Effective Doses and Excess Lifetime Cancer Risks from Absorbed Dose Rates Measured in Facilities of Two Tertiary Institutions in Nigeria

: This study was aimed at examining the radiation absorbed dose rate, annual effective doses and excess lifetime cancer risks of halls of residence, Radiotherapy Unit and Radiology Department of UI, UCH and UNIMEDTH respectively. Results of measurements show that the mean absorbed dose rate for male and female hall are 0.33 ± 0.05476 and 0.17 ± 0.05074 µSv h -1 respectively. The mean overall absorbed dose rates calculated for facilities studied are 0.269 ± 0.0992 µSv h -1 , 0.121 ±

Radiation is part of human existence and it is found almost everywhere (indoor, outdoor, rocks, Deep Ocean and intestinal track). It occurs naturally in the environment as a result of attempt by unstable elements to attain stability through emission of certain nuclear components. This is seen in the emission due to certain elements such as U-238, Th-232 and K-40 found in the earth crust. Radiation in the environment can also take its source from the cosmic rays whose origin is the outer space amidst galaxies. This usually affects high altitudes and spacecrafts. Another source of radiation found in the terrestrial environment originates from human and industrial activities. Human activities leading to the release of radiation to the environment could be from nuclear accident (Chernobyl), testing of atomic bomb, operation of nuclear reactor and certain therapeutic and diagnostic X-ray machines. A greater percentage of radiation dose received by man is from natural sources (Chongakar et al., 2003) and about 3.9 % are manmade. The naturally occurring background radiation comes from the decay of some primordial elements resulting from uranium-238, thorium-232 and potassium-40 (Jibiri and Famodimu, 2013). Background radiation distribution varies with geological make-up of the underlying rocks of the region, ongoing and past human activities in a given location. Since radiation cannot be detected by human senses, the knowledge of its presence and extent in an environment are important. It is essential to determine the level of radiation in human environment. This knowledge helps to determine the likely health effects such that informed decisions in the health sector can be made by local, international regulatory bodies and the institutions concerned with public health. Measurement of level of radiation is important when radiological or nuclear activity is carried out in an environment. This enables one to ascertain the safety indices of such environment. Benrhardson et al., (2012) observed that a serious events involving unwanted exposure to ionizing radiation will make it necessary to estimates the doses to the exposed individuals and the public in an attempt to support decision making. Example of such measurement was carried out in the Eastern Belarus affected by a major surface contamination of radionuclides from Chernobyl accident (Bernhardson et al., 2012). As a result of environmental assessment of dose in the region, it was classified as an area of strict radiation control (Balonov, 2007), and the inhabitants were offered the opportunity to resettle to others of less radiation contamination. In addition, environment where radiological activities such as treatment and diagnosis are carried out are expected to be regularly monitored to prevent unwanted radiation in the environment due to leakages of equipment and other radiation sources. This study was aimed at examining the indoor dose rate, annual effective doses and excess lifetime cancer risks (ELCR) in two Teaching Hospitals and halls of residence of a university in Southwestern Nigeria.

MATERIALS AND METHODS
The Global Positioning System (GPS) was used to measure the location of the rooms examined in this study. The study locations (University of Ibadan) has a mean elevation of 200 meters above sea level and lies between latitudes N7 o 26 l and N7 o 21 l , and longitudes E 3 o 53 l , and E 3 o 54 l . University of Ibadan (UI) is found in southwestern Nigeria and northern part of Ibadan metropolis along Oyo-Ilorin road (Egunyinka et al., 2009). It falls within the basement complex of geological setting of southwestern Nigeria characterized mainly by the metamorphic rock types of Precambrian age, but with few extrusion granite and porphyrites of Jurassic age (Jibiri and Okorie, 2006). The main forms of rocks found in UI are gneiss, quartzites and magnetite which originate from igneous and sedimentary rocks. The basement rocks are covered by superficial deposits which vary in thickness with location (Oresanya, 1984 ℎ −1 and 100 ℎ −1 . It has energy range of 48 keV to 2 MeV ± 4%. A sensitive calibrated digital Survey meter, RADOS Technology (RDS-30) was used to measure the scattered radiation in the halls of residence of University of Ibadan (UI). The meter has an accuracy of ± 5% of the reading in ceasium-137 exposure. During investigations, measurement of exposures in female and male halls were done in three different locations at a height of 1 meter above the floor in each room. A total of eight male and five male halls of residence are included in this study.
At UNIMEDTH, the Radiology Department housed both a conventional X-ray machine and a Computed Tomography (CT) unit. The RADTRACE dose rate meter was used to measure the scattered radiation around the facility. Nurses' Office, Reporting Room, Radiographers' Office, Toilet, Burns Unit, Intensive Care Unit (ICU), Reception and Main Entrance were examined. Other areas examined include: Control Room, Digitizer Room and Corridor (with exposures and without exposure). Also, dose rate were measured in the Teletherapy Suite of UCH Radiotherapy Department in the following locations: Entrance Door, Control Room, Reception and Cubicle when exposures were being carried out and when there was no treatment done. The Teletherapy unit houses the external beam machine and Brachytherapy machine. Measurement was also done in the Brachytherapy suite (Treatment Room, Control Room and Sitting Room) with exposure and without exposures.

Computation of Effective Dose and Excess Lifetime
Cancer Risk: An attempt was made to determine the radiological implications of indoor annual effective doses ( ). This was calculated from the measured absorbed dose rate, ( ℎ ) as seen in equation (1).
For a workplace such as UNIMEDTH and Teletherapy Department which requires eight hours work-hour per day, the occupancy factor is assumed to be = 0.33. By using equation (1) with occupancy factor of 0.33, the indoor annual effective doses were estimated. Additionally, excess lifetime cancer risk (ELCR) was calculated (equation 2) from indoor annual effective doses.

= 2
Where is the indoor annual effective dose, is average duration of life -for Nigerians, = 55.44 years (Microtrends, 2022). The value is the fatal cancer risk factor per sievert ( −1 ). For stochastic effects, ICRP 60 uses value of 0.05 as the risk factor for the public (ICRP, 1990).

RESULTS AND DISCUSSION
Tables 1 and 2 show the number of halls of residence, blocks and rooms examined in both male and female halls in University of Ibadan (UI). A total of 101 rooms were examined in male and 45 rooms in female halls. This consists of 30 and 21 blocks of rooms for male hall and female halls respectively. Table 3 is the indoor absorbed dose rate (µSvh -1 ) measured at various halls examined in University of Ibadan. The mean value of indoor dose rates ranges between 0.217 ± 0.0536 µSvh -1 (Zik Hall) and 0.406 ± 0.05911 µSv h -1 (PG Hall). The range of mean indoor absorbed dose rate (µSvh -1 ) measured in Female Halls is between 0.1068 ± 0.0289 (Queen Elizabeth Hall) and 0.2275 ± 0.015 µSvh -1 (Female PG Hall).  The difference in the dose rate can be attributed to the difference in the age of the two buildings. Both Zik Hall and Queen Hall (built about five decades ago) were built earlier than the PG Halls (male and female) with higher absorbed dose rates. The mean indoor dose rates for male and female halls of residences are 0.3337 ± 0.0547 and 0.16804 ± 0.05074 respectively. The mean dose rate found in the male halls is higher than the female hall by a factor of about 2 units. The mean overall indoor dose rate for all the halls in University of Ibadan is 0.2699 ± 0.0992 µSvh -1 . This overall mean for the thirteen halls is higher than the global population weighted average indoor gamma dose rate of 0.059 µSvh -1 in air (UNSCEAR, 2000) by a factor of 4.6 units. The overall mean found in this study is comparable with the mean value recorded in a study carried out in Abeokuta (Southwestern, Nigeria) by Okeyode et al (2019). Table 4 is the result of the mean indoor absorbed dose rate measured at the Radiology Department of University of Medical Sciences, Ondo City. The mean indoor absorbed dose rate measured in thirteen locations without exposures is 0.1083 ± 0.0208 µSvh -1 , while the mean absorbed dose rate measure within the facility when the machine was working is 0.175 ± 0.0423 µSvh -1 . The difference between the background absorbed dose rate and the dose rate due to exposure is 0.0667 µSvh -1 . This is an indication that there is an elevated absorbed dose rate when the machine is in use as expected. This suggests the need for appropriate shielding and to provide mechanism for radiation protection in the facility, essentially in the Control Room, Corridor and Reception where elevated absorbed dose rate was recorded during the operation of X-ray machines within the department.    Table 5 is the indoor absorbed dose rates measured at different locations in University College Hospital (Teletherapy and Brachytherapy Units), Ibadan. The overall mean exposure in the Teletherapy Unit is 0.123 ± 0.00936 µSvh -1 . This is greater than the population weighted global average. The mean indoor absorbed dose rate obtained from the Brachytherapy Unit when there was no exposure is 0.1075 ± 0.0106 µSvh -1 . The result of the mean indoor absorbed dose rate during exposures is 0.393 ± 0.2302 µSvh -1 . This is also greater than the population weighted global average.  female hall while that of the male hall has the highest mean annual effective dose of 2.67 mSv y -1 . Figure 2 is the result of mean annual effective dose (at UNIMEDTH) calculated from the indoor absorbed dose rate in the Control Room (0.37 mSv y -1 ), Corridor (0.44 mSv y -1 ) and Reception (0.27 mSv y -1 ) during exposures (EX) and absence of exposure (NE): Control Room (0.21 mSv y -1 ), Corridor (0.25 mSv y -1 ) and Reception (0.23 mSv y -1 ). The highest mean annual effective dose is found along the Corridor. In certain instances staff sit along the Corridor especially the Record Officers. Room are found to be 1.22 mSv y -1 and 0.94 mSv y-1 respectively. The Brachy Treatment Room has a relatively higher mean annual effective dose when the treatment is being carried out. This is however lower than the recommended annual dose limit. This calls for adequate shielding of the walls and doors of the treatment room. Other preventive mechanisms could be introduced especially during treatment. It is evident from the results of this study that annual effective doses recorded from background radiation level and during treatments process fall below the recommended annual dose limit. Table 6 is the results of mean lifetime cancer risk (ELCR-the difference between the lifetime risk for the exposed and the lifetime risk for the unexposed) calculated from mean annual effective dose. Results show that the mean ELCR for different facilities are 6.07 x 10 -3 ± 1.05    Table 7. The mean ELCR in the two halls are higher than the world average of 1.45 x 10 -3 and the standard value of 0.29 x 10 -3 (by factors not less than 1.97 units). The elevated ELCR is as a result of high dose rate recorded in the halls of residence. This might be attributed to the sources of building materials used, underlying rocks of the locations (Okeyode et al., 2019), and perhaps other human (mining, use of radioactive compounds) activities in the neighborhood. Result of earlier study on activity concentration of topsoil (outdoor) of UI (Egunyinka et al., 2009) shows that it falls within the acceptable value and does not pose any hazard. However, results of the present study indicate that the elevated dose rate could be as a result building materials used or perhaps certain recent radiological activities in the vicinity of the study location.The excess lifetime cancer risks calculated from the data of the two Teaching Hospitals (UCH and UNIMEDTH) are lower than the world average. However, this does not undermine the use of preventive measures to reduce the exposure of personnel and the public.
Conclusion: Dose rates of thirteen halls of residence in University of Ibadan, two units of Radiotherapy Department of University College Hospital and Radiology Department of University of Medical Sciences, Ondo were studied. The corresponding annual effective doses and excess lifetime cancer risks were determined. Results of the study show that the mean dose rate in the halls of residence, UCH and UNIMED are 0.2699 ± 0.0992 µSvh -1 , 0.121±0.036 µSvh -1 and 0.123 ± 0.00936 µSvh -1 respectively. The highest mean annual effective dose is found to be 1.22 mSv y -1 . The range of ELCRs calculated in this study is between 0.51 x 10 -3 and 6.07 x 10 -3 .