Fatima, Nosheen

Zaman, Maseeh Uz

English

eCommons@AKU

eCommons@AKUDepartment of Radiology Medical College, PakistanFebruary 201718FDG PET/CT: A Sensitive Tool That NeedsBetter Users' UnderstandingMaseeh Uz ZamanAga Khan University, maseeh.uzzaman@aku.eduNosheen FatimaAga Khan University, nosheen.fatima@aku.eduFollow this and additional works at: https://ecommons.aku.edu/pakistan_fhs_mc_radiolPart of the Radiology CommonsRecommended CitationZaman, M. U., Fatima, N. (2017). 18FDG PET/CT: A Sensitive Tool That Needs Better Users' Understanding. Journal of the College ofPhysicians and Surgeons Pakistan, 27(2), 64-65.Available at: https://ecommons.aku.edu/pakistan_fhs_mc_radiol/138Positron imaging tomography (PET) is the most powerfulfunctional imaging having very high sensitivity andspecificity depending upon the type of the radiolabelledprobes used. Initially, it was considered as a researchtool; however, over the last two decades it has becomea clinical need. In early 2000, it was further strengthenedby successful union with morphological computerisedtomography (CT), which has resulted in improveddiagnostic accuracy. Reasons for its popularity are theability of PET to qualitatively study the perfusion,metabolism, and various receptors functions withabsolute quantification as well. In PET imaging, we aredetecting two photons of 511 keV moving at 180degrees in opposite directions, produced as a resultof annihilation of a positron with a free electron.Simultaneous detection of these photons by nuclearmedicine detectors (scintillator or semiconductormaterials) is the basis of the coincidence imaging (PETimaging). The number of true coincidence (photons ofsame annihilation events) ensures good count statistic,low noise, and good image resolution.If we look over the evolution of imaging technology inmedicine, in early 70s we used to have fuzzy and lowresolution CT and PET images. However, over the lastthree decades there has been a tremendousdevelopment on technological frontiers resulting intoday's high resolution CT and PET images, individually.In 2001, medical imaging was further strengthened bythe arrival of first hybrid PET/CT scanner, which hasrevolutionised the oncology to a greater extent, andcardiology and neurology to a lesser extent.The paraphernalia of a PET/CT facility include: (1)cyclotron which is used to produce short-lived positronemitting isotope like Carbon-11 having half-life of 20minutes and Flourine-18 with half-life of 110 minutes; (2)radiochemistry unit which is used to label a chemicalsubstrate or probe with the short-lived positron emittingisotope produced by the cyclotron; (3) an uptake room inwhich patient stays for 55-75 minutes in a calm andquiet manner after getting PET isotope injection; and(4) an imaging suite equipped with PET/CT scanner.Fluorodeoxyglucose (18FDG) is the most commonlyused radiolabelled probe because of the longer half-life of 18F (110 minutes). Chemically, 18FDG is similar toglucose except that at carbon-2 level, hydroxyl group isreplaced with 18F. 18FDG competes with normal glucoseand follows the same metabolic pathway except once itis phosphorylated inside the cell by hexokinase, it is notfurther metabolised and stays within the cell whichenables in vivo imaging of 18FDG avid tissue. Primarypurpose of using 18FDG (i.e. radiolabelled glucose) isthat the most of malignant cells use glucose as thesentinel source of energy with overexpression ofhexokinase and glucose transporters (GLUT). Thisprocess of aerobic glycolysis by tumor cell is called theWarburg effect.Patient preparation is very important for PET/CTimaging as better 18FDG uptake in tumor cell needslower serum glucose and insulin levels at the time of itsinjection. Fasting for 4 - 6 hours for food is mandatory ashyperglycemia and hyperinsulinemia would result inenhanced localisation of 18FDG in non-tumor sites likeskeletal muscles. A fasting blood glucose (FBS) levelless than 200 mg/dl is used as a cut-off by manylaboratories; and procedure is rescheduled, if it is morethan 200 mg. To achieve a FBS < 200 mg/dl is a realissue in diabetics with inadequate glycemic control.Good hydration is important as it ensures prompt wash-out of 18FDG from kidneys because about 50 - 70% ofthe injected radioactivity is excreted through the kidneyswithin 1 hour after injection. Good hydration minimisesradiation exposure to urinary bladder and improvesimage quality as well. The usual injected dose of 18FDGis about 3 MBq/Kg (European recommendation focusedtowards low radiation exposure to patients) or 5 MBq/Kg(US practice). Once the 18FDG is injected, patient staysin a calm and quiet manner in an uptake room to allowbetter uptake of radiolabelled glucose in glucosedependent tumor cells. After 55 to 75 minutes of uptakeperiod, patient is asked to empty the urinary bladder andthen the PET/CT imaging is started. First, a scout CTimage is acquired for image planning, followed by a non-contrast enhanced CT (NCECT) examination frommidbrain to midthigh in most of oncological cases as perJournal of the College of Physicians and Surgeons Pakistan 2017, Vol. 27 (2): 63-65 63EDITORIAL18FDG PET/CT: A Sensitive Tool That NeedsBetter Users' UnderstandingMaseeh uz Zaman and Nosheen Fatima1 Department of Radiology, Section of Nuclear Medicine andPET/CT Imaging, The Aga Khan University Hospital (AKUH),Karachi, Pakistan.2 Department of Radiology, Section of Nuclear Medicine,Dr. Soliman Fakeeh Hospital, Jeddah, Saudi Arabia.Correspondence: Dr. Maseeh uz Zaman, Department of Radiology,Section of Nuclear Medicine and PET/CT Imaging, The AgaKhan University Hospital, Stadium Road, Karachi, E-mail: maseeh.uzzaman@aku.eduReceived: February 03, 2017;   Accepted: February 04, 2017.recommendation of recent guidelines to minimise theradiation exposure and contrast associated morbidities,and also to avoid possible over-estimation of tumor FDGuptake by computer algorithm. After the NCEC, PETimages are acquired which takes about 10 to 15 minutesdepending upon length of patient and time per bedposition required to cover the body. At the end ofimaging, the computer reconstructs CT and PET imagesand during this process CT images are used forattenuation correction of the PET images, which is amandatory step. CT images are also used for anatomicalmapping of PET images which lacks anatomicalinformation. Advantages of PET images include betterlesion characterisation and detection; and with CTimages, these have high diagnostic accuracy andclinical information.In clinical practice, 90% of the PET/CT burden is relatedto oncology and 5% each for brain and cardiac imaging.Recently, 18FDG PET/CT has also been used forevaluation of pyrexia of unknown origin (PUO). 18FDG-based PET/CT provides a wide spectrum of indication inclinical oncology. It may be used for the differentiationbetween benign and malignant tumors (although notvery popular) like evaluation of solitary pulmonarynodule. 18FDG-based PET/CT is an important step inmanaging paradigm of various malignancies (more than17 cancers as per National Comprehensive CancerNetwork guidelines, NCCN). It has an established role instaging, restaging of non-small-cell lung cancer,lymphoma, head and neck, esophagus, colorectal andfemale genital cancers. Use of PET/CT in the diagnosticworkup results in upstaging in about 20 - 40% cases(avoid under-treatment) and downstaging in 10% cases(avoid over-treatment, its cost and morbidity as well).18FDG-based PET/CT has also an established role inmonitoring therapy response and can stratify thepatients as responder and non-responder in the earlyphase of treatment. Therefore, treating oncologist canalways change the therapy regime as per availableinformation by a PET/CT exam. It also has a very strongprognostication value as a negative 18FDG PET/CTstudy has a significantly high negative predictive value(NPV) for overall and disease-free survivals. However,positive predictive value (PPV) is relatively low becauseof false-positive results caused by 18FDG uptake by non-malignant foci, like infection or inflammatory lesions. Inrecent days, the 18FDG-based PET/CT has also enteredinto the radiation oncology suite due to promising resultsof radiation therapy planning using metabolic tumorvolume (MTV) derived from 18FDG PET images. Intheranostics, the positron emitting isotope labelledpeptide and monoclonal antibodies imaging withPET/CT have been used to select cases of metastaticneuroendocrine tumors which can be treated usingsame probe labelled with particle emitters (Alpha emitteras Radium-223 or Lutitium-177 as beta emitter).A normal 18FDG PET/CT shows uniform distribution ofmild intensity of labelled glucose over mediastinum andliver with variable 18FDG uptake over the myocardium,depending upon the preparation of the patient. Presenceof tracer in kidneys and urinary bladder is a commonfinding; and hydration before and after the injection of the18FDG is recommended to have quick washout from thekidneys. Uptake of the 18FDG in the bowel is notuncommon and more commonly seen in diabeticpatients who have been on metformin for some unknownreasons. Abnormal focal 18FDG deposit is consideredwhen it is higher than the background activity(mediastinal or liver uptake); and reviewing with CTimages of same level provides correct localisation anddiagnostic input based on morphological abnormality (ifpresent). While interpreting 18FDG PET/CT images, thereader must be aware of the false-negative findingswhich are not unusual, indeed. Some well differentiatedcancers like thyroid, prostate, renal, and NET are not18FDG avid. Lesions smaller than 8 mm are beyond theresolution of the current scanners and most importantlyinadequate patient preparation must also be consideredas an important cause of false-negative results. False-positive findings are seen with areas of physiologicaluptake like intraocular muscles, laryngeal activity,cardiac uptake, or some infective or inflammatoryconditions which are not uncommon in patients withcancers. It is also not uncommon for the 18FDG to betaken up by benign tumors like hepatic adenomas orovarian follicles.In addition to the qualitative analyses mentioned above,semi-quantitative parameters have also been introducedand considered as one of the major strengths of PETimaging. The most commonly used semiquantitativeparameter in clinical practice is standardised uptakevalue (SUV). SUV is the ratio of the 18FDG, in the regionof interest, divided by the injected 18FDG, divided bypatient's weight. If it is assumed that the distribution of18FDG is uniform in the body and mass is equivalent toweight (gm = ml), then SUV is considered equal to 1.Various types of SUVs are being used like SUVmax,SUVmean and SUVpeak (depending upon ROI drawn).SUVmax is the most commonly used parameterworldwide due to its simplicity albeit lacks reproducibility.These SUVs are commonly used for responseevaluation in patients receiving chemotherapy orradiation therapy. Based on differential SUV valuesbetween baseline and follow-up scans, variousresponse evaluation criteria have been formulated.However, when comparing the SUV values of 2 PET/CTof a patient, reader must consider the confoundingfactors which could result in different values despite nosignificant change in tumor biology. These confoundingfactors include calibration of dose calibrator and PET/CTscanner, fasting blood glucose level, uptake timeMaseeh uz Zaman and Nosheen Fatima64 Journal of the College of Physicians and Surgeons Pakistan 2017, Vol. 27 (2): 63-65between two studies, partial volume effect for smalllesions, use of oral or intravenous contrast, size of theROI, and reconstruction algorithm used for two studies.To make PET/CT more meaningful, standardisation ofimaging protocols among imaging facilities worldwide isvery important.18FDG-based PET/CT is the most commonly usedhybrid imaging modality in oncology. It plays animportant role in staging, restaging, response evaluation,and prognostication of various cancers. Judicious andrationale use of this tool with high diagnostic accuracy,ensures optimal selection of therapy and tailoring thetherapy in non-responders in the early phase oftreatment. SUV (SUVmax most commonly used) is agood semiquantitative metric which is extensively beingused for response evaluation. However, one must notforget the impact of confounding factors upon thesecalculated values. To make PET/CT more meaningful,standardisation of imaging protocol among imagingfacilities worldwide is very important.18FDG PET/CT: A sensitive tool that needs better users' understandingJournal of the College of Physicians and Surgeons Pakistan 2017, Vol. 27 (2): 63-65 65

18FDG PET/CT: A Sensitive Tool That Needs Better Users\u27 Understanding

https://ecommons.aku.edu/cgi/viewcontent.cgi?article=1137&amp;context=pakistan_fhs_mc_radiol

18FDG PET/CT: A Sensitive Tool That Needs Better Users\u27 Understanding

Abstract

Similar works

Full text

Available Versions

eCommons@AKU