Background and Methods: Sinusoidal obstruction syndrome/veno-occlusive disease (SOS/VOD) is a severe complication of allogeneic stem cell transplant (allo-SCT). Given the increased use of allo-SCT and variability of SOS/VOD incidence in published reports, cases of allo-SCT from two major transplant centers in Ontario, Canada (2019–2021), were reviewed to identify risk factors prognostic for SOS/VOD onset and to assess outcomes. Results: This study included 536 allo-SCT cases, with a mean age of 53.4 years min–max: 17–76, including 322 male recipients and 214 female recipients. There were 17 SOS/VOD cases diagnosed during the first 100 days, representing 3% of allo-SCT cases, with a median age of 48 years 18–72 and equally distributed between genders. All cases were classical SOS/VOD, with onset prior to day 21 1–20. These cases were graded as one mild, six moderate, six severe, and four very severe cases. The mild case of SOS/VOD recovered after treatment with diuretics. In respect to the 16 cases graded as ≥moderate SOS/VOD, the average inpatient stay was 56 days 24–178, and eight patients were in the ICU for an average of 6 days 0–42, with a median of zero days. Five of the sixteen ≥moderate SOS/VOD patients died within 100 days 9–59—four from SOS/VOD. After day +100, five remained alive, and six died between days 125 and 419. Treatments for ≥moderate SOS/VOD included diuretics n = 15, steroids n = 3, and defibrotide n = 9. The nine patients treated with defibrotide were graded as moderate n = 2, severe n = 4, and very severe n = 3. Three of the nine patients treated with defibrotide died before day 100, and the other six survived beyond day 100. None of the six surviving patients died from SOS/VOD. Univariable regression analysis identified a higher baseline absolute neutrophil count (ANC) of 4.2 × 109/L compared to 2.6 × 109/L p = 0.035 and lower baseline platelet count of 104 × 109/L compared to 140 × 109/L p = 0.034 in SOS/VOD and non-SOS/VOD cases, respectively, as independent risks for ≥moderate SOS/VOD. Treatment with inotuzumab ozogamicin was also identified as a risk factor for ≥moderate SOS/VOD (p = 0.016). The absence of late-onset SOS/VOD in the cohort of 536 patients prompted a retrospective analysis of the data to identify potentially missed cases. Seven cases were identified as meeting the diagnostic criteria for SOS/VOD: four classical and three late-onset. One case would have been graded as severe, and the remaining six would have been graded as very severe. Six patients were reported to have died between days 11 and 107, with four deaths before day 100. The clinical diagnoses of patients meeting diagnostic criteria for SOS/VOD included infection (n = 3), graft-versus-host disease (GVHD) (n = 3), and pulmonary hemorrhage (n = 1). The inclusion of potentially missed cases in the analysis again suggested a lower baseline platelet count (p = 0.002) and prior treatment with inotuzumab ozogamicin (p = 0.003) as potential risk factors for SOS/VOD. The baseline ANC was lower in this combined cohort but did not reach statistical significance (p = 0.089) as it did in the confirmed SOS/VOD cohort (p = 0.035). Additional clinical features that were identified as statistically significant for the onset of SOS/VOD (potential and confirmed cases) included a lower Karnofsky Performance Status (p = 0.01), the presence of pulmonary hypertension (p = 0.012), lower baseline hemoglobin (p = 0.017), and higher baseline serum ferritin (p = 0.01). Conclusions: The incidence of classical SOS/VOD in this cohort was consistent with recent published reports and carried a high fatality rate. A higher ANC, lower platelet count at the start of the preparative regimen, and prior treatment with inotuzumab ozogamicin were identified as potential risk factors for diagnosed SOS/VOD. Hospital and intensive care unit stays were longer in SOS/VOD patients. There were no cases of late-onset VOD diagnosed within the first 100 days of allo-SCT transplant, which is inconsistent with recently reported incidence rates. Potentially missed cases of SOS/VOD were identified, suggesting that this disease may be under-diagnosed and underscoring the need for ongoing education and resources to allow for early intervention.
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