Are Ovarian Carcinoma Metastases the Same as Peritoneal Implants?
No, ovarian carcinoma metastases and peritoneal implants are not synonymous—the terminology depends critically on the histologic characteristics and invasive behavior of the peritoneal deposits.
Key Distinction Based on Invasiveness
The classification of peritoneal deposits in ovarian cancer hinges on whether they demonstrate invasive characteristics:
Non-Invasive Implants
- Non-invasive implants are peritoneal deposits that lack destructive invasion of underlying tissue and are typically associated with borderline ovarian tumors 1.
- These can be further subdivided into epithelial-type (papillary clusters on peritoneal surfaces without invasion) and desmoplastic-type (glands or papillary clusters within fibroblastic stroma without infiltration of adjacent tissue) 1.
- Non-invasive implants do not constitute metastatic carcinoma in the traditional sense 1.
Invasive Implants (True Metastases)
- Invasive implants demonstrate destructive invasion of underlying tissue with markedly crowded epithelial nests, glands, or micropapillary clusters in a haphazard arrangement 1.
- When invasive implants are diagnosed, they should be reported as extra-ovarian low-grade serous carcinoma, as endorsed by the 2014 WHO classification 1.
- These represent true metastatic disease and are staged accordingly in the FIGO system 1.
FIGO Staging Context
The FIGO staging system clarifies the relationship between peritoneal deposits and metastatic disease:
- Stage II disease involves pelvic extension and/or implants (which may be non-invasive) 1.
- Stage III disease requires microscopically or macroscopically confirmed peritoneal implants outside the pelvis—these are explicitly termed "peritoneal metastasis" in stages IIIA, IIIB, and IIIC 1.
- The staging system uses both terms ("implants" and "metastasis") but applies "metastasis" specifically to stage III and IV disease with confirmed spread beyond the pelvis 1.
Molecular Evidence of Clonal Relationship
Recent molecular studies provide insight into the origin of peritoneal deposits:
- The vast majority of implants are clonally related to the primary ovarian tumor, with all 10 invasive implants in one large population-based study showing the same mutational status (KRAS, BRAF, or wild-type) as the corresponding ovarian tumor 1.
- This suggests invasive implants represent metastases from the ovarian primary rather than independent peritoneal tumors, though the total number of invasive implants studied molecularly remains limited 1.
Clinical Implications for Terminology
In practice, the distinction matters for:
- Pathology reporting: Invasive implants must be designated as extra-ovarian low-grade serous carcinoma, while non-invasive implants are reported as such 1.
- Treatment decisions: Stage III disease with peritoneal metastases requires cytoreductive surgery followed by platinum-based chemotherapy (carboplatin plus paclitaxel for 6 cycles), identical to treatment for primary peritoneal cancer 2, 3.
- Prognostic assessment: The presence of invasive versus non-invasive implants significantly impacts prognosis and treatment planning 1.
Common Pitfall to Avoid
Do not use "peritoneal implants" and "metastases" interchangeably without histologic confirmation of invasion. When peritoneal deposits are identified, the pathologist must determine whether they represent non-invasive implants (which may occur with borderline tumors) or invasive implants (true metastatic carcinoma) 1. In cases where this distinction cannot be definitively made, the implants should be designated as "indeterminate type," though this terminology should be used sparingly and prompt specialist consultation 1.