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Opinion |
1 Department of Radiology, Massachusetts General Hospital and Harvard Medical School, 32 Fruit St., Boston, MA 02114.
Received July 7, 2000;
accepted after revision August 10, 2000.
Address correspondence to S. Saini.
Introduction
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The paradigm of radiologic assessment of drug efficacy is especially attractive in oncology patients because tumor size can be monitored serially from before treatment is started through the treatment period to any follow-up interval. Almost 20 years ago, the International Union Against Cancer and the World Health Organization (WHO) developed uniform guidelines [3] so that results of different trials could be compared. These guidelines included standardizing the tumor measurement technique and categorizing treatment response into one of four categories comprising complete response, partial response, stable disease, and progressive disease (Table 1). For more than two decades, these guidelines have been widely used. In 1994, the European Organization for Research and Treatment in Oncology, the National Cancer Institute of the United States, and the National Cancer Institute of Canada Clinical Trials Group set up a task force to review these guidelines. An important reason for initiating this analysis was to review the tumor measurement technique. After 5 years of deliberations, the revised guidelines, "Response Evaluation Criteria in Solid Tumors (RECIST)," were recently published [4] (Table 1).
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From a radiologic perspective the new guidelines have several noteworthy features. On the positive side, CT and MR imaging are recognized as providing more precise means of measuring tumor size than conventional radiography or sonography; optimal imaging protocols are to be used; and image analysis should be performed by independent expert reviewers (i.e., off-site radiologists as opposed to on-site oncologists). However, on the negative side, the revised guidelines recommend that a unidimensional tumor measurement technique consisting of longest lesion diameter in the axial plane replace the WHO requirement of lesion area, which is determined by multiplying the longest perpendicular diameters in the axial (cross-product) plane. This recommendation was based on analysis of data from 14 trials that showed response classifications from the two measurement techniques were not significantly different. Specifically, of the 4613 cases reviewed, complete response was seen in 205 patients with both measurement techniques and partial response was seen in 977 patients with the WHO area technique and 968 patients with the RECIST diameter technique. Hence, the overall response rate was almost identical at 25.6% using the WHO criteria and 25.4% with the RECIST criteria. Although they recognize the potential of volumetric measurements, the task force refrained from making such a recommendation because advanced imaging systems are not widely available worldwide and because the benefit of volumetric measurements over the more simple diameter measurements had not yet been scientifically validated.
These developments highlight the fact that to date radiology has played a limited role in formulating the role of medical imaging in oncologic trials. Indeed, there was not a radiologist on the RECIST task force. The RECIST report does not provide any details about the tumor measurement methodology in the individual trials to determine whether lesion measurement was performed properly; specifically, was tumor measurement performed by physicians who could ensure that between visits the same lesion was analyzed at a constant anatomic level or even that calibrated electronic calipers were used? The point is that if the quality of measurement was deficient, then neither technique will stand out.
Obviously, for perfectly spherical lesions, tumor diameter, cross-sectional area, and volume are mathematically related and any one can be used for measuring tumor size for response classification. Unfortunately, biology does not produce perfect geometric shapes, and tumors commonly develop invaginations into surrounding tissues, which makes tumor measurement as much of an art as a science. Indeed, prior studies have shown that when tumors develop into shapes in which the width is more than twice the length, which often occurs after treatment because of scarring, bidimensional measurements provide a more accurate classification of treatment response than diameter alone [5]. This analysis is important because differences among various tumor measurement techniques become critical when clinical trials are conducted in smaller test populations, which is an important goal of imaging-based clinical trials. For example, the largest difference between the WHO and RECIST measurement technique was noted in a lung cancer trial of the disease-progression rate in 24 patients; the bidimensional technique showed a 17% disease-progression rate compared with only 4% for the unidimensional technique [4].
These observations suggest that it is imperative that radiologists take a leadership role in formulating the science and the art of radiologic assessment of drug efficacy through novel application of modern imaging techniques and sophisticated image analyses algorithms for radiologic tumor measurement in particular and for imaging-based quantification of disease burden in general. For this we will need to not only foster research aimed at developing and validating imaging techniques for evaluating drug effect (necessary for acceptance by regulatory agencies such as the United States Food and Drug Administration) but also work closely with the pharmaceutical industry to increase awareness of medical imaging so that radiologic testing can be incorporated early in the drug development process. For example, with modern MR imaging and CT equipment, image analysis in the axial plane has become an arbitrary restriction and may not optimally display the disease process. In addition, advanced image analysis applications allow not only measurement of volumes of individual tumors but also measurement of tumor burden in an organ. This capability overcomes the difficulty associated with measuring the size of confluent lesions and eliminates the need to arbitrarily limit tumor measurement to five target lesions per organ as recommended by the RECIST guidelines. Newer developments that herald the onset of molecular and tumor perfusion imaging methodologies will require even more rigorous scientific analyses for validation of radiologic assessment of treatment response.
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