This study tests biopsy and tissue from patients who have been treated for primary rectal cancer at the Royal Marsden Hospital between 2011 and 2013, who have an mrTRG score at post-chemoradiotherapy MRI. It is a retrospective pilot study to determine the apoptotic and proliferative index count pre and post chemoradiotherapy.

The investigators hope to validate the apoptotic and proliferative index as a predictive response marker and lend support to mrTRG as a putative prognostic tool in the treatment pathway for rectal cancer patients. Even though this is a retrospective study, the investigators hope that pre-treatment apoptotic and proliferative scores will correlate to post treatment response and mrTRG scores. In doing so, the investigators can predict how patients will respond to treatment from initial diagnostic biopsies and foresee a patients individual treatment plan.

Inclusion Criteria:

• Patients over 18 years of age.
• Patients with primary adenocarcinoma of the rectum (diagnosed on tissue biopsy and disease spread assessed on CT and MRI).
• Tumours must be considered sufficiently high-risk to require pre-operative chemoradiotherapy followed by surgery.
• Patients must have had a pre- and post-treatment MRI scan of the rectum and pelvis.
• We must have access to stored tissue for each patient

Exclusion Criteria:

• No pre-operative radiotherapy
• Patients with synchronous tumours
• Patients under the age of 18 years

Patients with rectal cancer, diagnosed between 2011 and 2013, who have had an mrTRG score in a post-radiotherapy MRI scan.