Medical records of patients subjected to 18F-FDG PET/CT scan at Instanbul University- Cerrahpasa, Cerrahpasa Hospital Turkey were retrospectively analyzed for nearly 11 years from 04 Jan 2010 to 04 Nov 2020. This retrospective study was approved by the ethical committee of the institute at Cerrahpasa medical school. In our site, parameters of patients undergoing 18F-FDG administration like patients name, ID, indications, administered activity, date, and time of administration are available through the archiving system.
PET/CT scans were routinely acquired with cranially extended torso imaging protocol from top of head to midthigh following 18F-FDG administration. However, some pathologic conditions exclusively necessitate full-body scan like melanoma, bone cancer or metastases and inflammatory diseases. Two types of PET/CT systems are available in the department and were used. First, Siemens biograph 6 with 6.4 x 6.4 x 25 mm3 LSO scintillators integrated to 6 slices rotating CT scanner. The computed tomography (CT) scans were carried out using 130 kVp, tube current modulation ranging from ≈ 35-290 mA across the axial sections, 4 mm slice thickness, 0.921sec rotation time. The second PET/CT scanner (GE discovery 710) has been installed since early 2014 with 4.2 x 6.2 x 30 mm3 LYSO crystals and 585 ps coincidence timing. The CT scans were done using helical mode with 16 detectors rows. The imaging settings were: 120 kVp, tube current modulation typically with 35-215 mA, 0.7 sec rotation time, 18.8 mm table feed per rotation, 16×1.25 mm collimation width, and 0.938 pitch limit. Since 2014, GE scanner has become a workhorse making up 96 % of the performed PET/CT scans until the end of 2020.
Patients who were scanned multiple times at Cerrahpaşa hospital in a year were identified from patient ID. This helped to identify patients who were scanned frequently. The maximum number of scans made during every 1 year period for any patient was assessed by counting the number of repeated ID’s in a given year.
A set of commands was used in an excel sheet to extract the number of patients who were repeatedly scanned year by year among thousands of patients enrolled for 18F-FDG PET/CT over the study period. In addition to studying the frequency of patients undergoing multiple PET/CTs, we estimated the total number of scans for all patients who had more than one scan in any year.
The effective dose (E) from 18F-FDG was estimated using the dose conversion coefficient of 0.019 mSv/MBq as provided by ICRP Publication 128 [10].
For estimation of E from CT scans, the region-specific conversion coefficients (CFs) from dose length product (DLP) to E were summerized by Deak and co-authors [11]. The CF at 120 kVp was reported to be 0.0019, 0.0051, 0.0145, 0.0153 and 0.0129 for head, neck, chest, abdomen, and pelvis, respectively. Whereas, the corresponding CFs at 140 kVp were 0.0019, 0.0052, 0.0147, and 0.0155. Since the scan length of whole body PET/CT encompasses all of the above mentioned regions and 120 kVp was used for GE Discovery scanner, the average of the conversion coefficients for chest, abdomen and pelvis were used to estimate the E for patients scanned on GE discovery 710. Similarly, for Siemens PET/CT with 130 kVp, the average of the corresponding conversion coefficients was used. For head and neck, the corresponding coefficients were used assuming 15 cm and 12 cm scan length, respectively. The total scan length was set as 120 cm.
In accordance with the earlier assumptions, the formula below was used to estimate the total E from multiple 18F-FDG PET/CT scans:
Total E= E from PET part + E from CT part
Total E = [n × A (MBq) ×0.019 (mGy/MBq)] + [n × CF× DLP]
E: effective dose, n: number of scans, A: administered 18F-FDG activity, CF: conversion factor, DLP: dose linear product (mGy.cm2).