FDG-18 PET/CT - radiation dose and dose-reduction strategy Poster No.: C-1856 Congress: ECR 2014 Type: Authors: Keywords: DOI: Scientific Exhibit P. Nicholson, S. McSweeney, K. O'Regan; Cork/IE Radiation physics, Radioprotection / Radiation dose, Nuclear medicine, PET-CT, Dosimetry, Radiation safety, Dosimetric comparison 10.1594/ecr2014/C-1856 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply ECR's endorsement, sponsorship or recommendation of the third party, information, product or service. ECR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold ECR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies, ppt slideshows and any other multimedia files are not available in the pdf version of presentations. www.myesr.org Page 1 of 12
Aims and objectives The introduction of FDG-18 PET-CT into routine clinical practice has had a dramatic effect on both treatment plans and clinical outcomes worldwide. With the range of indications for PET-CT scanning expanding year-on-year, a common clinical question which comes up with increasing frequency is "What is the radiation dose for my patient from this scan?". The answer to this is not as straighforward as might first appear - there exists a relative paucity of data in the literature with regard to radiation dose in PET-CT. Some of the larger studies have documented average effective doses which range from 13.5-14.5 msv for a standard PET-CT. Effective dose, however, varies wildly between different deparments due to variation in the protocol used in image acquisition. Variations exist for both the PET part of the study (a consequence of both scanning technique and injected FDG dose) and for the CT part of the study. The culmination of these two separate parts of the scan results in the total mean effective dose. In our department, our PET-CT unit has been operational since June 2012. We sought to quantify the dose that patients receive from PET-CT scans in our institution, and analyse any factors which may affect this radiation dose. Following the initial measurement, we implemented a dose-reduction protocol and we reaudited our results six months later to look at any changes. Methods and materials We analysed body PET-CT studies obtained from a total of 100 patients were analysed. Images were acquired on a 64-slice GE Discovery PET-CT scanner. Page 2 of 12
Fig. 1: GE Discovery PET/CT Scanner References: www.gehealthcare.com In preparation for the examination, patients were fasted for a minimum of 6 hours. After the administration of the isotope, patients rested quietly for 45-60 minutes, and were asked to empty their bladder prior to entering the scan room. Next, a non-contrast CT scan was acquired - this data was later used for attenuation correction purposes and spatial co-ordination of the PET data, which was collected next. Helical mode was used for the CT part of the examination, with a rotation time of 0.5 seconds, 40mm detector coverage, 3.75 mm slice thickness, a pitch of 0.984, 39.37mm/ sec rotation speed, 120 kv, 80 mas. Patients were scanned in the supine position with their arms by their side - average time per bed-position was 3 minutes, and images were acquired at an average of seven table positions (range: 6-9). We recorded the following parameters for each patient: age, weight (kg), height (cm), Body Mass Index (kg/m 2 ), blood glucose (mmol/l), CT beam-current (ma) calculate CT dose index (CTDI Vol - mgy) and dose length product (DLP - mgy/cm) and injected FDG activity (mbq). Page 3 of 12
The effective dose of the injected FDG-18 was calculated using the factors provided in the ICRP 80 guidelines: Injected dose X (MBq) x 0.019 = x msv. In order to estimate the contribution of various body parts of a whole-body scan to a total effective dose, a dose contribution table published by the American Association of Physicists in Medicine (AAPM) was consulted: (Table 1): Body Part Conversion Factor Adult Head 0.0021 Neck 0.0059 Chest 0.014 Abdomen/Pelvis 0.015 To measure the average contribution of each of the above body parts (head, neck and so on) to the total scan volume, 10 random scans were chosen from our sample set of 100 scans. By consulting the scout view used to plan the scan, an average contribution of each body part - head, neck, chest, abdomen and pelvis - to the total scan length was calculated (Figure 2). Page 4 of 12
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Fig. 2: Scout image from standard PET-CT, used to estimate the contribution of various body parts to the total scan length. References: Radiology, Cork University Hospital - Cork/IE These fractions of the total scan length were then multiplied by the relevant conversion factors - from the aforementioned AAPM table - and a total effective dose was estimated for the CT part of the examination (ED CT ). This was then added to the whole-body effective dose (ED PET ) estimated from the injected FDG-18, and a total estimated whole-body effective dose was calculated for the PET-CT examination: ED TOTAL = ED CT + ED PET The EANM PET-CT guidelines recommend regular measurement of PET-CT radiation dose along with implentation of dose-reduction strategies, where possible. Thus, after our initial assessment of our doses, we implemented a weight-based FDG dosing protocol, in an effort to further reduce our doses. Patients who weighed les than 85kg received an injected dose of 340 MBq, those who weighed between 85kg and 100kg received a dose of 370 MBq, and those who weighed more than 100kg received a dose of 400 MBq. Following this change, we reaudited our doses. Images for this section: Page 6 of 12
Fig. 1: GE Discovery PET/CT Scanner Page 7 of 12
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Fig. 2: Scout image from standard PET-CT, used to estimate the contribution of various body parts to the total scan length. Page 9 of 12
Results We initially looked at 100 FDG-18 PET studies: there were 49 male patients and 51 female. The age range was 18-82 years, with a mean of 60.4 and a median of 63. The most common indications were lung carcinoma (n=31), followed by Non-Hogkin's lymphoma (n=18), colorectal (n=9) and oesophageal carcinoma (n=6). The mean injected dose (MBq) for the PET portion of the study was 366 (range: 215-415), yielding a mean effective dose of 6.95 msv (range: 4.1-7.89). The mean dose length product (DLP) for the CT portion of the investigation was 566 (range: 34.6-1031.08), yielding a mean effective dose of 7.4 msv (range: 2.3-13.6) for a total average mean effective dose from an FGD-18 PET-CT of 14.35 msv (range: 7.2-21). We also noted, as expected, a linear relationship between patient weight and effective dose (Figure 3). Because the same dose was obtained from the injected activity in all of these patients, this dose variance was therefore accounted for by the increased x-ray energy applied by the scanner - using the tube current modulation protocol described above - in order to yield a CT scan of acceptable quality. Fig. 3: Dose (msv) -v- Weight (kg) References: Radiology, Cork University Hospital - Cork/IE Page 10 of 12
When we reassessed our doses following introduction of the new weight-based protocol, we specifically looked at changes in overall effective dose. The average injected FDG dose was 352.06 (range 319-404) MBq, yielding an estimated effective dose of 6.69 - a 4% reduction. We also noted a decrease in the mean dose length product, which was now reduced to 529. This meant that the effective dose from the CT portion of the examination had decreased to 6.64 msv. In total, the estimated effective dose from the PET-CT examination was 13.33 msv following implementation of the dose-reduction protocol - a 7.5% overall reduction in effective dose. This was without any noted decrease in diagnostic image quality. Conclusion The effective radiation dose from PET-CT at our institution - 13.33 msv - is generally concordant with those doses published in the literature. Furthermore, implementation of a weight-based dose reduction strategy has resulted in a lower effective radiation dose without any compromise in diagnostic image quality, a process which can be reproduced in other institutions. Personal information References Boellaard, R. et al. FDG PET and PET/CT: EANM procedure guidelines for tumour PET imaging: version 1.0. Eur. J. Nucl. Med. Mol. Imaging 37, 181-200 (2010). Valentin, J. Radiation dose to patients from radiopharmaceuticals: ICRP Publication 80. Ann. ICRP 28, 1 (1998). 23, A. T. G. The Measurement, Reporting, and Management of Radiation Dose in CT (2008). at <http://medcontent.metapress.com/index/a65rm03p4874243n.pdf> Huang, B., Law, M. W.-M. & Khong, P.-L. Whole-body PET/CT scanning: estimation of radiation dose and cancer risk. Radiology 251, 166-74 (2009). Page 11 of 12
Willowson, K. P., Bailey, E. a & Bailey, D. L. A retrospective evaluation of radiation dose associated with low dose FDG protocols in whole-body PET/CT. Australas. Phys. Eng. Sci. Med. 35, 49-53 (2012). Page 12 of 12