Allogeneic Hematopoietic cell Transplantation Using Alemtuzumab in Asian Patients with Inborn Errors of Immunity.

Alemtuzumab is used with reduced-toxicity conditioning (RTC) in allogeneic hematopoietic cell transplantation (HCT), demonstrating efficacy and feasibility for patients with inborn errors of immunity (IEI) in Western countries; however, the clinical experience in Asian patients with IEI is limited. We retrospectively analyzed patients with IEI who underwent the first allogeneic HCT with alemtuzumab combined with RTC regimens in Japan. A total of 19 patients were included and followed up for a median of 18 months. The donors were haploidentical parents (n = 10), matched siblings (n = 2), and unrelated bone marrow donors (n = 7). Most patients received RTC regimens containing fludarabine and busulfan and were treated with 0.8 mg/kg alemtuzumab with intermediate timing. Eighteen patients survived and achieved stable engraftment, and no grade 3-4 acute graft-versus-host disease was observed. Viral infections were observed in 11 patients (58%) and 6 of them presented symptomatic. The median CD4+ T cell count was low at 6 months (241/µL) but improved at 1 year (577/µL) after HCT. Whole blood cells continued to exhibit > 80% donor type in most cases; however, 3/10 patients exhibited poor donor chimerism only among T cells and also showed undetectable levels of T-cell receptor recombination excision circles (TRECs) at 1 year post-HCT. This study demonstrated the efficacy and safety of alemtuzumab; however, patients frequently developed viral infections and slow reconstitution or low donor chimerism in T cells, emphasizing the importance of monitoring viral status and T-cell-specific chimerism. (238 < 250 words).

Impact of prophylactic echinocandin on the development of neurological complications in patients receiving busulfan-containing conditioning regimens for stem cell transplantation: A single-center retrospective study.

BACKGROUND: Although neurotoxicity is a major adverse event associated with busulfan, little information is available regarding the association between drug interactions and neurological symptoms during busulfan-based regimens. This study evaluated the association between prophylactic echinocandins and neurological complications in patients receiving busulfan-containing conditioning regimens for stem cell transplantation. METHODS: We retrospectively included consecutive patients who administered intravenous busulfan as a conditioning regimen at our facility between 2007 and 2022. Prophylactic echinocandin use was defined as the use of an echinocandin antifungal drug to prevent invasive fungal disease in SCT recipients. The primary outcome was the incidence of neurological complications within 7 days of busulfan initiation and was compared between the echinocandin group (patients received prophylactic echinocandin) and nonechinocandin group (patients received prophylactic antifungal drugs other than echinocandin and those without antifungal prophylaxis). RESULTS: Among the 59 patients included in this study, the incidence of neurological complications in the echinocandin (n = 26) and nonechinocandin groups (n = 33) was 30.8% and 63.6%, respectively. We observed a negative association between prophylactic echinocandin use and the development of neurological complications after adjusting for the propensity score for receiving prophylactic echinocandins (adjusted odds ratio 0.294, 95% confidence interval 0.090 to 0.959). We observed a lower incidence of neurological complications in the echinocandin group than in the nonechinocandin group. CONCLUSION: Our results suggested that the choice of antifungal prophylaxis is associated with busulfan neurotoxicity.

Association between conditioning intensity and height growth after allogeneic hematopoietic stem cell transplantation in children.

BACKGROUND: The present study aimed to examine the association between the conditioning intensity and height growth in pediatric patients who underwent allogeneic hematopoietic stem cell transplantation (allo-HSCT). METHODS: We reviewed the clinical records of 89 children with malignant diseases who underwent initial allo-HSCT between 2003 and 2021. Height measurements were standardized using standard height charts prepared by the Japanese Society for Pediatric Endocrinology to calculate standard deviation score (SDS). We defined short stature as a height SDS less than -2.0 in that reference. Myeloablative conditioning (MAC) comprised total-body irradiation at more than 8 Gy and busulfan administration at more than 8 mg/kg (more than 280 mg/m2 ). Other conditioning regimens were defined as reduced intensity conditioning (RIC). RESULTS: A total of 58 patients underwent allo-HSCT with MAC, and 31 patients received allo-HSCT with RIC. There were significant differences in the height SDS at 2 and 3 years after allo-HSCT between MAC and RIC group (-1.33 ± 1.20 vs. -0.76 ± 1.12, p = 0.047, -1.55 ± 1.28 vs. -0.75 ± 1.11, p = 0.022, respectively). Multivariate logistic regression analysis with the adjustments for potential confounding factors of patients less than 10 years of age at allo-HSCT and chronic graft-versus host disease demonstrated that MAC regimen was associated with a markedly increased risk of a short stature at 3 years after allo-HSCT (adjusted odds ratio, 5.61; 95% confidence interval, 1.07-29.4; p = 0.041). CONCLUSION: The intensity of conditioning regimen may be associated with short statures after allo-HSCT.

Epidural Spinal Cord Compression as the Presenting Manifestation of Acute Myeloid Leukemia: A Case Report and Literature Review.

We herein report a rare case of spinal cord compression due to epidural involvement of acute myeloid leukemia (AML). A 14-year-old boy presented with a 7-day history of back pain, paraplegia and hypoesthesia. Contrast-enhanced computed tomography revealed an epidural mass. Emergency laminectomy and resection of the mass were performed. Histopathologically, the resected mass was comparable to an extramedullary mass of AML. Chemotherapy was initiated, and complete remission was achieved. Neurological sequelae remained after the treatment. Based on the present and previous reports, spinal cord compression from epidural AML involvement may progress rapidly.

Incidence and Causative Organisms of Bacteriuria in Children With Cancer: A 9-Year Experience in a Tertiary Pediatric Center.

Although infection is common in patients with cancer, the distribution of causative agents and the proportion of resistant bacteria in patients with urinary tract infection remain unknown. The aim of this study was to describe the incidence, the causative agents, and the proportion of antimicrobial resistance in bacteria cultured from urine of hospitalized children with cancer. A single-center retrospective chart review of patients with cancer admitted between 2012 and 2020 was performed to identify patients with positive urine culture. Overall, 61 (0.9%) of 7107 patients were identified to have positive urine cultures. Among them, 25 patients (41%) had symptomatic bacteriuria. The most common pathogenic bacterium among all patients was Escherichia coli ( E. coli ) (n=15, 25%), followed by Enterococcus sp. (n=14, 23%), Klebsiella sp. (n=12, 20%), and Pseudomonas aeruginosa ( P. aeruginosa ) (n=12, 20%). Extended spectrum ß-lactamases-producing E. coli and Extended spectrum ß-lactamases-producing Klebsiella pneumoniae were detected in 1 patient (2%) and 4 patients (7%), respectively. No multidrug-resistant P. aeruginosa and vancomycin-resistant Enterococcus were detected. The incidence of bacteriuria was found to be low in children with cancer. The distribution of causative agents of bacteriuria in children with solid tumors and hematologic diseases may be different from that in previously healthy children.

Spindle cell sarcoma with KIAA1549-BRAF resembling infantile fibrosarcoma morphologically: A case report and literature review.

Infantile fibrosarcoma (IFS) commonly harbors ETS variant transcription factor 6 (ETV6)-neurotrophic receptor tyrosine kinase 3 (NTRK3) fusion. However, the recent accessibility to clinical next-generation sequencing (NGS) has revealed ETV6-NTRK3 negative spindle cell sarcomas resembling IFS morphologically, involving NTRK1/2, MET, RET and BRAF. The present report describes a pediatric case of spindle cell sarcoma with KIAA1549-BRAF resembling IFS morphologically. A 20-month-old female patient was referred to Kobe Children's Hospital (Kobe, Japan) for the treatment of intrathoracic spindle cell sarcoma. Pathologically, the intrathoracic tumor cells were composed of spindle cells with focal hemagiopericytomatous pattern. In immunohistochemistry analysis, the intrathoracic tumor cells focally expressed desmin and WT-1 and were negative for pan-tropomyosin receptor kinase (TRK), S-100 and CD34. Fluorescence in situ hybridization analysis for ETV6 and capicua transcriptional repressor revealed negative split signals. Although the patient was initially diagnosed with IFS morphologically, KIAA1549-BRAF fusion transcript was detected by comprehensive genomic profiling with NGS using intrathoracic tumor tissues and confirmed by reverse transcription-PCR. Chemotherapy induced a reduction in the tumor size. At present, the patient is alive with the disease and has been receiving therapy for 8 months since the initiation of chemotherapy. Review of BRAF-altered spindle cell sarcomas resembling IFS morphologically revealed the inconsistency in immunohistochemical expression patterns and the diversity of BRAF fusion genes and mutations. Therefore, the elucidation of genomic profiling by NGS may assist in making an appropriate diagnosis and selecting novel alternative therapies in ETV6-NTRK3-negative spindle cell sarcomas resembling IFS morphologically.

Association between muscle mass evaluated by computed tomography and the serum creatinine-cystatin C ratio in children with cancer: A cross-sectional study.

OBJECTIVE: There is limited knowledge about muscle-mass loss in childhood and adolescent patients with cancer. The aim of this study was to investigate the association between muscle mass evaluated by computed tomography (CT) and the serum creatinine-cystatin C ratio (CCR) in children and adolescents with cancer. METHODS: Patients age <18 y with cancer who underwent abdominal CT scans and blood sampling for serum creatinine and cystatin C within 1 wk before or after the CT scan between 2017 and 2019 at our hospital were retrospectively enrolled. A measurement was defined as a set of abdominal CT scans and serum creatinine and cystatin C levels. The psoas muscle cross-sectional area (PMCSA) was defined as the psoas major muscle area at the level of the fourth lumbar vertebra on axial CT. Statistical tests and modeling were performed as measurement-based analyses. RESULTS: A total of 109 patients and 204 measurements were included in the analysis. CCR showed a strong positive correlation with PMCSA (r = 0.75; P < 0.001). The association between CCR and PMCSA was observed in all age groups and both sexes. There was a clear trend of increased PMCSA across quartile categories of CCR (P < 0.001). On multivariate, generalized, estimating, equation, linear regression model analysis, examining factors associated with PMCSA, male sex, body surface area, and CCR were independent parameters for PMCSA. CONCLUSIONS: These results suggest that CCR may be a useful surrogate marker for muscle mass in cancer care for children and adolescents.

The comparison of acute toxicities associated with craniospinal irradiation between photon beam therapy and proton beam therapy in children with brain tumors.

INTRODUCTION: This study aimed to evaluate acute toxicities associated with irradiation between the X-CSI (photon beam craniospinal irradiation) and P-CSI (proton beam craniospinal irradiation) groups in children with brain tumors. METHODS: Sixty-two consecutive patients who received initial craniospinal irradiation (CSI) for brain tumors in our center between January 1, 2011 and May 31, 2021, were included in the study. Acute toxicities were retrospectively evaluated during CSI using Common Terminology Criteria for Adverse Events version 5.0. Maximum grades of fatigue, headache, insomnia, nausea, vomiting, dermatitis, constipation, abdominal pain, oropharyngeal mucositis, and hematological toxicities were evaluated. RESULTS: Thirty-six patients received X-CSI, and 26 patients received P-CSI. The median dose of CSI was 18.0 Gy in the X-CSI group and 23.4 Gy (relative biological effectiveness) in the P-CSI group (p < 0.001). The P-CSI group had a lower incidence of more than grade 2 nausea (11.5% vs. 69.4%, p = 0.008) and vomiting (7.7% vs. 38.8%, p < 0.001), compared with the X-CSI group. Multivariate logistic regression analysis with adjustments for potential confounding factors of doses of CSI showed that proton radiation therapy was associated with a marked reduced risk of more than grade 2 nausea and vomiting during CSI (adjusted odds ratio, 0.050; 95% confidential interval, 0.011-0.24; p < 0.001). CONCLUSION: The present study suggests that P-CSI reduces the acute gastrointestinal toxicities associated with irradiation.

Poorly Differentiated Chordoma of the Clivus With Loss of SMARCB1 Expression in a Pediatric Patient: A Case Report.

Poorly differentiated chordoma (PDC) is a rare, aggressive subtype of chordoma. A two-year-old girl presented with cervical pain, limb paralysis and respiratory failure. Magnetic resonance imaging and positron emission tomography-computed tomography revealed a tumor compressing the pons at the clivus and osteoblastic metastatic lesions of the left upper arm and right iliac bone. Her tumors shrank substantially after treatment with chemotherapy and proton beam therapy. Our initial diagnosis was an atypical teratoma/rhabdoid tumor, but final diagnosis of PDC was made on the basis of the immunohistochemical expression of brachyury. In addition, the detection of SMARCB1/INI1 mutation confirmed the diagnosis of PDC.

Individual and Nationwide Costs for Cancer Care During the First Year After Diagnosis Among Children, Adolescents, and Young Adults in Japan.

PURPOSE: There has been limited information on the economic evaluation of children, adolescents, and young adults (AYAs) with cancer. The aim of this study was to estimate the individual and nationwide costs for cancer care during the first year after diagnosis among children and AYAs in Japan. MATERIALS AND METHODS: We estimated the direct health care costs for children (0-14 years old) and AYAs (15-29 years old) from the perspective of the public payer. Children and AYAs with newly diagnosed cancer between April 2016 and March 2018 were identified from the Diagnosis Procedure Combination Study Group database to calculate the cost per patient. The nationwide cost was estimated by the bottom-up approach, using stratification by age group, sex, and cancer classification, based on Population Estimates and Cancer Statistics data. RESULTS: A total of 2,939 children and 5,512 AYAs were identified. The median 1-year cost per patient after diagnosis was 2,832,840 (interquartile range, 927,490-9,222,780) JPY (in USD: median, 28,047; interquartile range, 9,183-91,310). The median 1-year cost per patient was higher in children than in AYAs in all cancer classifications. Leukemia, treatment in cancer centers, and early death as well as longer hospital stay were identified to have an impact on 1-year cost per patient after diagnosis. The 1-year nationwide cost after diagnosis was estimated as 34.83 × 109 JPY (344.8 × 106 USD). CONCLUSION: We showed that cancer treatments for both children and AYAs were highly cost-intensive in Japan. Our results suggest the need for further financial and policy evaluation.

Effect of Monoammonium Glycyrrhizinate on the Development of Hepatotoxicity After Initial Intrathecal Chemotherapy for Leukemia.

BACKGROUND/AIM: Chemotherapy for acute leukemia includes agents known to cause hepatotoxicity. This study evaluated the role of monoammonium glycyrrhizinate for the prevention of hepatotoxicity after the first methotrexate-containing intrathecal chemotherapy (ITC) in children and adolescents with leukemia. PATIENTS AND METHODS: Patients with newly diagnosed acute leukemia (age 0-18 years) who received ITC during the first week of induction therapy at our hospital between April 2016 and March 2021 were enrolled. Intravenous monoammonium glycyrrhizinate (IVMG) was defined as the intravenous administration of monoammonium glycyrrhizinate initiated on the day before or the day of the first ITC. RESULTS: Overall, 39 of 118 patients (33%) developed grade 3-4 hepatotoxicity. The inverse probability of treatment weighting logistic regression model showed that IVMG was not associated with the development of grade 3-4 hepatotoxicity (OR=1.9, 95%CI=0.808-4.468). CONCLUSION: IVMG did not protect against the development of grade 3-4 hepatotoxicity after the first methotrexate-containing ITC for leukemia.

Clinical characteristics and risk factors for mortality in children with Pseudomonas aeruginosa bacteraemia: A retrospective review at a paediatric tertiary centre.

AIM: The objective of this study was to describe clinical features and to assess the risk factors associated with mortality in Pseudomonas aeruginosa bacteraemia in a tertiary Japanese paediatric care hospital. METHODS: Patients diagnosed with P. aeruginosa bacteraemia at our hospital between 2007 and 2018 were analysed in a retrospective case series. Inadequate initial therapy for P. aeruginosa bacteraemia was defined as initial treatment without antipseudomonal antibiotics or an administration of antipseudomonal agent to which the causative strain was resistant. Bacteraemia-related death was defined as all deaths occurring within 7 days after the onset of bacteraemia. RESULTS: Overall, 41 patients with 42 P. aeruginosa bacteraemia episodes were identified. The most common underlying condition was malignancy (27%), followed by congenital heart disease (20%) and preterm birth (17%). Among the 42 P. aeruginosa clinical isolates, 24% were resistant to at least one of the antipseudomonal agents and 10% were resistant to more than one agent. The susceptibility levels for piperacillin, fourth-generation cephalosporins and ciprofloxacin were higher than that for carbapenems. Bacteraemia-related death was observed in 43% of episodes. The 30-day all-cause mortality was 50% (standard error 8%). Neonates, intensive care, mechanical ventilation, afebrile episodes, septic shock, hypoxia, renal injury and inadequate initial therapy were associated with bacteraemia-related death episodes. CONCLUSIONS: We found that childhood P. aeruginosa bacteraemia is still a high mortality disease. Our results imply the importance of the identification of high-risk patients and the establishment of adequate empirical antibiotic therapy.

Muscle mass change during chemotherapy in children with high-risk neuroblastoma: a retrospective case series of 24 patients.

The clinical characteristics, cause, and risk factors of sarcopenia are unclear in children. The aim of this study was to describe the course of and identify the factors related to muscle mass change during chemotherapy in children with neuroblastoma. A total of 24 consecutive patients aged below 18 years with newly diagnosed high-risk neuroblastoma between 2010 and 2018 in our hospital were enrolled in a case-series study. The psoas muscle index (PMI) was calculated as a parameter of muscle mass based on computer tomography (CT) images of the psoas muscle. PMIs were evaluated at 4 time points (TPs): TP1, at the diagnosis of neuroblastoma; TP2, after the first cycle of chemotherapy; TP3, after the third cycle of chemotherapy; and TP4, at the end of the induction chemotherapy. PMI recovery was defined as an increase in PMI between TP2 and TP4. The mean PMI decreased by 15% between TP1 and TP2 (TP1 7.09 ± 0.99 vs. TP2 6.01 ± 0.98, P < 0.001) and by 10% between TP1 and TP4 (TP1 7.09 vs. TP4 6.35, P = 0.004). PMI recovery between TP1 and TP2 was observed in 7 (29%) patients. The median age of patients with PMI recovery was significantly lower (2 vs. 4 years, P = 0.028), and the proportion of boys was significantly higher in patients with PMI recovery (100% vs. 41%, P = 0.017).Conclusion: This study demonstrated that prominent PMI reduction occurs during the early time of chemotherapy, and a younger age and male sex may be predictive factors for PMI recovery. What is Known: • Sarcopenia is a common disorder in elderly people. • Several causes and risk factors have been reported in adults. • Children with previous hematological malignancies have decreased physical activity. What is New: • Prominent muscle mass loss was observed early in children with high-risk neuroblastoma during chemotherapy. • Age and sex were found to be potentially associated with muscle mass recovery.

Paranasal sinusitis at the initiation of chemotherapy is a risk factor for invasive fungal disease in children and adolescents with cancer.

BACKGROUND: The impact of paranasal sinusitis on the clinical outcome of patients with cancer remains unknown. The aim of this study was to determine whether paranasal sinusitis at the initiation of chemotherapy (SAI) affects the development of infectious complications in children and adolescents with cancer. METHODS: A retrospective cohort analysis of patients aged 0-20 years with cancer who received chemotherapy was performed. SAI was defined as the presence of a fluid level or mucosal swelling or total opacity on sinus computed tomography examination before the initiation of chemotherapy. The primary outcome measures were the incidence of bacteremia, septic shock, and invasive fungal disease (IFD, including proven, probable, and possible cases). RESULTS: SAI was observed in 57 (44%) of 130 enrolled patients. There were no significant differences in age, sex, and disease distribution between the patients with SAI (SAI group) and those without (non-SAI group). There was no significant difference in the 1-year cumulative incidence of bacteremia or septic shock after treatment initiation between the two groups (bacteremia, SAI group 33% vs. non-SAI group 35%, P = 0.53; septic shock, SAI group 4% vs. non-SAI group 4%, P = 0.87). The 1-year cumulative incidence of IFD was higher in the SAI group than in the non-SAI group (22% vs. 6%, P = 0.012). Cumulative incidence analysis after inverse probability of treatment weighting adjustment showed that the SAI group was more likely to develop IFD (HR: 3.5, 95% CI: 1.1-11.2, P = 0.033). CONCLUSIONS: Our findings suggest that patients with SAI may be at higher risk for IFD during chemotherapy.

Efficacy of liposomal amphotericin against febrile neutropenia in pediatric patients receiving prophylactic voriconazole.

BACKGROUND: The risk factors for invasive fungal infection have gradually become evident for pediatric patients with hematological diseases. Here we analyze the efficacy of liposomal amphotericin (L-AMB) for pediatric patients with febrile neutropenia using prophylactic voriconazole (VRCZ). METHOD: We administered L-AMB (2.5 mg/kg/day) in patients with febrile neutropenia who were receiving prophylactic VRCZ (10 mg/kg/day, orally) and were resistant to second-line antibiotics therapy. Thirteen patients (5 males, 8 females) with 19 febrile neutropenia episodes were targeted in this analysis. The median age of the patients was 14 years (range, 1-19 years). Eighteen out of 19 episodes occurred in patients with acute myeloid leukemia, with the remaining episode occurring in a patient with acute unclassified leukemia. RESULTS: The median period from start of L-AMB administration to resolution of fever was 4 days (1-27 days). In 15 out of 19 episodes, fever resolved within 5 days from commencement of L-AMB administration. Using criteria proposed by T. J. Walsh et al., the success rate of L-AMB for febrile neutropenia was 89.5% in this study. CONCLUSIONS: Although the sample size of our study was small, the extremely high efficacy of L-AMB warrants its administration in patients with febrile neutropenia who are receiving VRCZ.

Pharmacokinetic analysis for model-supported therapeutic drug monitoring of busulfan in Japanese pediatric hematopoietic stem cell transplantation recipients.

This prospective observational study analyzed the pharmacokinetics of busulfan in Japanese children and evaluated the predicting accuracy of previous pediatric PPK models of busulfan. This study enrolled five patients (aged 2-12 years, BW 14-48 kg) receiving a busulfan-based conditioning regimen for hematopoietic stem cell transplantation at our hospital between January 2017 and December 2018. All patients received a 2-hour intravenous busulfan infusion four times daily for a total of 16 doses. After the infusions, 51 plasma samples were collected with the plasma busulfan concentration measured by liquid chromatography-tandem mass spectrometry. PPK model fitting was analyzed using the (%MPE) and the (%MAPE). Limited sampling strategies for estimating busulfan AUC were also evaluated. High interpatient variability was observed in the PK parameters. The most suitable PPK model that reflected our data was McCune's two-compartment model (%MPE -8.7, %MAPE 19.3). A combination sampling method using the busulfan concentration at 2 and 6 hours after the start of the first busulfan dose was found to be able to estimate AUC4 day . These results provide useful information on busulfan therapeutic drug monitoring in the Japanese pediatric population.

Level of Seven Neuroblastoma-Associated mRNAs Detected by Droplet Digital PCR Is Associated with Tumor Relapse/Regrowth of High-Risk Neuroblastoma Patients.

Monitoring of several sets of neuroblastoma-associated mRNAs (NB-mRNAs) by real-time quantitative PCR (qPCR) can be used to evaluate minimal residual disease in NB patients. Droplet digital PCR (ddPCR) is an adaption of qPCR that potentially provides simpler and more reproducible detection of low levels of mRNAs. However, whether minimal residual disease in NB patients can be monitored by ddPCR using a set of NB-mRNAs is not yet tested. In this study, 208 bone marrow (BM) and 67 peripheral blood samples were retrospectively collected from 20 high-risk NB patients with clinical disease evaluation at two Japanese centers between 2011 and 2018, and level of each NB-mRNA (CRMP1, DBH, DDC, GAP43, ISL1, PHOX2B, and TH mRNAs) was determined by ddPCR. Level of 7NB-mRNAs (defined as the combined signature of each NB-mRNA) was higher in BM than peripheral blood, but correlated significantly with each other. In accordance with disease burden, it varied with disease status (remission, stable, or progression) and collection time point (diagnosis, treatment, post-treatment, or relapse). In 73 post-treatment BM samples, it was significantly higher in 17 relapsed/regrown samples than in 56 nonrelapsed/nonregrown samples. Furthermore, ddPCR had a better prognostic value than qPCR in detecting 7NB-mRNAs in the same 73 post-treatment BM samples. This study suggests that ddPCR detection of 7NB-mRNAs is significantly associated with tumor relapse/regrowth in high-risk NB patients.

Dynamics of Minimal Residual Disease in Neuroblastoma Patients.

Neuroblastoma is a common extracranial solid tumor of neural crest (NC) origin that accounts for up to 15% of all pediatric cancer deaths. The disease arises from a transient population of NC cells that undergo an epithelial-mesenchymal transition (EMT) and generate diverse cell-types and tissues. Patients with neuroblastoma are characterized by their extreme heterogeneity ranging from spontaneous regression to malignant progression. More than half of newly diagnosed patients present highly metastatic tumors and are stratified into a high-risk group with dismal outcome. As many as 20% of high-risk patients have residual disease that is refractory or progressive during induction chemotherapy. Although a majority of high-risk patients achieve remission, larger part of those patients has minimal residual disease (MRD) that causes relapse even after additional consolidation therapy. MRD is composed of drug-resistant tumor cells and dynamically presented as cancer stem cells (CSCs) in residual tumors, circulating tumor cells (CTCs) in peripheral blood (PB), and disseminated tumor cells (DTCs) in bone marrow (BM) and other metastatic sites. EMT appears to be a key mechanism for cancer cells to acquire MRD phenotypes and malignant aggressiveness. Due to the restricted availability of residual tumors, PB and BM have been used to isolate and analyze CTCs and DTCs to evaluate MRD in cancer patients. In addition, recent technical advances make it possible to use circulating tumor DNA (ctDNA) shed from tumor cells into PB for MRD evaluation. Because MRD can be detected by tumor-specific antigens, genetic or epigenetic changes, and mRNAs, numerous assays using different methods and samples have been reported to detect MRD in cancer patients. In contrast to the tumor-specific gene-rearrangement-positive acute lymphoblastic leukemia (ALL) and the oncogenic fusion-gene-positive chronic myelogenous leukemia (CML) and several solid tumors, the clinical significance of MRD remains to be established in neuroblastoma. Given the extreme heterogeneity of neuroblastoma, dynamics of MRD in neuroblastoma patients will hold a key to the clinical validation. In this review, we summarize the biology and detection methods of cancer MRD in general and evaluate the available assays and clinical significance of neuroblastoma MRD to clarify its dynamics in neuroblastoma patients.