Radiographically measurable or evaluable disease per RECIST v1.1. INCLUSION CRITERIA FOR SECOND-LINE THERAPY: Radiographically measurable disease INCLUSION CRITERIA FOR THIRD-LINE THERAPY: Radiographically measurable disease Pediatric patients with radiographically recurrent or radiographically progressive non-hematologic malignancies (central nervous system [CNS] or solid tumors) associated with activation of the RAS/RAF/MEK/ERK pathway will be eligible PHASE II: Patients with measurable radiographically recurrent or radiographically progressive disease that is measurable in at least two dimensions on imaging after standard up-front therapy as defined in the following three strata below will be eligible\r\n* Stratum 1: patients with radiographically recurrent or radiographically progressive low-grade gliomas with a BRAF KIAA1549 (or similar) truncated fusion duplication not previously treated with a BRAF or MEK inhibitor\r\n* Stratum 2: patients with NF1 and radiographically recurrent or radiographically progressive LGG (NF1 may be defined clinically or genetically) not previously treated with a BRAF or MEK inhibitor\r\n* Stratum 3: patients with radiographically recurrent or radiographically progressive tumors thought to involve the RAS/RAF/MEK/ERK pathway but not included in Stratum 1 or 2; this includes any radiographically recurrent or radiographically progressive LGG not included in Stratum 1 or 2 (i.e., any LGG without a BRAF truncated fusion in a patient without NF1), any CNS tumor other than LGG in a patient with NF1, and any other CNS or solid tumor (regardless of grade) with a documented activating BRAF, NRAS, or KRAS mutation Patients with clinical progression but without radiographically recurrent or radiographically progressive disease Part 2 only: Presence of radiographically measurable disease (defined as the presence of ?1 lesion that measures ?10 mm [?15 mm for lymph nodes]). Measurable disease that was previously radiated is only permitted if progressing. Radiographically measurable disease by RECIST v1.1 Patients must have radiographically measurable disease Patients must have radiographically measurable disease Patients with PTCL should have radiographically measurable disease >= 1.5 cm. Radiographically measurable lymphadenopathy or extranodal lymphoid malignancy. Documented disease that is radiographically measurable Presence of radiographically evaluable disease Radiographically measurable disease per mRECIST 1.1 All patients must have radiographically assessable disease Metastatic disease radiographically documented by CT or bone scan Radiographically measurable or evaluable disease per RECIST v1.1. Patients must have measurable disease within the orbit, either clinically and/or radiographically after biopsy confirmation of B cell lymphoma Radiographically measurable disease per RECIST 1.1 Radiographically measurable per RECIST v1.1. Radiographically measurable disease in the CNS documented ? 3 weeks prior to starting E6201 treatment Patients with M0 classic, non-desmoplastic medulloblastoma (R1) with radiographically measurable residual disease < 1.5 cm^2 are eligible Patients may have additional measurable and/or non-measurable but radiographically visible metastatic lesions (e.g. bone metastases) Radiographically documented measurable disease at study entry (as defined by the RECIST v1.1 criteria). Presence of radiographically measurable lymphadenopathy or extranodal lymphoid malignancy At least one radiographically measurable lesion Presence of radiographically measurable disease (defined as the presence of ? 1 lesion that measures ? 10 mm [? 15 mm for lymph nodes] At least one radiographically measurable lesion per RECIST 1.1 Subjects must have at least 1 extracranial, unresectable, non-bony lesion that is measurable radiographically (based on RECIST 1.1) Presence of radiographically measurable disease as defined by RECIST 1.1 Patients must have radiographically measurable disease Radiographically measurable disease acc. to RECIST 1.1 Radiographically-measurable disease based on RECIST 1.1 All patients must have radiographically assessable disease Subjects must have clinically or radiographically evident measurable disease at nodal stations Patients must have radiographically or clinically evaluable tumor All subjects must have radiographically assessable disease per RECIST v1.1 obtained by imaging within 28 days prior to registration. Histologically proven pancreatic adenocarcinoma with radiographically measurable OR evaluable metastatic disease Radiographically measurable or evaluable disease The participant has a radiographically measurable tumor. Evaluable disease is acceptable for Part 1 only Radiographically measurable or evaluable disease Radiographically measurable or evaluable disease (per RECIST v1.1) Radiographically measurable or evaluable disease Radiographically measurable or evaluable disease. Presence of radiographically measurable lymphadenopathy or extranodal lymphoid malignancy Disease progression either clinically or radiographically after 1-2 previous regimens. Radiographically measurable disease (>= 1 focus of lymphoma measuring >= 1.5 cm) Radiographically or clinically evaluable tumor. In the expansion phase, disease must be measurable according to RECIST 1.1. Radiographically measurable lymphadenopathy or extranodal lymphoid malignancy (defined as the presence of 1 or more lesions that measure at least 2.0 cm in the longest dimension (as assessed radiographically) Subjects must have had radiographically evaluable or measurable disease with standard magnetic resonance (MR) imaging To be enrolled in the dose escalation phase of the study, participants must have a radiographically or clinically evaluable tumor, but measurable disease as defined by RECIST version 1.1 [1] is not required for participation in this study.