There have been initial explorations on the treatment of peritoneal metastasis of gastric and colorectal cancer both at home and abroad. However, the comprehensive treatment plan of "LDRT + NIPEC + immunotherapy + systemic therapy" has not been reported either domestically or internationally. This study will explore the safety and efficacy of total abdominal low-dose radiotherapy followed by NIPEC and PD-1 treatment for peritoneal metastasis of gastric and colorectal cancer. 9-18 participants will be enrolled in this study. All will take part at Daping Hospital, Army Medical University.

This is a prospective, single-center, Ib-phase clinical study. At least 9 eligible participants will be recruited in this study. After all participants are enrolled, they will receive LDRT treatment once on the first day of each of the first 3 cycles, with LDRT treatment doses of 1.5Gy/3F, 3Gy/3F, and 4.5Gy/3F respectively. Then, NIPS Immunochemotherapy will be administered in sequence. The primary endpoint is safety and tolerability.

Inclusion Criteria:

1. Age between18 and 75 years old.
2. Histologically confirmed gastric cancer/colon cancer, and diagnosed as peritoneal metastasis of tumor through laparoscopy/puncture (patients must have metastatic tumors located within the peritoneal cavity).
3. An Eastern Cooperative Oncology Group (ECOG) performance status ≤2.
4. According to the Response Evaluation Criteria in Solid Tumors (RECIST) 1.1, patients must have measurable disease within the irradiated area.
5. Expected survival period≥3 months
6. Adequate cardiac function (Left Ventricular Ejection Fractions > 50%), hepatic function (total serum bilirubin ≤ 1.5 ×upper limit of normal, alanine aminotransferase or aspartate aminotransferase ≤ 2.5 × upper limit of normal), renal function (serum creatinine ≤ 1.5 × ULN or glomerular filtration rate > 60 ml/min, based on Cockcroft-Gault), and hematopoietic function (white blood cells ≥ 4.0 × 109 cells per L, neutrophils ≥ 1.5 × 109 cells per L, hemoglobin ≥ 90 g/L, platelets ≥ 100 × 109 cells per L).
7. Sign the informed consent and have good compliance.

Exclusion Criteria:

1. Presence of distant metastasis other than peritoneal metastasis (ovarian metastasis is allowed);
2. Presence of uncontrolled clinical symptoms or diseases of the heart, including but not limited to: (1) NYHA class II or above heart failure, (2) unstable angina pectoris, (3) myocardial infarction within 1 year, (4) clinically significant supraventricular or ventricular arrhythmias that have not been clinically intervened or remain uncontrolled after clinical intervention;
3. Upper gastrointestinal obstruction or physiological dysfunction, or suffering from malabsorption syndrome, which may affect the absorption of oral drugs;
4. Severe infection (CTCAE > grade 2) or other concomitant diseases within 4 weeks before the first use of the study drug, such as severe pneumonia, bacteremia, or infectious complications requiring hospitalization; baseline chest imaging shows active pulmonary inflammation, or symptoms and signs of infection within 14 days before the first use of the study drug, or requiring oral or intravenous antibiotic treatment, except for prophylactic use of antibiotics; active pulmonary tuberculosis infection is found through medical history or CT examination, or there is a history of active pulmonary tuberculosis within 1 year before enrollment, or there is a history of active pulmonary tuberculosis more than 1 year ago but without regular treatment;
5. History of immunodeficiency (including positive HIV test, or suffering from other acquired or congenital immunodeficiency diseases, or having a history of organ transplantation or allogeneic bone marrow transplantation);
6. Moderate or severe renal impairment (creatinine clearance ≤ 50 ml/min);
7. Allergy to paclitaxel, oxaliplatin, monoclonal antibodies or any component of the study drug;
8. Pregnant or lactating women;
9. Presence of clinically detectable second primary malignancy, or a history of other malignancies within 5 years, excluding adequately treated non-melanoma skin cancer, cervical carcinoma in situ, and superficial bladder tumors (non-invasive tumors, or carcinoma in situ, or T1);
10. Uncontrolled epilepsy, central nervous system disease or mental disorder, as judged by the investigator to be clinically significant enough to prevent signing the informed consent or affect the patient's compliance with drug treatment.