Radiology_May04 47 SA JOURNAL OF RADIOLOGY • May 2004 The World Health Organization (WHO), the National Cancer Institute and the European Organi- sation for Research and Treatment of Cancer have recently adopted a new set of tumour response criteria - Response Evaluation Criteria for Solid Tumours (RECIST). Monitoring response of tumours to treatment is an integral and increasingly impor- tant function of radiologists working in oncologic imaging. Imaging stud- ies play a pivotal, objective role in quantifying tumour response to a variety of physical and pharmaceuti- cal treatments. The RECIST criteria have been introduced to unify response assessment criteria, to define how to choose measurable lesions, and to enable the use of new imaging technologies (spiral computed tomography (CT) and magnetic reso- nance imaging (MRI)). Definitions and measurements The four categories of response are: (i) complete response (CR); (ii) par- tial response (PR); (iii) stable disease (SD); and (iv) progressive disease (PD). RECIST criteria rely on size change of lesions to make response assess- ments. Unidimensional measure- ments and the sum of the longest diameters are now used, instead of the conventional method using two mea- surements and the sum of the prod- ucts. At baseline, lesions are to be cate- gorised as measurable or non-mea- surable. Measurable lesions are defined as those that can be measured accurately in at least one diameter, that is ≥ 20 mm using conventional imaging techniques (including incre- mental CT) or ≥ 10 mm using spiral CT. Non-measurable lesions are dis- crete lesions with smaller dimensions. These lesions include bony metas- tases, leptomeningeal disease, ascites, pleural/pericardial effusions, inflam- matory breast cancer, lymphangitis carcinomatosa (cutis/pulmonis), abdominal masses that are not con- firmed and followed by imaging tech- niques, heavily calcified and cystic/ necrotic lesions. Interestingly, tumour lesions situ- ated in a previously irradiated area may also not be considered as mea- surable disease. The term ‘evaluable’ which refers to lesions that can be viewed but cannot be measured, has been dropped. After establishing that measurable disease exists, it is necessary to docu- ment ‘target’ lesions and non-target lesions. Measurable lesions up to a maximum of five lesions per organ and 10 lesions in total, representative of all involved organs, should be iden- tified as ‘target lesions’. These target lesions should be selected on the basis of size and suitability for accurate repeated measurements. A sum of the longest diameter of all target lesions constitutes the baseline sum longest diameter. Changes in the sum of the longest diameter are to be used to categorise ‘target tumour response’. Non-target lesions need not be measured on fol- low-up studies but any change should be noted. Final response should take into account changes in both target and non-target lesions as well as not- ing the presence or absence of new disease. It is noteworthy that for sta- ble disease and for progressive disease, the pre-treatment study no longer serves as the baseline study, instead the reference study is one where the smallest sum of the longest diameter was recorded. Baseline evaluations are to be per- formed as close as possible to the beginning of treatment, but not more PRACTICAL GUIDELINES Practical guidelines in the application of response evaluation criteria for solid tumours (RECIST) in oncology imaging Zarina Lockhat FFRad(D)SA Irma van de Werke FRCR Betsie van der Walt FCRad(D) Department of Radiology University of Pretoria REVIEW ARTICLE 48 SA JOURNAL OF RADIOLOGY • May 2004 than 4 weeks before treatment starts. With re-evaluation studies there is flexibility — it is recommended that follow-up every other cycle be per- formed — every 6 - 8 weeks. An end of treatment study enables overall treatment response assess- ment. In patients with partial response (PR) or complete response (CR) confirmatory imaging is required at 4 weeks after the criteria for CR or PR have been met. Practical imaging recom- mendations The same method of assessment and same technique should be used to characterise each identified and reported lesion at baseline and during follow-up. Imaging-based evaluation is pre- ferred to evaluation by clinical exami- nation when both methods have been used to assess the anti-tumour effect of a treatment. Chest X-ray Except for the chest radiograph, the role of radiography is not that important. Lesions are considered measurable when they are clearly defined and surrounded by aerated lung. However CT is preferable. Radiographs cannot be used to assess bone lesions because bony metastases are classified as non-measurable lesions. Ultrasound This is not used routinely to assess response of lesions because it is oper- ator-dependent and cannot be repro- duced for independent review at a later date. However ultrasound may be used as an alternative to clinical measurement — for superficially pal- pable lymph nodes, subcutaneous lesions and thyroid nodules. CT and MRI These are the best currently avail- able modalities and the most repro- ducible. For MRI assessment the same anatomical plane is used for subse- quent examinations. There are no specific sequence recommendations. For CT, when choosing measurable lesions, the basic rule to be followed is that the minimum size should be not less than double the slice thickness. This is to minimise partial volume averaging that can lead to underesti- mation of lesion size. The longest diameter of the target lesion should be Table I. Definition of best response according to WHO or RECIST criteria Best response WHO change in sum of products RECIST change in the sum of the longest diameter Complete response (CR) Disappearance of all target lesions Disappearance of all without any residual lesion; confirmed target lesions; confirmed at 4 weeks at 4 weeks Partial response (PR) 50% or more decrease in target lesions, At least 30% reduction in the sum without a 25% increase in any one of the longest diameter of target target lesion; confirmed at 4 weeks lesions, taking as reference the baseline study; confirmed at 4 weeks Stable disease (SD) Neither PR nor PD criteria are met Neither PR nor PD criteria are met, taking as reference the smallest sum of the longest diameter recorded since treatment started Progressive disease (PD) 25% or more increase in the size of At least 20% increase in the sum measurable lesion or appearance of of the longest diameter of target new lesions lesions, taking as reference the smallest sum of the longest diameter recorded since treatment started or appearance of new lesions 49 SA JOURNAL OF RADIOLOGY • May 2004 PRACTICAL GUIDELINES obtained in the axial plane only. For spiral CT the minimum size of a lesion may be 10 mm provided that a 10 mm collimation is used and reconstructions are at 5 mm intervals. For conventional incremental CT, the minimum size of lesions should be 20 mm with the use of contiguous 10 mm thick slices. Contrast medium usage is recom- mended. Oral contrast medium is used routinely — some studies have shown that water is a better contrast agent when evaluating stomach and bowel lesions. IV contrast is also used routinely - some lesions become measurable only after IV contrast administration; how- ever contrast may not be necessary when evaluating discrete lung disease. Another important recommenda- tion is that all images should be filmed, not just selected images of tar- get lesions. Lesions should be mea- sured on the same window setting at each examination. Conclusion With the introduction of RECIST criteria the role of imaging has become more important. CT exami- nations are performed at an increas- ing frequency, however the use of plain radiographs and ultrasound has declined. The method of assessing lesions, i.e. unidimensional measurement instead of bidimensional diameters has now been recommended. The measurement of lesions has previous- ly been regarded as laborious and time -consuming. Thus radiologists should be familiar with the RECIST criteria so that measurements are easier to make, more accurate, relevant and less time-consuming to guide clinical decision-making. Acknowledgement We would like to thank Annelize Gates for her assistance in the prepa- ration of this article. References 1. Padhani AR, Ollivier L. The RECIST criteria: Implications for diagnostic radiologists. Br J Radiol 2001: 74: 983 -986. 2. McHugh K, Kao S. Response evaluation criteria in solid tumours (RECIST) problems and need for modifications in paediatric oncology? Br J Radiol 2003: 76: 433-436. 3. Padhani AR, Husbano JE. Are current tumour response criteria relevant for the 21st century? Br J Radiol 2000: 73: 1031-1033. 4. Therasse P, Arbuck SG, et al. Special article: New guidelines to evaluate the response to treatment in solid tumours. J Natl Cancer Institute 2002; 92: 205-215.