Impact of CAR T cell therapy on thymus size in children and young adults with acute lymphoblastic leukemia

Chimeric Antigen Receptor (CAR) T-cell therapy has demonstrated efficacy in children and young adult patients with acute lymphoblastic leukemia (ALL). The purpose of our study was to investigate thymus size changes after CAR T-cell therapy, explore the associated clinical conditions, and assess survival differences of patients who underwent CAR T-cell therapy, we conducted a single-center retrospective study of children and young adult patients who underwent CAR T-cell therapy for ALL between April 2015 and October 2023.We measured the volume of the thymus on pre- and post-CAR T-cell chest CT scans of 20 patients (median [IQR] age, 18[11] years; 11 females). We divided patients into two groups, those who did (group 1) or did not (group 2) demonstrate increase in thymus size after therapy. Clinical and survival data were collected. We used the Wilcoxon signed-rank test or Fisher's exact test for group comparisons and analyzed event-free survival data. Seven of 20 patients (35%, group 1) showed increase in thymus volume (pre- vs. post-CAR T-cell thymus volume; 5.01 [2.18] cm³ vs. 20.87 [19.86] cm³, p = 0.01), while 13 patients (65%, group 2) showed no increase in thymus volume (pre- vs. post-CAR T-cell thymus volume; 3.01 [13.42] cm³ vs. 2.09 [8.34] cm³, p = 0.01). Patients in group 1 were younger (12 [8] years vs. 19[10] years, p = 0.028) and showed a higher rate of event-free survival compared to those in group 2 (p = 0.003). In children and young adults with ALL, increased thymus size after CAR T-cell therapy was associated with younger age and improved clinical outcomes.