Donor-derived del[20q] following allogeneic-hematopoietic cell transplantation: a case with 26-year follow-up and literature review.

Donor-derived cytogenetic abnormalities are a rare finding following allogeneic hematopoietic cell transplantation. Deletion of the long arm of chromosome 20 [del(20q)] is one of the more frequently observed structural abnormalities, but its significance in the post-transplant setting remains unclear. We describe a unique case of donor-derived del(20q) with 26 years of post-transplant follow-up, the longest reported to date. The recipient remains well with normal blood counts despite persistent del(20q) in both myeloid and lymphoid lineages and the presence of coexisting somatic mutations in DNMT3A and TP53. Retrospective analysis of the donor's marrow confirmed del(20q) and low-level DNMT3A and TP53 mutations at the time of transplant; the donor later developed therapy-related MDS after radiation therapy for thyroid cancer. To contextualize this case, we reviewed 20 published reports of donor-derived del(20q) post-transplant. The median time to detection was 16 months post-transplant, and 35% of cases progressed to donor-derived malignancy. Among those who progressed, the median time to malignancy diagnosis was 22 months post-transplant. Clinical outcomes across cases ranged from asymptomatic persistence and cytopenias to donor-derived myeloid malignancies, highlighting the need for long-term follow-up and potential use of molecular profiling to better define the neoplastic potential of donor-derived del(20q) after transplantation.