Journal of Medical Physics
ORIGINAL ARTICLE
Year
: 2022  |  Volume : 47  |  Issue : 3  |  Page : 219--224

Calculation of skin dose rate conversion factors due to surface contamination from frequently used radionuclides in local nuclear and medical facilities


Ian Pasquevich, Fiorela Merma Velasco, Pablo Andres 
 Division of Radiation Protection, Bariloche Atomic Centre, National Atomic Energy Commission, San Carlos De Bariloche, Argentina

Correspondence Address:
Mr. Ian Pasquevich
Division of Radiation Protection, Bariloche Atomic Centre, National Atomic Energy Commission, San Carlos De Bariloche
Argentina

Abstract

Purpose: The use of nonsealed radioactive sources can lead to skin contamination due to radiological accidents and staff oversight. This contamination has been shown to contribute considerably to the total skin dose received by nuclear medicine technicians and can easily exceed the limit of 500 mSv/year established by the current regulations. To assess the severity of contamination, it is necessary to estimate the skin dose through the use of suitable skin dose rate conversion factors. To determine the appropriate factors, it is important to study the influence of the contamination area, the epidermal thickness, and the percutaneous absorption on them. Materials and Methods: Monte Carlo simulations using the code PHITS 3.02 were carried out to study and quantify the dosimetry conversion factors of 15 frequently used radionuclides (11C, 18F, 36Cl, 54Mn, 60Co, 90Sr, 99 mTc, 123I, 131I, 137Cs, 153Sm, 177 Lu, 223Ra, 226Ra, and 241Am). Results: The absorbed dose to the skin is significantly influenced by epidermal thickness and percutaneous absorptions and can differ by up to two orders of magnitude with respect to the operational magnitude H'(0.07,0°). Conclusions: Skin dosimetry after a contamination incident may be complex because the absorbed dose delivered to the basal layer is influenced by the contamination area, the epidermal thickness, and the percutaneous absorption. Therefore, when an accident occurs, the dose should be quantified taking into account these parameters, especially the epidermal thickness, and the possible percutaneous absorption should be evaluated in cases where the contamination involves a dose approximately equivalent to the established limits.



How to cite this article:
Pasquevich I, Velasco FM, Andres P. Calculation of skin dose rate conversion factors due to surface contamination from frequently used radionuclides in local nuclear and medical facilities.J Med Phys 2022;47:219-224


How to cite this URL:
Pasquevich I, Velasco FM, Andres P. Calculation of skin dose rate conversion factors due to surface contamination from frequently used radionuclides in local nuclear and medical facilities. J Med Phys [serial online] 2022 [cited 2022 Dec 8 ];47:219-224
Available from: https://www.jmp.org.in/text.asp?2022/47/3/219/360598


Full Text

 Introduction



The use of nonsealed radioactive sources is a common practice in local nuclear and radiological facilities, such as nuclear medicine services and research reactors, which may lead to the overexposure of workers. In addition to the external dose received as a result of the use of sealed vials and syringes, radionuclides handling implies contamination risks as well. International organizations suggest an annual equivalent skin dose limit of 500 mSv,[1] which can be easily exceeded if no appropriate radiation protection measures and decontamination procedures are applied. Skin dose can be computed using specific conversion factors for each radionuclide.[2],[3]

A conversion factor links the protection and operational quantities to physical quantities characterizing the radiation field.[4] The absorbed dose can be determined by applying skin dose conversion factors, for which the activity delivered on the contamination area has to be measured. This measurement is sometimes conditioned by time or by the radiation monitors available, which affect the correct determination of the contamination area.[2] Conversion factors can be determined by software-based methods, expedient field methods, or experimental methods.[2],[4] Every single method has its advantages and disadvantages. Software-based methods are easier and cheaper to implement than experimental methods and the assessment of uncertainties can be reduced to a minimum.

The international commission on radiological protection (ICRP) suggest that the skin dose should be computed at a nominal depth of 70 μm,[5] since the basal layer can be found in a depth range between 50 μm and 100 μm in most parts of the body. However, the contamination with radionuclides is more likely to happen in the hands, where a usual epidermal thickness between 85 μm and 369 μm has been reported.[6] Due to the short range of alpha and beta particles, the bigger the epidermal thickness, the more important the changes in the absorbed dose delivered to the basal layer.

Previous research works have shown that several radionuclides can be rapidly absorbed by the skin, and as a result, an ineffective decontamination can lead to percutaneous permeability of these radionuclides. In case of a percutaneous absorption, the absorbed dose delivered to the basal layer can differ from situations where the radioactive source is placed on the skin surface.[7],[8],[9],[10]

In brief, the contamination area, the epidermal thickness, and the percutaneous absorption can have a big influence when computing the skin dose conversion factors. This work has focused on the study of these influencing factors in 15 radionuclides commonly used in local nuclear and medical facilities.

 Materials and Methods



All the calculations were performed using Monte Carlo simulations. In this work, Particle and Heavy Ion Transport Code System, version 3.02[11] was chosen to fulfill this task due to its advantages (free license cross-platform software, relevance in various radiation types, ability to model the transport of nuclides in materials). To study the influence of the contamination area and the epidermal thickness, the geometry illustrated in [Figure 1] was used. The absorbed dose was calculated in a cylindrical volume of cross-sectional area 1 cm2 and height 10 μm, centered on a 15 cm × 15 cm × 3 cm slab and located at a certain depth proportional to the epidermal thickness (70 μm, 140 μm, 220 μm, or 370 μm according to the study). The slab and the cylinder are composed of 'skin' material whose composition is given in [Table 1]. This composition is as recommended in ICRP Publication 110.[12] The contamination is simulated by a disk of variable area located at 1 μm from the surface of the slab.{Figure 1}{Table 1}

For the analysis of the percutaneous absorption, the geometry illustrated in [Figure 2] was used. The slab and the cylinder are still the same. However, in this case, the source is no longer a surface disk and becomes a pair of cylinders internal to the slab with dimensions equivalent to the thickness of the epidermis and dermis. As the distribution of the activity of each radioisotope after being absorbed is unknown, a uniform distribution was assumed. Two cases were considered separately, one with activity totally distributed in the epidermis and the other case with the totality in the dermis.{Figure 2}

Finally, for the calculation of the operational magnitude H' (0.07, 0°), the geometry used was quite similar to the [Figure 1], with only three differences. First, the dimension of the slab is now 30 cm × 30 cm × 15 cm; second, the cylinder's height is 50 μm and is located 50 μm from the surface of the slab (since the depth of the sensitive layer of most parts of the skin ranged from 50 μm to 100 μm[6]). Third, the composition of the slab and the cylinder is given by the International Commission on Radiation Units (ICRU) [Table 2].[4]{Table 2}

 Results and Discussion



[Table 3] shows the skin dose conversion factors computed at 70 μm deep when the contamination area varies. If the contamination must be quantified over a 1 cm2 area but the contaminated area differs in size, the conversion factors will be underestimated. When the contamination area is as large as 1 mm2, this underestimation can reach up to a maximum of 9%, which is linked to geometrical factors. This increase in the energy deposited is a consequence of the geometrical differences coming from the sources, since the larger the source area, the bigger the number of disintegrations outside the sensitive volume.{Table 3}

In case of a 100 cm2 contamination area, the skin dose conversion factors can reach up to a maximum of 20% compared to a 1 cm2 contamination area. This is mainly due to the contribution of high-energy electrons reaching the sensitive volume from outside the central area. In case of larger areas, beta disintegrations happening far beyond the sensitive volume will not reach it due to the electrons range. This is where gamma disintegrations become important. For example, when the 54Mn is analyzed, differences up to 51% can be found.

[Figure 3] shows the skin dose conversion factors as a function of the contamination area for five radionuclides (18F, 99 mTc, 60Co, 131I, and 241Am). The influence of the contamination area is described by asymptotic curves at both sides of the 1 cm2 area.{Figure 3}

When the influence of the epidermal thickness is analyzed, skin dose conversion factors values decrease rapidly as the basal layer depth increases, as can be seen in [Table 4]. A difference as large as 100% between the factors computed at 70 μm and those calculated at 370 μm may be observed. This difference is more evident in radionuclides such as 99 mTc, 123I, and 177 Lu, which could be explained by internal conversion low energy electrons (<150 keV). In this case, the absorbed dose delivered to the basal layer at 370 μm deep is caused mainly by photons, which only represent a small percentage at 70 μm deep. Similar results can be found in the literature.[13] In the rest of the radionuclides analyzed, the skin dose conversion factors values decrease smoothly because the energy deposited by electrons remains high.{Table 4}

In [Figure 4], the results obtained for different epidermal thicknesses can be seen. The 99mTc curve [Figure 4]b shows a pronounced decrease in the skin dose conversion factor value below 200 μm deep, a depth related to the range of 120 keV internal conversion electrons. Beyond 240 μm deep (range of the 140 keV internal conversion electrons), the absorbed dose is primarily due to photons.[13] When the 241Am is analyzed [Figure 4]e, this decrease is much more obvious because the range of the 5.4 MeV alpha particles is only 40 μm, which delivers high doses in shallow depths and negligible doses at deeper depths.{Figure 4}

The values in [Table 5] and [Table 6] show that the absorption of the radioisotope in the epidermis and dermis strongly influences the absorbed dose in the basal layer, given that these values are significantly higher than those obtained from a source in contact with the skin [Table 4]. When the activity is entirely absorbed in an epidermis of 70 μm, the difference is limited to a factor of two [column 1, [Table 4] vs. column 1, [Table 5]], although, in an epidermis of 370 μm, this difference can be up to two orders of magnitude [column 4, [Table 4] vs. column 4, [Table 5]]. It is worth noting the exception implied by the radioisotopes with -decay; in this case, the factors can increase up to four orders of magnitude due to the fact that there is no longer a complete attenuation of particles before the basal layer.{Table 5}{Table 6}

In the situation in which the activity is entirely absorbed in the dermis, the factors decrease reaching a maximum difference of 70% [Table 4] vs. [Table 6]. Hence, in conclusion, the absorption in the dermis and epidermis can also increase or decrease the factor in a significative way.

Finally, [Table 7] details the difference between the skin conversion factors and the conversion factors of H'(0.07,0°). In all the cases, the H'(0.07,0°) conversion factor overestimate in a small percentage (<12%) the skin conversion factor. This discrepancy is due the difference between the CSDA (continuos slowing down approximation) range of the ICRU and the “skin” material.[14]{Table 7}

 Conclusions



Skin dosimetry after a contamination incident may be complex because the absorbed dose delivered to the basal layer is influenced by several factors. One of these factors is the contamination area. Nevertheless, since the skin is usually contaminated by small splashes rather than large surfaces, the skin-absorbed dose might be slightly underestimated.

On the other hand, the skin-absorbed dose in the basal layer is also influenced by the epidermal thickness. Depending on where the contamination occurs, the conversion factors can be thoroughly modified (up to 100%). Therefore, a nominal deep of 70 μm will involve an important overestimation of the conversion factor when the contamination occurs in the hands.

When the percutaneous absorption is taken into account, the use of conversion factors might be limited if they are computed based in geometries where the radioactive source is located above the skin surface. In this work, both in the dermis layer and the epidermis layer, a uniform distribution of the activity was assumed. Even though this is a first approach, it shows that the skin absorption can both increase and decrease the absorbed dose delivered to the basal layer.

The use of the quantity H'(0.07,0°) might not be appropriate in case of skin contamination. This is because this contamination usually occurs in the hands, where a 70 μm deep is not justified.

The skin conversion factors computed here can be applied to estimate the skin dose in hands when the appropriate epidermal thickness is used. Nevertheless, these factors were computed by software simulations where the radioactive source was located over the skin surface. That is, the influence of a feasible percutaneous absorption was not taken into account.

Acknowledgment

This work i spart of an own thesis,[15] carried out at the Balseiro Institute with the aim of obtaining a master's degree in medical physics. Therefore, a special thanks to the professionals of the Balseiro Institute for the space and knowledge provided.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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