The purpose of this study is to see which participants have a better treatment response using PET/MRI imaging to study the removed tumor, after they have received a total neoadjuvant therapy (TNT). The treatment choice of long course chemo-radiotherapy treatment will be determined by institutional policy.

The researchers will also be looking at whether this study could significantly improve the future management and quality of life of rectal cancer patients.

Inclusion Criteria:

1. Patients with histologically proven adenocarcinoma of the rectum.
2. Clinical stage 2-3 rectal adenocarcinoma, cT3/4N0/M0 or Tx N1-2/M0, within 16 cm from anal verge (lesion below the sacral promontory).
3. Patients deemed suitable to undergo TNT followed by surgical resection.
4. Male or female ≥ 18 years of age.
5. ECOG/Zubrod status 0-2.
6. Able and willing to follow instructions and comply with the protocol.
7. Provide written informed consent prior to participation in the study.
8. Women of childbearing potential (WOCBP) must have a negative serum (or urine) pregnancy test at the time of screening if there is concern about pregnancy. WOCBP is defined as any female who has experienced menarche and who has not undergone surgical sterilization (hysterectomy or bilateral oophorectomy or bilateral salpingectomy) and is not postmenopausal.
9. If female of child-bearing potential and outside of the window of 10 days since the first day of the last menstrual period, concerned about pregnancy are not eligible for the study.
10. Females must not be breastfeeding (during treatment and at least 6 months from the last dose).
11. Male patients should agree that not donate sperm during the study (during treatment and at least 6 months from the last dose).
12. Patients of childbearing/reproductive potential should use highly effective birth control methods if indicated, as defined by the investigator, up to the period of finishing adjuvant chemotherapy. A highly effective method of birth control is defined as those that result in low failure rate (i.e., less than 1% per year) when used consistently and correctly.

Note: abstinence is acceptable if this is established and preferred contraception for the patient and is accepted as a local standard.

Exclusion Criteria:

1. Patient receiving short course radiotherapy alone for rectal cancer.
2. Patient receiving standard TNT.
3. Patients with metastatic disease.
4. Prior pelvic radiotherapy or chemotherapy.
5. Patients in whom MRI is contra-indicated as per prevailing institutional guidelines (e.g. pacemakers, aneurysm clips, cochlear implant, implanted cardiac defibrillator).
6. Prior or concurrent malignancy (except non-melanomatous skin cancer) unless disease-free for at least 5 years.
7. Inability to lay in supine position for approximately one hour.
8. Nursing or pregnant females.
9. Age <18 years.
10. Previous and concurrent, experimental, chemotherapy, or radiotherapy treatment for primary rectal carcinoma.
11. Patients with malabsorption syndrome, ulcerative colitis, inflammatory bowel disease, resection of the stomach or small bowel, or other disease significantly affecting gastrointestinal (GI) function.
12. Patients with a known history of documented upper GI bleeding or upper GI ulcerative disease.
13. Patients who have experienced untreated and/or uncontrolled cardiovascular conditions and/or have symptomatic cardiac dysfunction.
14. Known current alcohol abuse.
15. Patients with symptomatic inflammatory bowel disease.
16. Patients with uncontrolled hypothyroidism.
17. Patients with chronic liver disease.
18. Patients known to be suffering from infection with HIV, Tuberculosis, Hepatitis C or Hepatitis B.
19. Any contra-indications for intravenous contrast.
20. History of anaphylactic reaction to medications or drug allergy.