Randomized Study Comparing 4 and 6 Cycles of Chemotherapy With CHOP (Cyclophosphamide, Doxorubicin, Vincristine and Prednisone) at 21-day Intervals, Both With 6 Cycles of Immunotherapy With the Monoclonal Anti-CD20-Positive B-Cell Lymphoma Aged 18-60 Years Having no Risk Factor (Age-Adjusted IPI=0) and No Large Tumor Mass (Diameter <7,5cm) [FLYER 6-6-6-4 Study]