The risk of relapse of acute myeloid leukemia (AML) was shown to vary according to groups defined by the presence of various cytogenetic and molecular markers [1]. Leukemia relapse after allogeneic hematopoietic stem cell transplantation (allo-HSCT) usually occurs in the bone marrow (BM), but extramedullary relapse (EMR) also accounts for a significant proportion. Isolated EMR (iEMR) while morphologic remission in the marrow are still retained is relatively rare. The clinical significance and the most effective treatment of iEMR of AML after allo-HSCT compared with BM relapse is still unclear, particularly in children [2]. Leukemia is a group of clonal stem cell diseases characterized by the clonal proliferation of malignant stem cells. During or after chemotherapy, an uncommon event termed lineage switch is observed, but it is a rare event. The rate of conversion from acute lymphoblastic leukemia (ALL) to AML has been found to be 6-9%, but conversion from AML to ALL is extremely rare in children [3,4].

Herein, we report an unusual case of a 10-year–old male patient with AML who first relapsed as isolated mediastinal extramedullary granulocytic sarcoma after allo-HSCT and then subsequently experienced BM relapse with the cell lineage switched from AML to T-cell ALL.

A 10-year-old boy presented with prolonged high fever and extensive petechiae on both lower extremities. The patient appeared acutely illed, and he had slightly pale conjunctivae and palpable liver (3 cm). A blood cell count revealed a hemoglobin 7.8 g/dL, the white blood cell 74.2×10⁹/L and platelet 70×10⁹/L. Large myeloblasts were present at up to 80% in the differential count of peripheral blood cells. BM showed a cellularity of nearly 90% and a diffuse infiltration of myeloblasts (Fig. 1A). The blasts accounted for up to 83% of the BM nucleated cells and some blasts had an Auer rod (Fig. 1B). The flow cytometry showed the leukemic cells expressed MPO, CD33, CD13, CD117, CD34, and ectopic CD7. Conven-tional cytogenetics revealed complex numeric and structural abnormalities such as 48, XY, del(6)(q13q21),add(7) (q36),+8,+16,del(16)(q22). Recurrent genetic abnormalities including FLT3-ITD, C-kit, NPM1, and CEBPA mutation were not detected. The diagnosis was AML without matu-ration. After induction chemotherapy with cytosine arabinoside (Ara-C, 200 mg/m² for 7 days) and idarubicin (12 mg/m² for 3 days), the patient achieved complete remission (CR). Subsequently he received 2 courses of consolidation chemotherapies including high dose Ara-C (2.0 g/m² for 4 days). After the consolidation chemotherapy, the patient underwent haplo-identical stem cell transplantation (SCT) from his father. The conditioning regimen used was busulfan (4.0 mg/kg/d for 2 days), fludarabine (30 mg/m²/d for 6 days) and thymoglobulin (3.0 mg/kg/d for 3 days). The patient was treated with tacrolimus and mini dose methotrexate (10 mg/m²/d) for prevention of graft-versus-host disease (GVHD). After haplo-identical SCT, there were not acute or chronic GVHD or any infectious complications and his leukemia showed still in remission.

Figure 1. Initial bone marrow aspiration demonstrated many blasts showed large and relatively abundant cytoplasm (A, original magnification ×400, Wright-Giemsa stain) and some blasts revealed the presence of Auer rod (arrow) (B, original magnification ×400, Wright-Giemsa stain). The blasts at relapsed bone marrow aspiration showed small to medium sized and high nuclear-cytoplasm ratio compared to those of initial diagnosis (C, original magnification ×400, Wright-Giemsa stain).

Thirty eight months after haplo-identical SCT, he presented asymptomatic anterior mediastinal widening on routine follow-up chest X-ray (Fig. 2A). At that time, a blood count showed a hemoglobin 11.9 g/dL, a white cell 4.23×10⁹/L, and a platelet 267×10⁹/L. BM showed normocellular marrow without residual leukemic cells. Chimerism status using short tandem repeat analysis showed complete donor chimerism (97.5% of donor cells), suggesting complete remission, which was supported by the result of normal male karyotype (46, XY). Cerebrospinal fluid analysis was normal, showing white cell count of 1/mL with absence of blast. Chest CT showed huge anterior mediastinal mass (8.1×5.6 cm) (Fig. 2B). Ultrasound-guided core needle biopsy of anterior mediastinal mass was performed. The biopsy showed diffuse infiltration of myeloblasts, positive for LCA, CD34, CD117 (Fig. 2C), and negative for MPO, CD20, CD7 suggesting mediastinal iEMR. After re-induction chemotherapy with FLAG regimen (fludarabine 30 mg/m²/d and ara-C 2.0 g/m² both for 5 days), the anterior mediastinal mass showed only partial response. He received additional local radiation therapy of total dose of 24 Gy in 12 fractions. After radiotherapy, anterior mediastinal mass was gradually regressed. At that time, the patient presented with pale face and multiple petechiae on lower extremities. A blood count showed a hemoglobin 7.5 g/dL, a white cell 2.65×10⁹/L, and a platelet 38×10⁹/L. BM displayed elevation of lymphoid-appearing blasts (28.3% of all nucleated cells) (Fig. 1C). At BM relapse, the blasts showed negative for MPO on cytochemical stain and flow cytometry. We suspected a leukemic relapse with phenotypic change. Further examination by flow cytometry proved the blasts showed cytoplasmic CD3, CD5, CD7, and CD34. Conventional cytogenetics at relapse revealed same complex numeric and structural abnormalities seen at initial diagnosis of AML. Consequently, we made a diagnosis of leukemic relapse with lineage switched to T-cell ALL from initial AML. As re-induction chemotherapy, we used vincristine, prednisolone, L-asparaginase and idarubicin. Six weeks later after BM relapse, the leukemic blasts were increased to 70.4% of total nucleated cells with 30% of cellularity. Right now, the patient is on salvage chemotherapy with nelarabine, etoposide and cyclophosphamide for refractory T-cell ALL.

Figure 2. Anterior mediastinal widening on routine follow-up chest X-ray after 38 months of allo-HSCT for AML (A). Contrast-enhanced computed tomography showed large enhancing homogeneous mass (8.1×5.6 cm) involving anterior mediastinum, pericardium, and mediastinal pleura (B). Mediastinal mass biopsy showed the leukemic cells of mediastinal mass revealed the CD117 positivity (C, original magnification ×200, CD117 immunohistochemical stain).

Patients with relapsed AML may experience either BM relapse (BMR) or EMR, but there is a paucity of clinical data on EMR after allo-HSCT compared with BMR [2,5,6]. Isolated EMR after allo-HSCT in children is rare and clinical significance is not well characterized until now [7,8].

The 5-year cumulative incidence of isolated EMR after allo-HSCT in large pediatric population was reported 5.5%, similar to adult patients (3.1-14.1%) [2,5-7]. In studies evaluating long term follow up after allo-HSCT, EMR occurred later than BMR [2,7].

EMR occurs in diverse sites including the central nervous system (CNS), testis, bone, skin, neck, body cavity, sacral area, limbs, nasopharynx, breast and soft tissue. The CNS seems to be the most common extramedullary involvement site, but anterior mediastinum including thymus is a relatively rare site of EMR [6,7].

The mechanisms of iEMR after allo-HSCT remains unclear, but less graft versus leukemia (GVL) effect is one of the mechanisms responsible for the increased frequency and widespread distribution of EMR after allo-HSCT [2,7]. It has been thought that the GVL effect would protect patients from BMR and EMR. However the GVL process is less effective in preventing EMR than BMR [8-10]. Several risk factors have been identified as being associated with EMR after allo-HSCT, including previous chronic GVHD, donor lymphocyte infusion, younger age, extramedullary lesions before HSCT, advanced disease at HSCT [2,5,7]. Recently, the reduced intensity conditioning seems to play a role in the onset of EMR after allo-HSCT [6].

There is no established standard care for EMR after allo-HSCT. Although iEMR almost always results in systemic relapse which occurs within 1 to 12 months of the emergence of iEMR in patients who do not undergo transplantation, the impact of a secondary allo-HSCT on survival outcome for iEMR after 1st allo-HSCT remains unclear [6,7,9,10]. Another therapeutic option is the application of gemtuzumab ozogamycin which is an anti-CD33 monoclonal antibody conjugated to the antitumor antibiotic calicheamicin [11]. Recently, gemtuzumab ozogamycin has been reported to present excellent efficacy in salvage therapy of multiple relapse in EM. Our patient received systemic chemotherapy with FLAG regimen, but only partial response was observed. After we added local radiotherapy (24 Gy), the iEMR of the patient was completely regressed.

The prognosis of patients who developed iEMR after allo-HSCT remained poor. Although some studies reported that iEMR had a better prognosis than systemic relapse (3-year overall survival, 30.1% vs. 8.5%, P=0.002), others found a similar overall survival in these [2,7,10].

The pathogenesis for lineage switch in acute leukemia is currently unknown. But several hypotheses have been proposed [3,4,12]. Firstly, it is associated with clonal selection. Two or more different clones may exist when leukemia occurs, and the dominant leukemia clone at initial diagnosis might be eradicated by chemotherapy, the other subclone appears with a dominant phenotype. Secondary, a leukemia multipotent progenitor cell is capable of differentiating into more than one lineage. Thirdly, any exogenous factor such as chemotherapy could induce endogenous changes such as a new leukemogenic event. In our patient, the flow cytometric analysis showed that the leukemic cells expressed MPO, CD33, CD13, CD117, and also aberrant CD7 (87.9%) at initial diagnosis of AML. The domonant myeloid clone was eradicated by chemotherapy of AML. After several years, leukemic relapse was occurred. A subclone that expressed aberrant CD7 at initial diagnosis became a dominant clone at relapse. These findings suggest that clonal selection was the cause of lineage switch in this patient.

Lineage switch often occurs in leukemia relapse stage, and easily confused with therapy-related lymphoblastic leukemia [12,13]. Therapy-related ALL is one of the most serious long term complications for patients with prior cytotoxic chemotherapy. The majority of lineage switch retained the original genetic feature when leukemia is recurred, but therapy related ALL often acquires new cytogenetic abnormalities [12-14]. We performed karyotype analysis for our patient at initial diagnosis and also at BM relapse and did not find any cytogenetic alteration.

In our patient, unusual lineage switch subsequently developed just after combined chemotherapy and radiotherapy iEMR in AML. To our knowledge, the relationship between iEMR after allo-HSCT in AML and lineage switch on subsequent BM relapse has not been reported in literature. At this time, it is unclear whether this phenomenon represents a possible association or a mere coincidence between the two.

Because of the rarity of this lineage switch of AML to T-ALL, the prognosis of this phenomenon is still variable, but lineage switch in acute leukemia often lead to poor prognosis and resistance to therapy [3,4,15].

In conclusion, although iEMR or lineage switch of leukemia is relatively a rare event in children, we should always be careful of the possibility of iEMR or a lineage switch during the process of chemotherapy and allo-HSCT of leukemia in children.

This study was supported by a 2-Year Research Grant of Pusan National University.

The authors have no conflict of interest to declare.