Jun 19, 2014
By Al B. Benson III, MD, FACP, FASCO
Feinberg School of Medicine, Robert H.Lurie Comprehensive Cancer Center,Northwestern University
There is little question that advancesin imaging technology have revolutionizedcancer medicine, greatly enhancingprecision in screening, diagnosisand staging, treatment, and surveillance.Functional imaging has thepotential to refine staging accuracyand to provide a biologic characterizationof tumor behavior beyond anatomiclocation.
As is true of many evolving technologies,there is often incompleteevidence to guide the most optimaland cost-effective utilization of imaging—particularly when the clinician hasa number of tools available that canbe used in combination or in sequenceto diagnose and to evaluate patientsbefore, during, and after treatment.The uncertainty as to the best choiceof modality, worry over “missing” adiagnosis, and the availability of multiplereimbursable options has raisedconcern of inappropriate use or overuseof imaging, resulting in significantcosts, increased radiation exposure,and additional procedures based onfindings that are eventually determinedto be benign. Optimal imaging strategiesshould be further addressed inclinical trials and incorporated in clinicalguidelines.
In this issue of ASCO Connection, Dr.Michael MacManus and Dr. Garry Choyoffer contrasting perspectives summarizingthe pros and cons of positronemission tomography/computedtomography (PET/CT) versus contrast-enhancedCT, and examples of optimalimaging strategies from their respectivepoints of view.
Dr. Benson is a Professor of Medicine atFeinberg School of Medicine and AssociateDirector for Clinical Investigations at theRobert H. Lurie Comprehensive CancerCenter. He serves on the Editorial Boardsof ASCO Connection and ASCO University®,and is Co-Chair of the ASCO ColorectalCancer Surveillance Advisory Group.
Optimal Use of Contrast-Enhanced CT
By Garry Choy, MD, MBA
Massachusetts General Hospital
Contrast-enhanced CT is a reliable andhighly effective workhorse in cancerimaging. The benefits of contrast-enhancedCT alone are manifold—clinically, economically, and logistically.
Clinically, contrast-enhanced CToffers the ability to visualize anddetect lesions as well as characterizeinvolvement of surrounding structures,a highly applicable modality foroncology applications.1 Contrast-enhancedCT alone also results inthe use of lower total radiation dose.While radiation exposure is less of anissue in the balance between risk andbenefit in patients with cancer, PET/CT exposes patients to radiation fromboth the CT as well as the radioactiveFDG tracer. PET/CT may not alwaysbe the sole imaging modality one canrely on for a patient’s imaging followupprotocol. While PET/CT offerssuperior sensitivity to detect sites ofmalignancy, once initial disease burdenhas been established, continuedfollow-up surveillance scans could beperformed with contrast-enhancedCT alone and offer the same clinicalutility. Incidental vascular pathologyrelevant to patients with cancer mayalso be detected on contrast-enhancedCT, such as the occasional pulmonaryembolism or portal vein thrombosis.
Economically, a contrast-enhanced CTscan is much less costly than a PET/CTscan. PET/CT when typically performedresults in higher human capital andmaterial cost. More financial resourcesare required to pay for nuclearmedicine technologists, radiotracers,and scanner equipment.2 A PET/CTtakes at least 90 minutes to performand a significant amount of timefor the patient and patient’s familymembers who may be accompanyingthem. PET/CT may certainly result indetection of nonspecific FDG aviditythat requires contrast-enhanced CTfollow-up regardless.
Logistically, contrast-enhanced CTresults in a much shorter turn-aroundtime from patient scheduling to imageinterpretation. A contrast-enhancedCT alone is typically easier to schedulegiven the relative availability of CTscanner time. Most stand-aloneimaging centers and even academicmedical centers do not have anabundance of PET/CT scanners.(However, if one is fortunate to beat an institution focused on cancercare with numerous PET/CT scanners,this may not be an issue.) PET/CT isa lengthy procedure requiring bothemission (PET) and transmission (CT)scans. Image acquisition is muchfaster when obtaining CT imagesalone. Image analysis requiring fusionto register the PET and CT imagesmay also require significant time andeffort. Furthermore, downstream inthe imaging chain, fewer images fromCT alone results in a faster imageinterpretation and radiology reportavailability.
There likely will often be cases in whicha contrast-enhanced CT will do the jobwell and the modality offers severaladvantages. However, PET/CT doesoffer advantages in sensitivity as wellas specificity in cancer evaluation,as discussed by Dr. MacManus. PET/CT and contrast-enhanced CT are infact highly useful tools to leverage incancer imaging, and both techniquesoffer their strengths such that we canprovide the best care to our patients.
Dr. Choy is a radiologist in the Division ofThoracic and Cardiovascular Imaging atMassachusetts General Hospital, wherehis research and clinical interests includehealth information technology andquality of care. Follow him on Twitter@GarryChoy.
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- Saif MW, Tzannou I, Makrilia N, et al. Yale JBiol Med. 2010;83:53-65. PMID: 20589185.
Optimal Use of PET/CT
By Michael P. MacManus, MBBCh BAO,MD, MRCP, FRANZCR
Peter MacCallum Cancer Centre
Contrast-enhanced CT and otherstructural imaging modalities such asMRI provide basic clinical informationfor staging cancer and for widely usedtherapeutic response assessmentcriteria such as RECIST.1 However,single-modality structural imaging hassignificant limitations compared toPET, especially when PET is combinedwith CT to produce fused PET/CTimages.2 For most epithelial cancers,sarcomas, and hematologic tumors,18F-fluorodeoxyglucose (FDG) is thePET radiopharmaceutical of choice.Because uptake and retention ofFDG is higher in many tumors thanin the surrounding normal tissues,FDG-PET can image disease in siteswhere a radiologist reading a CT scanwould see no abnormality. In manyclinical scenarios, the use of PET/CTallows more accurate determinationof the true status of lymph nodes,gives additional information on thelocal extent of the primary tumor,and is more sensitive and specific forthe detection of distant metastasis.Furthermore, intensity of FDGuptake can give information on theaggressiveness of the disease. Forexample, high FDG uptake can indicatetransformation from follicular todiffuse large B-cell lymphoma.3
In common epithelial malignancies,such as non-small cell lung cancer(NSCLC), PET/CT is the imagingstandard of care for disease staging inpatients who are treated with curativeintent, with either surgery or radiationtherapy (RT). Meta-analyses haveconfirmed that PET adds significantlyto the accuracy of mediastinal lymphnode staging.4 Use of PET avoidsunnecessary thoracotomies inNSCLC,5 selects the best candidatesfor curative-intent RT, and increasesthe accuracy of RT delivery.6 A majorlimitation of CT in the assessment oflymph node status across all commoncancers is the use of lymph nodesize as the sole criterion for tumorinvolvement. Although a short axisdiameter larger than 1 cm is consideredpositive for tumor, reactive lymphnodes are commonly larger than this.On the other hand, tumor is oftenpresent in lymph nodes smaller than 1cm. The ability of PET to detect cancerin lymph nodes smaller than 1 cm andto more accurately categorize largernodes has led to an increasing trend toprefer it over CT for nodal staging ofmany epithelial malignancies, includingNSCLC, esophageal cancer,7 head andneck cancers,8 and cervical carcinoma.9
Another major advantage of PET isits ability to detect distant metastasisin organs outside the central nervoussystem when CT is negative. As asingle whole-body modality PET candetect disease in any organ, providedFDG uptake is sufficient and the tumoris large enough. Again, taking NSCLCas an example, PET and PET/CT candetect distant metastasis in morethan 20% of cases that are apparentlylocoregionally confined on CT-basedimaging.10 Furthermore, CT mustbe supplemented by other imagingmodalities such as bone scanning orliver ultrasound to compensate for itslimitations at specific anatomic sites.Accurate staging is essential forindividualized choice of curativetherapy in the lymphomas, bothfor Hodgkin (HL) and non-Hodgkinlymphomas (NHL).11 In early-stagedisease, limited chemotherapyand involved-field RT are standardtreatment choices. Upstaging ofpatients with lymphoma by PETfrom stage I-II to III-IV by detectingunsuspected lymphatic or extranodalsites has profound implications formanagement, often changing optimumtreatment to intensive systemictherapy.
Finally, PET has advantages over CTin therapeutic response assessment.PET response often precedes CTresponse and is more accurate.For example, in NSCLC, PET-basedresponse assessment is far superiorto CT response. In aggressive NHLand HL, early FDG-PET responseassessment after two to three cyclesof chemotherapy provides much moreaccurate prognostic information thanCT. Large clinical trials are in progressto determine the best way to use thisinformation.12
Although CT is superior in somespecific situations, such as detection ofsmall pulmonary nodules, in patientstreated with curative intent, PET/CToften provides incremental clinicalinformation, appropriately changesmanagement, and more than justifiesits greater initial cost.
Dr. MacManus is a radiation oncologistand Associate Director (Research) of theDepartment of Radiation Oncology at PeterMacCallum Cancer Center in Melbourne,Australia. An ASCO member since 1996,he has previously served on the Journal ofClinical Oncology Editorial Board.
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