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Histology of cancer spreading in the peritoneum

Metastasis from carcinoma of the colon or rectum

One can often find infiltrative growth and sometimes also central gland metastasis or metastasis to intra- or extra-abdominal organs, especially to liver and lungs. If tumor produces mucus (mucinous adenocarsinoma), the peritoneal spread is similar to peritoneal mucinous carcinomatosis (PMCA) as seen in pseudomyxoma peritonei.

Carcinomatosis from colon cancer. Click to enlarge. Metastatic mucus-producing andenocarcinoma. Click to enlarge. Metastatic adenocarcinoma of the colon, not mucus-producing. Click to enlarge.

Pseudomyxoma peritonei

Immunohistochemical and molecular genetic studies have shown that pseudomyxoma peritonei usually originate from tumors in the appendix and not from tumors in the ovary. This condition is usually due to low-grade tumors in the appendix, either cystadenoma with atypia or a low-grade mucus-producing adenocarcinoma. 

Appendix with a primary lesion in pseudomyxoma peritonei and a lot of mucinous material in the abdomen. Click to enlarge.

At the histological examination, the amount of atypical cells in relation to the amount of mucus should be considered, as well as cellular atypia, amount of glandular structures and amount of signet ring cells.

Pseudomyxoma peritonei differs from other types of metastatic mucus-producing carcinomas by the fact that the tumor cells do not infiltrate the peritoneal and omental tissue, but rather dissects into the tissue. Only the most malignant types infiltrate into deeper organs or produce lymph node metastases.

There is a classification into three subgroups based on the histological appearance as follows:

  • Disseminated peritoneal adenomucinosis (DPAM) - abundant mucus, sparse epthelium with slight or no atypia, a few mitotic figures and orginating from a cystadenoma.
  • Peritoneal mucinous carcinomatosis, intermediate type (PMCA- I/D) – predominantly DPAM with focal proliferating epithelium with moderate atypia, mitotic figures and originating from a cystadenoma or adenocarcinoma.
  • Peritoneal mucinous carcinomatosis (PMCA) – severe atypical epithelium/signet ring and originating from an adenocarcinoma. Infiltration and lymph node metastasis occurs.

Others combine PMCA-I/D with PMCA into a two-grade system. The grading is not included in WHO classification of intestinal tumors. WHO describe pseudomyxoma peritonei as a condition most probably caused by spreading from a low-grade adenocarcinoma.


This tumor does not produce mucus and infiltrates and spreads diffusely into the peritonal cavity. The main differential diagnosis is metastasis from adenocarcinoma. Immunohistochemical investigation can usually establish mesothelioma diagnosis using a few antibodies. In a few occasions there are lymph node metastasis in the cardiophrenic angle in the thoracic cavity.

Malignant mesothelioma in the peritoneum. Click to enlarge. Malignant mesothelioma growing on the colon. Click to enlarge.


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