OBJECTIVES: I. Estimate the overall survival, progression-free interval, and time to progression or recurrence in patients with nondisseminated glioblastoma multiforme or diffuse intrinsic brain stem tumors that are nonprogressive following surgery (if feasible) and involved-field irradiation and treated with intensive chemotherapy followed by autologous peripheral blood stem cell (PBSC) or autologous bone marrow (ABM) rescue. II. Estimate the toxicity of myeloablative chemotherapy with thiotepa (TSPA) followed by carboplatin (CBDCA) in these patients. III. Evaluate the pharmacokinetic interactions of high-dose CBDCA, TSPA, and triethylenephosphoramide (a metabolite of TSPA) and any impact on subsequent toxicity. IV. Evaluate the effectiveness of autologous stem cells in restoring hematopoiesis following myeloablative therapy.