A 62-year-old woman undergoes curative gastrectomy for gastric adenocarcinoma (pT3N2M0). Histopathology shows intestinal-type gastric cancer with signet-ring cell component. The surgical specimen shows a 5 cm lesion with invasion into the muscularis propria and perigastric lymph node involvement. Which investigation is most appropriate to assess for occult peritoneal metastases before adjuvant chemotherapy planning?

Explanation

## Detection of Occult Peritoneal Metastases in Gastric Cancer **Key Point:** Staging laparoscopy with peritoneal lavage cytology is the most sensitive method to detect occult peritoneal metastases (PM) and peritoneal carcinomatosis (PC) in locally advanced gastric cancer, which are present in 10–15% of patients and significantly alter prognosis and treatment intent. ### Why Peritoneal Metastases Matter in Gastric Cancer - **Prevalence:** 10–15% of locally advanced gastric cancers have occult PM not visible on imaging - **Prognostic impact:** PM converts the patient from M0 (potentially curable) to M1 (palliative intent) - **Imaging limitation:** CECT and EUS have low sensitivity (<50%) for small peritoneal deposits (<5 mm) - **Cytology significance:** Positive peritoneal cytology (PPC) alone, even without visible deposits, is classified as M1 in TNM 8 and indicates advanced disease ### Comparison of Methods for Peritoneal Assessment | Method | Sensitivity | Specificity | Invasiveness | Timing | Role | |---|---|---|---|---|---| | **Staging laparoscopy + lavage** | 85–95% | ~100% | Minimally invasive | Before surgery or after (if high-risk) | Gold standard for PM detection | | **CECT** | 40–60% | 90% | Non-invasive | Pre/post-op | Detects large deposits only | | **Diagnostic paracentesis** | Variable | High | Invasive | Only if ascites present | Not routine; therapeutic if symptomatic | | **Peritoneal scintigraphy** | Low | Low | Invasive | Rarely used | Obsolete for gastric cancer | **High-Yield:** In this case, the patient has **pT3N2M0** disease—locally advanced with nodal involvement. Preoperative or intraoperative staging laparoscopy is indicated to detect occult PM, which would change adjuvant therapy from chemotherapy alone to palliative intent or neoadjuvant/adjuvant peritoneal chemotherapy protocols (e.g., HIPEC, EPIC). ### Staging Laparoscopy Technique in Gastric Cancer ```mermaid flowchart TD A[Locally advanced gastric cancer]:::outcome --> B[Staging laparoscopy]:::action B --> C[Inspect peritoneum, omentum, liver surface]:::action C --> D[Peritoneal lavage with 500 mL saline]:::action D --> E[Send fluid for cytology]:::action E --> F{Cytology result?}:::decision F -->|Positive| G[M1 disease - Palliative/HIPEC consideration]:::urgent F -->|Negative| H[Proceed to curative gastrectomy]:::action G --> I[Adjuvant/peritoneal chemotherapy]:::action H --> J[D2 gastrectomy]:::action ``` **Clinical Pearl:** Positive peritoneal cytology (PPC) without visible peritoneal nodules is still classified as M1b in TNM 8th edition and carries a median survival of 6–12 months, similar to patients with visible peritoneal metastases. This justifies the aggressive preoperative assessment. **Mnemonic: LAP-PPC** — **L**aparoscopy, **A**ssess, **P**eritoneal **P**ositive **C**ytology = M1b disease. [cite:Harrison 21e Ch 297]