Colorectal Cancer

Indications for Testing

Laboratory testing for CRC is used to:

• Screen average-risk individuals
• Diagnose and inform prognosis in patients with suggestive signs and symptoms or a family history of CRC
• Monitor for recurrence
• Predict response to treatment

Laboratory Testing

Screening

Screening recommendations for CRC vary. ^-  Most guidelines recommend regular CRC screening for individuals 50-75 years of age.    The U.S. Preventive Services Task Force (USPSTF) also has a B grade recommendation for screening in individuals 45-49 years of age, and a C grade recommendation for selective screening in individuals 76-85 years of age.  Screening may involve imaging (eg, colonoscopy), laboratory testing, or a combination of techniques.

Diagnosis

Initial Workup

The initial laboratory workup for CRC appropriate for resection includes a CBC, chemistry profile, and CEA baseline measurement. 

The initial laboratory workup in patients with suspected metastatic synchronous adenocarcinoma also includes a CBC, chemistry profile, and CEA measurement.  Additionally, tumor KRAS/NRAS testing and BRAF testing are recommended (see RAS Somatic Testing and BRAF Somatic Testing). 

Histopathology

All polyps removed should be examined histologically. Cancer staging is generally performed via the tumor, node, metastases (TNM) system after surgical exploration and pathologic examination. 

Molecular Testing

Tissue from a primary, recurrent, or metastatic colorectal tumor is acceptable for somatic molecular testing because results are similar for these specimen types.  Formalin-fixed, paraffin-embedded tissue should be used.  Germline genetic testing for hereditary CRC syndromes should be based on clinical presentation and family history and should be performed in conjunction with genetic consultation.  A hereditary cancer multigene panel, single gene testing, or familial mutation testing may be appropriate if a hereditary cancer is suspected.

Microsatellite Instability and Mismatch Repair Somatic Testing

Testing for MSI via polymerase chain reaction (PCR) or for mismatch repair (MMR) protein status via immunohistochemistry (IHC) is recommended in all patients with a history of CRC to evaluate for LS risk. MSI (either low or high) or MMR deficiency identifies patients at high risk.  See the ARUP Consult Lynch Syndrome topic for more information.

MSI testing is also useful to determine whether to use adjuvant chemotherapy in stage II disease, and both MSI and MMR testing are useful in treatment selection in stage IV disease.  High MSI and MMR deficiency are both associated with a more favorable prognosis in stage II disease and decreased likelihood of metastases. 

Extended RAS Somatic Testing

Extended RAS gene testing, specifically KRAS (codons 12 and 13 of exon 2; codons 59 and 61 of exon 3; and codons 117 and 146 of exon 4) and NRAS (codons 12 and 13 of exon 2; codons 59 and 61 of exon 3; and codons 117 and 146 of exon 4), is recommended in all patients with CRC who have been diagnosed with stage IV disease  to help determine if anti-EGFR therapy is an option. Patients with any known KRAS variant in exons 2, 3, or 4 or NRAS variant in exons 2, 3, or 4 should not receive cetuximab or panitumumab.  No specific methodology is recommended for RAS testing.  Wild-type KRAS is associated with improved prognosis and increased lymph node retrieval. 

BRAF Somatic Testing

BRAF testing is recommended in all patients with metastatic CRC at the diagnosis of stage IV disease and should be considered in patients with wild-type KRAS/NRAS metastatic colon cancer.  Wild-type BRAF is associated with improved prognosis and increased lymph node retrieval, whereas BRAF mutations are associated with a poorer prognosis.  The presence of the BRAF V600E variant in patients with MMR-deficient CRC tumors with loss of MLH1 suggests sporadic CRC, although it does not rule out Lynch syndrome. Presence of the BRAF V600E variant also decreases the probability that treatment with panitumumab or cetuximab therapy will be effective without anti-BRAF therapy (eg, vemurafenib); combination therapy is recommended.  See the NCCN guidelines  for specific regimens.

HER2 Amplification Testing

Testing for HER2 amplifications is recommended in patients with metastatic CRC unless there is a known BRAF or KRAS/NRAS somatic variant.  Tumors with HER2 amplifications in the absence of RAS/BRAF variants may be responsive to HER2-targeted therapies and may be resistant to anti-EGFR therapy.  HER2 testing is not used to determine prognosis.  HER2 amplifications may be detected by IHC, florescence in situ hybridization (FISH), or next generation sequencing (NGS) testing; an NGS panel may be particularly useful to detect HER2 amplifications in conjunction with other biomarkers. 

Monitoring

Careful long-term monitoring is recommended following treatment to assess for complications or recurrence.  Recommended monitoring includes a combination of imaging (eg, colonoscopy), clinical assessment, and laboratory testing.

Carcinoembryonic Antigen

Laboratory test-based monitoring in CRC includes serum CEA. CEA should be regularly monitored for changes in concentration from a preoperative baseline. A serially elevated postoperative titer suggests recurrence and requires examination.  Stage II, III, and IV tumors warrant measurement of CEA every 3-6 months for 2 years after surgery, then every 6 months for a total of 5 years.  Routine CEA measurements are not recommended beyond 5 years because most recurrences take place within 5 years of treatment.  Follow-up for an elevated CEA should include imaging and clinical assessment until CEA concentrations stabilize or disease is discovered. 

Pharmacogenetics​

Fluorouracil Drugs

MSI-low (MSI-L) or microsatellite stable (MSS) tumors are associated with improved outcomes with 5-fluorouracil (5-FU) adjuvant therapy.  Patients with low-risk stage II MSI-high (MSI-H) tumors may also have a good prognosis but do not benefit from 5-FU adjuvant therapy.  For more information and guidelines on testing for 5-FU sensitivity, see the ARUP Consult Germline Pharmacogenetics topic.

Irinotecan​

Decreased UGT1A1 gene expression may lead to drug toxicity, including development of severe neutropenia, from irinotecan.  The UGT1A1*28 allele is associated with an increased risk of toxicity.  Patients who are heterozygous or homozygous for the *28 allele should receive a reduced starting dose of irinotecan  and be treated with caution. Pretreatment testing for UGT1A1*28 should be considered, although guidelines have yet to be established.  For additional information and guidelines on UGT1A1 testing, see the ARUP Consult Germline Pharmacogenetics topic.

Fluoropyrimidine​

MSI-H status and MMR deficiency are associated with a decreased benefit, and possible detrimental impact, from fluoropyrimidine adjuvant therapy in stage II disease.  Certain variants in the dihydropyrimidine dehydrogenase (DYPD) gene are associated with life-threatening toxicity from fluoropyrimidine.  These variants are thought to occur in 1-2% of the population; however, universal testing for these variants before fluoropyrimidine treatment is not currently recommended. 

Anti-EGFR Therapy

As mentioned above, patients with any known KRAS variant in exon 2, 3, or 4 or NRAS variant in exon 2, 3, or 4 should not receive cetuximab or panitumumab, and presence of the BRAF V600E variant also decreases the probability that treatment with these drugs will be effective without anti-BRAF therapy.  HER2 overexpression may also predict resistance to anti-EGFR treatment.  Appropriate treatment regimens are detailed in the NCCN guidelines.