. Here, some additional uncertainty is introduced by the nolens volens use of one and the same risk coefficient for both sexes and all age groups, since at present, available data is mostly from the epidemiological studies of men-miners.

The need for additional information is obvious, so as to estimate more precisely the lung cancer risk from radon inhalation. This refers both to the radiation factor and the health effect: disease, respectively death from lung cancer. Meanwhile, the carrying out of preventive activities is justified, moreover if these activities do not involve substantial additional expenses. Table 1
Attributable risk [%] of lung cancer from radon inhalation for the Bulgarian population according to age and sex

	Age groups (years)
Sex	0-9	10-19	20-29	>40	All ages
Male	-	<1.0	<1.0	1.5	1.0
Female	-	-	-	4.2	4.3
Both	-	<1.0	<1.0	2.3	2.1 (1.3-2.9)*

* 95% CI