Rectal Cancer Pathology Assessment
The preferred staging system (Tables 1 & 2) for rectal cancer is the TNM staging system proposed by the American Joint Committee on Cancer (AJCC) and the Union for International Cancer Control (UICC). This system consists of three categories: 1) tumor (T), which denotes the depth of mural invasion; 2) nodal involvement (N), and 3) distant metastasis (M). The combination of these categories based on clinical and/or pathologic data dictates the stage and this best correlates with the patient’s overall prognosis.
A key feature of rectal cancer staging is the importance of clinical stage in determining treatment recommendations. The Clinical TNM classification (cTNM) is assigned during initial evaluation of the patient using a combination of endoscopic (proctoscopy, flexible sigmoidoscopy, colonoscopy) and rectal cancer protocol MRI scans to assess tumor presence and penetration of the rectal wall, and presence of mesorectal lymph nodes, and CT assessment of chest, abdomen, and pelvis to determine the presence of distant metastases. Pathologic stage is only known after surgery and may be lower than the clinical stage, due to downstaging caused by neoadjuvant therapy. The symbol “p” refers to the pathologic version of the TNM classification, and is based on gross and histologic examination of endoscopic or surgical specimens. pT requires examination of the primary resected cancer specimen to assess the depth of mural invasion. pN requires resection or biopsy of nodal tissue, and pM requires histologic examination of lesions in distant organs.
Modifiers to TNM classifications do not affect the stage grouping, though they alert physicians to clinical features important to assessing cancer stage. The “y” prefix indicates specimens assessed during or following neoadjuvant therapy; the cTNM or pTNM category is thus modified by a “y” prefix. The “r” prefix represents a recurrent tumor when staged after a documented disease-free interval. The “m” suffix refers to the presence of multiple primary tumors in a single specimen and is recorded in parentheses (ex: pT(m)NM).
Pathologic analysis of the proctectomy specimen provides the basis for the pathologic stage. The College of American Pathologists (CAP) has provided a list of minimum reporting for each rectal cancer resection specimen that extends beyond the TNM stage of the tumor. Further, it is the responsibility of the pathologist to grade the quality of mesorectal excision, which is an important surrogate for the measure of the quality of surgical resection.
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