Monosomy 20 as a pointer to dicentric (9;20) in acute lymphoblastic leukemia

Twenty new cases of acute lymphoblastic leukemia (ALL) with the dicentric c hromosome dic(9;20)(p11 similar to 13;q11)are presented. This chromosomal a bnormality is difficult to identify from G-banding alone. It masquerades as monosomy 20 and is only accurately identified by fluorescence in situ hybr idization (FISH). Monosomy 20 was found in 59/2790 patients with successful karyotypes entered to the Leukaemia Research Fund/UK Cancer Cytogenetics G roup Karyotype Database in ALL (LRF/UKCCG Karyotype Database). FISH reveale d dic(9;20) in 20/25 cases with available material. Extra copies of chromos ome 21 were found in 8 of the 20 cases. Patients were 14 females and six ma les, aged 1-32 years (median 4 years), with leukocyte counts of 2-536 (medi an 23) x 10(9)/l and immunophenotypes of common or pre-B ALL (17 cases), T- ALL (one case) or unknown (two cases). Four patients relapsed at 2, 22, 28 and 47 months and two died at 49 and 63 months (median follow-up 37 months) . FISH studies on the remaining five patients showed one with monosomy 20 a nd four with other rearrangements of the chromosome. This study has increas ed the number of reported cases of dic(9;20) from 17 to 37. It has identifi ed dic(9;20) in one case of T-ALL and shows an association of this transloc ation with trisomy 21.