A 77-year-old male patient consulted us for tenesmus and diarrhoea with 2 months of evolution. Without general syndrome. Sister with colon cancer history. At physical examination identified a lump of 3-4 cm of the anal margin that occupied 3/4 parts of the circumference. Laboratory studies were normal.

The colonoscopy showed a neoformation from pectinea line up to 12 cm. Endorectal echography demonstrated a tumour from anal margin with circumferential growth. Grease affection was seen.

Thoraco-abdominal CT confirmed a mural thickening of the rectum, with nodular edges below the rectosigmoid union with longitudinal length of 6 cm and projection to mesorectal grease on the left side. Some small lymph nodes in path of the inferior mesenteric artery and right iliac primitive chain. No evidence of distant disease. (Fig. 1).

RMI reported an extensive rectal tumour (middle and inferior rectum), with a longitudinal extension of 8.7 cm. Its growth was homogenous and practically concentric. The thickening was more than 2 cm with an infiltration in mesorectal grease of 7 mm. The stage was concluded as T3N0.

The biopsy showed a non-Hodgkin diffuse large B-cell lymphoma. Bone marrow was negative for malignity.
The patient received chemotherapy with good results.

Primary rectal lymphoma is such an unusual disease that within primary neoplasms of colon and rectum it occurs in 0.5% and 0.01% respectively [1]. Gastrointestinal lymphoid tissue is found in the lamina propria and submucosa. The most common presentation is extranodal, Non-Hodgkin Lymphoma [2, 3, 4]. Histological verification is very important.

The higher incidence is in males between the fifth and seventh decade of life [5, 6]. The risk factors related include: States of immunosuppression, inflammatory bowel disease (is most frequently seen in ulcerative colitis), the use of biological agents, human immunodeficiency virus [7, 8].

The majority of patients present nonspecific symptoms, such as abdominal pain, altered bowel habit, anorexia, weight loss, rectal mass sensation or complications (intestinal perforation, obstruction, fistula recto-vesical). Colorectal adenocarcinoma has similar clinical presentation and it cannot be excluded. [5, 8, 3].

The imaging appearance at CT, MRI and echoendoscopy describe the characteristic radiological findings of lymphoma colorectal primary such as a homogeneous wall thickening, which tends to a circumferential deep growth rather than infiltrative injury. Some authors cite a wall thickness typical of lymphoma of colon of 7 to 12 cm from the lumen to the serosa [4]. Furthermore characterized by considerably decrease the intestinal lumen, however rarely causes obstruction (at least even in very advanced stages), this is because it does not cause a desmoplastic response and lymphoid infiltration in the submucosa weakens the muscle of the wall itself. This produces an image known as "rectal aneurysm"[9], which may be associated with the formation of fistulas, thickening of the adjacent elevator muscles and lymph nodes [1].

The characteristic radiological findings that can be oriented towards the primary rectal lymphoma are [9]:
• A mural mass homogeneous or a concentric marked thickening of the wall with obliteration of intestinal lumen.
• Affection of a huge segment on its longitudinal axis.
• Well-defined margins with preservation of fat planes
• No invasion into adjacent structures.
• Perforation with no desmoplastic reaction.
Currently there is not a treatment protocol because of its low prevalence. Multiple retrospective studies and bibliographical reviews are not conclusive in starting a surgical treatment or chemotherapeutic due to the shortage of patients.

In the case of our patient revealed good results with medical treatment, both clinical and radiological improvement in the first three months. It is recommended to assess through a multidisciplinary team with the patient comorbidities and the risks and benefits that can require different treatments.