Long-term outcome of STS in HS. (A) Hematologic response after STS: a significant increase in the Hb level (97 ± 1.9 grams per liter [g/L] SEM to 120 ± 1.8 g/L SEM; P < .0001) and a decrease in the reticulocyte count (392 ± 18 giga per liter [G/L] SEM to 291 ± 20 G/L SEM; P < .001) were observed 1 year after STS. These results were stable over time with a mean Hb level being even higher after 10 years (n = 23, P = .04). (B) Evolution of spleen volume after STS assessed by scintigraphy and/or ultrasonography providing 3-dimensional measurement. The remnant was considered as an ellipsoid using the following formula (length × width × thickness × 0.523). Measures were reported to the expected splenic volume for the weight.^18-21  Following an 88% reduction after STS (513 ± 60% SEM to 64 ± 5% SEM [n = 40], ****P < .0001), the remnant grew significantly at 1 year (180 ± 12% SEM [n = 50], ****P < .0001), continued to grow at a reduced rate for 5 years after STS (274 ± 29% SEM [n = 21], ***P < .001), and then remained stable (at 10 years: 285 ± 34% SEM [n = 29], not significant [NS]). (C) Spleen volume at 1 year following STS showing significantly faster splenic growth in patients with hematologic relapse (Hb level under 95 g/L) compared with patients with a sustainable hematologic response: median spleen volume at 1 year post-STS: 258 ± 33% SEM (n = 7) vs 165 ± 12% SEM (n = 43), **P = .008. (D) Time of TS according to clinical indication. TS was required earlier when the indication was a hematologic relapse (transfusion dependency and symptomatic anemia/Hb <95 g/L) than any other indications: mean delay 5.3 ± 1 year SEM vs 11.3 ± 1.6 years SEM, respectively (P = .02).