Accurate pre-therapeutic imaging of the tumour and regional lymph nodes, is - in addition to clinical examination and assessment for distant metastases -essential for prognostication and guiding treatment. There is, however, currently a lack of consensus as to which staging modality should be considered as the gold standard for local staging of the tumour (T) and detection of lymph node involvement (N) Local and international guidelines (NCCN, ESMO/ESTRO) vary. On the other hand, nodal Nodal staging has been validated as an important independent prognostic factor in anal cancer [EORTC22861; RTOG9811] and the nodal stage as well as the precise location of involved lymph nodes (mesorectum, presacral space, internal iliac lymph nodes, external iliac lymph nodes, ischiorectal fossa, inguinal lymph nodes) has significant bearing on informs the planning of radiotherapy treatment especially when using more conformal techniques such as intensity modulated radiotherapy. Therefore it It is therefore essential that to optimize and standardize pre-treatment staging is optimised and standardised imaging to provide accurate and reproducible information to guide treatment decision, development of radiotherapy atlases and clinical trial design.

Whereas there currently exist a lack of consensus as to which staging modality should be considered as the gold standard for local staging of the tumour (T) and detection of lymph node involvement (N), the use of multi-parametric Magnetic Resonance Imaging (MRI) of the pelvis has been endorsed by international guidelines (NCCN, ESMO/ESTRO).

An expert panel of radiologist and radiation oncologists specialising with expertise in anal cancer diagnosis and treatment identified by the Anal Staging Subgroup of the International Rare Cancer Incentive (IRCI) will be invited to participate. The panellists panelists will be assigned into groups based on the geographical location and local practice. Each panellist panelist will be provided with access to a series of anonymised anonymized MRI datasets of patients with newly diagnosed anal cancer ; either a combination of contrast enhanced computed tomography (CT) of the pelvis and Positron Emission Tomography (PET)/CT scans (Cases A1-30) OR a combination of contrast enhanced computer tomography (CT) of the pelvis and Magnetic Resonance Imaging (MRI) of the pelvis (Cases B1-30) and asked to provide a TNM stage as well as to identify sites of involved nodes (using a standardised proforma – Appendix1).