Behavior of the electron spin resonance signals in X-ray irradiated human fingernails for the establishment of a dose reconstruction procedure

Abstract

The retrospective dosimetry that follows accidental X-ray exposure is becoming more significant for improving radiation diagnosis and treatment. We investigated the dosimetric properties of electron spin resonance (ESR) signals in X-ray irradiated fingernails under conditions that resemble realistic situations. We collected fingernails from 12 Japanese donors between the ages of 30 to 70. The sampled fingernails were utilized for X-ray irradiation, mechanical stimulation and background measurements. We also collected 10 toenails from one of the donors to evaluate their differences from fingernails. Additionally, we prepared 15 samples from two donors to compare the signals generated by γ-rays to those by X-rays. After observing the linear dose-response for both X- and γ-ray irradiated samples, we found that the sensitivity of the air-absorbed dose of γ-ray irradiated samples was identical to that of X-ray irradiated samples. The effect from secondary electrons seemed to be small in fingernails. The inter-individual variation in the sensitivity was no greater than the intra-individual variation. The signal intensities in each measurement fluctuated about the linear response curve, and the size of the fluctuation was dependent on the sample. The average fluctuation corresponded to 1.7 Gy, and the standard deviation was 1.3 Gy. The signal induced by X-rays could be erased by soaking the samples in water and subsequently drying them for four days, which allowed us to estimate the signal intensity prior to the exposure. These characteristics of the ESR signal induced by X-rays facilitate the development of a feasible protocol for fingernail dose reconstruction.

Keywords: electron paramagnetic resonance (EPR); electron spin resonance (ESR); fingernail; retrospective dosimetry; toenail.

Fig. 1.

Typical examples of ESR spectrum from irradiated human nails with different modulation. Amplitude after linear baseline correction. Black, modulation amplitude = 1 mT; red, 0.25 mT; blue, 0.025 mT. Here, the y-axis is normalized by peak-to-peak amplitude to compare the spectrum shape and peak-to-peak width. We defined the signal intensity as the peak-to-peak amplitude with 1 mT modulation amplitude. It increased as given dose increased.

Fig. 2.

Typical dose–response curve for X-ray irradiation of signal from fingernails. Donor 1, right thumb. In this case, there is linearity in dose–response up to 93.6 Gy. The dotted line shows the best fit of the linear function to the data point.

Fig. 3.

The sensitivity distribution for all samples. The X-ray irradiated samples showed similar sensitivity to γ-ray irradiated samples for the same air absorbed dose.

Fig. 4.

The profile of absorbed dose along to depth of samples by 60 keV photons in PHITS calculation. The vertical axis is proportional to dose but in a.u. The build-up region is 0.8 mm and the absorbed dose increase 6% at the peak.

Fig. 5.

(a) The photon energy dependence of peak depth of build-up in the material assuming nail component in PHITS. The maximum was around 0.8 cm around 70 keV photon energy. (b) The photon energy dependence of build-up increment calculated by PHITS. One refers to the absorbed dose at the surface. The maximum increment was only 5.6% at peak with 60 keV case.

Fig. 6.

The ratio of absorbed dose in the virtual nail sample (1 cm long, 1 mm wide and 0.5 mm thick) by various energy photons to air-absorbed dose in the same radiation field.

Fig. 7.

The distribution of sensitivity for all donors in the case of the X-ray irradiation. The x-axis shows donor number. The inter-individual variation was not significantly larger than the intra-individual variation.

Fig. 8.

The distribution of sensitivity for finger position in the case of the X-ray irradiation. The x-axis shows finger position of fingernail samples (RT, right thumb; RI, right index; RM, right middle; RR, right ring; RB, right baby; LT, left thumb; LI, left index; LM, left middle; LR, left ring; LB, left baby). There was no significant effect of the fingernail positions on the sensitivity.

Fig. 9.

The ESR signal intensity versus total of additional cutting cross-section. The error size for cross-section was defined by propagation of the standard deviation of six measurements by the caliper. The open circle shows the result of sample A and the triangle shows the result of sample B.

Fig. 10.

The ESR signal intensity across four stages in our signal elimination experiment. We tried to eliminate both the RIS and MIS by soaking samples in water between stages 2 and 3. Stage 1—before irradiation; stage 2—just after 10 Gy X-ray irradiation; stage 3—after 4 days of drying; stage 4—after 6 days of drying. The signal sizes in stages 3 and 4 were expected to be the size of BKS.

Fig. 11.

The distribution of ESR signals of 144 non-irradiated fingernail samples from eight donors. The ESR signals should contain BKS and MIS generated at harvest. The MIS was increased for the storage time. The range we noted (0.17 ~ 0.28) in this histogram shows the rough estimation of the width of signal size when we take BKS, MIS and its increase under storage conditions into consideration.

Fig. 12.

The distribution of ESR signals of non-irradiated toenails from donor 2. The average was 0.11 and the standard deviation was 0.014. This distribution tended to be lower than that of fingernails, but it was still within the range of the fingernail distribution.

Fig. 13.

The distribution of fingernail and toenail sensitivity for X-rays from donor 2. The top histogram is for fingernails and the bottom is for toenails. The sensitivity of toenails was lower than that of fingernails.