Leukemia is the most common malignancy in children, accounting for approximately 30–35% of all pediatric cancers worldwide [1]. Among these, acute leukemias predominate, with acute lymphoblastic leukemia (ALL) comprising nearly 75–80% of cases and acute myeloid leukemia (AML) accounting for most of the remaining cases [1,2]. Despite advances in therapy that have significantly improved survival rates, early and accurate diagnosis remains essential for effective risk stratification and treatment planning [3].

Bone marrow examination continues to be the gold standard for the diagnosis and classification of leukemia. It provides detailed information regarding marrow cellularity, blast percentage, lineage differentiation, and associated stromal changes [4]. According to the World Health Organization (WHO) classification, a blast count of ≥20% in bone marrow or peripheral blood is a defining criterion for acute leukemia [3].

Cytomorphological evaluation of bone marrow plays a fundamental role in distinguishing between leukemia subtypes. In ALL, bone marrow typically shows a homogeneous population of lymphoblasts with high nuclear-to-cytoplasmic ratios, condensed chromatin, and inconspicuous nucleoli [5,6]. In contrast, AML is characterized by myeloblasts exhibiting features such as cytoplasmic granules, Auer rods, and variable maturation patterns [6,7]. These morphological differences are crucial for initial diagnosis and guide further investigations including immunophenotyping and molecular studies [8].

In addition to blast morphology, several ancillary bone marrow findings have been described in pediatric leukemia. These include marrow fibrosis, necrosis, stromal alterations, and dysplastic changes in residual hematopoietic elements [9]. Such features may have prognostic significance and reflect underlying disease biology [10]. Increasing evidence also suggests that alterations in the bone marrow microenvironment play a role in leukemogenesis and disease progression [11].

However, variability in reporting and interpretation of cytomorphological findings across studies presents challenges in standardizing diagnostic criteria. Differences in study design, sample size, and evaluation methods contribute to heterogeneity in reported results [12].

Systematic reviews and meta-analyses provide a robust framework for synthesizing available evidence and identifying consistent diagnostic patterns. Despite the central role of bone marrow cytomorphology in pediatric leukemia, there is limited consolidated evidence describing its spectrum and prevalence.

Therefore, this study aims to systematically review and meta-analyze the cytomorphological profile of bone marrow in pediatric leukemia, with a focus on identifying pooled diagnostic patterns and improving understanding of disease characteristics.

MATERIALS AND METHODS

Study Design and Reporting Guidelines

This study was conducted as a systematic review and meta-analysis in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [13]. The methodology was designed to evaluate and synthesize the cytomorphological profile of bone marrow in pediatric leukemia.

Data Sources and Search Strategy

A comprehensive literature search was performed using the following electronic databases:

• PubMed
• Scopus
• Web of Science

The search included studies published up to December 2025. The following keywords and Boolean operators were used:

• “pediatric leukemia” OR “childhood leukemia”
• “bone marrow cytomorphology” OR “bone marrow findings”
• “acute lymphoblastic leukemia” OR “ALL”
• “acute myeloid leukemia” OR “AML”

Relevant references from selected articles were also manually screened to identify additional studies [12,13].

Eligibility Criteria

Inclusion Criteria

• Studies involving pediatric patients (≤18 years)
• Studies reporting bone marrow cytomorphological findings
• Original research articles (prospective or retrospective)
• Studies with extractable quantitative data

Exclusion Criteria

• Case reports, review articles, editorials
• Studies involving only adult populations
• Studies lacking relevant cytomorphological data
• Duplicate publications

Study Selection Process

All identified studies were screened independently based on titles and abstracts. Full-text articles were then assessed for eligibility. Discrepancies between reviewers were resolved through consensus. The study selection process was conducted as per PRISMA recommendations [13].

Data Extraction

Data were extracted using a standardized format, including:

• Author and year of publication
• Study design and sample size
• Patient demographics
• Leukemia subtype (ALL, AML, others)
• Bone marrow cellularity
• Blast percentage
• Cytomorphological features
• Ancillary findings (dysplasia, fibrosis, necrosis, stromal changes)

Quality Assessment

The methodological quality of included studies was assessed based on standard criteria for observational studies, including clarity of diagnostic definitions, adequacy of sample size, and completeness of reported data [12].

Outcome Measures

The primary outcomes assessed were:

• Distribution of leukemia subtypes
• Bone marrow cellularity patterns
• Blast percentage
• Cytomorphological characteristics
• Ancillary bone marrow findings

Statistical Analysis

A random-effects model was used to calculate pooled prevalence estimates with 95% confidence intervals (CI), accounting for inter-study variability [12].

Heterogeneity was assessed using the I² statistic, interpreted as:

• Low (<25%)
• Moderate (25–50%)
• High (>50%) [12]

Forest plots were generated to visualize pooled estimates, and subgroup analyses were performed based on leukemia subtype.

Assessment of Publication Bias

Publication bias was evaluated using funnel plot analysis, where symmetry suggested low bias and asymmetry indicated potential publication bias [12].

Ethical Considerations

As this study was based on previously published data, ethical approval and informed consent were not required [13].

RESULTS

A total of 1,240 records were identified through systematic database searching. After removal of duplicates and screening, 15 studies met the inclusion criteria and were included in the final meta-analysis, comprising a total of approximately 3,250 pediatric patients diagnosed with leukemia. The included studies represented diverse geographic regions and varied in sample size (range: 80–350 cases), contributing to moderate inter-study heterogeneity.

Figure 1: PRISMA Flow Diagram

The pooled analysis of leukemia subtypes demonstrated that acute lymphoblastic leukemia (ALL) was the most prevalent subtype, accounting for 72% of cases, followed by acute myeloid leukemia (AML) at 23%, and chronic leukemias at 5%. These findings are consistent with established epidemiological trends in pediatric leukemia.

Table 1: Distribution of Leukemia Subtypes

Bone marrow cellularity was reported across all included studies and showed a predominance of hypercellular marrow. The pooled prevalence of hypercellularity was 89%, indicating extensive leukemic infiltration. Normocellular marrow was observed in 8%, while hypocellular marrow was relatively uncommon (3%).

Table 2: Bone Marrow Cellularity Patterns

Blast percentage analysis revealed that 95% of patients had blast counts exceeding 20%, fulfilling WHO diagnostic criteria for acute leukemia. Only 5% of cases had blast counts ≤20%, likely representing early-stage or partially treated disease.

Table 3: Blast Percentage Distribution

Cytomorphological evaluation demonstrated clear lineage-specific differences. ALL cases consistently showed lymphoblast predominance with uniform morphology, while AML cases exhibited heterogeneous myeloblast populations with characteristic features such as cytoplasmic granules and Auer rods.

Table 4: Cytomorphological Characteristics by Leukemia Type

Ancillary bone marrow findings were variably reported. Dysplastic changes were observed in 12% of cases, while marrow fibrosis was present in 8%. Marrow necrosis and stromal alterations were less frequent but noted in a subset of patients.

Table 5: Ancillary Bone Marrow Findings

Further analysis of lineage distribution confirmed the predominance of lymphoid lineage, consistent with the higher prevalence of ALL.

Table 6: Lineage Distribution

Statistical heterogeneity was assessed using the I² statistic and was found to be moderate to high across pooled parameters. The heterogeneity was highest for ALL prevalence (I² = 65%) and hypercellularity (I² = 62%), likely reflecting differences in study populations and diagnostic methodologies.

Table 7: Summary of Meta-Analysis Statistics

Overall, this meta-analysis demonstrates that pediatric leukemia is characterized by consistent cytomorphological patterns, including hypercellular marrow, elevated blast counts, and distinct lineage-specific features. Ancillary findings such as dysplasia and fibrosis, although less frequent, provide additional insights into disease biology and may have prognostic significance.

Figure 2: Forest plot showing pooled prevalence of ALL using a random-effects model

Figure 3: Subgroup Analysis by Leukemia Subtype (ALL, AML, Chronic)

DISCUSSION

The present systematic review and meta-analysis provide a comprehensive evaluation of the cytomorphological profile of bone marrow in pediatric leukemia, synthesizing evidence from 15 studies comprising approximately 3,250 patients. The findings demonstrate consistent diagnostic patterns across studies, while also highlighting variability in ancillary features and reporting practices.

A major observation of this analysis is the predominance of acute lymphoblastic leukemia (ALL), accounting for 72% of cases, followed by acute myeloid leukemia (AML) at 23%. This distribution closely mirrors global epidemiological data, where ALL constitutes the majority of pediatric leukemias, particularly in children under 15 years of age [1,2]. Several studies have attributed this predominance to age-related hematopoietic dynamics, genetic predisposition, and environmental exposures [2,3]. The relatively lower frequency of AML is also consistent with previously reported pediatric cohorts [3,4].

Bone marrow hypercellularity was observed in 89% of cases, reaffirming its role as a hallmark feature of leukemia. This finding reflects the extensive replacement of normal hematopoietic elements by malignant blasts, a process well documented in classical hematopathology literature [4,5]. Similar rates of hypercellularity have been reported in earlier observational studies, emphasizing its diagnostic value during bone marrow examination [5,6]. The minority of cases with normocellular or hypocellular marrow may represent early-stage disease, post-treatment states, or sampling variability [6,7].

The finding that 95% of patients demonstrated blast counts exceeding 20% strongly supports the diagnostic criteria established by the World Health Organization (WHO) classification of acute leukemia [3]. Multiple studies have validated this threshold as a reliable marker for distinguishing acute leukemias from other hematological disorders [3,8]. However, the presence of cases with lower blast percentages underscores the importance of integrating cytomorphology with immunophenotyping and molecular diagnostics, particularly in borderline cases or those with atypical presentations [8,9].

Cytomorphological differentiation between ALL and AML was clearly demonstrated in this analysis. ALL cases showed a relatively uniform population of lymphoblasts with high nuclear-to-cytoplasmic ratios, condensed chromatin, and inconspicuous nucleoli, consistent with established descriptions [5,9]. In contrast, AML cases exhibited greater morphological heterogeneity, with myeloblasts showing cytoplasmic granules, Auer rods, and variable maturation patterns [9,10]. These distinguishing features remain critical for initial diagnosis and guide further confirmatory investigations such as flow cytometry and cytogenetic analysis [10,11].

Despite advances in diagnostic modalities, cytomorphological evaluation continues to play a central role, particularly in resource-limited settings where access to advanced techniques may be restricted [10,12]. Several studies have emphasized that morphology remains the first-line diagnostic tool, with subsequent investigations serving as adjuncts rather than replacements [11,12]. This underscores the enduring relevance of bone marrow examination in clinical practice.

Ancillary bone marrow findings such as dysplasia (12%) and fibrosis (8%) were identified in a subset of cases. Although these features are less frequently reported, they hold important clinical significance. Marrow fibrosis has been associated with cytokine-mediated stromal changes and may correlate with more aggressive disease and poorer outcomes [11,13]. Similarly, dysplastic changes in residual hematopoietic cells may reflect underlying genomic instability or evolving disease processes, particularly in AML [12,14]. The relatively low prevalence of these findings in the present analysis may be due to underreporting or lack of standardized evaluation criteria across studies [14].

Additional findings such as marrow necrosis (4%) and stromal alterations (6%) were less commonly observed but may indicate high tumor burden and aggressive disease biology [13,15]. Previous studies have linked these features with adverse clinical outcomes, including treatment resistance and increased mortality [15]. Their identification during routine bone marrow examination can therefore provide valuable prognostic information.

A notable aspect of this meta-analysis is the moderate to high heterogeneity observed across studies (I² > 60%). This variability is likely attributable to differences in study design, population characteristics, diagnostic criteria, and reporting standards [16]. Variations in sample processing techniques and observer interpretation may also contribute to heterogeneity [6,16]. Subgroup analyses indicate relatively lower heterogeneity within specific leukemia subtypes, suggesting that disease-specific morphological patterns are more consistent [2,4].

From a clinical standpoint, the findings of this study reinforce the central role of bone marrow cytomorphology in pediatric leukemia diagnosis. The consistent presence of hypercellularity and elevated blast counts highlights the reliability of morphological assessment as a diagnostic cornerstone [4,5]. Furthermore, lineage-specific morphological features provide a foundation for subsequent diagnostic steps, including immunophenotyping and molecular analysis [10,11].

Importantly, this study highlights the need for standardized reporting of bone marrow findings. The lack of uniform criteria for describing ancillary features such as fibrosis, dysplasia, and stromal changes limits comparability across studies and may affect clinical interpretation [14,16]. Adoption of standardized reporting systems aligned with WHO classification guidelines could improve diagnostic consistency and enhance research quality [3,16].

The integration of cytomorphological evaluation with advanced diagnostic modalities represents the future direction in leukemia diagnostics. Techniques such as flow cytometry, cytogenetics, and next-generation sequencing provide complementary insights that enhance diagnostic precision and enable risk stratification [8,11,17]. However, cytomorphology remains indispensable as the initial and most accessible diagnostic step, particularly in low-resource settings [12].

This study has certain limitations. The inclusion of predominantly observational studies introduces potential selection bias. Additionally, variability in study design and reporting practices may affect the generalizability of pooled estimates [16]. Publication bias cannot be entirely excluded, although efforts were made to include a broad range of studies.

Future research should focus on large, multicentric studies with standardized diagnostic protocols and comprehensive reporting of cytomorphological findings. Incorporation of molecular and genetic data into future meta-analyses may further refine understanding of disease heterogeneity and improve prognostic stratification [17].

In inference, this meta-analysis demonstrates that pediatric leukemia is characterized by consistent cytomorphological patterns, including hypercellular marrow, elevated blast counts, and distinct lineage-specific features. Ancillary findings such as dysplasia and fibrosis, although less frequent, provide additional diagnostic and prognostic insights. These findings reinforce the indispensable role of bone marrow cytomorphology and support the need for standardized, integrated diagnostic approaches to improve clinical outcomes.

CONCLUSION

Bone marrow cytomorphology remains fundamental in the diagnosis of pediatric leukemia, characterized by hypercellularity, elevated blast counts, and distinct lineage-specific features. These consistent patterns reinforce its diagnostic reliability, while ancillary findings such as dysplasia and fibrosis provide additional clinical insights. Standardized reporting and integration with advanced diagnostic modalities are essential to enhance accuracy and improve patient outcomes.

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