Inclusion Criteria:

- Age > 18 years

- Locally invasive high-risk rectal adenocarcinoma as defined by the presence on MRI of
at least one of the following: i) Tumours within 1mm of mesorectal fascia i.e.
circumferential resection margin threatened or involved ii)T3 tumours at/below
levators iii)Tumours extending ≥5mm into peri-rectal fat iv)T4 tumours (including the
involvement of bladder or vagina if surgical resection is possible with clear
margins) v)Presence of extra-mural venous invasion (primary tumour is therefore at
least T3) vi)T2 N0/1/2 tumours requiring Abdomino-Perineal Excision, within 1mm of
mesorectal fascia i.e. circumferential resection margin threatened or involved

- The absence of malignant pelvic side-wall disease, local recurrence (either after TME
or wide local excision) or metastatic disease

- Completion of pre-operative long-course CRT

- No viable disease seen at MRI performed 4 weeks after long-course CRT, confirmed at 8
week MRI

- Evidence of partial response of rectal tumour to pre-operative long-course CRT at 4
week MRI which continues to show an incremental response at 8 week MRI.

- Histological diagnosis of adenocarcinoma of rectum.

- WHO performance status 0, 1 or 2.

- No evidence of metastatic disease as determined by CT scan of chest, abdomen, pelvis
or other investigations such as PET scan or biopsy if required.

- Informed written consent

Exclusion Criteria:

- Age < 18 years.

- Absence of concomitant chemotherapy.

- RT dose below 50Gy.

- Stable disease at 4 week MRI.

- Disease that demonstrates a partial response at 4 week MRI but shows no evidence of
an incremental response at 8 week MRI.

- Pregnancy or breast feeding

- Short course pre-operative radiotherapy

- Previous pelvic radiotherapy

- Medical or psychiatric conditions that compromise the patient's ability to give
informed consent

- Any contra-indication to MRI scanning, eg Cardiac Pacemaker or Hip prosthesis.

- Any patients within the EXPERT-C trial.

- Tumours which are mucinous (>50% mucin seen on MRI), as these are more likely to be
PET negative