Colorectal cancer (CRC) is the fourth most frequently diagnosed cancer and the second leading cause of cancer death in the United States.

The overall relative 5-year survival is about 50%-60% but is highly dependent on disease stage at the time of diagnosis ranging from approximately 80% to only 3%.

Curative treatment comprises resection of the primary tumour combined with adjuvant chemotherapy in selected patients. In recent years there has been an increasing role for curative intended surgical or ablative intervention in limited metastatic disease, i.e., solitary or few metastases to the liver and/or the lungs. Accurate preoperative staging is of paramount importance for directing the most appropriate therapeutic options, for indicating prognosis and outcome, and to avoid futile operations.

The use of imaging in the staging and restaging of colorectal cancers has been evolving and improving in the last two decades. Magnetic resonance imaging (MRI) is an accepted modality for staging of rectal cancer, allowing an accurate identification of transmural invasion of the mesorectal fat and mesorectal fascia involvement. Computed tomography (CT) significantly lacks resolution of soft tissue contrast so that its utility in T staging of rectal cancer is limited. Due to the higher sensitivity and specificity compared with CT,Positron Emission Tomography/ Computed Tomography with 18F-fluoro-2-deoxyglucose (FDG PET/CT) is recommended for use in a metastatic patients.

Pelvic MRI and 18F-FDG PET/CT are useful for staging and therapeutic management. Pelvic MRI allows for accurate definition of the distance to mesorectal fascia which is a predictor of the local recurrence rate, as well as for definition of the regional nodal status. 18F-FDG PET/CT also helps in evaluating the nodal status and is especially performed for the detection of distant metastases. MRI and 18F-FDG PET/CT are thus 2 complementary modalities for the initial staging of advanced rectal cancer.

Neoadjuvant therapies are performed for stage II and stage III rectal cancer. It has been shown that neoadjuvant therapy decreases local recurrence and increases survival. Therefore, correct preoperative staging has a critical role in determining whether patients should undergo neoadjuvant therapy.

The literature on the clinical use of FDG-PET/CT in colorectal cancer staging is fairly limited, but recent works have demonstrated some promise for optimizing the accuracy of initial staging by clarifying equivocal findings on conventional imaging in preoperative staging, and evaluating apparently limited metastatic disease before intervention .

It was reported that the use of FDG-PET/CT in this study changed the planned treatment strategy in a total of 30% of the patients. A change from palliative to curative or vice versa was seen in almost 10% of the patients.

PET/CT provides high accuracy for the detection and staging of liver lesions in CRC patients, PET led to a change in patient management in an average of 24 % of patients, including both exclusion from curative surgery and modification of the surgical approach.

Risk of local recurrence is substantial and correlates to the extension and grade of the tumor and to the nodal status at initial presentation. The guidelines also highlight the usefulness of PET/CT for restaging in the setting of a serially elevated carcinoembryonic antigen level (CEA), negative results on conventional imaging, and potentially resectable metachronous metastases documented by CT, MRI, or biopsy.It was reported that a high sensitivity and specificity of 18F-FDG PET/CT in detecting recurrent CRC, largely regardless of CEA levels, as well as they concluded that 18F-FDG PET/CT have high diagnostic value and can be used as the first choice in the detection of recurrent CRC in patients with unexplained rising CEA, even in patients with a recent normal routine CT.

Inclusion Criteria:

• Patients with pathologically proven colorectal cancer presented for pre-operative staging or restaging.
• Patients with treated colorectal cancer , suspecting recurrence .

Exclusion Criteria:

• Pregnancy.
• Patients with double primary.
• Expected life less than 3 months.
• Uncontrolled diabetes or elevated blood sugar more than 200 mg/dl.
• Severe medical condition.

patients daignosed with colorectal cancer , presented for pre-operative staging, so after doing the standard routine conevntional imaging , they will be sceduled to be imaged with F_18 FDG_PET/CT , hpothezied that the latter has potential promosing role in staging of those patients and lead to change of mangenent.

As well, patients with treated colorectal cancer , under follow up, suspecting recurrence , will be sceduled to be iamged with F-18 FDG-PET/CT for determing is role in those patients.