High-dose Chemotherapy and Stem Cell Transplantation, in Patients PET-2 Positive, After 2 Courses of ABVD and Comparison of RT Versus no RT in PET-2 Negative Patients

The purpose of this study is to define an improvement in patients: * To evaluate if patients resistant to the initial treatment for residual PET-positive masses after the first two courses of ABVD (PET-2 positive), can be salvaged by early shift to high-dose chemotherapy supported by stem cell rescue * To analyse if patients achieving early complete response (PET-2 negative), can be spared the adjuvant radiotherapy on areas of initial bulky disease, at the end of the planned six courses of ABVD. To answer this question, PET-2 negative patients will be randomized between radiotherapy versus no radiotherapy at the end of ABVD therapy.