The Role of PET-CT Fusion in Head and Neck Cancer

Positron-emission tomography(PET) and computed tomography(CT) fusion imaging is arapidly evolving technique that is usefulin the staging of non-small-celllung cancer (NSCLC), Hodgkin's disease,ovarian cancer, gastrointestinalstromal tumors, gynecologic malignancies,colorectal malignancies,and breast cancer. In their article,Rusthoven et al[1] describe the roleof PET-CT in head and neck malignanciesand include a review of allcurrently available literature. Accordingto the authors, PET-CT is usefulfor staging head and neck carcinomasand for target volume delineation duringradiation treatment planning.Staging Standard
PET-CT is quickly becoming thestandard of care for staging malignanciesat certain anatomic sites. Astudy by Antoch et al[2] comparedPET-CT with whole-body magneticresonance imaging (MRI). The overalltumor-node-metastasis (TNM)stage was correctly determined in 77%of patients with PET-CT and in 54%with MRI. Moreover, compared withMRI, PET-CT had a direct effect ondisease management in 12 patients.In a landmark study by Lardinois etal published in the New England Journalof Medicine,[3] PET-CT was foundto improve the accuracy of NSCLCstaging over PET and CT alone; thecombined modality provided additionalstaging information in 41% of patients.In head and neck tumors,PET-CT again appears to be superiorto PET alone, and probably also toPET and CT when both are assessedside by side for detection of tumorinvasion and staging accuracy.[4]Schoder et al[5] found that PET-CTwas more accurate in depicting cancerthan was PET alone, and PET-CT findingsresulted in a change in treatmentin 12 of 68 patients, further establishingthe higher efficacy of PET-CT overPET alone in recurrent head and neckcancer. Therefore, PET-CT may becomea "one-stop shop" for oncologicstaging of head and neck cancers.[6]Advantages of PET-CT
A major advantage of PET-CT overPET alone is the notable reduction inscanning time. A PET scan is composedof an emission scan, depictingthe distribution of fluorine-18 fluorodeoxyglucose(FDG) in the body, anda transmission scan that is used forattenuation correction. For PET, thetransmission scan can take approximately20 minutes, increasing the totalscanning time to approximately 50minutes.[7] In PET-CT, the CT dataare used for attenuation correction,and a whole-body scan can be performedin under 2 minutes.[8]An additional advantage of PETCTis that the intrinsic hardware provideshigh-quality images throughcoregistration of both image datasetsin a relatively fast acquisition time.[6]The coregistration of datasets obtainedby different techniques (ie, PET withCT or MRI) at different time pointsmay lead to inaccurate anatomic andphysiologic delineation of the tumorwith respect to normal tissues. Theseinaccuracies may be caused by anatomicchanges, neck repositioning, orhead and neck swelling.Feasibility studies have found thatthe use of PET-CT for planning threedimensional(3D) conformal radiationtherapy improves the standardizationof volume delineation compared withCT alone.[9,10] Rusthoven et al[1] reportthat since July 2002, PET-CT fusionimaging has been an integralplanning component for intensity-modulatedradiation therapy in patients withhead and neck cancer. Changes in theTNM stage have ranged from 14% to36% with PET-CT, and treatment volumeand dose have been altered in 14%and 11% of patients, respectively.Target Volumes
That said, the authors do not definethe appropriate threshold by whichphysiologic disease is correlated withanatomic disease. The resolution forclinical PET is approximately 5.0 to7.0 mm, and without pathologic correlationto help determine the true extentof gross and microscopic physiologicdisease, the radiation treatment volumescould be altered drastically. Furthermore,partial volume and misregistrationeffects can extend a portion of thePET-defined target volume into airspaces (ie, the larynx or trachea), whichmay alter the treatment volume.Rusthoven et al[1] also note that PETCTis not as sensitive in diagnosingtumors < 2 cm in diameter and mayresult in false-positive findings in inflammatorytissue or lesions.Institutional variability in definingthe threshold of malignant diseasewith physiologic imaging can have aprofound effect on the contoured biologictumor volume. By raising orlowering the threshold, the resultantsensitivity is altered, and the volumeof contoured disease decreases or increases,respectively. This may ultimatelyresult in the underdose oroverdose of the actual tumor volume.Recently, Scarfone et al[11] foundthat the threshold of PET images wasadjusted on a case-by-case basis to adequatelyvisualize FDG-avid lesionsrelative to the background, with theresultant "average" threshold being approximately50% of maximum imageintensity. They further expressed concernabout the use of PET-CT for radiationtreatment planning by pointingout that the optimal threshold neededto standardize the settings has yet to bedetermined. For this reason, we haveresisted the urge to modify treatmentplanning contours by incorporatingPET-CT in radiation treatment planningfor head and neck cancer at ourinstitution. Future studies confirminggross and microscopic pathologic diseasewith PET-CT will help define theappropriate threshold settings to betterdelineate target volumes.Conclusions
In the multidisciplinary managementof patients with cancer, PET-CT is anexciting and rapidly evolving techniquethat is improving our ability to makebetter treatment decisions. The use ofPET-CT for staging primary and recurrenthead and neck lesions is "ready forprime time," but its application in headand neck cancer treatment planningshould be viewed as investigational untilwe can better correlate our imagingfindings with gross and microscopicpathologic findings and resolve the issuesof variable FDG uptake by thetumor and nodal metastases as well asinstitutional threshold variability.

Disclosures:

The authors have nosignificant financial interest or other relationshipwith the manufacturers of any products orproviders of any service mentioned in this article.

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