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© 2004 Oxford University Press
CORRESPONDENCE |
PET–CT Modification of RECIST Guidelines
Correspondence to: Martin O’Connell, MD, Divisions of Nuclear Medicine and Abdominal Imaging, Department of Radiology, Duke University Medical Center, DUMC Box 3949, Durham, NC 27710 (e-mail: martinoconnell{at}hotmail.com)
The World Health Organization's 1981 criteria (1) for tumor size evaluation in response to cytotoxic treatment were simplified to the RECIST (Response Evaluation Criteria in Solid Tumors) guidelines (2) as a result of a consensus task force established in 1994. The purpose of this modification was to standardize measurements of lesion size in response to chemotherapy in adults (2,3). The RECIST guidelines were designed to allow accurate, reproducible measurements without excess time or effort. Consequently, one-dimensional measurements replaced previous two-dimensional measurements. Computed tomography (CT) provides anatomic information about the size and location of lesions. Axial CT measurements are used to assess tumor size. Positron emission tomography (PET) imaging is typically used to measure metabolic activity, which is increased in tumors. PET–CT systems have a number of advantages over CT or PET imaging alone that may allow more accurate assessment of tumor response by distinguishing scar tissue from residual tumor (4) and by facilitating easier orthogonal lesion measurements in three planes (5). Therefore, modification of RECIST guidelines to incorporate this new technology may have added benefit.
Increased availability of PET–CT systems makes tumor response assessment using a combination of anatomic CT and metabolic PET information practical. An alternate method of response evaluation to RECIST could combine CT measurements (one-dimensional or otherwise) with [18F]fluorodeoxyglucose (FDG)–PET activity assessment. A potential method of combining these data would to include the entire lesion CT measurement when any part of a target lesion demonstrates high metabolic activity. If a lesion has low metabolic activity, which means that it is likely to represent scar tissue (4), the lesion measurement is zero (Fig. 1). If a lesion has indeterminate uptake of FDG, the possibility of residual tumor cannot be excluded, and therefore the entire lesion measurement by CT would be included. PET imaging would also be useful in initial target lesion selection. Standardized uptake value measurements from PET imaging may be used to determine whether a lesion would be included for measurement, but these measurements are more time consuming than visual inspection and are subject to inaccuracy if quality control is not maintained. It is important that any high metabolic activity within a lesion be regarded as reflecting the biology of the lesion as a whole. Measurement of lesion size with CT is considerably more accurate than that achieved with PET imaging, and therefore CT evaluation when using size criteria remains essential.
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Despite the intuitive idea that two measurements are better than one, the RECIST criteria have proven to be reliable and equivalent to two-dimensional measurements in adults (6). However, estimating tumor size from the diameter measurement becomes inaccurate when the length of a lesion is more than twice its width. Most PET–CT systems have 8- or 16-slice multidetector CT capability, which makes routine coronal and sagittal reconstructions of acquired data practical and easy to perform. Derived images can be evaluated on standard workstations, with automated cross-referencing, making orthogonal measurements in three dimensions possible. This technology could potentially allow for new methods of size measurement, particularly in nonspherical lesions, for which one-dimensional measurements are subject to inaccuracy. It is anticipated that volumetric measurements would still be too time consuming to be of practical value (7).
Extensive investigation is needed to determine if PET–CT modification of RECIST guidelines is practical, reproducible, and will allow detection of response or disease progression. Assigning a zero measurement to lesions with low metabolic activity could potentially overestimate response to treatment. In addition, some tumors that are not FDG avid would not be suitable for this assessment method.
NOTES
Martin O’Connell is a radiology consultant to the Duke Protocol Office for CT examinations.
REFERENCES
1 World Health Organization. WHO handbook for reporting results of cancer treatment. Geneva (Switzerland): World Health Organization; 1979. p. 48.
2 Therasse P, Arbuck SG, Eisenhauer EA, Wanders J, Kaplan RS, Rubinstein L, et al. New guidelines to evaluate the response to treatment in solid tumors. J Natl Cancer Inst 2000;92:205–16.
3 Padhani AP, Ollivier L. The RECIST (Response Evaluation Criteria in Solid Tumors) criteria: implications for diagnostic radiologists. Br J Radiol 2001;74:983–6.
4 Stumpe KD, Urbinelli M, Steinert HC, Glanzmann C, Buck A, von Schulthess GK. Whole-body positron emission tomography using fluorodeoxyglucose for staging of lymphoma: effectiveness and comparison with computed tomography. Eur J Nucl Med 1998;25:721–8.[CrossRef][Web of Science][Medline]
5 Hany TF, Steinert HC, Goerres GW, Buck A, von Schulthess GK. PET diagnostic accuracy: improvement with in-line PET-CT system: initial results. Radiology 2002;225:575–81.
6 James K, Eisenhauer E, Christian M, Terenziani M, Vena D, Muldal A, et al. Measuring response in solid tumors: unidimensional versus bidimensional measurement. J Natl Cancer Inst 1999;91:523–8.
7 Sohaib SA, Turner B, Hanson JA, Farquharson M, Oliver RT, Reznek RH. CT assessment of tumour response to treatment: comparison of linear, cross-sectional and volumetric measures of tumour size. Br J Radiol 2000;73:1178–84.[Abstract]
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J Natl Cancer Inst 2004 96: 802.
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