In consideration of the higher incidence and disease-related death rates, accurate preoperative staging of rectal cancer is crucial. The staging must be reliable in order to select the appropriate treatment for patients. Treatment options alter according to preoperative stage. Surgery is the main curative treatment for local disease with/without limited liver metastases. Distant metastases from planned operation sites could affect the treatment strategy. The treatment of rectal cancer, especially in patients with a high risk of recurrence, has shifted from purely surgical treatment to multimodal therapy with neoadjuvant treatment . The purpose of preoperative staging is to evaluate the depth of the tumor’s local infiltration, degree of lymph node involvement, and presence of distant organ metastases. Conventionally, thoraco-abdomino-pelvic ceCT and pelvic MRI have been the choice of modalities for preoperative staging of rectal cancer. ceCT and MRI could provide information about the location, size, and local invasion of the primary tumor and also enlargement of regional lymph nodes. However, they have limited value during the evaluation of regional lymph node involvement because a normal-sized lymph node may have a tumor, and an enlarged lymph node may be reactive .
18F-FDG PET can show the metabolic activity of malignant tumors and is accepted as an important imaging method in the diagnosis and staging of many malignant diseases [12–18]. 18F-FDG PET allows for better staging of many cancers, such as esophageal and non-small cell lung cancer, and thus has contributed to an improvement in the treatment of patients . Some studies have reported that 18F-FDG PET imaging is effective in the evaluation of patients with suspected recurrent colorectal cancer [20–22] and patients with local advanced rectal cancer . Regarding primary rectal cancer studies, it has been reported that 18F-FDG PET imaging leads to changes in the cancer stage in one-third of patients . As a result of advances in the industry, hybrid devices have been designed to allow simultaneous imaging and interpretation of both anatomical (CT) and functional (18F-FDG PET) images . 18F-FDG PET/CT imaging was shown to be more effective in patients with locally recurring and metastatic colorectal cancer .
The role of 18F-FDG PET/CT in the primary staging of rectal cancer is controversial. First, 18F-FDG PET/CT could not give enough information about the T stage because of non-contrast-enhanced and low-dose CT images. Additionally, it has a limited role in the evaluation of millimeter-sized lung nodules and liver lesions because of the low spatial resolution. However, it has some advantages over conventional methods, especially in the evaluation normal-sized regional lymph nodes, nonspecified liver lesions, and small bone metastases. For these reasons, like in our study, the need for and additional role of 18F-FDG PET/CT for the primary staging of rectal cancer have been subjects of focus [19, 27].
Further, in 21.6% of the cases in our study, 18F-FDGPET/CT provided additional findings other than those previously observed by conventional tomography (p < 0.01). These new findings included metastases in the liver, lung, bone, and lymph nodes, invasions of adjacent organs, and synchronous tumors identified in the colon. The disease was upstaged in 11 cases. 18F-FDG uptake was detected two patients’ primary tumorsthose were normal in ceCT, local lymph node involvement was detected in normal-sized lymph nodes, 18F-FDG uptake was seen in suspected liver lesions, lung nodules, and distant lymph nodes, and there were bone metastases that were not detected by ceCT. Additionally, the disease was down staged in three patients whose suspected liver lesions and lung nodules were not 18F-FDG avid. In a total of 14 patients there was a need to change the treatment strategy or change the surgical intervention. Gearhart et al.  investigated the role of 18F-FDG PET⁄CT in the initial staging of rectal cancer and reported their results. They studied 37 patients for staging of rectal cancer and reported discordant findings in 38%, while 10 of the 37 patients (27%) underwent a change in stage after 18F-FDG PET⁄CT. Eglinton et al.  reported their results in 20 rectal cancer patients and found that 18F-FDG PET/CT detected discordant or incidental findings in about half of the patients, which in turn led to a change in staging in 30% of them. However, they did not observe significant results that impacted treatment decisions. Bassi et al.  reported that 18F-FDG PET/CT imaging changed staging in 16% of the patients and significantly increased the target volume in contouring radiotherapy. Recently, Davey et al.  evaluated the role of 18F-FDG PET⁄CT in 86 patients and reported a 31% change in stage due to 18F-FDG PET⁄CT. On the contrary, some studies advocate that preoperative staging is not necessary in primary rectal cancers . For instance, Kwak et al.  reported that preoperative 18F-FDG PET/CT and CT showed similar results in determining lymph node metastases.
In our patients, 18F-FDG PET/CT did not show primary rectal tumors in two patients. These patients showed histopathological subtypes of tumor that were mucinous and well-differentiated adenocarcinoma. Mucinous contentx and well-differentiated tumors are factors that decrease 18F-FDG uptake and could be the cause of false-negative results. Also, 18F-FDG PET/CT could not provide information about the degree of infiltration of the rectal wall because of the limited spatial resolution of the study. For this reason, 18F-FDG PET/CT is not recommended for T staging. In the future, contrast-enhanced 18F-FDG PET/CT examinations could solve this problem and could eliminate the need for ceCT imaging in a separate course. However, these false-negative results did not change the treatment management of these patients because primary rectal tumors had already been detected by biopsy.
In this study, in addition to comparing conventional tomography with 18F-FDG PET/CT, we also evaluated the serum CEA levels, localization of the tumor in the rectal wall (posterior, anterior, lateral, and all around), endoscopic assessment of the distance of the tumor from the anal canal (1–5 cm lower rectum, 6–10 cm mid-rectum, 11–15 upper rectum), and the degree of histopathological differentiation of the tumor. Moreover, we assessed the statistical relationship of the above with the compatibility of ceCT and 18F-FDG PET/CT, and, to that end, we did not find any relationship. Additional information obtained from 18F-FDG PET/CT seems to be independent from these factors.
We could not evaluate the sensitivity and specificity of 18F-FDG PET/CT because of the lack of histopathological confirmation of all FDG-avid lesions. In this study, we aimed to describe the need for and additional role of PET/CT in the staging and clinical management of our patient group.