The primary aims of this study is to characterize exosomal biomarker levels in patients with locally advanced rectal cancer undergoing neoadjuvant chemoradiation therapy. Compare rates of exosomal expression before during and after chemoradiation therapy with pathologic response rates at the time of APR or LAR. The researchers will also examine the functionality of these exosomes in malignant colonic organoids and mouse models of colorectal cancer.

All male and female patients, aged 18 and older with a histologically proven diagnosis of rectal adenocarcinoma, treated on this study will receive neo adjuvant chemotherapy (consisting of 5 FU or capecitabine) given concurrently with external beam radiation therapy to a dose of 50.4 Gy in 28 fractions. Radiation treatments will be administered daily, Monday thru Friday. Within one week prior to beginning chemotherapy and radiation treatments, a serum sample will be obtained from each patient. Patients will also have serum samples collected every two weeks during radiation, and on the last day of radiation therapy, as well as at their one month follow up visit after completion of radiation and prior to planned surgery. A final serum sample will be collected within one month after definitive surgery. Each serum sample collected will be less than 50 mL.

The study will involve obtaining serum samples from patients before during and after chemoradiation therapy. These serum samples will be analyzed to determine exosomal biomarker levels, which will be correlated with clinical, imaging, laboratory, and pathologic data obtained from the medical record.

Additional data from the patients' medical records will be collected, including demographic data, clinical information from notes in the medical record, radiology images and reports, results of diagnositic tests, and information from procedure notes and pathology reports.

Inclusion Criteria:

• Male and Female Patients aged 18 and older
• histologically proven diagnosis of rectal adenocarcinoma
• will be receiving neoadjuvant chemoradiation therapy prior to a planned definitive surgical resection

Exclusion Criteria:

• Age less than 18.
• Patients who are unable or unwilling to undergo definitive surgery.
• Patients with a prior history of pelvic external beam radiation, brachytherapy, or chemotherapy.
• Patients with a prior history of cancer (excluding non-melanoma skin cancers).
• Patients who are immunocompromised.

All patients seen in the Department of Radiation Oncology at The University of Kansas Health System that are over the age of 18 and have a histologically proven diagnosis of rectal adenocarcinoma and will be receiving neoadjuvant chemoradiation therapy before a planned, definitive surgical resection has been performed.