Review: Advances in Radiologic Staging of Colorectal Cancer
- CT colonography improves tumor staging in colorectal cancer (CRC), and in one study it outperformed colonoscopy in determining the segmental extent of colon cancer
- Three novel MRI prognostic factors in rectal cancer are being used in radiology clinics: clear circumferential resection margin, extramural vascular invasion and mucin content
- MRI has demonstrated superiority to CT and PET/CT for detecting metastases of CRC to the liver
- In a pilot study, PET/MRI was superior to MRI alone in detecting liver metastases of CRC, and in two other studies, a separate research team demonstrated the superiority of PET/MRI to PET/CT and rectal MRI for tumor and node staging of CRC
- Radiomics, the quantitative extraction of imaging features, is in the early stages of investigation in CRC for CT nodal staging and to facilitate the use of CT and MRI to detect microsatellite instability and mutations in KRAS, NRAS and BRAF
Compared with other cancers, colorectal cancer (CRC) is unusual in that the prognostic groups I–IV correlate only loosely with survival data. Once multidetector-row CT, high-resolution MRI and PET/CT became available, the importance of imaging in staging CRC increased substantially.
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In Clinical Radiology, Reece J. Goiffon, MD, PhD, a physician-scientist in the Division of Abdominal Imaging at Massachusetts General Hospital, and Mukesh G. Harisinghani, MD, director of Abdominal MRI, and a colleague review even more advanced imaging techniques that are guiding clinical practice now and show promise to be integrated into the international staging guidelines.
Despite the high spatial resolution of CT, accurate tumor staging of CRC is difficult due to poor soft-tissue contrast resolution. Diagnosis can be improved with CT colonography (CTC), which showed high sensitivity and specificity for T3 and T4 disease in a meta-analysis published in Surgical Oncology.
Moreover, in a study reported in Colorectal Disease, preoperative CTC outperformed colonoscopy in determining the segmental extent of colon cancer, allowing better surgical planning.
CT doesn't distinguish the nodal stage well when traditional sized-based predictors are used. Models using other features, such as shape and internal enhancement, have done better but are subjective. Radiomics, the quantitative extraction of imaging features, may solve that problem. Although still in early development, radiomic prediction of metastatic nodes has outperformed size thresholds.
High-resolution rectal MRI is an accurate predictor of clear circumferential resection margin, which has been linked to increased five-year survival in rectal cancer and decreased risk of recurrence, as reported in the Journal of Clinical Oncology.
Two other rectal MRI prognostic factors are currently being used in certain radiology clinics even though they have not yet been incorporated into TNM staging guidelines:
- Extramural vascular invasion, a predictor of poor rectal cancer outcomes that potentially outperforms current pathologic tumor and node staging
- The mucin content of rectal cancer, another independent poor prognostic factor, is included in several U.S. and European rectal cancer reporting templates
MRI has demonstrated superiority to CT and PET/CT for detecting metastases to the liver. One study, published in JMRI, concluded that diffusion-weighted MRI and T1-weighted imaging with gadolinium are similarly accurate in this respect. However, a systemic review in European Radiology determined that combining the techniques is more sensitive.
The role of PET/CT in the initial staging of CDC is limited to nodal and solid-organ metastatic assessment because uptake of (18)F-fluorodeoxyglucose in the primary tumor does not have significant prognostic value. A better hybrid technique may be PET/MRI.
In a pilot study of PET/MRI reported in Radiology, (18)F-fluorodeoxyglucose uptake by liver metastases of CRC was a significant prognostic indicator that could not be assessed by MRI alone. Another group has expanded PET/MRI to tumor and nodal staging, demonstrating improvements over PET/CT (Abdominal Radiology) and rectal MRI (European Journal of Nuclear Medicine and Molecular Imaging).
PET/MRI reduces radiation dosing by 20%–60% compared with PET/CT, which will be especially important if the incidence of CRC continues to increase among U.S. adults ages 20–49.
CT and MRI radiomics are being explored for their ability to detect microsatellite instability and mutations in KRAS, NRAS and BRAF. Although not incorporated into current TNM staging guidelines, these molecular tumor features affect prognosis and resistance to treatment, and many clinical guidelines consider them.
These emerging modalities may improve concordance between clinical and pathologic TNM staging, and consideration of new prognostic criteria may augment or supplant some current staging criteria. In the meantime, radiologists should keep abreast of new data justifying the use of imaging characteristics beyond those of TNM criteria and discuss local clinical preferences with their referring physicians.
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