Small cell lung cancer (SCLC) is highly malignant and aggressive, with most patients presenting with metastases at diagnosis. For postoperatively incidentally detected SCLC (identified unexpectedly in pathological examinations after surgery), conventional adjuvant chemoradiotherapy has shown limited efficacy, characterized by high recurrence rates and suboptimal long-term survival.Notably, advances in immunotherapy have transformed SCLC management. The EXTENTORCH trial, a pivotal study published in JAMA Oncology, demonstrated that toripalimab plus chemotherapy significantly improved outcomes in extensive-stage SCLC (ES-SCLC), achieving a reduced risk of progression or death (HR=0.67, 95% CI 0.54-0.82, P<0.001) with manageable safety, marking a substantial therapeutic breakthrough. Given the proven efficacy of toripalimab in ES-SCLC, there is a strong rationale to extend this strategy to postoperatively incidentally detected SCLC. This trial aims to evaluate the efficacy and safety of toripalimab as adjuvant maintenance therapy in this cohort, with the objective of reducing recurrence and enhancing long-term survival.

Inclusion Criteria:

1. The patient shall sign the Informed Consent Form;
2. Aged ≥ 18 years;
3. Patients with definitely incidental SCLC confirmed by pathological results after radical resection of lung cancer;
4. Eastern Cooperative Oncology Group (ECOG) performance status score of 0 or 1;
5. Life expectancy is at least 12 weeks;
6. Good function of other major organs (liver, kidney, hematological system, etc.);
7. Fertile female patients must undergo a pregnancy test within 7 days before the start of treatment with a negative result; and reliable contraceptive measures (such as intrauterine devices, contraceptives and condoms) should be used during the trial and within 30 days after its completion;
8. Fertile male subjects must use condoms for contraception during the trial and within 30 days after its completion.

Exclusion Criteria:

1. Patients with a malignancy other than SCLC within five years prior to the start of this trial;
2. Participants with any unstable systemic disease (including uncontrolled hypertension, severe arrhythmia, etc.);
3. With activate or suspectable autoimmune disease, or autoimmune paracancer syndrome requiring systemic treatment;
4. Participants who are allergic to the test drug or any auxiliary materials;
5. Participants with active hepatitis B, hepatitis C or HIV;
6. Participants with Interstitial lung disease currently;
7. Pregnant or lactating women;
8. Any malabsorption;
9. Participants suffering from nervous system diseases or mental diseases that cannot cooperate;
10. Other factors that researchers think it is not suitable for enrollment.

The researchers will consider whether IPD is available to other researchers only after the paper is published.