Real-World Study of Young Patients With Solid Tumors in China

Inclusion Criteria: 1. Confirmed Diagnosis: Patients with malignant solid tumors confirmed by histopathological or cytological examination. The primary tumor types of interest include, but are not limited to, breast cancer, malignant melanoma, and multiple primary tumors. 2. Age and Cohort Definitions: 2.1. Young Cohort (Core Population): Patients aged ≤40 years at the time of initial diagnosis of a malignant tumor. 2.2. Older Control Cohort: Patients aged \>40 years at the time of initial diagnosis of a malignant tumor. Patients will be selected proportionally according to the incidence of each tumor type and propensity score matching principles. 3. Eligible Time Window: The initial diagnosis or receipt of antitumor treatment at the participating center must have occurred between January 1, 2010, and May 31, 2026. 4. Treatment History: Patients must have received at least one standard antitumor treatment at a participating study center, including but not limited to curative or palliative surgery, neoadjuvant or adjuvant systemic therapy, and first-line or later-line chemotherapy, targeted therapy, or immunotherapy for advanced disease. 5. Availability of Multimodal Data: 5.1. Relatively complete clinical records must be available, including baseline demographic characteristics, medical records, treatment regimens, and follow-up records. 5.2. Baseline imaging data must be available, such as CT, MRI, or PET-CT scans. 5.3. Pathological tissue samples, such as formalin-fixed, paraffin-embedded (FFPE) blocks or unstained slides, must be available to support subsequent digital pathology and radiomic feature extraction. Exclusion Criteria: 1. Uncertain Diagnosis: Patients with an inconclusive histopathological or cytological diagnosis, or without clear documentation of tumor stage according to the TNM staging system. 2. Severe Missingness of Core Data: Patients with substantially incomplete clinical records, such as missing documentation of key treatment regimens, absence of any baseline imaging assessment, or no follow-up records whatsoever due to loss to follow-up. 3. Potential Confounding From Multiple Primary Malignancies: Patients with a concurrent malignancy or a history of another malignancy within the previous 5 years, except for cured basal cell carcinoma of the skin, cervical carcinoma in situ, or papillary thyroid carcinoma, to avoid confounding the analysis of survival outcomes, including overall survival (OS) and progression-free survival (PFS). 4. Deemed Unsuitable by the Investigator: Patients considered by the investigator, based on clinical judgment, to have other conditions that make them unsuitable for inclusion in this study.