On the follow-up of cancer patients to evaluate response to treatment, CT was the modality of choice with RECIST criteria as a widely used and accepted tool for assessment, yet it is depending on the changes in tumoral size which may show some inter-observer variability especially in irregular lesions. Also, CT has the inability to detect the changes in activity which occurred in response to treatment . It is difficult to distinguish necrotic tissue or fibrotic scar from residual tumor on CT scans . Response evaluation protocols that are depending on morphological changes are still limited as changes in tumor dimensions are not true markers of therapeutic efficacy because tumor tissue consists of different components that are not all completely regressed over time . This is why some studies found a discrepancy between the CT-based RECIST and the histopathological results among a significant number of the studied cases during follow-up after neoadjuvant chemotherapy .
So, more satisfactory methods of evaluation are needed to accurately measure tumor responses quantitatively. 18F-FDG PET thought to overcome these limitations and became a well-established quantitative method for the staging, follow-up, and detection of recurrence in patients with several malignancies [5, 11, 16]. With the development of 18F FDG PET/CT, which is integrated anatomic and metabolic imaging, a modified PET-based criteria (PERCIST) have been reported for assessing treatment response in both solid tumors and hematologic malignancies .
In our study, we investigated the concordance between the metabolic criteria and morphologic criteria for the assessment of end therapeutic response to chemotherapy in patients with malignant lymphoma determined using RECIST 1.1 and PERCIST 1.0 criteria.
We included 33 patients who were recently diagnosed with malignant lymphoma. Their end of treatment response was evaluated using RECIST 1.1 and PERCIST 1.0 criteria. Results showed a considerable discrepancy in the assessment of tumor responses between the morphologic criteria (RECIST 1.1) and metabolic criteria (PERCIST 1.0) with discordance in 39.4% of patients (13 patients) and P value < 0.001.
There is tendency for RECIST to downgrade the tumor response (ten patients classified as PR according to RECIST 1.1 were classified as CMR according to PERCIST 1.0, and two patients that classified as SD according to RECIST 1.1 were classified as PMR according to PERCIST 1.0). This lead to avoiding unnecessary anticancer treatments including chemotherapy or radiotherapy, which means that PERCIST 1.0 criteria were more sensitive and reliable than RECIST 1.1 for the detection of therapeutic response.
In a retrospective study conducted by Baratto et al. and compared therapeutic response in 38 patients with non-Hodgkin lymphoma by RECIST 1.1 and PERCIST criteria, evaluating both early treatment response as well as end of therapy concluded that the Deauville and PERCIST criteria were the most reliable for predicting end-of-treatment response, reporting an accuracy of 81.6%, being consistent with our results .
Our study results are also matched with other studies which included lymphoma and other different solid tumors. In a study done by Yanagawa et al. on 46 patients with esophageal cancer who received chemotherapy, 56.5% of patients showed discordant therapeutic response between RECIST 1.1 and PERCIST with Wilcoxon signed-rank test, P < 0.0001 . Other studies done by Ding et al. and Shang et al. which included 44 and 35 patients with non-small cell lung cancer, 34.1% and 62.9% of patients showed discordant response after application of RECIST 1.1 and PERCIST [14, 18]. A 61.5% discordant rate was achieved by Bang et al. in a study conducted on 39 patients with colorectal cancer after targeted therapy . Also, in a study done by Riedl et al. (included 65 patients with breast cancer who received therapy and evaluated with RECIST 1.1 and PERCIST criteria), a 52.3% discordant rate was found . Studies conducted by Agrawal et al. and Aras et al. included 43 and 60 patients with different types of solid tumors (breast cancer, lung cancer, PNET, head and neck cancer, sarcoma, non-Hodgkin lymphoma, GIT cancers, and others), 20% and 18.3% of patients showed contradictory response when evaluated using RECIST 1.1 and PERCIST criteria [21, 22].
These results mean that PERCIST is used among international observers more consistently, as it provides the needed standardization of the PET protocol. This is very important because availability of consistent criteria leads to accurate comparisons between different studies and facilitates advances in cancer treatment. Moreover, the use of computer-assisted reading application that is fully customized for use with PERCIST makes the procedure of evaluation easier and helps ensure more precise results.
There are few limitations of our study. First, the corresponding pathological results to our imaging findings has not been evaluated and further studies are needed to investigate the correlation of these findings with pathological examinations. Second, adequate follow-up of patients were not achieved to correlate our results with the patients’ progression-free survival or overall survival. The main limitation was the small sample size due to the high cost of the technique, and future studies with larger number of patients may be needed to obtain more accurate results.