??? About Rare type Large B Cell NHL...

Brother (late 30') has diagnosis of Large B Cell NHL, however, oncologist says he doesn't need rituxin as his CD20 test came back negative....given rituxin is fairly standard, we are concerned, but doc says he doesn't need it...however, he does have rare type where the cancer cells are dervied from a "plasma cystic differentiation" or its derived from the plasma cells. It's hard to find much intelligence on this in various web sites. His treatment will be with CHOP every 3 weeks as well as Methatrexite and Ara C. DOES ANYONE HAVE ANY INFORMATION/EXPERIENCE WITH THIS...my family is desperate to find out more if we can.