The patient was a 56-year-old male, with no personal history of interest, who consulted for self-limited episodes of monosymptomatic macroscopic haematuria of 2 months' duration.
On rectal examination, the prostate was well-defined and adenomatous in consistency (grade II/IV).
The following complementary examinations were performed:
- Blood count, biochemistry, urine culture, urine cytology and sediment: within normal limits. PSA: 1.46 ng/ml.
- Urological ultrasound: bilateral renal sinus cysts. Normal bladder.
- Intravenous urography: pyelocaliceal distortion due to the aforementioned cysts. Patent ureters. Normal bladder. Post-myocardial film showed the presence of a 'contrast leakage' into the abdominal cavity.
- Cystoscopy: small solid mass located in the bladder dome.

In view of these findings, transurethral resection (TUR) of the lesion was performed, with the histopathological result of adenocarcinoma infiltrating the bladder wall.
Following this diagnosis, an extension study was performed using chest X-ray (which was reported as normal) and abdominopelvic CT scan, showing a tubular structure extending from the bladder dome to the umbilicus with a thick wall, especially near the bladder (persistent urachus with possible infiltration of the wall by the tumour) and a moderate amount of free fluid in the peritoneal cavity.

One week before definitive surgical treatment, the patient was admitted with a high fever accompanied by micturition symptoms, abdominal pain, vomiting and anorexia.
On physical examination, the patient showed abdominal distension with some degree of peritoneal irritation. Laboratory tests showed leukocytosis with neutrophilia. After initiating broad-spectrum parenteral antibiotic treatment, the patient's symptoms improved, except for the abdominal distension. Abdominal ultrasound showed a large amount of ascitic fluid with septa.
With a suspected diagnosis of urachal adenocarcinoma, a wide partial cystectomy was performed, including the bladder dome, a large mass at the level of the urachus and umbilicus, as well as a pelvic lymphadenectomy. During surgery, we observed a large amount of mucoid material infiltrating the entire peritoneal cavity (several samples were taken from different locations for histological study). The anatomopathological diagnosis of the surgical specimen was mucinous adenocarcinoma of the urachus with extension to the urinary bladder and peritoneum. The histological report of the mucoid material sent was pseudomyxoma peritonei.

At present, after 1 year of follow-up, the patient is asymptomatic and with a disease-free extension study.