Prophylactic Nanocrystalline Megestrol Acetate Versus Placebo for Chemoradiotherapy-Related Anorexia in Nasopharyngeal Carcinoma

Inclusion Criteria: * Newly diagnosed, histologically or cytologically confirmed non-keratinizing nasopharyngeal carcinoma, clinical stage II-III according to the American Joint Committee on Cancer (AJCC) 9th edition staging system, and planned to receive concurrent chemoradiotherapy (CCRT). * No prior radiotherapy to the primary tumor. * Induction chemotherapy may be administered at the investigator's discretion according to local guidelines. * Aged 18-70 years, male or female. * Eastern Cooperative Oncology Group (ECOG) performance status of 0-1. * Able to comply with the study procedures and voluntarily provide written informed consent. * Adequate organ function, defined as follows: * Hematologic function: neutrophil count ≥1.5 × 10⁹/L, hemoglobin ≥90 g/L, and platelet count ≥100 × 10⁹/L. * Coagulation function: international normalized ratio (INR), prothrombin time (PT), or activated partial thromboplastin time (aPTT) ≤1.5 × the upper limit of normal (ULN). * Hepatic function: alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤2.5 × ULN, total bilirubin ≤2.0 × ULN, and serum albumin ≥30 g/L. * Renal function: serum creatinine ≤1.5 × ULN or calculated creatinine clearance (CrCl) ≥60 mL/min using the Cockcroft-Gault formula. Exclusion Criteria: * Prior antitumor therapy, including chemotherapy, radiotherapy, or surgery, other than induction chemotherapy permitted according to the study protocol. * Pregnant or breastfeeding women. * Inability to comply with regular follow-up because of psychological, social, familial, or geographical factors. * Concurrent participation in another clinical study involving investigational treatment during the study treatment period. * Known allergy to any chemotherapeutic agent that may be used in the study. * Severe uncontrolled infection or other serious uncontrolled medical illness. * Major organ dysfunction, including decompensated cardiac, pulmonary, renal, or hepatic failure, that would preclude tolerance of chemoradiotherapy. * Conditions that may substantially affect drug administration, distribution, metabolism, or excretion, including significant psychiatric disorders, central nervous system disorders, chronic diarrhea, ascites, or pleural effusion. * Long-term use of immunosuppressive agents following organ transplantation. * History of another malignancy before enrollment. * Conditions affecting gastrointestinal absorption or oral intake, including dysphagia, malabsorption, or uncontrolled vomiting; difficulty with food intake or requirement for enteral or parenteral nutritional support; anorexia nervosa or anorexia related to psychiatric illness; or difficulty eating because of pain. * Current or planned use of other medications that may increase appetite or body weight, including systemic corticosteroids (except short-term dexamethasone administered during chemotherapy), androgens, progestogens, thalidomide, olanzapine, anamorelin, or other appetite stimulants. * Cushing syndrome, adrenal insufficiency, or pituitary insufficiency; poorly controlled diabetes mellitus; or uncontrolled hypertension, defined as systolic blood pressure ≥160 mmHg or diastolic blood pressure ≥100 mmHg despite oral antihypertensive therapy. * Thromboembolic disease, ascites, or lower-extremity edema within 6 months before the first dose of study treatment; history of esophageal or gastric varices, severe gastrointestinal ulceration, gastrointestinal perforation and/or fistula, gastrointestinal obstruction (including incomplete intestinal obstruction requiring parenteral nutrition), intra-abdominal abscess, or acute gastrointestinal bleeding. * Known hypersensitivity to any component of the study drug. * Any other condition that, in the investigator's judgment, would make the patient unsuitable for participation in the study.