Sinusoidal Obstruction Syndrome After Allogeneic Hematopoietic Cell Transplantation With Post-Transplant Cyclophosphamide.

Sinusoidal obstruction syndrome/veno-occlusive disease (SOS/VOD) is a serious complication following allogeneic hematopoietic cell transplantation (HCT). Although post-transplant cyclophosphamide (PTCy) is increasingly used for graft-versus-host disease prophylaxis, data on its impact in the context of SOS/VOD remain limited. This study aimed to assess the incidence, clinical characteristics, prognostic factors, treatment approaches, and outcomes of SOS/VOD in HCT recipients receiving GVHD prophylaxis with PTCY, sirolimus or tacrolimus, and mycophenolate mofetil (MMF) across all donor types. This single-center observational study included all 532 consecutive adults who underwent HCT with PTCy between January 2017 and February 2024. Patient demographics, transplant procedures, toxicities, and complications were prospectively collected. Clinical charts were reviewed as needed to address inconsistencies or missing information. Myeloablative conditioning was administered to 96% of recipients, who received grafts from matched sibling donors (MSD, 36%), matched unrelated donors (MUD, 34%), haploidentical donors (26%), or mismatched unrelated donors (MMUD, 4%). SOS/VOD was diagnosed in 35 patients and classified according to EBMT criteria as probable (n = 10), clinical (n = 23), or proven (n = 2). Classical SOS/VOD occurred in 21 patients (60%), while 14 (40%) had late-onset disease. EBMT severity grading showed 3% mild, 20% moderate, 37% severe, and 40% very severe cases. The 100-day cumulative incidence was 6.6% (95% CI:4.7 to 8.9). Multivariable analysis identified prior transplantation, prior antibody-drug conjugates and higher CD3+ cell dose as risk factors. Patients with very severe or severe SOS/VOD (based on clinical features but not those upgraded solely due to the presence of risk factors) received defibrotide. Four patients (11.4%) died from SOS/VOD; all classified as very severe, and three of them had undergone prior transplantation (two autologous, one allogeneic). Despite high severity, SOS/VOD analyzed as a time-dependent variable showed no association with overall survival or nonrelapse mortality. SOS/VOD remains a challenging but clinically manageable complication with a modest incidence following HCT with PTCy. Although many cases were classified as severe or very severe, the associated mortality rate was relatively low. The identification of key risk factors, such as prior transplantation, antibody-drug conjugate exposure, and higher CD3⁺ cell doses, along with the notable proportion of late-onset cases, underscores the need for vigilant monitoring and individualized management strategies.