Highlighting on postoperative patients with colorectal carcinoma is to detect whether recurrence and metastasis exist or not, which can be detected by traditional imaging only when lesion sites present morphological changes. But it is difficult to distinguish early recurrence and metastasis from the inflammatory process and anatomic changes caused by operation and radiotherapy. A quarter of the recurrent colorectal carcinoma confines to the original operative site, and these patients can be cured by a second operation. The only way to reduce metastasis is early detection of recurrence. 18F-FDG PET shows metabolism changes that happen before morphological changes, which makes it detect recurrence earlier than conventional imaging [2].
In the current study, we evaluated 45 patients for the presence of recurrence and/metastasis. According to the recommendations of ASCO, carcinoembryonic antigen is considered the best marker for monitoring metastatic colorectal cancer, and its measurement can be useful in the detection of asymptomatic recurrences. After surgical resection, it was found that the tumor has recurred in almost 90% of patients who have elevated postoperative CEA levels [10]. In the current study, 30 patients had elevated serum CEA levels. The sensitivity, specificity, and positive and negative predictive values of FDG PET/CT in detecting CRC recurrence/metastasis in patients with elevated serum CEA as the following: 96.9%, 83.3%, 94.2%, and 90%. That was in accordance with Metser et al. [10] who reported that the sensitivity ranges from 79–100% and the specificity ranges from 50–100%. The results of the current study fall within this range, so recurrent CRC was detected by FDG PET/CT with high sensitivity and specificity (Figs. 2, 3, 4, 5, and 6).
Agarwal et al. [9] reported that FDG PET/CT is more sensitive than conventional imaging in the evaluation for recurrence of CRC patients with rising CEA levels. In the current study, we also concluded that PET/CT is useful in the detection of recurrence in patients with normal CEA values but with suspicious clinical symptoms or radiologic findings. In the current study, two patients had normal serum CEA levels with positive PET/CT findings.
The diagnosis of local recurrence and differentiating it from post-operative and/or radiotherapy changes constitute a diagnostic challenge for conventional imaging as these changes may appear on CT images for many years and be indistinguishable from tumor recurrence [11]. In the current study, the detection of local recurrence by PET/contrast-enhanced CT was higher compared with enhanced CT. Ten lesions were detected as local recurrence and/or operative bed findings by CECT while PET/CT detect 17 lesions. So, PET/CT revealed additional information by detecting occult malignant lesions which were not detected in CECT. These results agree with Kitajima et al. [4] and Chiewvit et al. [12] which showed that the sensitivity of detecting local recurrence by PET/contrast-enhanced CT was relatively high compared with enhanced CT (96.7 vs 80.0%). Also, the study done by the study of Kitajima et al. [11] demonstrated that FDG-PET/CT seems to be the method of choice in the evaluation of presacral fibrotic tissue in patients who have undergone rectal amputation [12]. In the current study, out of the 16 patients diagnosed with recurrence, PET/CT detected 4 patients with pararectal soft tissue masses and 3 patients with presacral soft tissue lesions.
Liver metastasis is very common in patients with colorectal cancer with an incidence of approximately 50–60% [11]. The presence of hepatic metastasis is the main determinant of survival and guides the therapeutic strategy, particularly in patients with colorectal carcinoma [13]. In the current study, 12 patients proved to have metastatic hepatic lesions with higher detection rate of PET/CT over CECT in the detection of metastatic hepatic deposits. Fifty lesions were detected by CT and 65 lesions were detected by PET/CT which detected occult metastatic hepatic focal lesions. The superiority of PET/CT over CECT owes to the latter’s inability to diagnose small masses (less than 15 mm in diameter) as benign or malignant. These results are comparable to those of Ali and Abd Elkhalek [14] and Agarwal et al. [9] which demonstrated that FDG PET is highly sensitive for the detection of liver metastases, and the routine FDG-PET/CT assessment of patients with hepatic metastasis has a significant impact on disease staging and selection of candidates suitable for solitary liver metastasis resection.
PET/CT could also be used in monitoring the treatment efficacy of local therapy of isolated lung and liver metastatic lesions. The local therapy approaches of hepatic metastasis are radiofrequency ablation (RFA), transcatheter arterial chemoembolization (TACE), and cryoablation. Traditional imaging techniques could not discriminate between tissue edema and tumor remnant. However, PET provides a functional imaging of the levels of metabolism within the suspected foci and also the whole body, so it substantially helps the clinicians in recognizing tumor remnants [15]. In the current study, one patient underwent radiofrequency ablation for hepatic metastasis which showed successful ablation of the focal hepatic lesion with no residual FDG uptake. This is also comparable with the results of Veit el al [16]. which showed that PET/CT plays a distinctive role in the follow-up of patients undergoing RFA of the liver lesions.
In the current study, eight patients proved to have metastatic peritoneal deposits. The number of detected lesions by CECT was 9, and 14 lesions were detected by PET/CT which showed the superiority of PET/CT over CECT which detected occult peritoneal deposits.
Lymph node metastasis is an important prognostic factor in patients with colorectal cancer [17]. In the current study, 17 patients proved to have metastatic lymph nodes with higher detection rate by PET/CT over CECT as regards the number of the detected lesions. Thirty-eight lymph nodes were detected by CECT and 58 lymph nodes were detected by PET/CT which detected 18F-FDG avid subcentimetric lymph nodes; these additional sites of metastatic involvement would have been missed if assessment was done using CT alone, and this result in up-staging and consequently change the management plan in these patients. CT’s strengths are its accurate depiction of anatomic abnormalities and its ability to define structures that are below the resolution of conventional nuclear medicine. A major limitation of CT however is its reliance on anatomic criteria in order to identify the pathological condition, for example using a 1-cm short axis dimension threshold for pathological lymphadenopathy.
However, lymph nodes smaller than 1 cm can contain malignant cells and, in addition to reactive or inflammatory processes, can result in nodes larger than 1 cm [1].
Nodal metabolic activity assessment using FDG PET is not directly reliant on the nodal size to determine the presence or absence of malignancy. Nodes that are not enlarged can contain tumor on FDG PET. However, PET can give false-positive results due to inflammation. In addition, some cystic or mucinous nodal metastasis may not show significant FDG uptake [18].
Bone metastasis from colorectal carcinoma is relatively uncommon [1]. In the current study, patients had metastatic bone deposits with higher detection rate by PET/CT over CECT. Six lesions were detected by CECT and 13 were detected by PET/CT with retrospective detection of 7 occult lesions missed on CT. That was in accordance with Ali and Abdelkhalek [19].
PET scanning can identify osseous metastases at an early stage of growth before host reactions to tumor cells occur (active osseous deposits without structural abnormalities) [18]. Early malignant bone marrow infiltration can be depicted by 18F-FDG PET because of the very early increased metabolism of glucose in the neoplastic cells. The combined PET/CT is unique as it can scan the whole body in one session, and it can provide us with the functional and anatomic data in co-registered images. Combined PET/CT improved the CT ability to detect and characterization of metastatic osseous deposits, which is essential for proper staging and further management planning [19].
The overall comparison between CECT and PET/CT in the current study showed significant superiority of PET/CT over CECT.
The limitations of our study were as follows: