This multicentre randomized controlled trial aims to investigate whether an abbreviated MRI is comparable to a combined single venous phase CT with an additional 3 min equilibrium phase of the liver, in the pretreatment radiological workup in patients with rectal cancer.

The incidence of rectal cancer in the European Union is approximately 125 000 per year, constituting a third of the total colorectal cancer incidence(1). In Sweden approximately 2000 individuals are diagnosed with rectal cancer each year. Evaluation of patients and possible treatment strategies (including neo-adjuvant chemo/radiotherapy, choice of surgical technique and adjuvant chemotherapy) are based on tumor specific characteristics together with patient factors such as general condition and comorbidity. These characteristics include tumor, lymph node and metastasis classification (cTNM), height of tumor (distance from anal verge), engagement of circumferential margin or mesorectal fascia, signs of extramural invasion and the presence of distant metastases. Clinical examination and endoscopy together with magnetic resonance imaging (MRI) of the pelvic region are essential in the local staging process, where MRI provides the most accurate radiological assessment, especially for rectal tumors in stage T3 and T4. The recommended methods regarding evaluation of metastases (M stage) include contrast enhanced computed tomography (CT) or MRI of the liver/abdomen, CT of the thorax, and in selected cases positron emission tomography (PET)- CT, if extensive extramural vascular invasion is present.

Previous studies have investigated the efficiency of MRI compared to other radiological methods and found that MRI of the liver is superior to contrast enhanced CT, in terms of specificity and sensitivity for diagnosis of liver metastasis (both per patient and per lesion).

Even though MRI has been recommended as the first line modality for evaluating colorectal cancer liver metastases in patients who have not previously undergone therapy the Swedish national guidelines, as well as the European Society for Medical Oncology (ESMO), include recommendations of single venous phased contrast enhanced CT of the liver/abdomen in the pre-treatment radiological workup. Progress within diagnostics has led to development of CT examination with an additional 3 min equilibrium phase of the liver that may be added to the single venous phase in order to increase the sensitivity and specificity of the diagnosis of liver metastases. However, this has not yet been included in guidelines nor implemented on a national basis in the pre-treatment radiological workup in patients with colorectal cancer.

Objective The primary objective of the present study is to compare percentage of patients requiring further radiological examinations after index examination (due to inconclusive results) between patients randomized to an abbreviated MRI liver protocol and patients randomized to a combined single venous phase CT with an additional 3 min equilibrium phase of the liver. A secondary aim is to compare time to treatment initiation, survival and development of liver metatstases between patients randomized to either an abbreviated MRI or a combined single-venous phase CT with an additional 3 min equilibrium phase of the liver.

The study is a multicentre randomized controlled trial where the hypothesis is that there is no clinical difference between the two groups, and the study is designed as a non inferiority trial.

Inclusion Criteria:

• Adults >18 years of age
• Endoscopically or clinically diagnosed with (suspected) rectal cancer and considered for further radiological investigation
• Are able to understand trial information and provide oral and written consent

Exclusion Criteria:

• Patients unable to undergo MRI (causes may include claustrophobia, presence of pacemaker or certain metal components in their body).
• Patients diagnosed with benign disease of the rectum or squamous cell carcinoma
• Patients diagnosed with recurrent rectal cancer
• Patients unable to understand the trial information

It will not be possible to mask for either patients nor participating health care professionals if patients are enrolled in the control- or intervention group. Hence, the study will not be blinded. When the final data analysis is performed for the primary outcome measure the person performing the analysis will not be aware of which group each patient was a part of.