Radiation Protection Dosimetry (2015), Vol. 165, No. 1–4, pp. 424 –429 Advance Access publication 9 April 2015
doi:10.1093/rpd/ncv130
PATIENT DOSES FROM HYBRID SPECT– CT PROCEDURES S. Avramova-Cholakova1,*, M. Dimcheva2, E. Petrova3, M. Garcheva3, M. Dimitrova4, Y. Palashev5 and J. Vassileva1 1 National Centre of Radiobiology and Radiation Protection, Sofia, Bulgaria 2 Sofia City Cancer Centre, Sofia, Bulgaria 3 University Multiprofile Hospital for Active Treatment ‘Alexandrovska’, Sofia, Bulgaria 4 Specialized Hospital for Active Treatment in Oncology, Sofia, Bulgaria 5 University Multiprofile Hospital for Active Treatment ‘St Ivan Rilski’, Sofia, Bulgaria *Corresponding author: [email protected]
INTRODUCTION Hybrid single-photon emission computed tomography and computed tomography (SPECT–CT) procedures are related to high radiation doses to patients due to the combination of two methods using ionising radiation and delivering relatively high doses. The first hybrid system was installed in Bulgaria only 5 y ago, and nowadays four SPECT– CT facilities are in operation. The contribution of CT modality to patient doses from SPECT–CT has not been studied in the country thus far. The aim of this work is to estimate patient doses from SPECT–CT procedures and to explore potential for optimisation.
MATERIALS AND METHODS All four SPECT–CT systems contributed to the patient dose study: two Symbia 2 T (Siemens Medical Solution) equipped with a 2-detector row CT, denoted further in the text as SI and SII; one Symbia T16 (Siemens Medical Solution) with a 16-detector row CT, denoted as SIII; and one Discovery NM/CT670 (GE Healthcare) with 16-detector row CT, denoted as SIV. Standard data collection form was developed and distributed to all departments, including patient data (age, sex, weight, height), data related to exposure from SPECT (radiopharmaceutical, radionuclide and the administered activity in MBq) as well as CTrelated data [dose indexes CTDIvol and dose length
product (DLP) available from the display, tube voltage, mAs, rotation time, slice thickness, etc.]. All data were retrospectively extracted from the archive of the examinations. Ten types of examinations were considered, representing all diagnostic procedures performed in SPECT–CT facilities. Administered activities of radiopharmaceuticals (RF) were averaged for all patients in a given system for a particular examination. Effective doses from the SPECT component were calculated multiplying the ICRP 53 and ICRP 80 conversion coefficients by averaged activities(1, 2). Computed tomography dose index (CTDI) and DLP values were averaged for all patients for a particular examination and system, separately for men and women. Effective doses from the CT component were calculated with CT-Expo software (version 2.1, Medizinische Hochschule, Hannover, Germany) for both ICRP 60(3) and ICRP 103(4) tissue weighting factors, using average CTDI and DLP as an input quantity. For a particular scanner, typical tube voltage (kV), tube current (mA), time of rotation (s), pitch, beam collimation and anatomic region scanned were used for calculation, separately for men and women. Parallel estimation of CT contribution was performed applying the National Radiological Protection Board (NRPB) conversion coefficients where applicable(5). In this simple method, effective dose is estimated by multiplication of DLP by a conversion coefficient dependent on the anatomic region scanned. These coefficients are derived for ICRP 60 tissue weighting factors(3) and are not separated by
# The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
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The aim of this work is to estimate patient doses from hybrid single-photon emission computed tomography (SPECT) and computed tomography (CT) procedures. The study involved all four SPECT– CT systems in Bulgaria. Effective dose was estimated for about 100 patients per system. Ten types of examinations were considered, representing all diagnostic procedures performed in the SPECT– CT systems. Effective doses from the SPECT component were calculated applying the ICRP 53 and ICRP 80 conversion coefficients. Computed tomography dose index and dose length product were retrospectively obtained from the archives of the systems, and effective doses from the CT component were calculated with CT-Expo software. Parallel estimation of CT component contribution with the National Radiological Protection Board (NRPB) conversion coefficients was performed where applicable. Large variations were found in the current practice of SPECT– CT imaging. Optimisation actions and diagnostic reference levels were proposed.
Table 1. The number of patients for which data were provided, type of radiopharmaceutical and administered activity, CTDI and DLP per type of examination and system. Examination
Bone imaging Somatostatin receptor Imaging Thyroid metastasis
Parathyroid imaging Old protocol New protocol Lung perfusion Myocardial perfusion stress þ rest Stress
Radiopharmaceutical
I II III I III IV I II I I II I III
64 9 18 42 35 13 43 11 12 7 10 14 10
99m
I II I II II II III IV II
10 20 7 10 10 20 14 19 21
99m
III
25
99m
Tc-MIBI Tc-MIBI 99m Tc-Tetrofosmin 99m Tc-MDP 99m Tc-MDP 99m Tc-MDP 99m Tc-Tektrotyd 99m Tc-Tektrotyd 131 I-NaI 131 I-NaI 131 I-NaI 99m Tc-Pertechnetate 99m Tc-Tetrofosmin 99m
Tc-Nanocoll Tc-Nanocoll Tc-Pertechnetate þ 99mTc-MIBI 99m Tc-Pertechnetate þ 99mTc-MIBI 99m Tc-MIBI 99m Tc-MAA 99m Tc-MAA 99m Tc-MAA 99m Tc-Tetrofosmin 99m 99m
Tc-Tetrofosmin
Amean (min., max.); SD (MBq) 705 (629, 740); 47 650 (555, 740) 370 (370, 370); 0 446 (259, 740); 167 592 (370, 740); 161 512 (370, 740); 154 733 (629, 740); 28 444 (444, 444); 0 167 (74, 185); 42 167 (74, 185); 42 74 (74, 74); 0 74 (74, 74); 0 370 (370, 370); 0 148 (148, 148); 0 56 (56, 56); 0 74 þ 714 (555, 740); 70 74 þ 555 (555, 555); 0 555 (555, 555); 0 104 (92, 111); 9.6 185 (185, 185); 0 185 (185, 185); 0 370 þ 555 (555, 555); 0 555 (555, 555); 0
CTDImean (min., max.); SD (mGy)
DLPmean (min., max.); SD (mGy cm)
4.1 (2, 6.5); 1.1 2 (1.5, 3); 0.5 1.9 (1.5, 2.7); 0.3 1.7 (1.5, 6.2); 1 2.3 (2.3, 2.3); 0 13.1 (5.7, 25.8); 5.4 4.1 (1.8, 6.5); 1.2 1.5 (1.5,1.5); 0 3.92 (2.2, 6.0); 1 1.5 (1.5, 1.5); 0 7 (5.4, 7.8); 0.8 4.7 (2.4, 6.6); 1.4 2.1 (1.6, 3.3); 0.5
155 (72, 244); 42 83 (65, 122); 20 73 (48, 109); 14 88 (48, 344); 49 99 (91, 121); 6 523 (212, 1147); 235 142 (69, 250); 43 66 (58, 73); 6 156 (85, 234); 42 82 (67, 131); 22 120 (87, 163); 22 165 (97, 239); 44 86 (59, 123); 19
3.7 (2.7, 5.2); 0.8 2.4 (1.5, 4.9); 1 5.7 (4.6, 7.5); 1.1 8 (2.1, 9.8); 2.3 4.9 (1.5, 6.8); 1.8 2.6 (1.2, 4.3); 0.7 1.8 (1.5, 2.8); 0.4 15 (7.3, 27.1); 6 1.5 (1.5, 1.5); 0
130 (91, 163); 27 97 (54, 226); 48 192 (109, 237); 42 155 (78, 195); 34 124 (65, 196); 39 81 (36, 132); 23 60 (38, 87); 18 458 (210, 850); 177 27 (22, 31); 3
3.2 (1.5, 6.2); 1.1
69 (28, 153); 27
Mean values are presented, minimum and maximum values in brackets, and standard deviation (SD).
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425
Thyroid imaging Tumour imaging of thyroid Lymphatic system
No. of patients
DOSES FROM HYBRID SPECT–CT PROCEDURES
Breast imaging
SPECT–CT system
S. AVRAMOVA-CHOLAKOVA ET AL.
sex(5, 6). This method was not applicable when conversion coefficient for the scanned anatomic region was not available. Different body regions were scanned for some patients for a particular examination, with two or three scans per patient in some cases. In this case, patient data were grouped according to the region, and separate calculations were performed, summing then the corresponding dose to obtain total E. Similar approach was used in cases of two applications of RF per patient (e.g. rest and stress myocardial perfusion). RESULTS AND DISCUSSION
Figure 1. Effective doses from SPECT– CT examinations for all systems (SI– SIV) per type of examination. The first column on the graphs shows effective dose in millisievert from CT component, calculated with NRPB conversion coefficients. The second column shows effective dose calculated with CT-Expo applying ICRP 60 tissue weighting factors, the third column with CT-Expo applying ICRP 103 factors, the fourth column shows effective dose from the SPECT component and the fifth shows effective dose from the whole examination, taking into account CT contribution with CT-Expo and ICRP 103.
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Effective dose E was estimated for at least 100 adult patients per system, except for the SIV, in which only 32 patients were considered. The number of patients and RF used per examination type in every system are presented in Table 1. Mean, minimum and maximum values and standard deviation are presented for administered activity, CTDI and DLP. The values of calculated typical effective doses are shown in Figure 1 per type of examination. The first columns on the graphs depict E from CT component determined with NRPB method, the second column with CT-Expo and ICRP 60 tissue weighting factors, the third with CT-Expo and ICRP 103, the fourth depicts E from SPECT component and the fifth from the total effective dose, taking into account the CT contribution calculated with CT-Expo and ICRP 103. For breast imaging, the E values from the CT component, estimated with CT-Expo and ICRP 103 tissue weighting factors (Figure 1), are twice higher for the SI (3.2 mSv), compared with SII and SIII (1.7 and 1.5 mSv, respectively). The administered activities and hence E from SPECT for the SIII are much lower (370 MBq vs. 650 and 705 MBq mean values for SI and SII, respectively), and the type of RF is different (Table 1). This determines the total E with the lowest value of 4.3 mSv for the SIII, 9.5 mSv for the SI and 7.6 mSv for the SII. For bone imaging, CT doses in SI and SIII are similar (1.2 and 1.8 mSv, respectively) and much higher, 7.2 mSv, for the SIV. Different anatomical regions were scanned—SI reported chest or pelvis, SIII scanned chest or abdomen and SIV pelvis. The SPECT doses are similar (the lowest value of 2.5 mSv for SI; 3.4 and 2.9 mSv for SIII and SIV, respectively). The high CT dose is the reason for the highest value of total E on SIV—10.1 mSv (3.8 mSv for SI and 5.1 mSv for SIII). Larkin et al. estimated 3.8 mSv from CT for
this examination and 6.3 mSv from SPECT with the same RF; the latter is about two times higher than those on the three systems(7). The CT contribution is higher than those on SI and SIII, but quite lower compared with SIV. The total E from Larkin is 10.1 mSv, the same value like on SIV. Sharma et al. reported 4.2 mSv from CT and 4.1 mSv from SPECT(8). Mhiri et al. announced 3.5 mSv from CT and 4.2 mSv from SPECT(9). These doses are about two times higher than those from this survey with SI and SIII. Somatostatin receptor imaging is related to higher CT doses in SI (5.1 mSv vs. 1.2 mSv in SII). On one side, this is due to higher-dose CT protocol used (both CTDI and DLP are higher, as seen from Table 1). On the other side, in the SI, 68 % of the patients had undergone two CT scans, 21 % had three scans and only 11 % had only one scan. In the SII, longer anatomical area was scanned at once, and lower activities of RF were used. In result, the total E for SII is much lower—3.7 mSv, vs. 9.1 mSv for SI. Another difference is the use of automatic exposure control in the SI and fixed mAs with the SII, both of the same model. Thyroid metastases are examined with two different CT protocols in the SI. For some patients, the protocol for lymph nodes (related to lower doses, 0.5 mSv) and not for thyroid (1 mSv) was used. In the SII, the CT contribution was 2.4 mSv. Only administered activities are presented on the graph (Figure 1), as this examination is performed after therapy of patients with different residue of the thyroid, which can lead to high uncertainty in the estimation of effective dose. The activities in SII are much lower—74 MBq, vs. 167 MBq in SI. Thyroid imaging is related to 3.6 mSv CT dose in SI and 1 mSv contribution of SPECT. The total E is 4.6 mSv (Figure 1). Tumour imaging of thyroid delivered 1.5 mSv from CT in SIII and 2.8 mSv from SPECT. The total E is 4.3 mSv. Lymphatic system is examined with slightly higher CT doses in SI (2.8 mSv vs. 2.1 mSv in SII). SPECT doses are significantly lower in SII (0.3 mSv in SI vs. 0.1 mSv in SII), and the total E is higher in SI compared with SII (3.1 and 2.2 mSv, respectively). For parathyroid imaging, SI has higher CT dose (4.1 mSv) and SPECT dose (7.4 mSv) with a total E of 11.5 mSv compared with the SII. Two different CT protocols were used in SII. The first protocol was used till the end of 2013. It was related to higher CT contribution (2.3 mSv), including two SPECT scans with
DOSES FROM HYBRID SPECT–CT PROCEDURES
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427
S. AVRAMOVA-CHOLAKOVA ET AL. 99m
Comparison of CT effective dose calculation methods CT doses estimated with NRPB coefficients were slightly lower than those from CT-Expo, ICRP 60. Larger differences, up to 15 –20 %, were observed for breast imaging and lymphatic system. This can be explained with the fact that patients are females, taken into account the sex-specific calculation with CT-Expo, against the average NRPB coefficients(5, 6).
Other possible reason is different mathematical phantoms used for modelling(10). When the body regions scanned differ from the standard(11), CT-Expo provides more precise estimation of effective dose as it is providing selection of the scan range. This study confirmed the comment in CT-Expo User’s guide(10) that the effective doses calculated using the American Association of Physicists in Medicine conversion coefficients(12), which are the same as NRPB coefficients(5), tend to be too low (Figure 1). Diagnostic reference levels Based on the present results, annual surveys in nuclear medicine(13) and expert’s opinion of specialists in nuclear medicine, diagnostic reference levels (DRL) can be established. Proposed values are presented in Table 2. Activity values relate to both SPECT and SPECT–CT procedures performed in the country. The DRL of 700 MBq 99mTc-MIBI/ Tetrofosmin for breast imaging is slightly lower compared with the current Bulgarian DRL for SPECT (740 MBq)(14). Compared with the typical activities reported by Mettler et al.(15), the values from the present study are much lower in some cases. Mettler reported 1100 MBq typical activity (6.3 mSv effective dose) for bone imaging with 99mTc-MDP, while the DRL proposed in this study is 600 MBq, which is lower than the DRL in Finland (700 MBq) and the current DRL in Bulgaria (740 MBq)(14). The proposed DRL of 185 MBq 131I-NaI for thyroid metastases after ablation is higher than the current Bulgarian DRL of 90 MBq and lower than the Finnish (200 MBq). Although this examination is performed with the SI and SII, and SI reported higher activities from 74 to 185 MBq with mean 167 MBq, while in SII 74 MBq is used (Table 1), it was judged by the nuclear medicine specialists that the current DRL might be increased. The typical value for thyroid imaging with 99m Tc-Pertechnetate given by Mettler et al. is 370 MBq; the Finnish DRL is 150 MBq, and the actual
Table 2. DRLs proposed. Examination
Breast imaging Bone imaging Thyroid metastasis Thyroid imaging Lymphatic system Parathyroid imaging Lung perfusion Myocardial perfusion stress and rest Myocardial perfusion stress
Radionuclide
99m
Tc Tc 131 I 99m Tc 99m Tc 99m Tc 99m Tc 99m Tc 99m Tc 99m
Radiopharmaceutical
MIBI/Tetrofosmin MDP Iodide Pertechnetate Nanocoll MIBI MAA Tetrofosmin Tetrofosmin
428
DRL A (MBq)
CTDI (mGy)
DLP (mGy cm)
700 600 185 74 74 700 120 950 370
3 3 4 4 4 6 2.6 3 3
120 200 160 170 120 160 100 70 70
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Tc-Pertechnetate and 99mTc-MIBI and SPECT dose from both RF of 6 mSv (Table 1). Since 2014, the new protocol was introduced, related to lower CT dose (1 mSv) and performing only one SPECT scan with 99mTc-MIBI (5 mSv). This optimisation led to a decrease in the total E from 8.3 to 6 mSv. Larkin et al. found 5.4 mSv CT contribution and 8.3 mSv from SPECT for the same examination (total 13.7 mSv)(7). The effective dose reported by Sharma et al. is 1.6 mSv from CT and 6.4 mSv from SPECT (total 8 mSv)(8). Mhiri et al. found 2.3 mSv of CT dose and 8.3 mSv of SPECT (total 10.6 mSv) for parathyroid imaging with the same RF(9). The values in SI are lower than those announced by Larkin, but higher compared with the data from Sharma and Mhiri. The lowest doses are in SII, especially after introduction of the new protocol. Doses from lung perfusion are similar for SII and SIII (1.4 and 1.3 mSv from CT and 1.1 and 2 mSv from SPECT, respectively). In SIV, doses are much higher—8.5 mSv from CT and 2 mSv from SPECT. This resulted in a high total E of 10.5 mSv, compared with SII (2.5 mSv) and SIII (3.3 mSv). Rest and stress myocardial perfusion is performed in SII, while in SIII only stress is applied. Consecutively, each patient has two CT scans in SII, and the RF is applied twice, versus once in SIII. Despite the two CT scans for each patient, the CT contribution in SII is lower (0.8 mSv) than in SIII (1.3 mSv). As expected, the SPECT contribution is higher in SII. The total E is 7.6 mSv in SII and 5.2 mSv in SIII.
DOSES FROM HYBRID SPECT–CT PROCEDURES
CONCLUSIONS Large variations were found in the current practice of SPECT–CT imaging. Different CT protocols and activities were applied for the same examinations performed with systems of the same model. A potential for optimisation was found. Except for one system, lower doses for bone imaging and parathyroid imaging were found compared with other similar studies. DRLs for hybrid imaging studies were established.
REFERENCES 1. International Commission on Radiological Protection. Radiation dose to patients from radiopharmaceuticals. ICRP Publication 53. Ann. ICRP 18 (1987). 2. International Commission on Radiological Protection. Radiation dose to patients from radiopharmaceuticals. ICRP publication 80. Ann. ICRP 28 (1998). 3. International Commission on Radiological Protection. 1990 Recommendations of the International Commission on Radiological Protection. ICRP Publication 60. Ann. ICRP 21 (1991). 4. International Commission on Radiological Protection. The Recommendations of the International Commission on Radiological Protection. ICRP Publication 103. Ann. ICRP 37 (2007). 5. Shrimpton, P. C. Assessment of patient dose in CT. Chilton, NRPB-PE/1/2004 (2004). 6. Cristy, M. and Eckerman, K. F. Specific absorbed fractions of energy at various ages from internal photon sources. I.Methods. Oak Ridge National Laboratory (1987). 7. Larkin, A. M., Serulle, Y., Wagner, S., Noz, M. and Friedman, K. Quantifying the increase in radiation exposure associated with SPECT/CT compared to SPECT alone for routine medicine applications. Int. J. Mol. Imaging 2011(Article ID 897202), 1– 5 (2011). 8. Sharma, P., Sharma, S., Ballal, S., Bal, C., Malhotra, A. and Kumar, R. SPECT– CT in routine clinical practice: increase in patient radiation dose compared with SPECT alone. Nucl. Med. Commun. 33(9), 926 – 932 (2012). 9. Mhiri, A., Slim, I., Ghezaiel, M. and Slimene, M. Estimation of radiation dosimetry for some common SPECT-CTexams. Int. J. Biotech. Well. Indus. 1, 266–269 (2012). 10. CT-Expo v.2.1. A tool for dose evaluation in computed tomography. User’s guide. Medizinische Hochschule (2012). 11. Huda, W., Ogden, K. and Khorasani, M. Converting dose-length product to effective dose at CT. Radiology 248(3), 995– 1003 (2008). 12. American Association of Physicists in Medicine. The measurement, reporting, and management of radiation dose in CT. AAPM Report 96, Task group 23. AAPM (2008). 13. Vassileva, J. et al. National survey of patient doses in diagnostic and interventional radiology and nuclear medicine 2002– 2013. NCRRP (2013), in Bulgarian. 14. Korpela, H., Bly, R., Vassileva, J., Ingilizova, K., Stoyanova, T., Kostadinova, I. and Slavchev, A. Recently revised diagnostic reference levels in nuclear medicine in Bulgaria and in Finland. Radiat. Prot. Dosim. 139(1–3), 317– 320 (2010). 15. Mettler, F., Huda, W., Yoshizumi, T. and Mahesh, M. Effective doses in radiology and diagnostic nuclear medicine. Radiology 248(1), 254–263 (2008).
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Bulgarian DRL is 100 MBq. The current study sets a lower value of 74 MBq. For lymphatic system, no change in the current DRL is proposed (74 MBq, 99m Tc-Nanocoll), as well as for parathyroid imaging (700 MBq, 99mTc-MIBI). The Finnish DRL for the latter exam is 800 MBq, and the DRL reported by Mettler is 740 MBq. Lung perfusion is performed with 185 MBq according to Mettler, and the current Bulgarian DRL is 150 MBq, the same as in Finland. The DRL in this study is set to 120 MBq. The actual Bulgarian DRL for myocardial perfusion rest and stress with 99mTc-MIBI/Tetrofosmin is 1100 MBq, the same as in Finland, and the newly proposed is 950 MBq. For myocardial perfusion stress, the DRL is set to 370 MBq. CTDIs were not compared with the current Bulgarian DRLs for CT, as in most cases, low-dose CT was performed for SPECT–CT. Most of the exams included CT of the same body regions, exceptions were bone and somatostatin receptor imaging. In the SI, 53 % of the patients undergoing bone imaging had chest CT and 47 % had pelvis CT. Two different CT protocols were used depending on the preferences of physician performing the examination. For chest, the mean CTDI and DLP were 1.8 mGy and 93 mGy cm and for pelvis 1.5 mGy and 70 mGy cm correspondingly. In the SIII, 64 % of patients had chest CT, with mean CTDI 2.3 mGy and DLP 99 mGy cm, and 36 % had abdomen CT using the same CT protocol, with CTDI 2.3 mGy and DLP 100 mGy cm. In the SIV, most patients had pelvis (only few cases with chest were not included in the calculations) with the same CT protocol. The highest values of CTDI 13.1 mGy and DLP 523 mGy cm were found for this system. Based on these results, conclusion was made to propose common DRL for all body regions scanned in bone imaging. Because of the small number of SPECT–CT facilities in the country, DRLs proposed for the CT part of the examinations were chosen based on expert’s judgement and on the results from this survey, excluding the highest values observed (Table 2).
doi:10.1093/rpd/ncv130
PATIENT DOSES FROM HYBRID SPECT– CT PROCEDURES S. Avramova-Cholakova1,*, M. Dimcheva2, E. Petrova3, M. Garcheva3, M. Dimitrova4, Y. Palashev5 and J. Vassileva1 1 National Centre of Radiobiology and Radiation Protection, Sofia, Bulgaria 2 Sofia City Cancer Centre, Sofia, Bulgaria 3 University Multiprofile Hospital for Active Treatment ‘Alexandrovska’, Sofia, Bulgaria 4 Specialized Hospital for Active Treatment in Oncology, Sofia, Bulgaria 5 University Multiprofile Hospital for Active Treatment ‘St Ivan Rilski’, Sofia, Bulgaria *Corresponding author: [email protected]
INTRODUCTION Hybrid single-photon emission computed tomography and computed tomography (SPECT–CT) procedures are related to high radiation doses to patients due to the combination of two methods using ionising radiation and delivering relatively high doses. The first hybrid system was installed in Bulgaria only 5 y ago, and nowadays four SPECT– CT facilities are in operation. The contribution of CT modality to patient doses from SPECT–CT has not been studied in the country thus far. The aim of this work is to estimate patient doses from SPECT–CT procedures and to explore potential for optimisation.
MATERIALS AND METHODS All four SPECT–CT systems contributed to the patient dose study: two Symbia 2 T (Siemens Medical Solution) equipped with a 2-detector row CT, denoted further in the text as SI and SII; one Symbia T16 (Siemens Medical Solution) with a 16-detector row CT, denoted as SIII; and one Discovery NM/CT670 (GE Healthcare) with 16-detector row CT, denoted as SIV. Standard data collection form was developed and distributed to all departments, including patient data (age, sex, weight, height), data related to exposure from SPECT (radiopharmaceutical, radionuclide and the administered activity in MBq) as well as CTrelated data [dose indexes CTDIvol and dose length
product (DLP) available from the display, tube voltage, mAs, rotation time, slice thickness, etc.]. All data were retrospectively extracted from the archive of the examinations. Ten types of examinations were considered, representing all diagnostic procedures performed in SPECT–CT facilities. Administered activities of radiopharmaceuticals (RF) were averaged for all patients in a given system for a particular examination. Effective doses from the SPECT component were calculated multiplying the ICRP 53 and ICRP 80 conversion coefficients by averaged activities(1, 2). Computed tomography dose index (CTDI) and DLP values were averaged for all patients for a particular examination and system, separately for men and women. Effective doses from the CT component were calculated with CT-Expo software (version 2.1, Medizinische Hochschule, Hannover, Germany) for both ICRP 60(3) and ICRP 103(4) tissue weighting factors, using average CTDI and DLP as an input quantity. For a particular scanner, typical tube voltage (kV), tube current (mA), time of rotation (s), pitch, beam collimation and anatomic region scanned were used for calculation, separately for men and women. Parallel estimation of CT contribution was performed applying the National Radiological Protection Board (NRPB) conversion coefficients where applicable(5). In this simple method, effective dose is estimated by multiplication of DLP by a conversion coefficient dependent on the anatomic region scanned. These coefficients are derived for ICRP 60 tissue weighting factors(3) and are not separated by
# The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at Tufts University on December 8, 2015
The aim of this work is to estimate patient doses from hybrid single-photon emission computed tomography (SPECT) and computed tomography (CT) procedures. The study involved all four SPECT– CT systems in Bulgaria. Effective dose was estimated for about 100 patients per system. Ten types of examinations were considered, representing all diagnostic procedures performed in the SPECT– CT systems. Effective doses from the SPECT component were calculated applying the ICRP 53 and ICRP 80 conversion coefficients. Computed tomography dose index and dose length product were retrospectively obtained from the archives of the systems, and effective doses from the CT component were calculated with CT-Expo software. Parallel estimation of CT component contribution with the National Radiological Protection Board (NRPB) conversion coefficients was performed where applicable. Large variations were found in the current practice of SPECT– CT imaging. Optimisation actions and diagnostic reference levels were proposed.
Table 1. The number of patients for which data were provided, type of radiopharmaceutical and administered activity, CTDI and DLP per type of examination and system. Examination
Bone imaging Somatostatin receptor Imaging Thyroid metastasis
Parathyroid imaging Old protocol New protocol Lung perfusion Myocardial perfusion stress þ rest Stress
Radiopharmaceutical
I II III I III IV I II I I II I III
64 9 18 42 35 13 43 11 12 7 10 14 10
99m
I II I II II II III IV II
10 20 7 10 10 20 14 19 21
99m
III
25
99m
Tc-MIBI Tc-MIBI 99m Tc-Tetrofosmin 99m Tc-MDP 99m Tc-MDP 99m Tc-MDP 99m Tc-Tektrotyd 99m Tc-Tektrotyd 131 I-NaI 131 I-NaI 131 I-NaI 99m Tc-Pertechnetate 99m Tc-Tetrofosmin 99m
Tc-Nanocoll Tc-Nanocoll Tc-Pertechnetate þ 99mTc-MIBI 99m Tc-Pertechnetate þ 99mTc-MIBI 99m Tc-MIBI 99m Tc-MAA 99m Tc-MAA 99m Tc-MAA 99m Tc-Tetrofosmin 99m 99m
Tc-Tetrofosmin
Amean (min., max.); SD (MBq) 705 (629, 740); 47 650 (555, 740) 370 (370, 370); 0 446 (259, 740); 167 592 (370, 740); 161 512 (370, 740); 154 733 (629, 740); 28 444 (444, 444); 0 167 (74, 185); 42 167 (74, 185); 42 74 (74, 74); 0 74 (74, 74); 0 370 (370, 370); 0 148 (148, 148); 0 56 (56, 56); 0 74 þ 714 (555, 740); 70 74 þ 555 (555, 555); 0 555 (555, 555); 0 104 (92, 111); 9.6 185 (185, 185); 0 185 (185, 185); 0 370 þ 555 (555, 555); 0 555 (555, 555); 0
CTDImean (min., max.); SD (mGy)
DLPmean (min., max.); SD (mGy cm)
4.1 (2, 6.5); 1.1 2 (1.5, 3); 0.5 1.9 (1.5, 2.7); 0.3 1.7 (1.5, 6.2); 1 2.3 (2.3, 2.3); 0 13.1 (5.7, 25.8); 5.4 4.1 (1.8, 6.5); 1.2 1.5 (1.5,1.5); 0 3.92 (2.2, 6.0); 1 1.5 (1.5, 1.5); 0 7 (5.4, 7.8); 0.8 4.7 (2.4, 6.6); 1.4 2.1 (1.6, 3.3); 0.5
155 (72, 244); 42 83 (65, 122); 20 73 (48, 109); 14 88 (48, 344); 49 99 (91, 121); 6 523 (212, 1147); 235 142 (69, 250); 43 66 (58, 73); 6 156 (85, 234); 42 82 (67, 131); 22 120 (87, 163); 22 165 (97, 239); 44 86 (59, 123); 19
3.7 (2.7, 5.2); 0.8 2.4 (1.5, 4.9); 1 5.7 (4.6, 7.5); 1.1 8 (2.1, 9.8); 2.3 4.9 (1.5, 6.8); 1.8 2.6 (1.2, 4.3); 0.7 1.8 (1.5, 2.8); 0.4 15 (7.3, 27.1); 6 1.5 (1.5, 1.5); 0
130 (91, 163); 27 97 (54, 226); 48 192 (109, 237); 42 155 (78, 195); 34 124 (65, 196); 39 81 (36, 132); 23 60 (38, 87); 18 458 (210, 850); 177 27 (22, 31); 3
3.2 (1.5, 6.2); 1.1
69 (28, 153); 27
Mean values are presented, minimum and maximum values in brackets, and standard deviation (SD).
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Thyroid imaging Tumour imaging of thyroid Lymphatic system
No. of patients
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Breast imaging
SPECT–CT system
S. AVRAMOVA-CHOLAKOVA ET AL.
sex(5, 6). This method was not applicable when conversion coefficient for the scanned anatomic region was not available. Different body regions were scanned for some patients for a particular examination, with two or three scans per patient in some cases. In this case, patient data were grouped according to the region, and separate calculations were performed, summing then the corresponding dose to obtain total E. Similar approach was used in cases of two applications of RF per patient (e.g. rest and stress myocardial perfusion). RESULTS AND DISCUSSION
Figure 1. Effective doses from SPECT– CT examinations for all systems (SI– SIV) per type of examination. The first column on the graphs shows effective dose in millisievert from CT component, calculated with NRPB conversion coefficients. The second column shows effective dose calculated with CT-Expo applying ICRP 60 tissue weighting factors, the third column with CT-Expo applying ICRP 103 factors, the fourth column shows effective dose from the SPECT component and the fifth shows effective dose from the whole examination, taking into account CT contribution with CT-Expo and ICRP 103.
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Effective dose E was estimated for at least 100 adult patients per system, except for the SIV, in which only 32 patients were considered. The number of patients and RF used per examination type in every system are presented in Table 1. Mean, minimum and maximum values and standard deviation are presented for administered activity, CTDI and DLP. The values of calculated typical effective doses are shown in Figure 1 per type of examination. The first columns on the graphs depict E from CT component determined with NRPB method, the second column with CT-Expo and ICRP 60 tissue weighting factors, the third with CT-Expo and ICRP 103, the fourth depicts E from SPECT component and the fifth from the total effective dose, taking into account the CT contribution calculated with CT-Expo and ICRP 103. For breast imaging, the E values from the CT component, estimated with CT-Expo and ICRP 103 tissue weighting factors (Figure 1), are twice higher for the SI (3.2 mSv), compared with SII and SIII (1.7 and 1.5 mSv, respectively). The administered activities and hence E from SPECT for the SIII are much lower (370 MBq vs. 650 and 705 MBq mean values for SI and SII, respectively), and the type of RF is different (Table 1). This determines the total E with the lowest value of 4.3 mSv for the SIII, 9.5 mSv for the SI and 7.6 mSv for the SII. For bone imaging, CT doses in SI and SIII are similar (1.2 and 1.8 mSv, respectively) and much higher, 7.2 mSv, for the SIV. Different anatomical regions were scanned—SI reported chest or pelvis, SIII scanned chest or abdomen and SIV pelvis. The SPECT doses are similar (the lowest value of 2.5 mSv for SI; 3.4 and 2.9 mSv for SIII and SIV, respectively). The high CT dose is the reason for the highest value of total E on SIV—10.1 mSv (3.8 mSv for SI and 5.1 mSv for SIII). Larkin et al. estimated 3.8 mSv from CT for
this examination and 6.3 mSv from SPECT with the same RF; the latter is about two times higher than those on the three systems(7). The CT contribution is higher than those on SI and SIII, but quite lower compared with SIV. The total E from Larkin is 10.1 mSv, the same value like on SIV. Sharma et al. reported 4.2 mSv from CT and 4.1 mSv from SPECT(8). Mhiri et al. announced 3.5 mSv from CT and 4.2 mSv from SPECT(9). These doses are about two times higher than those from this survey with SI and SIII. Somatostatin receptor imaging is related to higher CT doses in SI (5.1 mSv vs. 1.2 mSv in SII). On one side, this is due to higher-dose CT protocol used (both CTDI and DLP are higher, as seen from Table 1). On the other side, in the SI, 68 % of the patients had undergone two CT scans, 21 % had three scans and only 11 % had only one scan. In the SII, longer anatomical area was scanned at once, and lower activities of RF were used. In result, the total E for SII is much lower—3.7 mSv, vs. 9.1 mSv for SI. Another difference is the use of automatic exposure control in the SI and fixed mAs with the SII, both of the same model. Thyroid metastases are examined with two different CT protocols in the SI. For some patients, the protocol for lymph nodes (related to lower doses, 0.5 mSv) and not for thyroid (1 mSv) was used. In the SII, the CT contribution was 2.4 mSv. Only administered activities are presented on the graph (Figure 1), as this examination is performed after therapy of patients with different residue of the thyroid, which can lead to high uncertainty in the estimation of effective dose. The activities in SII are much lower—74 MBq, vs. 167 MBq in SI. Thyroid imaging is related to 3.6 mSv CT dose in SI and 1 mSv contribution of SPECT. The total E is 4.6 mSv (Figure 1). Tumour imaging of thyroid delivered 1.5 mSv from CT in SIII and 2.8 mSv from SPECT. The total E is 4.3 mSv. Lymphatic system is examined with slightly higher CT doses in SI (2.8 mSv vs. 2.1 mSv in SII). SPECT doses are significantly lower in SII (0.3 mSv in SI vs. 0.1 mSv in SII), and the total E is higher in SI compared with SII (3.1 and 2.2 mSv, respectively). For parathyroid imaging, SI has higher CT dose (4.1 mSv) and SPECT dose (7.4 mSv) with a total E of 11.5 mSv compared with the SII. Two different CT protocols were used in SII. The first protocol was used till the end of 2013. It was related to higher CT contribution (2.3 mSv), including two SPECT scans with
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S. AVRAMOVA-CHOLAKOVA ET AL. 99m
Comparison of CT effective dose calculation methods CT doses estimated with NRPB coefficients were slightly lower than those from CT-Expo, ICRP 60. Larger differences, up to 15 –20 %, were observed for breast imaging and lymphatic system. This can be explained with the fact that patients are females, taken into account the sex-specific calculation with CT-Expo, against the average NRPB coefficients(5, 6).
Other possible reason is different mathematical phantoms used for modelling(10). When the body regions scanned differ from the standard(11), CT-Expo provides more precise estimation of effective dose as it is providing selection of the scan range. This study confirmed the comment in CT-Expo User’s guide(10) that the effective doses calculated using the American Association of Physicists in Medicine conversion coefficients(12), which are the same as NRPB coefficients(5), tend to be too low (Figure 1). Diagnostic reference levels Based on the present results, annual surveys in nuclear medicine(13) and expert’s opinion of specialists in nuclear medicine, diagnostic reference levels (DRL) can be established. Proposed values are presented in Table 2. Activity values relate to both SPECT and SPECT–CT procedures performed in the country. The DRL of 700 MBq 99mTc-MIBI/ Tetrofosmin for breast imaging is slightly lower compared with the current Bulgarian DRL for SPECT (740 MBq)(14). Compared with the typical activities reported by Mettler et al.(15), the values from the present study are much lower in some cases. Mettler reported 1100 MBq typical activity (6.3 mSv effective dose) for bone imaging with 99mTc-MDP, while the DRL proposed in this study is 600 MBq, which is lower than the DRL in Finland (700 MBq) and the current DRL in Bulgaria (740 MBq)(14). The proposed DRL of 185 MBq 131I-NaI for thyroid metastases after ablation is higher than the current Bulgarian DRL of 90 MBq and lower than the Finnish (200 MBq). Although this examination is performed with the SI and SII, and SI reported higher activities from 74 to 185 MBq with mean 167 MBq, while in SII 74 MBq is used (Table 1), it was judged by the nuclear medicine specialists that the current DRL might be increased. The typical value for thyroid imaging with 99m Tc-Pertechnetate given by Mettler et al. is 370 MBq; the Finnish DRL is 150 MBq, and the actual
Table 2. DRLs proposed. Examination
Breast imaging Bone imaging Thyroid metastasis Thyroid imaging Lymphatic system Parathyroid imaging Lung perfusion Myocardial perfusion stress and rest Myocardial perfusion stress
Radionuclide
99m
Tc Tc 131 I 99m Tc 99m Tc 99m Tc 99m Tc 99m Tc 99m Tc 99m
Radiopharmaceutical
MIBI/Tetrofosmin MDP Iodide Pertechnetate Nanocoll MIBI MAA Tetrofosmin Tetrofosmin
428
DRL A (MBq)
CTDI (mGy)
DLP (mGy cm)
700 600 185 74 74 700 120 950 370
3 3 4 4 4 6 2.6 3 3
120 200 160 170 120 160 100 70 70
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Tc-Pertechnetate and 99mTc-MIBI and SPECT dose from both RF of 6 mSv (Table 1). Since 2014, the new protocol was introduced, related to lower CT dose (1 mSv) and performing only one SPECT scan with 99mTc-MIBI (5 mSv). This optimisation led to a decrease in the total E from 8.3 to 6 mSv. Larkin et al. found 5.4 mSv CT contribution and 8.3 mSv from SPECT for the same examination (total 13.7 mSv)(7). The effective dose reported by Sharma et al. is 1.6 mSv from CT and 6.4 mSv from SPECT (total 8 mSv)(8). Mhiri et al. found 2.3 mSv of CT dose and 8.3 mSv of SPECT (total 10.6 mSv) for parathyroid imaging with the same RF(9). The values in SI are lower than those announced by Larkin, but higher compared with the data from Sharma and Mhiri. The lowest doses are in SII, especially after introduction of the new protocol. Doses from lung perfusion are similar for SII and SIII (1.4 and 1.3 mSv from CT and 1.1 and 2 mSv from SPECT, respectively). In SIV, doses are much higher—8.5 mSv from CT and 2 mSv from SPECT. This resulted in a high total E of 10.5 mSv, compared with SII (2.5 mSv) and SIII (3.3 mSv). Rest and stress myocardial perfusion is performed in SII, while in SIII only stress is applied. Consecutively, each patient has two CT scans in SII, and the RF is applied twice, versus once in SIII. Despite the two CT scans for each patient, the CT contribution in SII is lower (0.8 mSv) than in SIII (1.3 mSv). As expected, the SPECT contribution is higher in SII. The total E is 7.6 mSv in SII and 5.2 mSv in SIII.
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CONCLUSIONS Large variations were found in the current practice of SPECT–CT imaging. Different CT protocols and activities were applied for the same examinations performed with systems of the same model. A potential for optimisation was found. Except for one system, lower doses for bone imaging and parathyroid imaging were found compared with other similar studies. DRLs for hybrid imaging studies were established.
REFERENCES 1. International Commission on Radiological Protection. Radiation dose to patients from radiopharmaceuticals. ICRP Publication 53. Ann. ICRP 18 (1987). 2. International Commission on Radiological Protection. Radiation dose to patients from radiopharmaceuticals. ICRP publication 80. Ann. ICRP 28 (1998). 3. International Commission on Radiological Protection. 1990 Recommendations of the International Commission on Radiological Protection. ICRP Publication 60. Ann. ICRP 21 (1991). 4. International Commission on Radiological Protection. The Recommendations of the International Commission on Radiological Protection. ICRP Publication 103. Ann. ICRP 37 (2007). 5. Shrimpton, P. C. Assessment of patient dose in CT. Chilton, NRPB-PE/1/2004 (2004). 6. Cristy, M. and Eckerman, K. F. Specific absorbed fractions of energy at various ages from internal photon sources. I.Methods. Oak Ridge National Laboratory (1987). 7. Larkin, A. M., Serulle, Y., Wagner, S., Noz, M. and Friedman, K. Quantifying the increase in radiation exposure associated with SPECT/CT compared to SPECT alone for routine medicine applications. Int. J. Mol. Imaging 2011(Article ID 897202), 1– 5 (2011). 8. Sharma, P., Sharma, S., Ballal, S., Bal, C., Malhotra, A. and Kumar, R. SPECT– CT in routine clinical practice: increase in patient radiation dose compared with SPECT alone. Nucl. Med. Commun. 33(9), 926 – 932 (2012). 9. Mhiri, A., Slim, I., Ghezaiel, M. and Slimene, M. Estimation of radiation dosimetry for some common SPECT-CTexams. Int. J. Biotech. Well. Indus. 1, 266–269 (2012). 10. CT-Expo v.2.1. A tool for dose evaluation in computed tomography. User’s guide. Medizinische Hochschule (2012). 11. Huda, W., Ogden, K. and Khorasani, M. Converting dose-length product to effective dose at CT. Radiology 248(3), 995– 1003 (2008). 12. American Association of Physicists in Medicine. The measurement, reporting, and management of radiation dose in CT. AAPM Report 96, Task group 23. AAPM (2008). 13. Vassileva, J. et al. National survey of patient doses in diagnostic and interventional radiology and nuclear medicine 2002– 2013. NCRRP (2013), in Bulgarian. 14. Korpela, H., Bly, R., Vassileva, J., Ingilizova, K., Stoyanova, T., Kostadinova, I. and Slavchev, A. Recently revised diagnostic reference levels in nuclear medicine in Bulgaria and in Finland. Radiat. Prot. Dosim. 139(1–3), 317– 320 (2010). 15. Mettler, F., Huda, W., Yoshizumi, T. and Mahesh, M. Effective doses in radiology and diagnostic nuclear medicine. Radiology 248(1), 254–263 (2008).
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Bulgarian DRL is 100 MBq. The current study sets a lower value of 74 MBq. For lymphatic system, no change in the current DRL is proposed (74 MBq, 99m Tc-Nanocoll), as well as for parathyroid imaging (700 MBq, 99mTc-MIBI). The Finnish DRL for the latter exam is 800 MBq, and the DRL reported by Mettler is 740 MBq. Lung perfusion is performed with 185 MBq according to Mettler, and the current Bulgarian DRL is 150 MBq, the same as in Finland. The DRL in this study is set to 120 MBq. The actual Bulgarian DRL for myocardial perfusion rest and stress with 99mTc-MIBI/Tetrofosmin is 1100 MBq, the same as in Finland, and the newly proposed is 950 MBq. For myocardial perfusion stress, the DRL is set to 370 MBq. CTDIs were not compared with the current Bulgarian DRLs for CT, as in most cases, low-dose CT was performed for SPECT–CT. Most of the exams included CT of the same body regions, exceptions were bone and somatostatin receptor imaging. In the SI, 53 % of the patients undergoing bone imaging had chest CT and 47 % had pelvis CT. Two different CT protocols were used depending on the preferences of physician performing the examination. For chest, the mean CTDI and DLP were 1.8 mGy and 93 mGy cm and for pelvis 1.5 mGy and 70 mGy cm correspondingly. In the SIII, 64 % of patients had chest CT, with mean CTDI 2.3 mGy and DLP 99 mGy cm, and 36 % had abdomen CT using the same CT protocol, with CTDI 2.3 mGy and DLP 100 mGy cm. In the SIV, most patients had pelvis (only few cases with chest were not included in the calculations) with the same CT protocol. The highest values of CTDI 13.1 mGy and DLP 523 mGy cm were found for this system. Based on these results, conclusion was made to propose common DRL for all body regions scanned in bone imaging. Because of the small number of SPECT–CT facilities in the country, DRLs proposed for the CT part of the examinations were chosen based on expert’s judgement and on the results from this survey, excluding the highest values observed (Table 2).
