Primary peritoneal clear cell carcinoma. A case report
Primary peritoneal malignant tumors are exceptional. Among them, clear cell carcinoma is extremely rare, being only thirteen cases previously reported in the literature since 1990.

Here presents a case of a 48-year-old woman with progressive abdominal pain over the last seven months, with the initial diagnosis of renal-ureteral colic.

Ultrasound and computed tomography of the abdomen and pelvis revealed a 25 × 15 cm, well-defined cystic lesion with papillary projections, centrally located in the abdomen. The radiology report suggested a primary ovarian tumor versus peritoneal implant as the first option. The patient underwent an exploratory laparotomy showing a large cystic mass located in the urinary bladder peritoneum, firmly attached to the mesentery. The entire abdominal tumor was completely excised, and total hysterectomy with bilateral salpingo-oophorectomy and infra-colical omentectomy were performed.

The final histological study revealed a new case of primary peritoneal clear cell carcinoma located in the urinary bladder peritoneum, firmly attached to the mesentery. Grossly, it was well-circumscribed and multicystic with papillary growth involving part of the inner wall. Microscopically, it showed tubulocystic and papillary patterns with highly atypical tumor cells. After an extensive immunohistochemical analysis, the most relevant finding was an ARID1A loss that was corroborated by molecular analysis showing an ARID1A deletion. The patient received systemic chemotherapy with carboplatin and paclitaxel protocol . Patient follow-up after the eighth month showed peritoneal implants predominantly in the right diaphragmatic cupule that were histologically confirmed as recurrence. She has received another six cycles of chemotherapy with carboplatin and paclitaxel.

In conclusion, CCC is a very rare tumor in the peritoneum. It may also be influenced by its lack of recognition, being diagnosed as another neoplasia more frequent in this location. Therefore, in addition to the typical histological findings, immunohistochemistry is a very useful tool for the main differential diagnosis.