Inclusion Criteria:
1. Children aged 6 months to 18 years at the time of randomization;
2. Diagnosed of solid tumor by cytological or histological examination;
3. Going to initiate MEC/HEC chemotherapy;
4. PS score ≤ 2 points;
5. predicted life expectancy ≥3 months and weight greater than 6Kg;
6. Patient's parent or guardian signs informed consent
Exclusion Criteria:
1. Has vomited in the 24 hours prior to chemotherapy initiation on Treatment Day 1 ;
2. Has a symptomatic primary or metastatic central nervous system (CNS) malignancy with nausea and/or vomiting (asymptomatic participants may participate in study) ;
3. Will be receiving stem cell rescue therapy within 14 days following administration of ondansetron ;
4. Has experienced High emetic chemotherapy within two weeks ;
5. Has received or will receive total body irradiation to the abdomen or pelvis in the week prior to Treatment Day 1 and/or during the diary reporting period (120 hours following initiation of chemotherapy) ;
6. Has had benzodiazepine, opioid or opioid like therapy initiated within 48 hours prior to study drug administration, or is expected to receive within 120 hours following initiation of chemotherapy except for single doses of midazolam, temazepam or triazolam ;
7. Has started on systemic corticosteroid therapy within 72 hours prior to study drug administration or is expected to receive a corticosteroid as part of the chemotherapy regimen ;
8. Allergic to Ondansetron and dexamethasone ;
9. Has an active infection (e.g., pneumonia), congestive heart failure, bradyarrhythmia, any uncontrolled disease (e.g., diabetic ketoacidosis, gastrointestinal obstruction) except for malignancy ;
10. Is mentally incapacitated or has a significant emotional or psychiatric disorder ;
11. Has a known history of QT prolongation or is taking any medication that is known to lead to QT prolongation ;
12. Abnormal liver function (alanine aminotransferase or aspartate aminotransferase ≥ 2 times higher than the upper bound of the normal value) or abnormal renal function (serum creatinine ≥ 2.5 times higher than the upper bound of the normal value) ;
13. Is currently taking, or has taken within 48 hours of Treatment Day 1 the following drugs with antiemetic properties: 5-hydroxytryptamine 3 (5-HT3) antagonists (e.g., ondansetron), benzamides (e.g., haloperidol), cyclizine, domperidone, herbal therapies with potential antiemetic properties, olanzapine, phenothiazines (e.g., prochlorperazine), scopolamine (this is not an exhaustive list) ;
14. Has ever participated in a previous study of ondansetron or has taken an investigational drug with the last 4 weeks ;
15. other situations in which the researchers believe that they cannot be included in the group.
