TLD environmental monitoring of new scanner facilities at the Nuclear Medicine Department of the Taiwan Medical University Hospital

1.1PET/CT facility

2-deoxy-2-[18F]-fluoro-d-glucose, 18F-FDG injections must still be prepared manually due to a large number of patients, so extra doses to staff performing the work are unavoidable. The main application of 18F is in PET/CT, administered as 18F-FDG in the injection room (I) to allow glucose cell metabolism that in turn detects tumor processes. PET/CT is considered among the best diagnostic imaging techniques, since it provides both morphological and functional information about the organ. 18F is a positron emitter that has characteristic annihilation photons, 511 keV.

Prior to diagnosis using 18F-FDG, each patient was asked to wait in the activation box (Act 1–3) for 45–60 minutes to ensure thorough distribution. The patient remained in the exclusive toilet (T) for approximately two minutes to empty the bladder before being moved to the imaging room (C1).

The patient was positioned on the PET/CT camera bed of the facility (C1) for the transmission scan. The PET/CT scans took 20 minutes and were performed twice in each patient. The second scan was performed almost 3–4 hour after the first, and the patient was asked to wait in the activation box during the interval. PET/CT scans took 20 minutes in the PET/CT imaging room (C1), and the patient spent more time inside the PET/CT facility. Finally, the patient could leave the NM of CSMUH.

1.2SPECT/CT facility

Consequently, new occupational radiation safety problems had to be resolved. The most widely used radionuclide in the NM is 99⁢mTc, not only in the acquisition of heart and skeletal images but also those of the bladder, liver and lungs. Patients can also be diagnosed using 99⁢mTc-methylene diphosphate (99⁢mTc-MDP), which emits 140 keV photons during an SPECT/CT scan [6]. Moreover, 67Ga is the main radionuclide used to diagnose inflammatory diseases. In addition, 201Tl is used to obtain myocardial images. 131I can be used in the diagnosis as well as treatment of the thyroid gland.

The thyroid uptake and whole-body retention of 131I were measured by SPECT. Regions of interest could be drawn over the thyroid organ on the each scan. Patient whole-body scanning was connected and the NaI (Tl) detector was positioned at fixed distances of 5 cm from the body. An approximate 30 minute acquisition time yielded whole-body scintigraphy images in the imaging room (A1) [4].

Additionally, prior to SPECT/CT scans using 99⁢mTc-MDP, the injection took 30–60 seconds in the injection room (I). In the waiting room (W) and in the consultation room (F), each patient was asked to wait about 20 minutes to four hours prior to meeting with a medical doctor, respectively. The administered activities of 201Tl and 69Ga range from 3 to 5 mCi (11.8 to 18.5 MBq).

In this study, thermoluminescence dosimetry (TLD) was used to measure environmental radiation among the NM areas. This high energy radiation poses different radiation hazard problems. Therefore, it became necessary to evaluate a possible increase in environmental radiation rates due to the operation of the new ITC, SPECT, and DXA. Special attention is necessary to optimize radiation safety among these facilities of NM to minimize environmental radiation for the safety of personnel and the public.

2.1TLD-100 system

TLD-100H has been widely used to detect environmental radiation, since it indicates long-term accumulation during the last 30 years [7]. The TLD-100H was composed of LiF: Mg, Cu, P, 3.2 × 3.2 × 0.9 mm3. The Harshaw 3500 reader could be used to measure irradiated TLDs. These chips automatically fall under the one month survey [7]. In addition, a highly sensitive TLD-100H was used to map the photon leakage dose rate within the NM of Lin Hsin Hospital during the Fukushima nuclear plant accident. Three TLDs were put in one bag to eliminate the fluctuation in the TLD-100H; thus, 120 TLDs could represent 40 measurement locations by reusable TLDs.

The TLDs were calibrated using known activities, 10 mCi (37 MBq) 131I capsule (Global Medical Solutions Taiwan, Ltd., Taiwan). First, the ion chamber [PTW Farmer chamber type 30013 (0.6 cm3 made by Best Medical Com., USA)] was placed at 0.5–6 meters away from the 131I source and exposed to earn radiation rates. Second, the TLDs were irradiated with this source for 62 hours.

2.2Statistical analysis and weighting factor

In order to compare the fluctuations of the TLDs, three TLDs were put in one bag, so that 120 TLDs could be represent 40 measured locations. To ensure batch homogeneity, the sensitivity of each TLD (Si) was described as:

where Si is the sensitivity of the ith TLD; Ri is the reading on the ith TLD in this bag, and R¯ is the average of all TLD readings.

These radiation rates can be evaluated from various measured locations under the same conditions. The weighted mean (Rw) of R is expressed by [11]:

Standard error (σi) of the ith TLD (Ri) can be expressed as [11]:

The weighting factor (Wi) of ith TLD is derived from standard error (σi), as:

The weighted standard error of each bag σ(Aw) is given by:

2.3Surveying environmental radiation at the NM center

To evaluate environmental radiation, TLD was randomly suspended at one meter above the floor to represent exposure doses for a person standing among the NM [10]. The measurements were taken for a one month survey and were repeated three times from 1 October to 31 December 2018. Two bags (six TLDs) were used to monitor the environmental radiation rates in Taichung, 750 meters and 3000 meters away from NM, respectively, which were labeled TC1 and TC2. Four bags (12 TLDs) were used to monitor background radiation in the low background lab of CSMU [3, 7]. The results for each bag were averaged. The fluctuations, calibration, and counting statistics of the Harshaw 3500 reader for the calibrated TLD-100H were within 10% [7, 12].

3.1TLD-100H calibration

The irradiated TLD chips were read by Harshaw 3500 reader automatically on the following reading cycle. First, TLD is pre-heating temperature 135∘C. Second, pre-heating time is 10 sec, and holding 1 s. Third, then acquisition temperature is 240∘C at linear heating rate 10∘C/s. The acquisition time is 23 s. Before irradiation, the TLD-100H was heated up to 240∘C for 10 min in a furnace 47900 [3, 7]. The Y (mSv) = 0.0078 × x (nC) -0.1304 is the conversion factor of TLD-100H. In addition, R2, the square of the correlation coefficient, was equal to 0.9998. The TLD approach was chosen due to its high sensitivity and easy availability for environmental measurements due to a high published R2 value [3, 7].

3.2Environmental radiation

The TLDs measurements among the SPECT, DXA, PET/CT, and SPECT/CT facilities show a huge gradient that environmental radiation rates varied from 0.15 ± 0.04 to 1.00 ± 0.15 mSv per month, corresponding to the dataset of 40 measured points. All measurement data were not only obtained at one meter above the floor but also on the ceiling above the “O” ring of the scanners.

From the SPECT imaging room (A1), an explicit photon dose rate of 0.90 ± 0.15 mSv per month was measured, which is 5.88 times those measured at the nurse station (N) (0.16 ± 0.04 mSv per month). Hence, these imaging rooms (A1–D1) are classified as “hot” areas, with maintenance access only as absolutely necessary. Thus, RC/Pb equivalent walls can suppress extra radiation leakage. The high X-ray field and 364 keV show that the public and staff must be careful to avoid extra radiation during SPECT operations.

Following the three month survey, the average environmental radiation in the vicinity of the ceiling “O” ring of the SPECT ranged from 0.28 ± 0.04 to 0.90 ± 0.15 mSv per month in the SPECT imaging room (A1). The dose rate decreased markedly with increasing distance from the gantry head of the 670 DR Discovery.

The highest environmental radiation rates came from patient 18F-FDG as well as X-ray leakage from the PET/CT facility, because the measured environmental radiation was approximated from the background radiation in the activation rooms (Act1-3), and in our low background lab were far from these facilities (Fig. 1). Furthermore, the effectiveness of the RC/Pb in the PET/CT control room (C) was 0.90/0.15 = 6.00.

Table 2 lists the environmental radiation in the NM at the 40 measured points. The highest radiation was obtained in the imaging room of the SPECT/CT facility (D1). The individual environmental radiation at points in D1 ranged from 0.76 ± 0.12 to 1.00 ± 0.15 mSv per month.

Shielding ranged from 0.13 ± 0.02 to 0.90 ± 0.15 mSv per month by the CR/Pb shield in the SPECT/CT control room (D). The radiation rates among radioimmunoassay laboratory (E), corridor (R1-10), consultation room (F), and the nurse station (N) ranged from 0.13 ± 0.02 to 0.19 ± 0.03 mSv per month. Staff, patients, and relatives in these locations were not at significant hazard of exposure. This is primarily due to the fact that RC/Pb effectively attenuates penetrating photons. The estimated radiation reasonably agrees with that in TC1 and TC2. The environmental radiation in the PET/CT facility on the upper floor surface which is immediately above C1 and on the lower floor of basement 2 which is below C1 were 0.14 ± 0.03 and 0.15 ± 0.04 mSv per month, respectively. This is in accordance with published data, and the ambient doses determined herein must meet the local radiation protection regulations. The highest evaluated values are far lower than the occupational dose limit for controlled areas, as those are 50 mSv per year and 100 mSv per 5 years [4, 13]. Comparisons with previous data show no main public health impact at the Taiwan Medical University Hospital. No significant contributions were measured.

3.3Minimum detectable dose (MDD) of TLD method

The values of radiation rates have a confidence level of 95% and were evaluated by the minimum detectable dose (MDD). When the environmental radiation rates were measured at several points at the same distance, the average environmental radiation rate at these points was used. All measurements were performed three times. These TLDs were subsequently counted and analyzed to evaluate the distribution of environmental radiation within the NM. Many of the detected points of environmental radiation rates were uncertain because the corresponding photon doses were under the MDD [12, 14]. The net counts (Δ𝑛𝑒𝑡) of the MDD values in the evaluation of the glow curve were obtained using a Harshaw 3500 reader. The Δ𝑛𝑒𝑡 has significant meaning, kα is the standard deviation of the net count rate which is given as a single-tail area of α. kβ is the standard deviation with an area β to the left tail. B presents the gross background count. In the statistical results, either α or β is equal to 0.025, which implies the 95% confidence level. Therefore, kα and kβ were both equal to 1.96. Δ𝑛𝑒𝑡 and can be simplified to Eq. (6) [14].

Kα= 1.645; Kβ= 1.645; B= Gross background count

Figure 2.

Both peak A or B corresponded to nearly the same gross count. The gross count is equal to background count (B) adds net count (Δ𝑛𝑒𝑡) which is written in the region of interest, ROI.

Accordingly, the Δ𝑛𝑒𝑡 for fulfilling the criteria of MDD increases with the background counts. Figure 2 indicates the probability density of the gross background count as well as the Δ𝑛𝑒𝑡 in identifying the measured radiation rates written from a Harshaw 3500 counter. Either peak A or B correspond to nearly the same gross count (gross count is equal to background count adds Δ𝑛𝑒𝑡 which is written in the region of interest, ROI). Additionally, the data obtained from peak B is more reliable than that from peak A.

The derived measurements could be re-evaluated to obtain these MDD. A total of 82 of 120 analyzed results to identify radiation from the TLDs failed, due to the 95% confidence level. The unqualified glow curves from the TLDs were mostly around the free and controlled areas within the NM, indicating that only a negligible amount of γ-rays were emitted from the radiopharmaceuticals (18F, 67Ga, 99⁢mTc, 131I, and 201Tl) within patients and from the scanners in the NM.

3.4Environmental radiation for personnel

The “hot” points in the radiology personnel control rooms were near the PET/CT control room (A). The extra environmental radiation rates a year for a radiologist who was stationed in the control room (B) of the PET/CT facility was:

This extra radiation dose rate is far lower than that of the annual limit of 50 mSv per year recommended by the ICRP 60 [10]. The results show that the RC/Pb shielding can adequately protect hospital personnel. Hence, the penetrating photons cause no health concerns for medical personnel in the CSMUH during the operation of these scanners. However, due to the ALARA principle, the actual extra radiation in the vicinity outside the NM is worth measuring in future work.