Myeloablative Vs. Non-Myeloablative Consolidation for Primary Central Nervous System Lymphoma: Results of Alliance 51101.

While it is evident that standard dose whole brain radiotherapy as consolidation is associated with significant neurotoxicity, the optimal consolidative strategy for primary central nervous system lymphoma (PCNSL) is not defined. We performed a randomized phase 2 clinical trial via the U.S. Alliance cancer cooperative group to compare myeloablative consolidation supported by autologous stem cell transplantation with non-myeloablative consolidation after induction therapy for PCNSL. This is the first randomized trial to be initiated that eliminates whole brain radiotherapy as a consolidative approach in newly-diagnosed PCNSL. Patients, age 18-75 years, were randomly assigned in a 1:1 manner to induction therapy (methotrexate, temozolomide, rituximab and cytarabine) followed by consolidation with either thiotepa plus carmustine and autologous stem cell rescue versus induction followed by non-myeloablative, infusional etoposide plus cytarabine (EA) The primary endpoint was progression-free survival (PFS). 113 patients were randomized and 108 (54 in each arm) were evaluable. More patients in the non-myeloablative arm experienced progressive disease or death during induction (28% versus 11%, p = 0.05). Thirty-six patients received autologous stem cell transplant and 34 received non-myeloablative consolidation. The estimated 2-year PFS was higher in the myeloablative versus non-myeloablative arm (73% versus 51%; p= 0.02). However, a planned secondary analysis, landmarked at start of consolidation, revealed that the estimated 2-year PFS in those who completed consolidation therapy was not significantly different between the arms (86% versus 71%; p = 0.21). Both consolidative strategies yielded encouraging efficacy and similar toxicity profiles. Clinicaltrials.gov (NCT01511562).

Predictors of clinical outcome in myeloproliferative neoplasm, unclassifiable: A Bone Marrow Pathology Group study.

OBJECTIVES: Myeloproliferative neoplasm, unclassifiable (MPN-U, revised to MPN, not otherwise specified in the fifth edition of the World Health Organization classification) is a heterogeneous category of primary marrow disorders with clinical, morphologic, and/or molecular features that preclude classification as a more specific MPN subtype due to stage at diagnosis, overlapping features between MPN subtypes, or the presence of coexisting disorders. Compared with other MPN subtypes, the contribution of the mutational landscape in MPN-U in conjunction with other clinical and morphologic biomarkers to prognosis has been less well investigated. METHODS: We performed a multicenter, retrospective study of MPN-U (94 cases) to better define the clinicopathologic features, genetic landscape, and clinical outcomes, including subgroups of early-stage, advanced-stage, and coexisting disorders. The Dynamic International Prognostic Scoring System (DIPSS) plus scoring system was applied to assess its relevance to MPN-U prognosis. RESULTS: Multivariate analysis demonstrated bone marrow blast count and DIPSS plus score as statistically significant in predicting overall survival. Univariate analysis identified additional potential poor prognostic markers, including abnormal karyotype and absence of JAK2 mutation. Secondary mutations were frequent in the subset analyzed by next-generation sequencing (26/37 cases, 70.3%) with a borderline association between high molecular risk mutations and overall survival. CONCLUSIONS: This study, as one of the largest of MPN-U studies incorporating both clinicopathologic and molecular data, moves toward identification of biomarkers that better predict prognosis in this heterogeneous category.

Aggressive B-cell non-Hodgkin lymphomas: a report of the lymphoma workshop of the 20th meeting of the European Association for Haematopathology.

Aggressive B-cell non-Hodgkin lymphomas are a heterogeneous group of diseases and our concepts are evolving as we learn more about their clinical, pathologic, molecular genetic features. Session IV of the 2020 EAHP Workshop covered aggressive, predominantly high-grade B-cell lymphomas, many that were difficult to classify. In this manuscript, we summarize the features of the submitted cases and highlight differential diagnostic difficulties. We specifically review issues related to high-grade B-cell lymphomas (HGBCLs) with MYC and BCL2 and/or BCL6 rearrangements including TdT expression in these cases, HGBCL, not otherwise specified, large B-cell lymphomas with IRF4 rearrangement, high-grade/large B-cell lymphomas with 11q aberration, Burkitt lymphoma, and pleomorphic mantle cell lymphoma. Since the workshop, the 5th edition of the WHO Classification for Haematolymphoid Tumours (WHO-HAEM5) and International Consensus Classification (ICC) 2022 were published. We endeavor to use the updated terminology.

Pathologist-trained machine learning classifiers developed to quantitate celiac disease features differentiate endoscopic biopsies according to modified marsh score and dietary intervention response.

BACKGROUND: Histologic evaluation of the mucosal changes associated with celiac disease is important for establishing an accurate diagnosis and monitoring the impact of investigational therapies. While the Marsh-Oberhuber classification has been used to categorize the histologic findings into discrete stages (i.e., Type 0-3c), significant variability has been documented between observers using this ordinal scoring system. Therefore, we evaluated whether pathologist-trained machine learning classifiers can be developed to objectively quantitate the pathological changes of villus blunting, intraepithelial lymphocytosis, and crypt hyperplasia in small intestine endoscopic biopsies. METHODS: A convolutional neural network (CNN) was trained and combined with a secondary algorithm to quantitate intraepithelial lymphocytes (IEL) with 5 classes on CD3 immunohistochemistry whole slide images (WSI) and used to correlate feature outputs with ground truth modified Marsh scores in a total of 116 small intestine biopsies. RESULTS: Across all samples, median %CD3 counts (positive cells/enterocytes) from villous epithelium (VE) increased with higher Marsh scores (Type 0%CD3 VE = 13.4; Type 1-3%CD3 VE = 41.9, p < 0.0001). Indicators of villus blunting and crypt hyperplasia were also observed (Type 0-2 villous epithelium/lamina propria area ratio = 0.81; Type 3a-3c villous epithelium/lamina propria area ratio = 0.29, p < 0.0001), and Type 0-1 crypt/villous epithelial area ratio = 0.59; Type 2-3 crypt/villous epithelial area ratio = 1.64, p < 0.0001). Using these individual features, a combined feature machine learning score (MLS) was created to evaluate a set of 28 matched pre- and post-intervention biopsies captured before and after dietary gluten restriction. The disposition of the continuous MLS paired biopsy result aligned with the Marsh score in 96.4% (27/28) of the cohort. CONCLUSIONS: Machine learning classifiers can be developed to objectively quantify histologic features and capture additional data not achievable with manual scoring. Such approaches should be further investigated to improve biopsy evaluation, especially for clinical trials.

Tumor-Infiltrating Normal B Cells Revealed by Immunoglobulin Repertoire Clonotype Analysis Are Highly Prognostic and Crucial for Antitumor Immune Responses in DLBCL.

PURPOSE: Tumor-infiltrating B lymphocytes (TIL-B) have demonstrated prognostic and predictive significance in solid cancers. In this study, we aimed to distinguish TIL-Bs from malignant B-cells in diffuse large B-cell lymphoma (DLBCL) and determine the clinical and biological significance. EXPERIMENTAL DESIGN: A total of 269 patients with de novo DLBCL from the International DLBCL R-CHOP Consortium Program were studied. Ultra-deep sequencing of the immunoglobulin genes was performed to determine B-cell clonotypes. The frequencies and numbers of TIL-B clonotypes in individual repertoires were correlated with patient survival, gene expression profiling (GEP) data, and frequencies of DLBCL-infiltrating immune cells quantified by fluorescent multiplex IHC at single-cell resolution. RESULTS: TIL-B abundance, evaluated by frequencies of normal B-cell clonotypes in the immunoglobulin repertoires, remarkably showed positive associations with significantly better survival of patients in our sequenced cohorts. DLBCLs with high versus low TIL-B abundance displayed distinct GEP signatures, increased pre-memory B-cell state and naïve CD4 T-cell state fractions, and higher CD4+ T-cell infiltration. TIL-B frequency, as a new biomarker in DLBCL, outperformed the germinal center (GC) B-cell-like/activated B-cell-like classification and TIL-T frequency. The identified TIL-B-high GEP signature, including genes upregulated during T-dependent B-cell activation and those highly expressed in normal GC B cells and T cells, showed significant favorable prognostic effects in several external validation cohorts. CONCLUSIONS: TIL-B frequency is a significant prognostic factor in DLBCL and plays a crucial role in antitumor immune responses. This study provides novel insights into the prognostic determinants in DLBCL and TIL-B functions with important therapeutic implications.

The spectrum of nodular lymphocyte predominant Hodgkin lymphoma: a report of the lymphoma workshop of the 20th meeting of the European Association for Haematopathology.

Session 4 of the 2021 European Association of Haematopathology/Society for Hematopathology Workshop focused on nodular lymphocyte predominant Hodgkin lymphoma (NLPHL). First, the spectrum of immunophenotypic variations in NLPHL and the defining criteria for classic Hodgkin Lymphoma (CHL) were discussed. The added value of further immunophenotypic characterization of both tumor cells and microenvironment to support the differential diagnosis was presented. Next, unusual cases with combined growth patterns and evolution of morphological features over time were presented to explore the clinicopathological impact of presumed high-risk patterns. Based on a large collection of cases, the defining morphological, immunophenotypical, and gene expression features of T-cell/histiocyte-rich large B-cell lymphoma (THRLBCL) and THRLBCL-like NLPHL (pattern E) were reviewed to explore this challenging differential diagnosis and critically evaluate whether aggressive behavior and transformation of NLPHL can be predicted in practice.

Mediastinal large B cell lymphoma and surrounding gray areas: a report of the lymphoma workshop of the 20th meeting of the European Association for Haematopathology.

Session 3 of the 2021 European Association for Haematopathology/Society for Hematopathology Workshop focused on mediastinal large B cell lymphomas and surrounding gray areas. One half of the session was dedicated to primary mediastinal large B cell lymphoma (PMBL) and included cases with classic clinicopathologic features, as well as cases with either morphologic or immunophenotypic variation, and PMBL-like cases with primary extramediastinal disease. The role of additional immunophenotyping and/or molecular testing to aid in the diagnosis of PMBL was discussed. The second half of the session focused on mediastinal and non-mediastinal gray zone lymphomas (GZL) with features intermediate between diffuse large B cell lymphoma (DLBCL) and classic Hodgkin lymphoma (CHL). Several cases illustrating the current challenges in separating this entity from PMBL/DLBCL and CHL were presented. There was discussion regarding the clinical and genetic differences between mediastinal and non-mediastinal GZLs. Rare cases of PMBL and GZL associated with EBV or follicular lymphoma were reviewed. Finally, several cases included in the session highlighted composite or sequential CHL and PMBL/DLBCL and/or GZL, highlighting challenges in separating such cases from GZL.

CD6-targeted antibody-drug conjugate as a new therapeutic agent for T cell lymphoma.

T cell lymphomas (TCL) are heterogeneous, aggressive, and have few available targeted therapeutics. In this study, we determined that CD6, an established T cell marker, was expressed at high levels on almost all examined TCL patient specimens, suggesting that CD6 could be a new therapeutic target for this life-threatening blood cancer. We prepared a CD6-targeted antibody-drug conjugate (CD6-ADC) by conjugating monomethyl auristatin E (MMAE), an FDA-approved mitotic toxin, to a high-affinity anti-human CD6 monoclonal antibody (mAb). In contrast to both the unconjugated anti-CD6 mAb, and the non-binding control ADC, CD6-ADC potently and selectively killed TCL cells in vitro in both time- and concentration-dependent manners. It also prevented the development of tumors in vivo in a preclinical model of TCL. More importantly, systemic or local administration of the CD6-ADC or its humanized version, but not the controls, significantly shrank established tumors in the preclinical mouse model of TCL. These results suggest that CD6 is a novel therapeutic target in TCLs and provide a strong rationale for the further development of CD6-ADC as a promising therapy for patients with these potentially fatal lymphoid neoplasms.

Lymphomas with plasmablastic features: a report of the lymphoma workshop of the 20th meeting of the European Association for Haematopathology.

Lymphomas with plasmablastic features are a heterogeneous group of aggressive and mostly uncommon neoplasms of varied aetiologies, presenting in immunocompetent individuals as well as in immunodeficiency, associated with EBV and Kaposi sarcoma virus infections, and some as progression from indolent B-cell lymphomas. They show overlapping diagnostic features and pose a differential diagnosis with other aggressive B-cell lymphomas that can downregulate the B-cell expression programme. The spectrum of rare reactive proliferations and all lymphomas defined by plasmablastic features, together with an expanding range of poorly characterised, uncommon conditions at the interface between reactive lymphoid proliferations and neoplasia submitted to the session V of the 20th European Association for Haematopathology/Society for Hematopathology lymphoma workshop are summarised and discussed in this paper.

Significant Variability in the Identification and Reporting of Band Neutrophils by Participants Enrolled in the College of American Pathologists Proficiency Testing Program: Time for a Change.

CONTEXT.­: Increased band neutrophils in blood smear differential counts ("bandemia") are entrenched in medicine as a flag for sepsis. However, laboratory hematology experts have long advocated for discontinuation of reporting bands separately from segmented neutrophils because of poor sensitivity and specificity, poor interobserver agreement, and availability of alternative biomarkers for sepsis. OBJECTIVE.­: To describe band neutrophil reporting practices and reproducibility of band classification among laboratories participating in the College of American Pathologists (CAP) proficiency testing (PT) program. DESIGN.­: A survey questionnaire was distributed to hematology PT participants. A subsequent morphologic challenge included 12 preselected cell identifications of segmented neutrophils, bands, and metamyelocytes, and a 100-cell manual differential count of a digitally scanned blood smear. RESULTS.­: Among laboratories that reported manual differentials, most respondents reported bands (4554 of 5268; 86.4%). Only 3222 of 4412 respondents (73.0%) provided band reference ranges. Though participants classified "easy" band neutrophils well (78.0%-98.3%), categorization of cell identifications for "moderate" and "difficult" bands was poor (3.1%-39.0% of laboratories), with classification instead as segmented neutrophils. This pattern was seen regardless of laboratory demographic characteristics. Marked variability in band counts was observed on the 100-cell differential count for both CAP PT participants and CAP Hematology and Clinical Microscopy Committee (HCMC) members (coefficients of variation, 55.8% and 32.9%, respectively). Variability was significantly improved when segmented and band neutrophils were grouped together (coefficients of variation, 6.2% and 5.0%, respectively). CONCLUSIONS.­: Most CAP PT-participating laboratories report band counts, many without reference ranges. The survey confirms significant interlaboratory variability of band enumeration when bands are separately identified from segmented neutrophils. This study reaffirms the CAP Hematology and Clinical Microscopy Committee's strong recommendation to group segmented and band neutrophils together in manual differential counts.

Mutations associated with progression in follicular lymphoma predict inferior outcomes at diagnosis: Alliance A151303.

Follicular lymphoma (FL) is clinically heterogeneous, with select patients tolerating extended watch-and-wait, whereas others require prompt treatment, suffer progression of disease within 24 months of treatment (POD24), and/or experience aggressive histologic transformation (t-FL). Because our understanding of the relationship between genetic alterations in FL and patient outcomes remains limited, we conducted a clinicogenomic analysis of 370 patients with FL or t-FL (from Cancer and Leukemia Group B/Alliance trials 50402/50701/50803, or real-world cohorts from Washington University School of Medicine, Cleveland Clinic, or University of Miami). FL subsets by grade, stage, watch-and-wait, or POD24 status did not differ by mutation burden, whereas mutation burden was significantly higher in relapsed/refractory (rel/ref) FL and t-FL than in newly diagnosed (dx) FL. Nonetheless, mutation burden in dx FL was not associated with frontline progression-free survival (PFS). CREBBP was the only gene more commonly mutated in FL than in t-FL yet mutated CREBBP was associated with shorter frontline PFS in FL. Mutations in 20 genes were more common in rel/ref FL or t-FL than in dx FL, including 6 significantly mutated genes (SMGs): STAT6, TP53, IGLL5, B2M, SOCS1, and MYD88. We defined a mutations associated with progression (MAP) signature as ≥2 mutations in these 7 genes (6 rel/ref FL or t-FL SMGs plus CREBBP). Patients with dx FL possessing a MAP signature had shorter frontline PFS, revealing a 7-gene set offering insight into FL progression risk potentially more generalizable than the m7-Follicular Lymphoma International Prognostic Index (m7-FLIPI), which had modest prognostic value in our cohort. Future studies are warranted to validate the poor prognosis associated with a MAP signature in dx FL, potentially facilitating novel trials specifically in this high-risk subset of patients.

Guide to the Diagnosis of Myeloid Neoplasms: A Bone Marrow Pathology Group Approach.

OBJECTIVES: The practicing pathologist is challenged by the ever-increasing diagnostic complexity of myeloid neoplasms. This guide is intended to provide a general roadmap from initial case detection, often triggered by complete blood count results with subsequent blood smear review, to final diagnosis. METHODS: The integration of hematologic, morphologic, immunophenotypic, and genetic features into routine practice is standard of care. The requirement for molecular genetic testing has increased along with the complexity of test types, the utility of different testing modalities in identifying key gene mutations, and the sensitivity and turnaround time for various assays. RESULTS: Classification systems for myeloid neoplasms have evolved to achieve the goal of providing a pathology diagnosis that enhances patient care, outcome prediction, and treatment options for individual patients and is formulated, endorsed, and adopted by hematologists/oncologists. CONCLUSIONS: This guide provides diagnostic strategies for all myeloid neoplasm subtypes. Special considerations are provided for each category of testing and neoplasm category, along with classification information, genetic testing requirements, interpretation information, and case reporting recommendations based on the experience of 11 Bone Marrow Pathology Group members.

Ki67 proliferation index in follicular lymphoma is associated with favorable outcome in patients treated with R-CHOP.

Follicular lymphoma (FL) is a common, indolent small B-cell lymphoma. While the Follicular Lymphoma International Prognostic Index is widely used, reliable prognostic and predictive biomarkers are needed. A recent study suggested that architectural patterns of CD10, BCL6, and Ki67 expression may correlate with progression-free survival (PFS) in FL patients treated with chemotherapy-free regimens. We examined the prognostic and predictive utility of architectural patterns of CD10, BCL6, Ki67, and FOXP1 in 90 patients treated with immunochemotherapy (bendamustine-rituximab [BR] and R-cyclophosphamide, doxorubicin, vincristine, prednisone [CHOP]). We found that high follicular Ki67 (≥30%) was associated with longer PFS in the subgroup of patients treated with R-CHOP but not among those treated with BR. Validation of this biomarker may support routine use of Ki67 as a predictive marker in FL.

Acute leukemias with complex karyotype show a similarly poor outcome independent of mixed, myeloid or lymphoblastic immunophenotype: A study from the Bone Marrow Pathology Group.

Mixed phenotype acute leukemia (MPAL) is a heterogenous group of acute leukemias characterized by leukemic blasts that express markers of multiple lineages. The revised 4th edition WHO classification of MPAL excludes AML with myelodysplasia related changes (AML-MRC), including those with complex karyotype (CK), from a diagnosis of MPAL. Abnormal karyotype is frequent in MPAL with the reported rate of CK in MPAL ranging from 19% to 32%. Due its rarity, the clinical and genetic features of MPAL with CK remain poorly characterized. This study aims to further characterize the genetic features of MPAL with CK in comparison to cases of AML and ALL with CK. Cases of de novo MPAL, AML, and B- and T-ALL patients with CK were collected from 8 member institutions of the Bone Marrow Pathology Group. We found no significant difference in overall survival between MPAL with CK compared to AML and ALL with CK. AML with CK was more strongly associated with TP53 mutations, however the presence of TP53 mutations conferred a worse prognosis regardless of lineage. ALL with CK seems to show increased IKZF1 mutation rates which is known to confer a worse prognosis in ALL. Additionally, MPAL with CK showed similarly poor outcomes regardless of whether a lymphoid or myeloid chemotherapy regimen is chosen. Our results suggest that acute leukemias with complex karyotype show a similarly poor outcome regardless of lineage differentiation and that mutation in TP53 confers a poor prognosis in all lineages. Our results support the exclusion of immunophenotypic MPAL with CK from MPAL and appear to confirm the approach proposed in the revised 4th edition WHO to include them as AML with myelodysplasia-related changes and similar myelodysplasia-related AML categories of newer classifications.

Myeloid sarcoma with NPM1 mutation may be clinically and genetically distinct from AML with NPM1 mutation: a study from the Bone Marrow Pathology Group.

Myeloid sarcoma (MS) is currently considered equivalent to de novo acute myeloid leukemia (AML); however, the relationship between these entities is poorly understood. This retrospective multi-institutional cohort study compared 43 MS with NPM1 mutation to 106 AML with NPM1 mutation. Compared to AML, MS had more frequent cytogenetic abnormalities including complex karyotype (p = .009 and p = .007, respectively) and was enriched in mutations of genes involved in histone modification, including ASXL1 (p = .007 and p = .008, respectively). AML harbored a higher average number of gene mutations (p = .002) including more frequent PTPN11 mutations (p < .001) and mutations of DNA-methylating genes including DNMT3A and IDH1 (both p < .001). MS had significantly shorter overall survival (OS) than AML (median OS: 44.9 vs. 93.2 months, respectively, p = .037). MS with NPM1 mutation has a unique genetic landscape, and poorer OS, compared to AML with NPM1 mutation.

First study comparing genetic profiles of MS and AML with a common disease-defining lesion.NPM1^Mut MS may be genetically distinct from NPM1^Mut AML.NPM1^Mut MS may have inferior overall survival compared to NPM1^Mut AML.

The clinicopathologic significance of NPM1 mutation and ability to detect mutated NPM1 by immunohistochemistry in non-AML myeloid neoplasms.

NPM1 mutated non-AML myeloid neoplasms (MN; <20% blasts) are characterized by an aggressive clinical course in a few studies. In this retrospective study, we evaluate the clinicopathologic and immunohistochemical features of non-AML MN patients with NPM1 mutations. We assessed NPM1 mutation by targeted next generation sequencing (NGS). Cytoplasmic NPM1 expression was assessed by immunohistochemistry (IHC) on formalin-fixed, formic acid-decalcified bone marrow biopsy specimens. We evaluated 34 non-AML MN patients with NPM1 mutations comprising MDS (22), MPN (3) and MDS/MPN (9). They commonly presented with anemia (88%), thrombocytopenia (58%) and leukopenia (50%). Bone marrow dysplasia was common (79%). The karyotype was often normal (64%). NGS for MN-associated mutations performed in a subset of the patients showed a median of 3 mutations. NPM1 mutations were more often missense (c.859C > T p. L287F; 65%) than frameshift insertion/duplication (35%) with median variant allele frequency (VAF; 9.7%, range 5.1%-49.8%). Mutated NPM1 by IHC showed cytoplasmic positivity in 48% and positivity was associated with higher VAF. The median overall survival (OS) in this cohort was 70 months. Nine patients (26%) progressed to AML. OS in patients who progressed to AML was significantly shorter than the one of patients without progression to AML (OS 20 vs. 128 months, respectively, log rank p = 0.05). NPM1 mutated non-AML MN patients commonly had cytopenias, dysplasia, normal karyotype, mutations in multiple genes, and an unfavorable clinical outcome, including progression to AML. Our data demonstrated that IHC for NPM1 can be a useful supplementary tool to predict NPM1 mutation in some non-AML MN; however, genetic testing cannot be replaced by IHC assessment.

Biallelic TET2 mutations and canonical ASXL1 mutations are frequent and cooccur in Blastic Plasmacytoid Dendritic Cell Neoplasm (BPDCN): An institutional experience and review of literature.

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is recurrently mutated in epigenetic pathway genes. We studied the myeloid-related genetic mutations in a cohort of five patients with BPDCN and one paired relapse case at our institution and identified a high frequency of biallelic TET2 and canonical ASXL1 (c.1934dupG) mutations. The number of cases is small, but the variant allele fraction (VAF) sums of the TET2 mutations, as well as the persistence of TET2 mutations in a case of relapsed BPDCN, suggest an ancestral/founder nature of TET2 clones in the cases. Further literature review shows a high frequency of biallelic TET2 mutations in reported cases of BPDCN.

Clinicopathologic characteristics of myeloproliferative neoplasms with JAK2 exon 12 mutation.

The presence of JAK2 exon 12 mutation was included by the 2016 World Health Organization (WHO) Classification as one of the major criteria for diagnosing polycythemia vera (PV). Few studies have evaluated the clinical presentation and bone marrow morphology of these patients and it is unclear if these patients fulfill the newly published criteria of 5th edition WHO or The International Consensus Classification (ICC) criteria for PV. Forty-three patients with JAK2 exon 12 mutations were identified from the files of 7 large academic institutions. Twenty patients had complete CBC and BM data at disease onset. Fourteen patients met the diagnostic criteria for PV and the remaining six patients were diagnosed as MPN-U. At diagnosis, 9/14 patients had normal WBC and platelet counts (isolated erythrocytosis/IE subset); while 5/14 had elevated WBC and/or platelets (polycythemic /P subset). We found that hemoglobin and hematocrit tended to be lower in the polycythemia group. Regardless of presentation (P vs IE), JAK2 deletion commonly occurred in amino acids 541-544 (62 %). MPN-U patients carried JAK2 exon 12 mutation, but did not fulfill the criteria for PV. Half of the patients had hemoglobin/hematocrit below the diagnostic threshold for PV, but showed increased red blood cell count with low mean corpuscular volume (56-60 fL). Three cases lacked evidence of bone marrow hypercellularity. In summary, the future diagnostic criteria for PV may require a modification to account for the variant CBC and BM findings in some patients with JAK2 exon 12 mutation.