Imaging methods

06/2017

MUDr. Šárka Bohatá, Ph.D.; prof. MUDr. Vlastimil Válek, CSc., MBA

Klinika radiologie a nukleární medicíny FN a LF MU, Brno

SUMMARY

The golden standard for primary diagnosis of colorectal cancer is colonoscopy together with biopsy. Exact staging and the preoperative assessment are crucial for optimal management of colorectal carcinoma, influencing decision making in case of neoadjuvant treatment and also determining the prognosis. EUS depicts the anatomic layers of the rectal wall with a high degree of accuracy and thus enables precise determination of the tumor extent in relation to the different wall layers (T staging). EUS accuracy is about 85% for T staging and 70-75% for nodal staging. Compared with the other commonly used techniques, CT scan is more largely accessible, faster, inexpensive and less operator-dependent, however CT is not the first choice method for local staging, because the definition of the bowel wall layers is not possible, but is one of the preferred tools to evaluate mainly distant spread, because of its high reproducibility and availability. High-resolution MRI in hands of experienced radiologist plays a pivotal role and has become almost mandatory in the pretreatment assessment of primary rectal cancer. MRI is currently the only imaging modality that is highly accurate (with specificity 92%) in predicting whether or not it is likely that a tumor-free margin can be achieved and in determination of the local invasion depth. Especially in distal tumors MRI provides important information for surgeon's decision whether sphincter-sparing surgery is possible. In patients with advanced rectal cancer extramural vascular invasion of the tumor can be depicted, used as an independent negative prognostic factor for local and distant recurrence rate and shorter overall survival rate. MRI can determine which patients can be treated with surgery alone and which will require radiation therapy to promote tumor regression. MRI can differentiate patients with good therapy response from non-responders. Tumor regression grade assessment is an independent predictor of overall survival and overall symptom-free survival rates and also plays substantial role in patients with the local recurrence. In patients with rectal cancer who have received concurrent chemotherapy and radiation therapy before surgery, all methods have lower accuracy in prediction of the pathologic stage owing to overstaging or understaging. The factors related to this problem include fibrosis, desmoplastic reaction, oedema, inflammation, and viable tumor nets at a fibrotic scar from a previous tumor.

KEYWORDS

computed tomography, endoscopic ultrasound, magnetic resonance imaging, PET/CT, PET/MRI, colorectal cancer, staging, therapy response