Hematopoietic Stem Cell Transplantation; Infertility, Male/etiology; Neoplasms/complications; Spermatogenesis; Survivors; Testis/pathology; Adolescent; Allografts; Antineoplastic Combined Chemotherapy Protocols/administration & dosage; Antineoplastic Combined Chemotherapy Protocols/adverse effects; Busulfan/administration & dosage; Busulfan/adverse effects; Child; Child, Preschool; Combined Modality Therapy; Cranial Irradiation/adverse effects; Cyclophosphamide/administration & dosage; Cyclophosphamide/adverse effects; Follicle Stimulating Hormone/blood; Hormone Replacement Therapy; Humans; Infant; Infertility, Male/blood; Infertility, Male/pathology; Infertility, Male/prevention & control; Male; Myeloablative Agonists/administration & dosage; Myeloablative Agonists/adverse effects; Neoplasms/drug therapy; Neoplasms/surgery; Organ Size; Postoperative Complications/blood; Postoperative Complications/etiology; Postoperative Complications/pathology; Puberty; Radiation Injuries/blood; Radiation Injuries/etiology; Radiation Injuries/pathology; Semen Analysis; Testis/drug effects; Testis/radiation effects; Testosterone/therapeutic use; Transplantation Conditioning/adverse effects; Whole-Body Irradiation/adverse effects