The detection of circulating tumor DNA or ctDNA, tumor-derived fragmented DNA in the bloodstream that is not associated with cells, after neoadjuvant (preoperative) chemo-radiotherapy – a therapy combining chemotherapy and radiation therapy – was associated with poor response and increased risk of metastases in patients with locally advanced rectal cancer.
Each year, approximately 44,180 patients are diagnosed with rectal cancer in the United States. Globally the incidence of rectal cancers depends on the region, with higer incidences in Western nations compared to Africa and South-Asia.
“Responses to neoadjuvant chemo-radiotherapy are variable in patients with rectal cancer,” said senior author David Cunningham, MD, director of clinical research and consultant medical oncologist at The Royal Marsden in London, United Kindom.
“If we can accurately differentiate the good responders from the poor responders, we may be able to adapt treatment and move away from a one-size-fits-all approach,” Cunningham added.
Today, neoadjuvant chemo-radiotherapy has become the standard treatment for locally advanced rectal cancer. The approach not only reduces tumor size and recurrence, it has also demonstrated to increase the tumor resection rate and anus retention rate with very slight side effect.
Assessing the response to neoadjuvant chemo-radiotherapy includes imaging and direct examination of surgically removed tissue. However, the traditional methods for assessing response on imaging can be challenging due to the scarring of tumor tissue arising from treatment.
In addition, Cunningham explained that in “15% to 20% of patients have no evidence of cancer remaining in the tissue after neoadjuvant treatment. Establishing whether these patients could have been spared from surgery if their responses had been detected beforehand is an area of active research.”
The potential of ctDNA
In the study, supported by the National Institute for Health Research Biomedical Research Center at The Royal Marsden/The Institute of Cancer Research, Cunningham and colleagues assessed the potential of ctDNA to predict tumor response and disease progression in patients with rectal cancer. The study enrolled 47 patients with locally advanced rectal cancer
undergoing chemo-radiotherapy, as well as patients with non-metastatic rectal cancer who were recommended to receive chemo-radiotherapy.
The researchers analyzed 243 blood samples used to look for ctDNA and if present, to measure levels of ctDNA.
The researchers collected samples before, during, and after chemo-radiotherapy, as well as after surgery. For patients who did not receive surgery, blood samples were collected every three to six months after chemo-radiotherapy until within three months of tumor regrowth. Up to three somatic variants were tracked in plasma using droplet digital PCR.
A total of forty-seven patients were evaluable for ctDNA and primary tumor response. Thirty-three percent of patients with poor tumor responses, as measured by tumor regression grade on magnetic resonance imaging, had detectable levels of ctDNA after chemoradiotherapy, compared to 5% of patients with good tumor responses. 
Three patients with pathologic complete responses (PCR) had detectable ctDNA prior to chemo-radiotherapy and undetectable ctDNA from mid-treatment onwards. The researchers observed that the detection of ctDNA was not associated with tumor response measured by Response Evaluation Criteria In Solid Tumors (RECIST), a set of published rules that define when cancer patients improve (“respond”), stay the same (“stable”) or worsen (“progression”) during treatments. 
Development of metastases
Cunningham and his team also observed that the detection of ctDNA after chemo-radiotherapy and persistence of ctDNA during treatment was also associated with the development of metastasis. Seven of the 10 patients (70%) with detectable ctDNA upon completion of chemo-radiotherapy went on to develop metastases, compared to four of the 37 patients (11%) who did not have detectable ctDNA after chemo-radiotherapy.
Furthermore, metastasis-free survival was significantly shorter in patients with detectable ctDNA upon completion of chemo-radiotherapy and in those with ctDNA that persisted throughout treatment [HR 7.1; 95% confidence interval (CI), 2.4–21.5; P < 0.001].
The authors also found that detection of ctDNA after surgery was associated with relapse. All three patients with detectable ctDNA post-surgery experienced relapse, compared to zero relapses among the 20 patients without detectable ctDNA post-surgery (P = 0.001).
“Our results indicate that ctDNA can be used to identify patients who have a greater risk of developing metastases or experiencing relapse,” noted Shelize Khakoo, MD, a medical oncologist at The Royal Marsden and the first author of the published article.
The researchers are currently recruiting colon and rectal cancer patients into a multicenter study that will use the presence or absence of ctDNA to guide treatment decisions after surgery. “If the associations we’ve observed during the neoadjuvant period are confirmed, detection of ctDNA could also be used to tailor treatment after surgery in an attempt to prevent cancer spread,” said Cunningham.
One of the limitation of the study is the small sample size.
 Merker JD, Oxnard GR, Compton C, et al. Circulating Tumor DNA Analysis in Patients With Cancer: American Society of Clinical Oncology and College of American Pathologists Joint Review. J Clin Oncol. 2018;36(16):1631–1641. doi:10.1200/JCO.2017.76.8671 [Pubmed]
 Khakoo S, Carter PD, Brown G, et al. MRI Tumor Regression Grade and Circulating Tumor DNA as Complementary Tools to Assess Response and Guide Therapy Adaptation in Rectal Cancer [published online ahead of print, 2019 Dec 18]. Clin Cancer Res. 2019;10.1158/1078-0432.CCR-19-1996. doi:10.1158/1078-0432.CCR-19-1996 [Pubmed][Article]