IV PCA With or Without Continuous Dose vs Oral Opioid to Maintain Analgesia for Severe Cancer Pain After Successful Titration

Inclusion Criteria 1. Age 18 to 80 years; 2. Histologically or cytologically confirmed malignant solid tumor; 3. Persistent severe cancer-related pain (≥7 at rest on the 11-point numeric rating scale \[NRS\], where 0=no pain and 10=excruciating pain) in the 24 hours before screening; 4. No radiotherapy to the painful area prior to randomization; 5. No radiotherapy, chemotherapy, hormone therapy, targeted therapy, or bisphosphonate therapy within 7 days before randomization; 6. Successful IPCA-HM titration within the past 24 hours; 7. No history of psychiatric disorders; 8. Ability to complete questionnaires; 9. Ability to correctly understand and follow medication guidance from doctors and nurses; 10. ECOG performance status ≤ 3; 11. Provided written informed consent. Exclusion Criteria 1\) Patients with non-cancer-related pain; 2) Patients with paralytic ileus; 3) Patients with brain metastases; 4) Patients with hypersensitivity to morphine or hydromorphone; 5) Abnormal laboratory results: creatinine ≥ 2-fold of upper limit of normal (ULN) value, Alanine Aminotransferase (ALT) or Aspartate Aminotransferase (AST) ≥ 2.5-fold of the ULN value (≥ 5-fold for subjects with liver metastasis or primary liver cancer), or Child-Pugh class C liver function; 6) Patients unable to take oral medication; 7) Patients with uncontrolled nausea or vomiting; 8) Prior use of hydromorphone, morphine, or PCA devices within 14 days before screening; 9) Use of monoamine oxidase inhibitor drugs (MAOID) within the two weeks before randomization; 10) Women who are pregnant, lactating, or planning to be pregnant within one month after the trial completion; 11) Patients who abuse alcohol; 12) Patients with any other medical condition or reason, in the investigator's judgment, that would make them unsuitable to participate in the clinical trial.