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Primary sinonasal diffuse large B-cell lymphoma (PSDLBCL) is a rare aggressive lymphoma. Recently, genetic classification using Next Generation Sequencing (NGS) demonstrated that PSDLBCL largely consists of the MCD genotype, which has a poor prognosis mainly driven by MYD88L265P and CD79Bgene abnormalities. This study investigated the prevalence and clinicopathological significance of MYD88L265P and CD79BY196 mutations using droplet digital PCR in 55 patients with PSDLBCL, as well as the translocation of BCL2/BCL6/c-Mycwith FISH. We found mutations in MYD88L265P (29/55, 52.7%) and CD79BY196 (20/55, 36.4%). The MCD-like subtype, defined by the mutation of MYD88and/or CD79B, was found in 32 out of 55 cases (58.2%). This subtype largely consists of non-GCB type (31/32, 96.9%; P<0.01) and double-expressor cases (20/32, 62.5%; P=0.01) compared with the MYD88/CD79Bco-wild type, with BCL6translocation in a small subset (2/32, 6.3%) and no translocations of BCL2(0/32) or c-Myc(0/32). The MCD-like subtype tended to relapse in specific sites such as the central nervous system, testis, and/or skin compared with the co-wild type (P=0.03), showing poorer outcomes in overall survival (P=0.02) and progression-free survival (P=0.01). In conclusion, our study highlights a high prevalence of MYD88and CD79Bmutations in PSDLBCL, identifying an aggressive MCD-like subtype with a distinct relapse pattern. This molecular subclassification can be helpful for both prognostic prediction and therapeutic strategy in patients with PSDLBCL.