T. Sairir, BDS,MS,Dip. ABOMR,*  l.A.N. Al Agil, PhD,**
V. Manoharan, BE***
* Division of Radiology, College of Dentistry, King Saud University
** College of Science, Department of Physics, King Saud University
*** Department of Biomedical Engineering, King Khalid University Hospital

Thermoluminescent dosimetry was employed to measure the efficiency of the lead barriers in the Dental X-ray operatories. A monthly radiation dose of 2.7 milli rad detected by one dosime­ ter, located outside an operatory. The remaining forty seven dosimeters placed in the corridors did not register any dose. The incorporated lead thickness in the walls of the operatories was found to be 2mm, ten times than the required lead thickness. The annual occupational radia­ tion exposures around the operatories was estimated to be well below the backround radiation levels in City of Riyadh.

With the advent of 20th century, the detrimental effects of radiation became apparent. Many pioneers in this field became victims of skin, bone, and blood cancers.1 In the middle of the century, the consensus developed among the scientific community that the detrimenta! effects of ionizing radiations were dose dependent.2 Recommenda­ tions were made for minimizing the exposure to the patient undergoing a procedure and the operator conducting the procedure. This awareness of radi­ ation hygiene and carefulness has resulted in marked decrease of human radiation intake through the skin during an intra-oral examination from 1130 milliRoentgen (mR) to 300 mR since  1970.3 On the other hand, the total number of den­ tal X-ray exposures made on the U.S. population has increased from 52 million in 1970 to 200 mil-lion in 1977.4 Extensive dental X-ray exposures constitute significant radiation hazard to the dental profession.

Presently, controversies exist regarding quantifi­ cation, recognition, and expression of this hazard. The radiation-induced malignancies are indistin­ guishable from the natural ones in their clinical behavior. Detrimental effects of chronic low-dose exposures have been studied through animal experiments, tissue cultures, and exposed human populations. The latter group constitutes the indi­ viduals who were exposed in warfare, accidents, or radiation therapy. Information gained from exposures of animals, such as fruit flies and rats, cannot be extrapolated to the human beings.5 The epidemiological studies of exposed human popu­ lations firmly establish the relationship of car­ cinogenesis, or genetic mutations, to the absorbed radiation doses. The exposure is usually above 100 rad in these populations (400 rad is the mean lethal dose to the humans).6 Diagnostic radiological pro­ cedures incur exposures from few m.rads to approximately 5 rad for the different anatomical parts. There is no epidemiological data available in this range which shows causal relationship between dose and effect. The problem has been addressed by extrapolating information from the known linear dose-effect relationships of higher dose region to that of the vaguely understood lower dose region. It was assumed that the linear dose- effect relationship at higher doses also remains true for lower doses, and that there is no threshold dose. The linear non-threshold model postulates that any miniscule amount of radiation may result in cancer or genetic abnormalities.This model is on the conservative side, and it, definitely, overestimates the risk. Even though the speculation exists that there may be a threshold dose for carcinogenesis, and the notion that radiation doses below 10 rad may not be detrimental. Prudence, however, demands that, in the absence of definite proofs at the present time, the existing protective radiation measures must continue to be relied upon.5

Annual maximum permissible doses (M.P.D.) have been recommended for the whole body, and various other organs for the individuals who work with or in the vicinity of an X- ray generator. It is stated to be 5 rem for the operator and 0.5 rem for non-operator.7 Whalen and Baiter8 explained that M.P.D. recommendations were based on accept­able stochastic risk levels. An occupation is considered to be safe if the risk of mortality is below 10-4. If an operator has received his annual thyroid M.P.D. (5rem/yr.)for40years, his chance of dying from radiogenic thyroid carcinoma is one in 10,000.8 M.P.D. should not be thought as safe levels, but levels above which the radiation risks are considered unacceptable.

The radiation pollutant in dental X-ray facility is generated by primary, scatter, and leakage radia­ tions. The leakage through the tube housing shield is negligible at the maximum kVp and mA usages. The scatter radiation from the patient travels in a direction other than the primary beam. It depends on the beam characteristics, and the nature of the interacting matters.

The radiological facilities should be designed and operated to control exposure from the scatter and primary beam radiations. This can be accomplished by incorporating radiation absorb­ ing materials, such as lead, in the walls of the operatory.9 A 2.5 mm thick lead sheet will absorb 99.99% of incident radiation generated by a 90 kVp dental X-ray unit.10 The National Council on Radiation Protection (NCRP) Report No. 35 facili­ tates computation of required lead thickness for optimum design.11

The purpose of this study was to test the safety of the X-ray operatories in the newly- commissioned building of the College of Dentistry at King Saud University (KSUCD) in Riyadh, Saudi Arabia.  

The dental X-ray facility at KSUCD has twelve operatories. They are located in two sections, designated A and B. All the rooms are identical in
size {175 cm X 310 cm). All rooms, including the doors, are of modular W-inch steel fabrication. A
2 mm thick lead sheet is incorporated in the walls extending up to the ceiling and in the floors of the operatories. The wall facing the corridors has lead glass window (75 cm x 45 cm) for patient visuali­ zation during a procedure. No weak areas were detected in the walls using an Americium 241 source.

There are ten Siemens Heliodent 70, X-ray machines with Siemens Dentotime timer units.*
The exposure switches for individual X-ray machines are provided outside the rooms. Various characteristics of each X-ray unit were measured (Table 1). The room layouts were identical so it was decided to survey the two adjacent commonly used rooms in section B (Table 1). An environmental dose rate survey was conducted using ther-moluminiscent dosimeters. Six sites were selected inside and six outside each room, as shown in Fig. 1. The sites were piano-parallel to the floor, and one and a half meter above the floor. The plastic plate carrying four dosimeters for each site were attached to the walls with a strong adhesive tape. The dosimeters were left in place for four weeks in March 1987. At the end of the survey the dose received by dosimeters were determined by Harshaw thermoluminiscent counter.**

The results are shown in Table 2. Forty-eight dosimeters were placed outside the operatories. None of these dosimeters registered any measura­ ble exposure, with the exception of one dosimeter located at site (1) in Room B4. It received a dose of 2.7 m.rad in four weeks equivalent to approxi­mately 0.1 miilirads per day above background level. Section B operatories are used more often because of their close proximity to the technician room. The sites adjacent to the dental chair had registered the maximum exposures. The primary beam is usually directed towards either side walls during intraoral radiography. The graph in Fig. 2 demonstrates the number of dental films exposed per month for two consecutive years. It shows that the maximum number of films exposed in a particu­ lar months was about 10,500, which was about 2,600 films per week. This work was spread out in ten X-ray operatories (260 films per room). Work­ load (mAs generated per week) was estimated by multiplying number of films exposed per week with seven (7) milliamperage and 0.32 second, which is the exposure factors used for maxillary molar area in this institution. The workload was found to be 582 mAs/week in that month (March 1987).
According to NCRP Report No. 35, this workload will require incorporation of 0.1 mm thickness of lead in the walls located 3.5 ft from the X-ray unit and receiving direct radiation. If the non-radiological personnel are working in this area, 0.2 mm thickness of lead will be required for the walls for controlling the exposure.

The occupational radiation exposure depends upon the quality of the energy and the frequency of the exposure. It is also essential to consider the nature and place of work of the individuals staying in the area.

The National Council on Radiation Protection (NCRP) Report No. 35, recommends guidelines for designing X-ray facilities. The X-ray facility, which is solely accessible to X- ray technicians, is desig­ nated as "controlled area". The annual occupational exposure should not exceed more than 5 rem.

An area called "non-controlled" is where non-radiological workers perform their duties. These workers are receptionists, accountants, and prosthetic technicians. The radiation exposure in such area should not exceed 0.5 rem in a year, which is ten (10) times less than that of the "control­ led area". Ideally, every X-ray facility may be designed by providing adequate shielding to
achieve exposure levels of "non-controlled" areas. The walls at which the X-ray beam is directed are called primary barriers, and the walls exposed to scattered radiation are called secondary barriers.
The primary barriers require more shielding. In a dental X-ray operatory, all the walls are considered primary barriers. This is explained on the basis of diversities of various radiographic examinations possible in dental clinical situations. The thickness of the shielding material also depends upon the workload, and distance between X-ray unit and the place to be protected.

The dental X-ray facility in this institution is radia­ tion "non-controlled" area. The central sterilization unit is about 7 ft from the X-ray operatories. It is always occupied by the attendants. The general dental clinics are located on the other side of the operatories at a distance of 7 ft from the X-ray units. The corridors around the facility are frequented by students and the dental auxilaries, which are about 3 ft distance from the X-ray units.The results of this study demonstrated that the 0.2 mm thick lead sheet should be incorporated in the walls separating the corridors from the X-ray units. In reality, the total thickness of the lead sheets, incorporated in the walls, was found to be 2 mm or ten times the recommended thickness.

The maximum workload estimated in a month, in years 1986 and 1987, was found to be 5,800
 mAs/week (2,600 exposed films in a week). Suppose that all films were exposed with one X-ray unit instead of 10 functioning units located in the facility, then the recommended thickness of lead for the walls next to the corridors would be 0.5 mm. This value is four times less than the actually incorporated thickness for the non-controlled area.The walls of the operatories consists of pre-fabricated steel sheets which will further act as radiation attenuator.

The estimated dose level of 0.1 mrad is a very low level. This amount is about one-third of the
daily background radiation level, which is 0.3 mrad per day. A worker in this facility may accumulate 24 mrad of occupational radiation exposure in a year, which is below the fluctuations of the background radiation levels worldwide.

These levels may vary from 20 mrad per year to 1,200 mrad per year in certain regions on the earth. This cumulated exposure is about 200 times less than recommended annual exposure (5 rem).

Cohen and Lee12 estimated actuarial life shorten­ ing to which an individual might be exposed from different risks. It can be reduced from their data that an exposure of 24 mrad may entail estimated loss of 36 minutes of life of the exposed individual, which may be of smaller magnitude, as compared to loss of 7,000 minutes incurred by buying a small car in USA or loss of 10 minutes by smoking a single cigarette. Crabtree et al13 surveyed 231 dental offices, and detected radiation exposures ranging from 5 mrad to 60 mrad in a month. In the present study, the radiation level in the corridors was found to be 2.7 mrad per month.

Presently, X-ray operatory designs are based on the principle of ALARA (as low as reasonably achievable). This principle advocated implementa­ tion of all the possible methods to minimize occu­ pational exposures below the recommended levels (MPD) without any monetary constraints to the institution. The facility design in this institution was based on the de minimis dose levels, an ideal,
which every institution aspires for. The de minimis level of radiation is considered very low in that it will be impossible to demonstrate any bioeffects from such levels. Proposals for the de minimis level range from 100 mrad per year (corresponding to background variations in the USA) to 0.1 mrad per year (corresponding to one brief air trip per year).8 The magnitude of radiation exposure in private dental offices may be quite different from that of the teaching institution due to the volume of work. Exposure of 90 films per week with a Siemens Heliodent X-ray unit in "controlled area" may require 0.1 mm thick lead barrier for primary walls at one (1) meter distance from the source. A 0.2 mm thick lead sheet will be required for changing it into "non-controlled area". This protection can be easily gained by installing four thicknesses of 5/8
inch dry walls. It is roughly equivalent to 0.3 mm thick lead sheet.10

Incorporation of 2 mm thick lead sheets in the walls of a 70 kVp dental X-ray operatory reduces the occupational radiation exposure to undetectable levels.

Radiation risks to the workers in the KSUCD facilities are negligible and theoretical in nature.

Address reprint requests to: Or. T.Saini, College of Dentistry,
King Saud University, P.O.Box 60169, Riyadh 11545, Saudi Arabia.

Table 1. Characteristics of Dental X-Ray Units installed in sections of Radiology, College of Dentistry (King Saud University)