The goal of this observational study or is to explore the correlation between dynamic changes in postoperative circulating tumor DNA (ctDNA) and recurrence risk using a novel, tumor-informed ctDNA detection technique, thus laying the foundation for further promoting ctDNA detection to guide postoperative adjuvant therapy decisions. This study focuses on patients with esophageal squamous cell carcinoma who did not achieve pCR following neoadjuvant therapy combined with surgical resection. The main question it aims to answer is:

Is there a correlation between postoperative ctDNA changes and tumor recurrence?

Participants will be asked to undergo postoperative ctDNA detection.

Inclusion Criteria:

1. Operable locally advanced esophageal squamous cell carcinoma;
2. Have received neoadjuvant chemoradiotherapy ± immunotherapy and have not achieved pCR after R0 resection;
3. Can tolerate postoperative adjuvant immunotherapy;
4. Sufficient baseline tumor tissue and blood samples available for NGS testing;
5. Eastern Cooperative Oncology Group (ECOG) performance status score of 0-1;
6. Consents to and is capable of complying with the scheduled study visits, treatment plans, laboratory tests, and other study procedures
7. Expected postoperative survival time of ≥ 6 months.

Exclusion Criteria:

1. Concurrent other malignant tumors;
2. Lack of a detailed histopathological diagnostic report for determining the nature of the lesion;
3. Severe and/or poorly controlled medical conditions that, in the investigator's judgment, could affect participation or interfere with study results;
4. Any social/psychological issues that, in the investigator's judgment, make the patient unsuitable for participation;
5. Unable to undergo long-term, regular postoperative follow-up at the current medical institution;
6. Unwilling or unable to comply with the study protocol.

Patients with esophageal squamous cell carcinoma who have not achieved pCR following neoadjuvant chemoradiotherapy and surgery.