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ORIGINAL ARTICLE
Year : 2013  |  Volume : 9  |  Issue : 3  |  Page : 477-483

Estimation of patient dose in 18 F-FDG and 18 F-FDOPA PET/CT examinations


1 Department of Cyclotron and Radiopharmaceutical Sciences, Institute of Nuclear Medicine and Allied Sciences, Brig. S.K. Mazumdar Road, Timarpur, Delhi, India
2 Department of PET Imaging, Institute of Nuclear Medicine and Allied Sciences, Brig. S.K. Mazumdar Road, Timarpur, Delhi, India
3 Department of Radiation Biosciences, Institute of Nuclear Medicine and Allied Sciences, Brig. S.K. Mazumdar Road, Timarpur, Delhi, India

Date of Web Publication8-Oct-2013

Correspondence Address:
Bilikere S Dwarakanath
Division of Radiation Biosciences, Institute of Nuclear Medicine and Allied Sciences, Brig. S.K. Mazumdar Road, Timarpur, Delhi
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0973-1482.119354

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 > Abstract 

Purpose: To estimate specific organ and effective doses to patients resulting from the 18 F-FDG ( 18 F-2-deoxy-D-glucose) and 18 F-FDOPA (6-fluoro-( 18 F)-L-3, 4-dihydroxyphenylalanine) PET/CT examinations for whole body and brain.
Materials and Methods: Three protocols for whole body and three for brain PET/CT were used. The CTDI values were measured using standard head and body CT phantoms and also computed using a software CT-Expo for dose evaluation from the CT component. OLINDA software based on MIRD method was used for estimating doses from the PET component of the PET/CT examination.
Results: The organ doses from 18 F-FDG and 18 F-FDOPA whole body and brain PET/CT studies were estimated. The total effective dose from a typical protocol of whole body PET/CT examination was 14.4 mSv for females and 11.8 mSv for male patients from 18 F-FDG, whereas it was 11 mSv for female and 9.1 mSv for male patients from 18 F-FDOPA. The total effective doses from a typical protocol for PET/CT studies of brain was 6.5 mSv for females and 5.1 mSv for males from 18 F-FDG whereas it was 3.7 mSv for females and 2.8 mSv for males from 18 F-FDOPA.
Conclusions: The effective radiation doses from whole body PET/CT examination was approximately 4-8 times higher than the background radiation dose from both 18 F-FDG and 18 F-FDOPA scans, while it was 1-3 times the background radiation dose from PET/CT scans of brain.

Keywords: 18F-FDG, 18F-FDOPA, CTDI, effective dose, organ dose, PET/CT


How to cite this article:
Kaushik A, Jaimini A, Tripathi M, D'Souza M, Sharma R, Mishra AK, Mondal A, Dwarakanath BS. Estimation of patient dose in 18 F-FDG and 18 F-FDOPA PET/CT examinations. J Can Res Ther 2013;9:477-83

How to cite this URL:
Kaushik A, Jaimini A, Tripathi M, D'Souza M, Sharma R, Mishra AK, Mondal A, Dwarakanath BS. Estimation of patient dose in 18 F-FDG and 18 F-FDOPA PET/CT examinations. J Can Res Ther [serial online] 2013 [cited 2019 Nov 21];9:477-83. Available from: http://www.cancerjournal.net/text.asp?2013/9/3/477/119354


 > Introduction Top


The combined positron emission tomographic (PET) and computed tomographic (CT) PET/CT scanner plays an important role in diagnosis and staging of human disease. [1] It allows the quasi-simultaneous acquisition of anatomic (CT) and functional (PET) information of a patient within a single examination and hence provides co-registered images of anatomic and functional information. [2],[3] Since a PET/CT examination leads to exposure of patients from the internally administered PET radiopharmaceuticals and externally from the X-rays generated by the CT, it is one of the more challenging and interesting areas of radiation safety in diagnostic medicine. [4] Whole body 18 F-FDG PET/CT examinations are generally accompanied by substantial radiation dose that may enhance the cancer risk. [3],[5],[6] It is very important therefore to estimate the radiation doses associated with PET/CT examination, which may vary from one center to other because the radiation doses associated with such examinations depend on the type of PET pharmaceutical, the amount of radioactivity, the physiology of the patient and also on the parameters used for CT scanning.

The present study was undertaken to estimate the organ doses and effective doses to patients resulting from the PET/CT examinations carried out using 18 F-FDG and 18 F-FDOPA that have bearing on the long-term risk.


 > Materials and Methods Top


All studies were performed on a 16-slice PET/CT system (Discovery STE, M/s GE), which has Bismuth Germinate (BGO) crystals. The PET/CT system has Lightspeed 16 as the model of the CT system.

Subjects

A total of 300 patients were included in this study of which 100 were for 18 F-FDG PET/CT whole body studies, 100 for 18 F-FDG PET/CT brain studies, 50 for 18 F-FDOPA whole body PET/CT studies and 50 for 18 F-FDOPA brain PET/CT studies. The mean age of male patients was 45 ± 8 years (age range, 18-72 years) and for female patients was 45 ± 6 years (age range, 22-75 years). The average weight of the male patients was 60.7 ± 5.4 kg and for female patients it was 59.4 ± 6.3 kg. The use of patient's data for the purpose of estimating radiation doses was approved by the institutional human ethics committee.

For each imaging study, the total length of the scanned region, the start and stop location of each series was recorded. The technical parameters like the kV p , mA, beam collimation, rotation time and pitch were recorded for the CT scan. The amount of 18 F-FDG and 18 F-FDOPA activity administered for each patient was recorded for the PET scan.

Protocol for PET/CT Scan

The protocol for 18 F-FDG and 18 F-FDOPA PET/CT examinations comprised (a) a scout scan, for positioning; (b) a spiral CT scan; (c) a 3D PET scan over the same axial range as the CT scan. For the whole body PET/CT scan, the patients were scanned from the top of the skull to mid thigh and the scanned length was equivalent to six (89.6 cm) to seven (102 cm) bed positions for the PET study. For the PET/CT scan of brain, the scanned length was equivalent to one bed position (15 cm).

External Dosimetry (CT)

Three protocols for whole body CT scanning (A, B and C) and three protocols for CT scanning of brain (D, E and F) were used [Table 1]. Protocols B and C for whole body and the protocols E and F for brain were used for patients with larger body habitus and also when diagnostic CT scans were performed for contrast enhanced studies. Protocol A for whole body and protocol D for brain scan are the most frequently used protocols in our unit.
Table 1: Parameters for the CT Protocols

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 > Computed Tomography Dose Index (CTDI) Top


The principal dosimetric quantity used in CT is Computed Tomography Dose Index (CTDI). [7] This is defined as the integral along a line parallel to the axis of rotation (z) of the dose profile [D(z)] for a single rotation and a fixed table position, divided by the nominal thickness of the X-ray beam. CTDI can be assessed using a pencil ionization chamber with an active length of 100 mm, so as to provide a measurement of CTDI 100 , expressed in terms of absorbed dose to air (mGy). [8]



Where n is the number of tomographic slices, each of nominal thickness T, imaged during a single rotation. CTDI, expressed in terms of air kerma in milligray, are obtained at the periphery (CTDI p ) and at the center of the phantom (CTDI c ). The weighted CTDI w is obtained as the sum of one-third of CTDI c and two-thirds of CTDI p . [9] The other quantity derived from CTDI is volume CTDI (CTDI vol ), which is CTDI w divided by the pitch where pitch is defined as the table increment distance per X-ray tube rotation divided by the nominal beam width at the scanner isocenter. [10] CTDI vol represents the average dose (air kerma) within a specified CT dosimetry phantom. [11] Dose length product (DLP) is obtained as the product of CTDI vol and the scan length and is expressed as milligray-centimeter.

In the present study, CTDI 100 measurements were performed using CT dosimetry acrylic phantoms with diameters of 16 cm (i.e. head) and 32 cm (i.e. body). A pencil ionization chamber with a calibrated dosimeter (10x6-3 CT ionization chamber with Accu-Dose Dosimeter, Radcal Corporation, USA) was used for making the measurements. [Figure 1] CTDI values were measured for the standard scan parameters of 120 kV, 100 mAs, slice collimation 10 mm and single rotation using standard body phantom and 140 kV, 100 mAs, slice collimation 10 mm and single rotation using head phantom. Normalized weighted CTDI ( n CTDI w ) value was calculated in terms of mGy (mAs) -1 . CTDI values were also measured for the scan parameters listed in [Table 1]. CTDI vol and then DLP were calculated from measured CTDI w . The effective dose was estimated by using DLP to effective dose conversion factors for head (2.2 μSv (mGy.cm) -1 ) and body (18 μSv (mGy.cm) -1 ). [12]
Figure 1: Instrumentation used for the measurement of Computed Tomography Dose Index (CTDI). (a) ionization chamber; (b) acrylic CTDI phantom; (c) digitizer and (d) dosimeter

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CT-Expo

On the basis of scan parameters used for the CT scan [Table 1], a software CT-Expo (Version 2.0, Germany) was used to estimate organ doses and effective doses to patients from the CT component of the 18 F-FDG and 18 F-FDOPA PET/CT examination. CT-Expo is a novel MS Excel application for assessing the radiation doses delivered to patients undergoing CT examinations, based on computational methods. It provides gender-specific dose calculation and is applicable to all existing scanner models. [13]

The evaluation of effective dose by CT-Expo software was carried out on the basis of characteristics of the model of CT system, Lightspeed 16 stored in the software database. Scan parameters and scan range related to each patient were entered as variables. The adult female model EVA and male model ADAM was used for organ dose and effective dose calculation.

Internal Dosimetry (PET)

The internal radiation dose from 18 F-FDG and 18 F-FDOPA PET scan is estimated by using the method recommended by Medical Internal Radiation Dose (MIRD) Committee. As per the MIRD method, [14] the generic equation for the absorbed dose in an organ is given by:



Where D = absorbed dose (Gy); Γ = cumulated activity (MBq s); n i = number of particles with energy E i emitted per nuclear transition; E i = energy per particle (MeV); f i = fraction of energy absorbed in the target; m = mass of target region (kg) and k = proportionality constant (Gy kg/MBq s MeV). The equation for absorbed dose in the MIRD system can be simply expressed as:



Where τ is the residence time and S is given by



The S values for various radionuclides are available in MIRD pamphlets.

In the present study, the organ doses and in turn the effective doses from each PET series were computed using the OLINDA software (Version OLINDA/EXM 1.0, Vanderbilt University, Nashville, TN, USA). [15] An absorbed dose estimation study from 18 F-FDG by Hays et al.[16] was used as kinetic model for effective dose and organ dose calculations. For 18 F-FDOPA, the biokinetic data provided by Harvey et al.[17] and Dhawan et al.[18] was used to estimate the radiation doses. Adult male and female models in OLINDA's phantom library were utilized to generate effective dose conversion factor (mSv/MBq). Patient-specific effective dose conversion factors (mSv/MBq) were obtained by modifying the input data for phantom with the mass of brain of each patient and his/her total body weight. The average activity of 18 F-FDG and 18 F-FDOPA administered to patients was 336.3 ± 49.4 MBq and 98.6 ± 35.4 MBq, respectively for whole body PET/CT scan. The average activity of 18 F-FDG and 18 F-FDOPA administered to patients was 300 ± 44.02 MBq and 94.4 ± 26.9 MBq, respectively for the PET/CT scan of brain. The effective doses were also estimated by using the dose coefficients provided by ICRP in its publication no. 106. [19]


 > Results Top


The effective dose computed from the CT component of PET/CT examination using the software CT-Expo and derived from the measured values of CTDI correlated reasonably well. All the measured/calculated and computed values of CT dose parameters using CT Expo are tabulated in [Table 2] for the scan parameters of all the protocols. The percentage difference between the measured and calculated values of CTDI w and CTDI vol , respectively and those computed using CT-Expo is 19%-24% for the whole body scan and 8%-11% for brain scans. The normalized weighted CTDI ( n CTDI w ) value was 0.094 mGy (mAs) -1 as per CT-Expo software, whereas the value calculated from the measured value of CTDI for whole body protocols was 0.084 mGy (mAs) -1 leading to the variation of 10% between the two. The n CTDI w was 0.182 mGy (mAs) -1 for head region as per CT-Expo software, whereas it was 0.26 mGy (mAs) -1 as calculated from the measured values of CTDI w . The variation between the two values is 30%. The variation in the values of DLP calculated using CTDI vol and computed using CT-Expo is 21%-25%. The variation in the values of effective dose from whole body scan calculated from DLP and computed by CT-Expo was 8.2% to 11.7%, whereas the variation in the values of effective dose from head scan derived from DLP and computed by CT-Expo was 38%-39%.
Table 2: Measured and computed values of CT dose estimation quantities

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The average effective dose from the internally administered 18 F-FDG and 18 F-FDOPA for the whole-body PET/CT examination was 6.3 mSv and 2.9 mSv, respectively for females and 4.9 mSv and 2.2 mSv, respectively for males whereas the average effective dose from the internally administered 18 F-FDG and 18 F-FDOPA for the brain PET/CT examination was 5.6 mSv and 2.8 mSv, respectively for females and 4.4 mSv and 2.1 mSv, respectively for males. The effective dose computed using dose coefficients for adults provided in ICRP 106 for 18 F-FDG was 6.4 mSv and 2.5 mSv for 18 F-FDOPA. The contribution to effective dose from the CT component of 18 F-FDG and 18 F-FDOPA PET/CT scans are tabulated in [Table 3]. The total effective dose of the combined PET/CT studies calculated by summing the effective doses of PET and CT scanning from 18 F-FDG whole body PET/CT examination was 14.4 ± 0.8 mSv for female patients and 11.8 ± 0.7 mSv for male patients from a typical Protocol A. On the other hand, it was 11 ± 0.2 mSv for female and 9.1 ± 0.2 mSv for male patients from 18 F-FDOPA whole body PET/CT studies from the same protocol. The total effective doses from 18 F-FDG PET/CT studies of brain were 6.5 ± 0.8 mSv for females and 5.1 ± 0.7 mSv for males from a typical protocol D. The total effective doses from 18 F-FDOPA PET/CT studies of brain were 3.7 ± 0.8 mSv for females and 2.8 ± 0.7 mSv for males. The total effective dose from other protocols listed in [Table 1] is tabulated in [Table 3]. Since the scan range is from skull to mid thigh, a number of organs received substantial dose. A detailed assessment of organ dose distribution within the human body from protocol A for whole body is given in [Figure 2] and from protocol D for brain PET/CT scan is given in [Figure 3]. The dose to uterus, which is often used to estimate exposure to embryo in the early stage of pregnancy was 13 mSv and 9.4 mSv for whole-body 18 F-FDG and 18 F-FDOPA PET/CT examinations, respectively.
Table 3: Effective doses from the PET/CT scans

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Figure 2: Organ Dose Distribution for Whole Body PET/CT Examination (a) Adult Female: 18F-FDG (b) Adult Male: 18F-FDG (c) Adult Male: 18F-FDOPA and (d) Adult Female: 18F-FDOPA

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Figure 3: Organ Dose Distribution for PET/CT Scan of Brain (a) Adult Male: 18F-FDG (b) Adult Female: 18F-FDG (c) Adult Male: 18F-FDOPA and (d) Adult Female: 18F-FDOPA

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 > Discussion Top


With the increasing use of ionizing radiation in the nuclear medicine and radio-diagnostic examinations, estimates of patient (absorbed) dose receives greater importance as the projected risk of late radiation effects, particularly cancer does not seem to have any threshold. [5],[6] Since the estimated patient dose is dependent on several factors viz. machine parameters, diagnostic protocol employed, tissue weighting factors and the analysis algorithms used, studies on patient dose estimations should critically address these issues for arriving at a realistic implication of the results obtained.

The dose received by the subjects from hybrid modalities like PET/CT scans is not only from the internally administered PET pharmaceutical but also from CT, which is considered to be a high-dose imaging modality. As a result, there is associated risk of radiation-induced late effects like cancer. The risk from PET/CT examinations could be best quantified by the radiation protection quantity, effective dose. [20],[21] In the present study, the effective dose from the PET/CT examination of whole body was approximately 4-8 times the worldwide average effective dose from background radiation over one year, which is estimated to be 2.4 mSv. [22]

The CT doses estimated in the study for whole body and brain scans using the Software CT-Expo were validated by phantom measurements using an ionization chamber for different scan protocols. The scanner specific n CTDI w values were measured and compared with the corresponding standard values used for dose calculation in the software CT-Expo. The derived value of effective dose from whole body CT scan correlated well with the effective dose computed using CT-Expo, whereas the difference in the derived and computed value of effective dose for head scan was higher (38%-39%). This may be because the dosimetric parameters computed in CT-Expo are at a tube voltage of 120 kV for head scans, whereas the measurements using 16 cm diameter acrylic cylinder (head phantom) were performed at a tube voltage of 140 kV. The effective doses from the CT component of whole body PET/CT studies computed in this study (7-13 mSv) are comparable to the earlier findings. [2],[23] However, marginal differences are noted, which is because of the differences in the scan parameters, the type of PET/CT scanner, and the limitations associated with measurements and computational techniques. The organ doses computed in this study using CT-Expo were comparable to the doses reported earlier using thermoluminescent dosemeters (TLDs) and ImPACT Calculator. [3],[24]

As expected, the organ dose from the internally administered 18 F-FDG and 18 F-FDOPA was highest to the bladder in both the cases, as both these pharmaceuticals are excreted through the renal excretory system. In case of 18 F-FDG, there is significant dose to heart, brain, liver and lungs because of relatively higher uptake in these organs due to their higher metabolic activity and hence rapid blood supply. In case of 18 F-FDOPA, after the bladder, the genitalia receive the highest dose of radiation. Most of this is due to gamma irradiation from the bladder and can be reduced by minimizing the amount of radioactive urine retained during the first two hours of a study by proper hydration and frequent voiding. [17] The organ-specific dose coefficient for 18 F-FDOPA for bladder is higher than 18 F-FDG as one-half of the 18 F-FDOPA, injected collects in the bladder, while the other half is distributed throughout the body. On the other hand, 18 F-FDG is taken up by many other organs in larger fractions and hence only 0.24 fraction of the injected dose of 18 F-FDG gets collected in the bladder. [19] Because of this differential excretion, the radiation dose to the bladder from 18 F-FDOPA exceeds that from 18 F-FDG. For this reason, the amount of activity administered for 18 F-FDOPA scan is less as compared to 18 F-FDG scan. [18] The effective dose from internally administered 18 F-FDG and 18 F-FDOPA computed using OLINDA software and ICRP coefficients match reasonably well. However, the limitation with the use of ICRP coefficients for effective dose estimation is that these coefficients are provided for hermaphrodite phantoms. The difference in the total effective doses to male and female patients from whole body 18 F-FDG and 18 F-FDOPA PET/CT examinations is because of higher radiosensitivity of breast (and therefore higher tissue weighting factor) [25] and also distribution of pharmaceuticals in male and female patients linked to differences in the physiology. [26] The organ doses and the effective doses from PET/CT examination estimated in the study are reasonably high as compared to the background radiation dose and from many diagnostic examinations. Repeated PET/CT scan on the same subject would still lead to more exposure of the subject thereby proportionately increasing the risk of radiation induced cancer.

Although the doses to various organs computed using various techniques from PET/CT investigations estimated here and reported earlier provide an estimate of the exposure levels, it is well understood that the realistic picture of biological consequences to the subjects/patients (potential damage to the various organs) is best provided by the biological dosimetry. If the examination is repeated on the same individual, the radiation dose to the individual is added and proportionately the radiation risk is increased. The organ and effective doses estimated in the study would be very useful in estimating risk of radiation-induced cancer from PET/CT scans.


 > Conclusions Top


The effective radiation doses from 18 F-FDG whole body PET/CT examination was approximately 5-8 times higher than the background dose and 4-7 times the background dose from 18 F-FDOPA whole body PET/CT examination. The effective radiation dose from 18 F-FDG PET/CT scan of brain was approximately 2-3 times the background dose and 1-2 times from 18 F-FDOPA PET/CT scan of brain. The values are comparable with the values reported earlier. However, the doses can be further reduced by optimizing the protocol for PET/CT scans by administering the amount of activity as per the body weight of the subject and modulating the CT scan parameters as per the size and weight of the patient.

 
 > References Top

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    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]


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