Localized Irradiation of the Skin: Estimation of Dose, Excess Cancer Risk, and 

Probability of Causation/Assigned Share of Skin Cancer 

David C. Kocher and A. Iulian Apostoaei 

Oak Ridge Center for Risk Analysis 

Oak Ridge, Tennessee 

January 15, 2014 

A recent report by Mauro and Behling (2013) raised concerns about use of the Interactive 

RadioEpidemiological Program (IREP) to estimate the probability of causation/assigned share 

(PC/AS) of diagnosed skin cancers when irradiation of the skin is localized. Such a scenario 

would occur, for example, when the dose to skin is due to deposition of radioactive particles on 

the hands, face, or neck. Mauro and Behling (2013) also raised concerns about methods used by 

the National Institute for Occupational Safety and Health (NIOSH) to estimate the dose to skin 

due to localized deposition of radioactive particles on the body surface. 

The purpose of this report is twofold. First, we discuss the use of IREP to estimate 

PC/AS of diagnosed skin cancers when irradiation of the skin is localized. Second, we discuss 

the approach used by NIOSH to estimate the dose to skin and its uncertainty in cases of localized 

deposition of radioactive particles on the body surface (ORAUT, 2005). 

1. Use of IREP to Estimate PC/AS in Cases of Localized Irradiation of Skin 

PC/AS of a diagnosed cancer in an individual due to exposure to ionizing radiation is 

defined as: 

PC/AS = EAR/(EAR + B) , 

where EAR is the excess absolute risk (probability) of the cancer type of concern due only to that 

individual’s radiation exposure and B is the baseline risk (probability) of that cancer due to all 

other causes. This is the representation of PC/AS discussed by Mauro and Behling (2013). 

In IREP, PC/AS of a diagnosed cancer in an exposed individual is calculated on the basis 

of an estimate of the excess relative risk (ERR) due to radiation, rather than EAR (Land et al., 

2003; Kocher et al., 2008). ERR, which is not a probability, is defined as RR – 1, where RR is 

the relative risk (i.e., the total risk of the cancer type of concern in an exposed individual relative 

to the baseline risk) given by (EAR + B)/B. The relationship between ERR and EAR then is 

ERR = EAR/B , 

and PC/AS is calculated in IREP as 

PC/AS = ERR/(ERR + 1) . 

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In IREP, ERR for skin cancer (and many other cancer types) is assumed to be a linear 

function of dose and is estimated as the product of a risk coefficient (ERR per Sv), denoted by 

ERR/Sv, and the dose to skin (Sv). The assumed risk coefficients for skin cancer as a function of 

an individual’s age at the time of exposure and attained age (age at diagnosis) are based on data 

in Japanese atomic-bomb survivors, who received uniform exposures of the whole body. 

The basic issue with using IREP to calculate PC/AS of a diagnosed skin cancer when 

irradiation of the skin is localized that was raised in Section 6.0 of the report by Mauro and 

Behling (2013) can be stated as follows: 

Given that estimates of ERR/Sv for skin cancer in IREP are based on estimates of excess 

risks of skin cancer, regardless of their location on the body surface, due to uniform 

exposure of the whole body and estimates of baseline risks of skin cancer, also regardless 

of location, can those risk coefficients be used to calculated PC/AS when irradiation of 

the skin is localized? Or, is some adjustment of the ERR/Sv for skin cancer in IREP 

required to give a proper estimate of PC/AS in cases of localized irradiation? 

The following argument suggests that estimates of ERR/Sv for skin cancer in IREP can 

be used to estimate PC/AS in cases of localized irradiation of the skin or, more generally, in 

cases of non-uniform irradiation. If the area of the skin in which a localized irradiation occurs is 

a fraction fskin of the total area of the skin, estimates of EAR and B for skin cancers at all 

locations, which are used to estimate ERR/Sv for skin cancer in IREP, can be reduced by the 

fraction fskin to give estimates appropriate to the location of interest; i.e., in cases of localized 

irradiation, EARlocal = fskin × EAR and Blocal = fskin × B. From these relationships, it follows that 

ERRloc = EARloc/Bloc = EAR/B is independent of fskin and, therefore, estimates of ERR/Sv in 

IREP can be applied to any localized or non-uniform irradiation of the skin. 

This conclusion comes with a potentially important caveat. An assumption that estimates 

of ERR/Sv for skin cancer in IREP can be used to estimate PC/AS of a diagnosed skin cancer for 

any localized or non-uniform irradiations of the skin is strictly correct only if either of the 

following two conditions is met: (1) EAR and B are uniform over the body surface; or (2) EAR 

and B vary with location in the same way, i.e., the ratio of EAR to B is the same at all locations. 

In either case, ERR = EAR/B would be the same at any location. 

It is doubtful, however, that either condition described above is met. The first condition 

(EAR and B are uniform over the body surface) clearly is not met in Caucasians for whom 

baseline rates of skin cancer on the face, head, and neck, which comprise a small fraction of the 

total area of the body surface, are much higher than baseline rates in all other regions combined 

(Scotto et al., 1996). The non-uniformity of baseline rates of skin cancer over the body surface 

in Caucasians is due to the primary importance of ultraviolet (UV) radiation in causing skin 

cancer and the effect of clothing in shielding much of the body surface from UV radiation. 

In regard to the second condition (EAR and B are not uniform over the body surface but 

EAR/B is independent of location), unless there is a strong multiplicative interaction between 

ionizing and UV radiation in causing skin cancer (i.e., EAR due to radiation is roughly 

proportional to B), the distribution of EAR for skin cancer over the body surface presumably 

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would be more uniform than the distribution of B, in which case ERR = EAR/B would be lower 

in skin of the face, head, or neck than elsewhere. This difference would be substantial if, for 

example, EAR due to radiation were approximately uniform over the body surface. 

The relationship between excess risks of skin cancer due to radiation and baseline risks in 

different regions of the body in Japanese atomic-bomb survivors was investigated by Ron et al. 

(1998); see also Kishikawa et al. (2005) and Preston et al. (2007). For non-melanoma skin 

cancers including basal cell carcinomas (BCCs), which are the most common in Caucasians 

(Scotto et al., 1996), EARs in atomic-bomb survivors were rather similar in UV-exposed and 

UV-shielded regions and, thus, ERRs were significantly higher in shielded regions. 

Current data on induction of non-melanoma skin cancers in Japanese atomic-bomb 

survivors appear to be inadequate to investigate in detail the relationship between EAR due to 

radiation and the baseline risk in different regions of the body surface. Given that the estimated 

number of excess non-melanoma skin cancers in Japanese atomic-bomb survivors is only about 

12% of the total number of cases (Preston et al., 2007), we believe that it would be difficult to 

evaluate the dependence of EAR on location on the body surface with a meaningful degree of 

confidence, even if the locations of all such skin cancers in exposed and unexposed groups of 

survivors were known. 

Given the difficulties in estimating the relationship between EAR and B for skin cancers 

over the body surface, we believe that the only reasonable option at the present time is to assume 

that ERR = EAR/B does not vary substantially over the body surface and can be used to estimate 

PC/AS of skin cancer in cases of localized or non-uniform irradiation of the skin. To the extent 

that EAR is independent of B (i.e., the interaction between ionizing and UV radiation in causing 

skin cancer is additive), as suggested in studies of atomic-bomb survivors, this assumption 

should result in overestimates of ERR = EAR/B and PC/AS in regions, such as the hands, face, 

or neck, that usually are not covered by clothing and where baseline risks are the highest. These 

are the regions where localized irradiation due to deposition of radioactive particles on the body 

surface is most likely to occur. Therefore, although estimates of ERR/Sv for skin cancer in IREP 

are not biased (i.e., they are based on estimates of EAR for uniform whole-body exposure and B 

for all skin cancers regardless of location), we believe that use of IREP to calculate PC/AS for 

skin cancer in cases of localized irradiation of the hands, face, or neck as prescribed by NIOSH 

(ORAUT, 2005) is claimant-favorable. 

We caution, however, that use of estimates of ERR/Sv for skin cancer in IREP to 

calculate PC/AS in cases of localized irradiation of skin in regions of the body surface that 

usually are covered by clothing probably is not claimant-favorable. This situation would occur, 

for example, when radioactive particles are deposited and retained on clothing. If, as indicated in 

studies of atomic-bomb survivors, EAR due to radiation is more uniform over the body surface 

than the baseline risk, ERR/Sv in regions of the body surface that normally are covered should 

be greater than estimates in IREP and, therefore, PC/AS of diagnosed skin cancers in those 

regions should be underestimated. We also note, however, that doses to skin from contamination 

of clothing should be less than doses from the same level of contamination on bare skin, due to 

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such factors as the frequent changing and washing of clothing and the shielding effect of clothing 

in reducing dose rates from beta particles.1 

2. Estimation of Dose in Cases of Localized Irradiation of Skin 

The approaches used by NIOSH to estimate radiation doses to skin in cases of localized 

deposition of radioactive particles on the body surface or other partial-body irradiations are 

described in Section 3.0 of ORAUT (2005). Three different scenarios are considered: 

• it is known that a diagnosed skin cancer occurred within an area of contamination or 

partial-body irradiation; 

• it is known that a diagnosed skin cancer did not occur within an area of contamination or 

partial-body irradiation; 

• it is not known whether or not a diagnosed skin cancer occurred within an area of 

contamination or partial-body irradiation. 

The approaches to estimating doses to skin in the first two scenarios are straightforward. 

In the first, the dose to skin at the location of a skin cancer is calculated as prescribed by NIOSH 

and entered into IREP, without adjusting for the area of the skin of interest relative to the total 

area of the skin over the entire body. As described in the previous section, estimates of ERR/Sv 

in IREP are considered appropriate for use in estimating PC/AS in such cases and should be 

claimant-favorable when the area of contamination or partial-body irradiation usually is not 

covered by clothing. In the second scenario, the assigned dose in the area where a diagnosed 

skin cancer occurred due to contamination or partial-body irradiation in another area is zero.2 

The third scenario is more challenging, because the “true” dose is either the dose within 

the area of contamination or partial-body irradiation or it is zero (or a lower dose due, for 

example, to an additional uniform whole-body exposure), but it is not known which of the two 

doses is correct. In such cases, an estimate that best represents the state of knowledge of the true 

but unknown dose is the dose to skin averaged over the whole body. The “correct” 

representation of this average dose is a binomial probability distribution with a weight fskin given 

to the dose Dlocal, where Dlocal is the dose to skin in the irradiated region and fskin again is the 

fraction of the total area of the skin in that region, and a weight (1 – fskin) given to the dose in all 

other regions, Dother. The mean of this distribution is a dose of fskinDlocal + (1 – fskin)Dother. 

A binomial probability distribution to represent an uncertain dose cannot be entered into 

IREP at the present time.

3 The approach used by NIOSH is to assume an equivalent lognormal 

1

 Irradiation of the basal layer of the skin due to deposition of alpha-emitting radionuclides on the body 

surface also could be important, depending on the thickness of the epidermis in the contaminated area and 

the energies of alpha particles. Models and data that can be used to estimate dose to skin due to 

deposition of alpha-emitting radionuclides on the body surface are discussed in Section 4.1.5 of NCRP 

Report No. 163 (NCRP, 2009). 

2 The assigned dose of zero assumes that a radiation-induced skin cancer can occur only in areas that are 

irradiated, i.e., that abscopal effects do not occur. 

3

 IREP could be modified to accept a binomial probability distribution of dose or any other distribution 

that would better represent the state of knowledge of an uncertain dose than currently available options. 

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probability distribution of the dose (ORAUT, 2005). The mean of the binomial probability 

distribution calculated as indicated above is assumed to be the median (50th percentile) of a 

lognormal probability distribution. The geometric standard deviation (GSD) of that distribution 

is defined such that the 100(1 – fskin)-th percentile is approximately at the dose Dlocal; i.e., the 

probability that the dose is Dlocal or greater is assumed to be approximately fskin. 

For example, if we assume, as in a case discussed by Mauro and Behling (2013), that 

(1) the dose to skin on the head, face, and hands is 16 mrem, (2) the skin in those regions 

comprises 14% of the total area of the skin, and (3) the dose in other regions is zero, the mean 

dose to the skin is (0.14)(16 mrem) = 2.2 mrem, and this dose is assumed to be the median of a 

lognormal probability distribution. The GSD then is defined by setting the dose of 16 mrem at 

the 86th percentile of the lognormal distribution, which gives a GSD of 6.3. 

Rather than estimate the GSD of an assumed lognormal probability distribution of the 

dose for each case on the basis of the appropriate fraction of the total area of the skin that is 

irradiated, NIOSH simplifies the method by defining ranges of the ratio of the non-irradiated 

(other) area of the skin to the irradiated (localized) area [the ratio (1 – fskin)/fskin] over which the 

GSD is assigned integer values that approximate the condition described above (ORAUT, 2005). 

For example, if the ratio of the non-irradiated area of the skin to the irradiated area is in the range 

of 1 to 10, a GSD of 6 is assumed, and if that ratio is in the range of 10 to 100, a GSD of 8 is 

assumed. We verified by example calculations that the integer values of the GSD assigned by 

NIOSH for defined ranges of the ratio (1 – fskin)/fskin are approximately equal to or greater than 

the GSD that would be calculated by setting the 100(1 – fskin)-th percentile of the assumed 

lognormal probability distribution at approximately the dose Dlocal.

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In agreement with a conclusion in the ORAUT (2005) report, we believe that the 

lognormal probability distributions of dose used by NIOSH when the location of a skin cancer in 

relation to the area of a localized irradiation is not known are claimant-favorable; i.e., they are 

more likely than not to result in overestimates of PC/AS of diagnosed skin cancers. One source 

of conservatism in the lognormal probability distributions of dose used by NIOSH is that the 

mean dose, which is the expectation value, is always greater than the median; i.e., the mean of an 

assumed lognormal probability distribution of dose is always greater than the mean of the 

“correct” binomial distribution. 

A second source of conservatism is a result of the assumption used by NIOSH to define 

the GSD of the equivalent lognormal probability distribution of dose. In the example discussed 

by Mauro and Behling (2013) and described above, the probability distribution used by NIOSH 

assumes that there is a probability of about 14% that the dose to skin in the irradiated region is 

greater than 16 mrem. In that case, for which NIOSH assumes a GSD of 6, the upper tail of the 

4

 In the example discussed by Mauro and Behling (2013) and described above, in which the irradiated 

area comprises 14% of the total area of the skin, the calculated GSD of 6.3 is slightly greater than the 

integer value of 6 assigned by NIOSH. However, we do not believe that the difference between the two 

GSDs is significant, given that there is uncertainty in the fraction of the total area of the skin that is 

irradiated (fskin) and the dose to skin in that region (Dlocal). When the ratio (1 – fskin)/fskin is 10 or higher, we 

found that the integer value of the GSD assigned by NIOSH is always greater than the value that would 

be calculated using that ratio. 

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assumed lognormal probability distribution extends well above 16 mrem; e.g., the 95th percentile 

is about 40 mrem. This assumption clearly is conservative if the estimated dose of 16 mrem in 

the irradiated region is accurate, and it would result in an increase in the 99th percentile of the 

probability distribution of PC/AS used in adjudicating claims for compensation. 

In general, the degree of conservatism in the mean dose using the NIOSH approach (i.e., 

the ratio of the mean of an assumed lognormal probability distribution to the “true” mean of a 

“correct” binomial probability distribution when the latter is assigned as the median of the 

lognormal distribution) increases as the assumed GSD of the lognormal distribution (i.e., the 

ratio of the non-irradiated area to the irradiated area) increases. However, the degree of 

conservatism embodied in the upper tail of the assumed lognormal probability distribution (i.e., 

the fraction of the probability distribution that extends beyond the dose Dlocal) decreases as the 

assumed GSD increases (i.e., as the irradiated area decreases). 

On the basis of these considerations, we conclude that the approach used by NIOSH to 

estimate the dose to skin and its uncertainty when it is not known whether or not a diagnosed 

skin cancer occurred within an area of contamination or partial-body irradiation is reasonable 

and is unlikely to result in underestimates of PC/AS at the 99th percentile. 

3. Conclusions 

Discussions in this report have led to two conclusions, which are summarized as follows. 

• The assumption by NIOSH that estimates of ERR/Sv for skin cancer in IREP can be used 

to calculate PC/AS of diagnosed skin cancers in cases of localized irradiation of skin on 

the hands, face, or neck (ORAUT, 2005) should be claimant-favorable. 

This conclusion is based on two considerations: (1) estimates of ERR/Sv for skin cancer in IREP 

are based on data on excess risks due to uniform whole-body exposure and baseline risks 

regardless of location; and (2) data in atomic-bomb survivors indicate that EAR for skin cancer 

due to radiation is more uniform over the whole body than the baseline risk, which is due 

primarily to exposure to UV radiation in areas of the skin that normally are uncovered. 

Consequently, ERR = EAR/B in regions of the body surface that normally are uncovered should 

be less than estimates obtained using IREP, and PC/AS should not be underestimated. 

We caution, however, that the conclusion stated above probably is not valid when a 

diagnosed skin cancer occurs in a region of the body surface that normally is covered by 

clothing. In such cases, ERR = EAR/B should be greater than estimates obtained using IREP, 

and PC/AS should be underestimated. 

• In cases of localized deposition of radioactive particles on the body surface or other 

partial-body irradiations, the approach used by NIOSH to estimate the median dose to 

skin and its uncertainty when it is not known whether or not a diagnosed skin cancer 

occurred within the irradiated area should be claimant-favorable. 

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This conclusion is based on three considerations: (1) the “true” mean dose to skin should be 

overestimated; (2) the extent to which the dose to skin in an irradiated area could be exceeded 

should be overestimated; and (3) the assigned integer values of the GSD of an assumed 

lognormal probability distribution should be approximately equal to or greater than values that 

would be calculated by assuming that the 100(1 – fskin)-th percentile of the distribution is 

approximately at the dose in the irradiated area. 

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References 

Kishikawa, M., Koyama, K., Iseki, M., Kobuke, T., Yonehara, S., Soda, M., Ron, E., Tokunaga, 

M., Preston, D.L., and Tokuoka, S. (2005). “Histologic Characteristics of Skin Cancer in 

Hiroshima and Nagasaki: Background Incidence and Radiation Effects,” Int. J. Cancer 

117, 363–369. 

Kocher, D.C., Apostoaei, A.I., Henshaw, R.W., Hoffman, F.O., Schubauer-Berigan, M.K., 

Stancescu, D.O., Thomas, B.A., Trabalka, J.R., Gilbert, E.S., and Land, C.E. (2008). 

“Interactive RadioEpidemiological Program (IREP): A Web-Based Tool for Estimating 

Probability of Causation/Assigned Share of Radiogenic Cancers,” Health Phys. 95(1), 

119–147. 

Land, C., Gilbert, E., Smith, J.M., Hoffman, F.O., Apostoaei, I., Thomas, B., and Kocher, D.C. 

(2003). Report of the NCI-CDC Working Group to Revise the 1985 

Radioepidemiological Tables, NIH Publication No. 03-5387, U.S. Department of Health 

and Human Services, Washington, DC. 

Mauro, J., and Behling, U.H. (2013). Discussion Points Regarding Reconstructing Localized 

Skin Dose Associated with Direct Deposition of Uranium Oxide Dust and Flakes on 

Exposed Skin, Draft Report, SC&A, Inc., Vienna, Virginia. 

NCRP (2009). National Council on Radiation Protection and Measurements. Radiation Dose 

Reconstruction: Principles and Practices, NCRP Report No. 163 (National Council on 

Radiation Protection and Measurements, Bethesda, Maryland). 

ORAUT (2005). Interpretation of Dosimetry Data for Assignment of Shallow Dose, ORAUTOTIB-0017, Oak Ridge Associated Universities Team, Cincinnati, Ohio. 

Preston, D.L., Ron, E., Tokuoka, S., Funamoto, S., Nishi, N., Soda, M., Mabuchi, K., and 

Kodama, K. (2007). “Solid Cancer Incidence in Atomic Bomb Survivors: 1958–1998,” 

Radiat. Res. 168, 1–64. 

Ron, E., Preston, D.L., Kishikawa, M., Kobuke, T., Iseki, M., Tokuoka, S., Tokunaga, M., and 

Mabuchi, K. (1999). “Skin Tumor Risk Among Atomic-Bomb Survivors in Japan,” 

Cancer Causes and Control 9, 393–401. 

Scotto, J., Fears, T.R., Kraemer, K.H., and Fraumeni, J.F., Jr. (1996). “Nonmelanoma Skin 

Cancer,” pages 1313–1330 in Cancer Epidemiology and Prevention, Second Edition, ed. 

by Schottenfeld, D., and Fraumeni, J.F., Jr., Oxford University Press, New York. 

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