This clinical trial is focused on studying a type of cancer called Classical Hodgkin Lymphoma in children and young adults. The trial is testing a treatment that includes a medication called pembrolizumab, also known by its code name MK-3475. Pembrolizumab is used in combination with chemotherapy for patients who have not responded well to initial treatments. The chemotherapy drugs involved in this study include doxorubicin, vinblastine, dacarbazine, cyclophosphamide, vincristine, prednisone, and prednisolone. These medications are administered through an intravenous infusion, which means they are given directly into a vein.

The purpose of the study is to evaluate how well the combination of pembrolizumab and chemotherapy works in treating patients with slow early response to initial treatment. Participants will receive the study treatment over a period of time, and their response to the treatment will be monitored. The study will also compare the effectiveness of different chemotherapy combinations, such as AVD (doxorubicin, vinblastine, and dacarbazine) and COPDAC-28 (cyclophosphamide, vincristine, prednisone, and dacarbazine), in achieving a negative result on a PET scan, which is a type of imaging test that helps doctors see how the cancer is responding to treatment.

Throughout the study, researchers will track various outcomes, including the rate of response to treatment, survival rates, and any side effects experienced by participants. The study aims to provide valuable information on the best treatment options for children and young adults with Classical Hodgkin Lymphoma who do not respond well to initial chemotherapy. The trial is expected to continue until 2028, allowing researchers to gather comprehensive data on the long-term effects and benefits of the treatment.