The purpose of this study is to determine if positron emission tomography (PET) imaging with an imaging agent called 18F-fluorodeoxythymidine([18F]-FLT) will allow investigators to measure how well tumor(s) respond to treatment without taking a tissue sample (biopsy). Additionally, the investigators want to determine if it is possible to predict how well tumor(s) might respond to treatment with [18F]-FLT PET imaging.

Inclusion Criteria:

• Subjects with known rectal cancer.
• Subjects must have signed an approved consent form.
• Subjects must be 18 years of age or older.

Exclusion Criteria:

• Children less than 18 are excluded.
• Pregnant women and women who are breast feeding will be excluded from this study. A serum beta HCG will also be performed for each pre-menopausal female subject.
• Patients who are acutely ill who are deemed by their treating physician as not suitable candidates for this study