SOLID is a multicenter, open-label, randomized, controlled, phase III non-inferiority trial comparing anatomic segmentectomy with lobectomy in patients with deeply located, solid-dominant, clinical stage IA non-small cell lung cancer (NSCLC) measuring <=2 cm. Eligible participants will undergo centralized imaging review and will be randomized preoperatively in a 1:1 ratio before induction of anesthesia.

The primary endpoint is 5-year overall survival. The key supportive secondary endpoint is 5-year recurrence-free survival. The study will enroll 1,200 participants and will use intention-to-treat analysis for the primary endpoint. An independent Data and Safety Monitoring Board and an independent Endpoint Adjudication Committee will oversee safety, surgical quality, data quality, and endpoint adjudication.

Segmentectomy has been established as an accepted treatment option for selected small peripheral NSCLC, but high-level randomized evidence is limited for tumors located in the inner two-thirds of the lung parenchyma. Deep, solid-dominant nodules may have higher oncologic risk, more complex anatomy, and greater difficulty achieving conventional linear surgical margins.

This trial evaluates whether standardized anatomic segmentectomy, including surgical quality control and lymph-node dissection requirements, is non-inferior to lobectomy for overall survival in patients with deeply located, solid-dominant, clinical stage IA NSCLC <=2 cm. The trial incorporates centralized pre-randomization imaging review, mandatory recording of margin and surgical quality metrics, systematic or lobe-specific lymph-node dissection, independent endpoint adjudication, and DSMB safety and quality oversight.

Participants randomized to the segmentectomy arm will undergo anatomic segmentectomy when feasible. Conversion to lobectomy is required for intraoperative frozen-section N1/N2 nodal metastasis and may occur for inadequate margins, anatomic difficulty, or safety concerns. Participants randomized to the lobectomy arm will undergo standard anatomic lobectomy. Intraoperative findings and final pathology do not change the original randomized assignment for the primary intention-to-treat analysis.

Inclusion Criteria:

1. Age 18 to 80 years.
2. Solitary pulmonary nodule with maximum diameter <=2 cm confirmed by preoperative thin-slice CT (<=1 mm) and 3D reconstruction.
3. Solid-dominant nodule with consolidation-to-tumor ratio (CTR) >0.5, including pure-solid nodules.
4. Deeply located lesion, defined as the lesion center located in the inner two-thirds of the lung field on axial, coronal, and sagittal MPR images.
5. Clinically definite suspected primary NSCLC after complete preoperative work-up and MDT discussion, with estimated malignancy probability >=95%. Preoperative pathologic confirmation is not mandatory, but preoperative or intraoperative pathology should be obtained when technically safe and feasible.
6. Complete staging with thin-slice contrast-enhanced chest CT, whole-body PET-CT, brain contrast-enhanced MRI or CT, and serum tumor markers as appropriate; clinical stage IA (cT1a-bN0M0) without regional nodal or distant metastasis.
7. Cardiopulmonary function sufficient to tolerate both lobectomy and segmentectomy, with FEV1 and DLCO >=60% predicted.
8. Written informed consent, including special disclosure that final pathology may be benign.
9. Centralized imaging review confirmation by the lead-center technical committee before randomization.

Exclusion Criteria:

1. Lesion located in the right middle lobe.
2. Lesion located in the inner two-thirds but adjacent to the lung apex or base and judged suitable for wedge resection.
3. Two or more lesions requiring anatomic resection. One secondary lesion requiring only non-anatomic wedge resection is allowed; two main lesions in the same segment are excluded.
4. Prior ipsilateral lung surgery or another malignancy within 5 years.
5. Severe cardiovascular or cerebrovascular disease or other comorbidity making lobectomy or conversion to lobectomy intolerable.
6. Planned basal segmentectomy (S7-10); proper segmentectomy is permitted.