Editorial orientation provided by MedAPI — not part of the coding instruction.
C20 covers a primary malignant neoplasm of the rectum, including the rectal ampulla. Rectal cancers are clinically distinguished from colon and anal canal cancers because local staging, radiation planning, and surgical management differ.
Common Histologies
- Adenocarcinoma is the predominant histology, usually arising from colorectal-type glandular epithelium.
- Mucinous and signet-ring cell variants are less common and may have distinct prognostic features.
- Squamous, adenosquamous, and other rare carcinomas may occur.
- Malignant carcinoid tumor of the rectum is excluded from C20 and classified in the malignant carcinoid tumor category.
Common Symptoms
- Rectal bleeding or blood mixed with stool (K62.5)
- Change in bowel habits (R19.4), stool caliber (R19.4), urgency (R15.2), or tenesmus (R19.4)
- Sensation of incomplete evacuation (R15.0)
- Pelvic or rectal discomfort (R10.20)
- Iron-deficiency anemia (D50.9), fatigue (D50.9), or weight loss (E61.1)
- Obstructive symptoms in advanced disease (C34.90)
Some tumors are detected through colorectal cancer screening before symptoms develop.
Diagnosis
Diagnosis generally requires endoscopic visualization and biopsy. Evaluation commonly includes:
- Digital rectal examination and colonoscopy or flexible sigmoidoscopy
- Histopathologic confirmation
- Pelvic MRI for local tumor and nodal assessment
- CT of the chest, abdomen, and pelvis for distant disease
- Endorectal ultrasound in selected early lesions
- Baseline carcinoembryonic antigen (CEA) for prognosis and surveillance, recognizing that it is not diagnostic by itself
Staging
Rectal cancer is staged using the TNM system, based on depth of invasion, regional lymph-node involvement, and distant metastasis. Important local features include mesorectal fascia involvement, extramural vascular invasion, sphincter involvement, and the relationship of the tumor to the peritoneal reflection and anal verge.
Molecular Markers
Tumor testing may include:
- Mismatch-repair proteins or microsatellite instability (MMR/MSI)
- KRAS/NRAS and BRAF in advanced disease
- HER2 amplification and rare actionable fusions such as NTRK, when clinically appropriate
MMR/MSI findings may affect immunotherapy selection and can prompt assessment for Lynch syndrome. Broader testing is particularly relevant in recurrent or metastatic disease.
Common Treatments
Management depends on stage, tumor location, resectability, molecular findings, and patient factors. Options may include:
- Local excision for carefully selected early tumors
- Radical resection with total mesorectal excision
- Neoadjuvant chemoradiation, short-course radiotherapy, or total neoadjuvant therapy for many locally advanced tumors
- Systemic chemotherapy
- Immunotherapy for eligible MMR-deficient or MSI-high cancers
- Targeted therapy in selected advanced cancers
- Nonoperative surveillance after a complete clinical response in highly selected patients managed through experienced multidisciplinary programs
- Palliative systemic therapy, radiation, surgery, diversion, or stenting when appropriate for advanced disease or symptoms
Scope Note
C20 identifies the primary anatomic site as the rectum, including the rectal ampulla. It does not encode histologic subtype, grade, TNM stage, molecular profile, treatment status, recurrence, or metastatic sites. It should not be used to represent a secondary malignant deposit in the rectum. Malignancies of the rectosigmoid junction and anal canal fall under different site families, and malignant carcinoid tumor of the rectum is specifically excluded and classified separately.