From the Guidelines
Rectal thickening on CT scan should be further evaluated with a colonoscopy with biopsy as the gold standard for diagnosis, and a PET scan may be helpful in detecting metabolically active cancer cells and assessing potential metastatic disease, but its use should be determined on a case-by-case basis in consultation with specialists. The initial evaluation of rectal thickening on CT scan should prioritize colonoscopy with biopsy to obtain a definitive diagnosis, as it allows for direct visualization of the rectum and collection of tissue samples 1. Additional imaging, such as MRI of the pelvis, may be useful for local staging of rectal cancer if malignancy is confirmed, due to its superior soft tissue resolution 1.
Key Considerations
- A PET scan can detect metabolically active cancer cells and assist in determining if the rectal thickening is malignant, but its sensitivity and specificity are limited, particularly for small lesions or mucinous tumors 1.
- The use of PET-CT for initial staging is not recommended, but it may be useful in conjunction with other imaging modalities to assess features associated with a high risk of metastases 1.
- Blood tests, including complete blood count, liver function tests, and carcinoembryonic antigen (CEA), may also be ordered as part of the diagnostic workup.
Clinical Context
The decision to proceed with a PET scan should be made in consultation with specialists, considering the overall clinical picture and results of initial diagnostic tests. The utility of PET scans depends on the clinical context and findings from other investigations, and they are most valuable for evaluating potential metastatic disease rather than for initial diagnosis 1.
Diagnostic Approach
The diagnostic approach to rectal thickening on CT scan should prioritize a colonoscopy with biopsy, followed by additional imaging and testing as needed to determine the extent of disease and guide treatment decisions. A PET scan may be a useful adjunct in certain cases, but its use should be tailored to the individual patient's clinical context and should not replace other diagnostic modalities.
From the Research
Evaluation of Rectal Thickening using PET Scan
- A PET scan can be helpful in evaluating rectal thickening on CT scans for possible malignancy, as it provides metabolic information that can help identify cancerous tissues 2, 3.
- The study by 2 found that PET scans had the most significant impact on changing management plans in patients with rectal cancer, particularly in those with metastatic disease.
- Another study by 4 reported a case where a PET/CT scan led to the diagnosis of a PSMA-positive rectal adenocarcinoma in a patient with prostate cancer, highlighting the potential role of PET scans in detecting occult primary malignancies.
Comparison with other Imaging Modalities
- PET scans can be used in combination with other imaging modalities such as CT and MRI to evaluate rectal thickening 2, 3, 5.
- The study by 5 found that CT colonography provides important information for the preoperative assessment of T staging, while MRI is more accurate than CT for the evaluation of liver metastases.
- However, the study by 6 noted that FDG PET/CT interpretations can be false-positive for presacral recurrent rectal cancer, and additional MRI may be diagnostically useful in such cases.
Clinical Applications
- PET scans can be used for initial diagnosis, assessment of local treatment response, and evaluation for metastatic disease in colorectal and anal cancers 3.
- The study by 2 found that changes in disease stage post-neoadjuvant chemoradiation were observed in 50% of patients, and the management plans of 15% of patients were changed based on the results of the PET scan.
- However, the study by 5 noted that there is no definite evidence to support the routine clinical use of PET/CT colonography, and its use should be carefully considered in each individual case.