Diffusion-weighted Magnetic Resonance Imaging in Characterization and Staging of Rectal Cancer

Rectal cancer is one of the most common clinical malignant tumors, which accounts for about 15% of all malignant tumors (1). It is the third most common cancer in men and the second most common cancer in women (2). Adenocarcinomas compromise approximately 96% of all colorectal cancers, whereas the uncommon malignancies include lymphoma, gastrointestinal stromal tumors and carcinoid (3). The prognosis of rectal cancer is directly related to tumor infiltration into the mesorectum and the ability to surgically achieve negative circumferential resection margins (CRMs) (4). The use of total mesorectal excision (TME) as the standard treatment of rectal cancer and the adoption of neoadjuvant chemo-radiotherapy for patients with locally advanced rectal cancers, diagnosed on the basis of MRI features, has led to substantial improvement in local disease control (5). There is a need for an accurate clinical staging of rectal cancer to optimize individualized treatment (6). Rectal cancer is staged based on TNM classification system where T stage refers to local tumor extent, N stage refers to regional lymph node status, and M stage refers to the presence or absence of distant metastatic disease (7). Currently, MRI is the preferred imaging modality for local staging of rectal cancer (8). High-resolution T2-weighted images are the gold standard for evaluating rectal cancer. Proper planning of high resolution T2 imaging sequences is essential in staging accuracy (9). However, structural imaging techniques have shown clear limitations in tumor evaluation. Different functional and molecular imaging techniques such as DWI and dynamic contrast enhanced (DCE) imaging are useful tools for providing insights into tumor phenotype and improving the assessment of tumor response to treatment (10, 11). DW-MRI enables a noninvasive characterization of biologic tissues on the basis of their water diffusion properties (12). Turkish Journal of Physiotherapy and Rehabilitation; 32(3) ISSN 2651-4451 | e-ISSN 2651-446X 29363 www.turkjphysiotherrehabil. org Pathology Rectal Cancer Colorectal cancer is the third most common cancer and a major cause of morbidity and mortality throughout the world (13). Successful multimodal treatment of rectal cancer requires accurate diagnosis and staging, which guides optimal treatment strategies (14). The diagnosis is usually established by means of clinical examination (rectal digital examination), endoscopy (sigmoidoscopy and colonoscopy), double-contrast barium enema examination and histologic confirmation, supplemented by biochemistry (tumor markers eg, blood carcino-embryonic antigen measurement). All of these techniques are poor indicators of the depth of invasion and lymph node involvement, which are both important features for prognosis (14, 15). Accurate preoperative assessment of these prognostic factors is an important first step in assigning patients to one of the available treatment strategies which include surgical excision or course of preoperative chemotherapeutic and radiation therapy aimed at downstaging the tumor and decreasing the risk of post-operative tumor recurrence (16). Histologic Types: Most primary rectal cancers are adenocarcinomas, of which most are conventional glandforming tumors. However, other special types need to be addressed as they exhibit different behavior and/or molecular phenotype such as mucinous adenocarcinoma with more than 50 % of the lesion composed of pools of extracellular mucin, signet-ring cell carcinoma, medullary carcinoma, adeno-squamous carcinoma and undifferentiated carcinoma (17). Tumor grading: Histopathologic Adenocarcinomas are graded predominantly based on the extent of glandular appearance. Grade 1: (well-differentiated): Lesions exhibit glandular structures in >95 % of the tumor. Grade 2: (moderately differentiated): Adenocarcinoma has 50–95 % glands). Grade 3: (poorly differentiated): Adenocarcinoma has 5–50 % glands. Grade 4: (undifferentiated): < 5 % of tumor with glandular differentiation (17). Staging of Rectal Cancer with MRI: In the context of primary tumor staging, performing rectal MRI is important for the evaluation of tumor location and morphology, T category, anal sphincter complex involvement, circumferential resection margin status (CRM Status), involvement of the pelvic sidewall, extramural vascular invasion (EMVI), and N category. These features should be included in the rectal MRI report (8) (Table1) . Turkish Journal of Physiotherapy and Rehabilitation; 32(3) ISSN 2651-4451 | e-ISSN 2651-446X 29364 www.turkjphysiotherrehabil. org Standardized reporting template for dictation MRI studies for rectal cancer: Table (1): Standardized reporting template for MRI studies for rectal cancer (18).