Primary Intraosseous Spindle Cell Rhabdomyosarcoma: A Case Report in an Unusual Location.

Spindle cell/sclerosing rhabdomyosarcoma is an infrequent subtype of rhabdomyosarcoma according to the World Health Organization Classification of Soft Tissue and Bone Tumours, which includes a novel category of intraosseous spindle-cell rhabdomyosarcomas (ISCRMS) with EWSR1:: or FUS::TFCP2 fusions. We report a case of ISCRMS with EWSR1::TFCP2 fusion presenting in the femur mimicking osteosarcoma in this unusual primary location. We present an 18-year-old male with relapsed widely metastatic sarcoma, morphologically identical to osteosarcoma responding poorly to chemotherapy, initially presenting in the distal femur. Sections showed a high-grade malignant neoplasm with sheets of epithelioid and spindled cells without obvious rhabdomyoblastic differentiation morphologically containing focal areas resembling new bone/osteoid formation. Molecular sequencing identified t(12;22) EWSR1::TFCP2. The tumor cells were diffusely positive for pancytokeratin, MyoD1, and ALK by retrospective immunohistochemistry. Desmin and SATB2 were focally positive. Myogenin was negative, and INI-1 expression was retained. ISCRMS commonly involves craniofacial and pelvic bones, but rarely originates in long bones, as in this case. Initially, osteosarcoma was the primary diagnostic consideration based on distal long bone location, patient age, and evidence of osteoid formation. Distinction between the two entities may be nearly impossible on morphologic grounds alone, which presents a diagnostic pitfall without molecular or extensive immunoprofiling data.

Clinical relevance of inflammation on rectal biopsy for Hirschsprung disease: An outcomes analysis.

OBJECTIVES: Inflammation on diagnostic rectal biopsy for children with suspected Hirschsprung disease (HSCR) is reported on pathology, and its significance is unknown. We describe the management and outcomes of a cohort with inflammation on rectal biopsy compared to those without. Specifically, to address the hypothesis that inflammation on diagnostic biopsy is associated with increased complication rates irrespective of intervention type and timing. METHODS: A single institution retrospective review of children with HSCR who underwent biopsy and endorectal pull-through (ERPT) from 2010 to 2020 was performed. The primary outcome was overall complications at 30-days following ERPT. Secondary outcomes included timing and type of operative intervention as well as postoperative enterocolitis diagnosed within 6-months of ERPT. RESULTS: Forty-nine children were identified; inflammation was present on diagnostic biopsy for 17 children. Those with inflammation were more likely to have clinical evidence of enterocolitis at the time of biopsy (p = 0.001) and were more likely to undergo leveling colostomy before ERPT (p = 0.01). Children with inflammation had a higher anastomotic leak rate (p = 0.04). Subgroup analysis of patients with inflammation undergoing primary ERPT versus leveling colostomy demonstrated no significant difference in outcomes following definitive ERPT. CONCLUSIONS: Our study suggests inflammation on diagnostic rectal biopsy for HSCR is associated with increased anastomotic leak rates. While additional prospective studies are indicated, attention to methods of mitigating inflammation and confirming its resolution before definitive pull-through may be of benefit for improving clinical outcomes in patients found with inflammation on diagnostic rectal biopsy.

Well-Differentiated Neuroendocrine Tumors of the Appendix in Children and Adolescents: A Clinicopathologic Study.

BACKGROUND AND AIMS: Pediatric neuroendocrine tumors (NET) of the GI tract are rare and appendiceal NET are typically incidental. Few studies have been done in the pediatric population and practice guidelines are mainly based on adult data. There are currently no diagnostic studies specific for NET. Our study aimed to identify clinical, radiological, and pathological findings in pediatric appendiceal NET, test criteria for follow up surgical treatment, review potential prognostic pathological findings, and possible pre-operative diagnostic radiological studies. MATERIALS AND METHODS: A retrospective data search was conducted for well-differentiated NET of the appendix in patients ≤21 years between 1/1/2003 and 7/1/2022. Available clinical, radiologic, pathological, and follow-up information was recorded. RESULTS: Thirty-seven patients with appendiceal NET were identified. No masses were reported in the patients who underwent presurgical imaging. Appendectomy samples showed NET (0.2->4 cm), most located in the tip. Most cases were WHO G1 (34/37), with negative margins (n = 25). Sixteen cases extended to the subserosa/mesoappendix (pT3). Lymphovascular (6), perineural (2), and both lymphovascular and perineural invasion were also noted (2). The specified tumor stages were pT1 (10/37), pT3 (16/37), and pT4 (4/37). Patients who underwent laboratory testing for chromogranin A (20) and urine 5HIAA (11) had normal limits. Subsequent surgical resection was recommended in 13 cases and performed in 11. To date, all patients have no recurrent or additional metastatic disease. CONCLUSIONS: Our study showed that all pediatric well-differentiated appendiceal NET were incidentally found as part of acute appendicitis management. Most NET were localized with low-grade histology. Our small cohort support the previously suggested management guidelines with follow up resection in certain cases. Our radiologic review didn't identify a best modality for NET. Comparing cases with and without metastatic disease, no tumors under 1 cm had metastasis, but serosal and perineural invasion along with G2 status were associated with metastasis in our limited study.

Secretory Carcinoma in Children and Young Adults: A Case Series.

Secretory carcinoma (SC), previously known as mammary analogue secretory carcinoma, is a rare salivary gland neoplasm that typically presents as a slow-growing painless lesion in the head and neck. SC occurs mainly in adults but has been described in children with the youngest reported patient diagnosed at five years of age. In children the gender distribution has been reported as female to male ratio of 1:1.2. SC is generally considered a low-grade malignancy with characteristic morphological features and immunological profile. SC also harbors ETV6-NTRK3 fusion (t(12;15)(p13:q25)). Surgical resection with or without lymph node dissection is the standard treatment, with generally favorable clinical outcomes. Here we present a single institution case series of six patients (ages 9-21) with SC and a review of the previously described pediatric cases. Our small series showed male predominance in pediatric patients with predominantly low-grade and stage tumors. All cases underwent complete surgical resections and when follow up is available there was no evidence of recurrences or metastases. To the best of our knowledge, this is the only SC case series comprised exclusively of pediatric and youth patients.

Case report of heart transplantation for giant cell myocarditis in a patient with common variable immunodeficiency.

BACKGROUND: Solid-organ transplantation in patients with common variable immunodeficiency (CVID) is controversial due to the risk for severe and recurrent infections. Determining transplantation candidacy in CVID patients is further complicated by the presence of CVID-related non-infectious complications that can reduce overall survival and also recur in the transplanted organ. Data regarding solid organ transplantation in patients with CVID are limited, particularly in heart transplantation. CASE SUMMARY: A 32-year-old female with CVID presented with new heart failure after 3 months of dyspnoea on exertion. Her echocardiogram showed severe global systolic dysfunction with an ejection fraction of approximately 10%, and her right heart catheterization revealed severe biventricular pressure overload and severely reduced cardiac output. Endomyocardial biopsy revealed giant cells and mononuclear infiltrate consistent with giant cell myocarditis (GCM). Despite medical management, she developed progressive cardiogenic shock and underwent uncomplicated orthotopic heart transplantation on hospital Day 38. After 2 years of follow-up, she has had no major infectious complications and continues to have normal graft function with no recurrence of GCM. CONCLUSION: We report a case of successful heart transplantation for GCM in a patient with CVID, with no major infectious complications after 2 years of follow-up. CVID should not be considered an absolute contraindication for heart transplantation.

Case Report: A Relatively Mild Presentation of Unilateral Congenital Pulmonary Lymphangiectasia.

Pulmonary lymphangiectasia (PL) is a rare congenital disorder of pulmonary lymphatic development. Although it was traditionally a fatal disorder of infancy, some cases in later childhood have been reported, suggesting a spectrum of severity. We present an unusual case of unilateral, congenital pulmonary lymphangiectasia. Our patient presented with neonatal respiratory distress, a chronic wet cough and recurrent episodes of bronchitis. Chest CT revealed thickening of the interlobular septae of the right lung. A lung biopsy confirmed the diagnosis of lymphangiectasia. His clinical course is characterized by chronic coughing and recurrent bronchitis but normal growth and development. This case illustrates a relatively mild presentation of unilateral PL, which, along with other reports, suggests variability in the presentation and severity of this disorder. In the absence of imaging and histological examination, mild presentations may go undiagnosed.

Single nucleotide polymorphism array and cytogenetic analyses of ovarian teratomas in children.

Teratomas are the most common tumors in the ovary during childhood. Previous studies suggested that they may be derived from germ cells at any developmental stage from premeiotic oogonia through meiotic oocytes to post-meiotic ova. The majority of mature teratomas reveal normal karyotypes and immature teratomas show higher frequency of chromosomal abnormalities. We analyzed fresh tissue samples from 25 primary ovarian teratomas and three extraovarian deposits using whole genome single nucleotide polymorphism (SNP) array and karyotype. SNP array detected five patterns of copy neutral loss of heterozygosity (CN-LOH): failure of meiosis I (type I) in 12 tumors, failure of meiosis II (type II) in six tumors, endoreduplication of a haploid ovum (type III) in two tumors, premeiotic error (type IV) in four tumors, and both meiotic I and meiotic II errors in one tumor (type V). Three tumors with type I error had a single chromosome showing meiotic II error, and two tumors with type II error had a single chromosome showing premature sister-chromatid separation in meiosis I. Lack of recombination in multiple chromosomes in meiosis I were common, chromosomes 17, 7, 8, 21, and 22 were most commonly involved. Abnormal karyotypes were observed in four teratomas including +3, del(3q), +7, +8, +12, and i(18q). The extraovarian deposits revealed the same CN-LOH pattern as the primary teratoma. In summary, SNP array reveals the origin of ovarian teratoma and we propose a new mechanism that consecutive meiotic I and II errors occur frequently in ovarian teratomas.

Overlap of lymphatic dysplasia in Fontan-associated protein-losing enteropathy and Mucosa-Associated Lymphoid Tissue (MALT lymphoma): implications for management of protein-losing enteropathy.

Lymphatic vessel dysplasia is associated with Fontan-associated protein-losing enteropathy. Extra nodal non-Hodgkin lymphomas including mucosa-associated lymphoid tissue (MALT lymphoma) are associated with lymphatic vessel dysplasia. Here, we describe the case of a 7-year-old with Fontan-associated protein-losing enteropathy who developed MALT lymphoma with a clinical course indicative of interaction between these pathologies and improvement in protein-losing enteropathy after MALT lymphoma treatment. This case suggests a pathophysiologic overlap which has implications for the management of Fontan-associated protein-losing enteropathy.

A Case of 45,X/46,XY Mosaicism Presenting as Swyer Syndrome.

BACKGROUND: Swyer syndrome is a difference of sex development that is typically associated with mutations in genes responsible for testicular development. It is speculated that some cases may result from cryptic 45,X/46,XY mosaicism leading to abnormal gonadal development. The presence or absence of a 45,X lineage is important for prognosis and management. CASE: We present a case of apparent Swyer syndrome associated with a 46,XY chromosomal complement in lymphocytes and 45,X/46,XY mosaicism on analysis of her noncancerous gonad. Gonadal histology was consistent with a 45,X phenotype. SUMMARY AND CONCLUSION: This case demonstrates the clinical variability in the presentation of 45,X/46,XY mosaicism and highlights the importance of thorough genetic testing that includes consideration of chromosomal mosaicism. We will discuss the implications of this diagnosis for management.

Recurrent endobronchial inflammatory myofibroblastic tumors: Novel treatment options.

Endobronchial inflammatory myofibroblastic tumors (IMTs) rarely occur in children younger than 10 years of age and have intermediate malignant potential. A 7-year-old girl initially presented with pneumonia. After failing outpatient treatment, she re-presented in status asthmaticus. Computed tomography showed a left mainstem endobronchial mass which was resected bronchoscopically. Pathology was consistent with IMT. Surveillance bronchoscopy identified a recurrence. Despite a left upper lobectomy, recurrence led to further treatment with celecoxib and argon plasma coagulation. Follow-up bronchoscopy revealed complete resolution. She remains disease and symptom-free at her six-year follow-up.

Malignant risk of indeterminate pediatric thyroid nodules-An institutional experience.

BACKGROUND: Few studies focus on pediatric thyroid nodules categorized under indeterminate diagnostic categories. The current study was conducted to assess the risk of malignancy of indeterminate pediatric thyroid nodules. METHODS: A search of the institutional electronic pathology database from 01/2011 to 09/2018 was performed to identify pediatric (<21 years old) thyroid nodules that were interpreted as follicular lesion of undetermined significance (FLUS), suspicious for follicular neoplasm (SFN), or suspicious for malignancy (SFM) and subsequently managed with surgery, repeat fine-needle aspiration (FNA), or ≥ 6 months of clinical/imaging monitoring. Results of follow-up (F/U) surgical resections and repeat FNA/Afirma tests, and clinical and radiologic data were collected. RESULTS: We identified 46 cases from 42 patients (11-20 years old, 33 females and 9 males), including 30 FLUS, 10 SFN, and 6 SFM. Twenty-five FLUS, ten SFN, and six SFM cases underwent surgery. The histology revealed carcinomas in 36% of FLUS, 20% of SFN, and 100% of SFM categories; follicular adenomas in 32% of FLUS and 80% of SFN categories; and benign nodules in 32% of FLUS category. All five nonsurgically treated FLUS cases were considered benign based on the findings of repeat FNA/Afirma tests (n = 3, 3-22 months F/U) or clinical/radiologic exams (n = 2, 8-12 months F/U). CONCLUSIONS: Based on a limited study cohort, malignancy was identified in 36%, 20%, and 100% of surgically managed pediatric thyroid nodules categorized as FLUS, SFN, and SFM, respectively; suggesting a markedly higher malignant rate than the implied malignant risk for FLUS and SFM categories in adults.

Adolescent Vulvar Angiokeratoma Associated with Lichen Sclerosus.

BACKGROUND: We present an adolescent with multiple vulvar angiokeratomas within a background of lichen sclerosus. CASE: A 13-year-old girl presented with vulvar pruritus and wart-like vulvar lesions. Four lesions were resected because of discomfort and uncertainty of the diagnosis. Pathology revealed angiokeratomas with chronic inflammation suggestive of lichen sclerosus. Postoperatively, pruritus continued in the largest excised lesion, which was associated with lichen sclerosus, and symptoms were treated successfully with topical steroids. SUMMARY AND CONCLUSION: Vulvar angiokeratomas are asymptomatic red papular lesions and are rare in the female adolescent population. In this case, the pathology revealed the rare co-occurrence of angiokeratomas and lichen sclerosus. Biopsies of vulvar vascular lesions in symptomatic adolescents are recommended. Vulvar angiokeratomas might manifest rare genetic disease in otherwise asymptomatic female patients and warrant further follow-up.

Guidelines for synoptic reporting of surgery and pathology in Hirschsprung disease.

BACKGROUND/PURPOSE: Synoptic, or standardized, reporting of surgery and pathology reports has been widely adopted in surgical oncology. Patients with Hirschsprung disease may experience morbidity related to surgical factors or underlying pathology and often undergo multiple operations. Our aim is to improve the postoperative outcome and care of patients with Hirschsprung disease by proposing a standardized set of data that should be included in every surgery and pathology report. METHODS: Members of the American Pediatric Surgical Association Hirschsprung Disease Interest Group and experts in pediatric pathology of Hirschsprung disease participated in group discussions, performed literature review and arrived at expert consensus guidelines for surgery and pathology reporting. RESULTS: The importance of accurate operative and pathologic reports and the implications of inadequate documentation in patients with Hirschsprung disease are discussed and guidelines for standardizing these reports are provided. CONCLUSIONS: Adherence to the principles of reporting for operations and surgical pathology may improve outcomes for Hirschsprung disease patients and will facilitate identification of correlations among morphology, function, genetics and outcomes, which are required to improve the overall management of these patients. LEVEL OF EVIDENCE: V.

Splenic development and injury in premature lambs supported by the artificial placenta.

INTRODUCTION: The purpose of this study is to evaluate splenic effects during artificial placenta (AP) support. METHODS: AP lambs (118-121 d, n = 14) were delivered and placed on the AP support for a goal of 10-14 days. Cannulation used right jugular drainage and umbilical vein reinfusion. Early (ETC; 115-120 d; n = 7) and late (LTC; 125-131 d; n = 7) tissue controls were delivered and immediately sacrificed. Spleens were formalin fixed, H&E stained, and graded for injury, response to inflammation, and extramedullary hematopoiesis (EMH). CD68 and CD163 stains were used to assess for macrophage activation and density. Clinical variables were correlated with splenic scores. Groups were compared using Fisher's Exact Test and descriptive statistics. p < 0.05 indicated significance. RESULTS: Mean survival for AP lambs was 12 ±â€¯5 d. There was no necrosis found in any of the groups. Vascular congestion and sinusoidal histiocytosis did not significantly differ between AP and control groups (p = 0.72; p = 0.311). There were significantly more pigmented macrophages (p = 0.008), CD163 (p = <0.001), and CD68 (p = <0.001) stained cells in the AP group. ETC and LTC demonstrated more EMH than AP spleens (p = <0.001). CONCLUSIONS: During AP support, spleens appear to develop normally and exhibit an appropriate inflammatory response. After initiation of AP support, EMH transitions away from the spleen. STUDY TYPE: Research Paper/Therapeutic Potential. LEVEL OF EVIDENCE: N/A.

Refractory and metastatic infantile fibrosarcoma harboring LMNA-NTRK1 fusion shows complete and durable response to crizotinib.

Infantile fibrosarcoma (IFS) is a rare soft-tissue sarcoma, which classically presents as an aggressive and rapidly enlarging tumor over the distal extremities of children in their first year of life. The presence of ETV6 and NTRK3 gene rearrangement is characteristic of IFS, which can be detected on routine fluorescence in situ hybridization (FISH) testing. Patients with IFS typically respond well to surgical resection and chemotherapy and have an overall survival of ∼90%. In this report, we outline the use of integrative clinical sequencing (ICS) including RNA-seq in a patient with refractory, metastatic IFS to reveal an unusual fusion (LMNA-NTRK1), not detected by routine FISH testing, which was treated with oral crizotinib and resulted in a complete and durable long-term response. This study highlights the utility of ICS in identifying cryptic gene fusions, especially in refractory malignancies, and demonstrates how such information can be used to select targeted therapies in patients with actionable molecular alterations.

An Artificial Placenta Protects Against Lung Injury and Promotes Continued Lung Development in Extremely Premature Lambs.

An artificial placenta (AP) utilizing extracorporeal life support (ECLS) could protect premature lungs from injury and promote continued development. Preterm lambs at estimated gestational age (EGA) 114-128 days (term = 145) were delivered by Caesarian section and managed in one of three groups: AP, mechanical ventilation (MV), or tissue control (TC). Artificial placenta lambs (114 days EGA, n = 3; 121 days, n = 5) underwent venovenous (VV)-ECLS with jugular drainage and umbilical vein reinfusion for 7 days, with a fluid-filled, occluded airway. Mechanical ventilation lambs (121 days, n = 5; 128 days, n = 5) underwent conventional MV until failure or maximum 48 hours. Tissue control lambs (114 days, n = 3; 121 days, n = 5; 128 days, n = 5) were sacrificed at delivery. At the conclusion of each experiment, lungs were procured and sectioned. Hematoxylin and eosin (H&E) slides were scored 0-4 in seven injury categories, which were summed for a total injury score. Slides were also immunostained for platelet-derived growth factor receptor (PDGFR)-α and α-actin; lung development was quantified by the area fraction of double-positive tips of secondary alveolar septa. Support duration of AP lambs was 163 ± 9 (mean ± SD) hours, 4 ± 3 for early MV lambs, and 40 ± 6 for late MV lambs. Total injury scores at 121 days were 1.7 ± 2.1 for AP vs. 5.5 ± 1.6 for MV (p = 0.02). Using immunofluorescence, double-positive tip area fraction at 121 days was 0.017 ± 0.011 in AP lungs compared with 0.003 ± 0.003 in MV lungs (p < 0.001) and 0.009 ± 0.005 in TC lungs. At 128 days, double-positive tip area fraction was 0.012 ± 0.007 in AP lungs compared with 0.004 ± 0.004 in MV lungs (p < 0.001) and 0.016 ± 0.009 in TC lungs. The AP is protective against lung injury and promotes lung development compared with mechanical ventilation in premature lambs.

Short bowel mucosal morphology, proliferation and inflammation at first and repeat STEP procedures.

BACKGROUND: Although serial transverse enteroplasty (STEP) improves function of dilated short bowel, a significant proportion of patients require repeat surgery. To address underlying reasons for unsuccessful STEP, we compared small intestinal mucosal characteristics between initial and repeat STEP procedures in children with short bowel syndrome (SBS). METHODS: Fifteen SBS children, who underwent 13 first and 7 repeat STEP procedures with full thickness small bowel samples at median age 1.5 years (IQR 0.7-3.7) were included. The specimens were analyzed histologically for mucosal morphology, inflammation and muscular thickness. Mucosal proliferation and apoptosis was analyzed with MIB1 and Tunel immunohistochemistry. RESULTS: Median small bowel length increased 42% by initial STEP and 13% by repeat STEP (p=0.05), while enteral caloric intake increased from 6% to 36% (p=0.07) during 14 (12-42) months between the procedures. Abnormal mucosal inflammation was frequently observed both at initial (69%) and additional STEP (86%, p=0.52) surgery. Villus height, crypt depth, enterocyte proliferation and apoptosis as well as muscular thickness were comparable at first and repeat STEP (p>0.05 for all). Patients, who required repeat STEP tended to be younger (p=0.057) with less apoptotic crypt cells (p=0.031) at first STEP. Absence of ileocecal valve associated with increased intraepithelial leukocyte count and reduced crypt cell proliferation index (p<0.05 for both). CONCLUSIONS: No adaptive mucosal hyperplasia or muscular alterations occurred between first and repeat STEP. Persistent inflammation and lacking mucosal growth may contribute to continuing bowel dysfunction in SBS children, who require repeat STEP procedure, especially after removal of the ileocecal valve. LEVEL OF EVIDENCE: Level IV, retrospective study.

DICER1 Mutations in the Era of Expanding Integrative Clinical Sequencing in Pediatric Oncology.

PURPOSE: DICER1 syndrome is a recently described inherited cancer predisposition syndrome caused by pathogenic variants in DICER1. With the recent increase in integrative clinical sequencing for pediatric patients with cancer, our understanding of the DICER1 syndrome continues to evolve, as new and rare pathogenic variants are reported. As the frequency of integrative clinical sequencing increases, discussions regarding challenges encountered in the interpretation of sequencing results are essential to continue to advance the field of cancer predisposition. The purpose of this work was to identify patients with somatic and/or germline DICER1 variants in our patient population and to discuss sequencing interpretation and the clinical recommendations that result from the integrative clinical sequencing results. METHODS: Patients were enrolled in the PEDS-MIONCOSEQ study. This integrative clinical sequencing study includes paired tumor/normal whole-exome sequencing and tumor transcriptome sequencing. Patients identified as having DICER1 variants were included. RESULTS: We report a DICER1 variant of unknown clinical significance in a patient with a highly unusual response to therapy. Two patients had diagnoses clarified once the integrative clinical sequencing revealing a DICER1 variant was available. We also discovered a patient with low-level DICER1 mosaicism and the challenges encountered in the sequencing interpretation for this patient. In addition to the sequencing data and result interpretation, this work also highlights testing and screening recommendations made to patients with DICER1 variants and their families on the basis of these results. CONCLUSION: This work serves to extend the DICER1 phenotype and advance the utility of clinical integrative sequencing in the fields of pediatric oncology and cancer genetic predisposition.

Multiple Cavitary Lung Lesions in an Adolescent: Case Report of a Rare Presentation of Nodular Lymphocyte Predominant Hodgkin Lymphoma.

A 14-year-old male patient presented with a nonproductive cough, weight loss, fatigue, and malaise. A chest radiograph showed large bilateral cavitary lung lesions in both upper and lower lobes that failed to improve with antibiotics and anti-inflammatory medications. Infectious and rheumatologic work-ups were negative. Thoracoscopic lung biopsies were diagnostic for nodular lymphocyte predominant Hodgkin lymphoma (NLPHL). The patient received combination chemotherapy and immunotherapy based on current treatment standards with an excellent clinical response. NLPHL is a rare B-cell lymphoma that typically presents as peripheral lymph nodal disease, clinically distinct from classical Hodgkin lymphoma. The prognosis of NLPHL in children is favorable, although relapse rates are high. This case details several unique features of NLPHL and describes the presentation, diagnosis, and treatment of an adolescent male with a rare pulmonary and cervical NLPHL, the first such case described in a pediatric patient.

The artificial placenta: Continued lung development during extracorporeal support in a preterm lamb model.

PURPOSE: An artificial placenta (AP) utilizing extracorporeal life support (ECLS) could avoid the harm of mechanical ventilation (MV) while allowing the lungs to develop. METHODS: AP lambs (n = 5) were delivered at 118 days gestational age (GA; term = 145 days) and placed on venovenous ECLS (VV-ECLS) with jugular drainage and umbilical vein reinfusion. Lungs remained fluid-filled. After 10 days, lambs were ventilated. MV control lambs were delivered at 118 ("early MV"; n = 5) or 128 days ("late MV"; n = 5), and ventilated. Compliance and oxygenation index (OI) were calculated. After sacrifice, lungs were procured and H&E-stained slides scored for lung injury. Slides were also immunostained for PDGFR-α and α-actin; alveolar development was quantified by the area fraction of alveolar septal tips staining double-positive for both markers. RESULTS: Compliance of AP lambs was 2.79 ±â€¯0.81 Cdyn compared to 0.83 ±â€¯0.19 and 3.04 ±â€¯0.99 for early and late MV, respectively. OI in AP lambs was lower than early MV lambs (6.20 ±â€¯2.10 vs. 36.8 ±â€¯16.8) and lung injury lower as well (1.8 ±â€¯1.6 vs. 6.0 ±â€¯1.2). Double-positive area fractions were higher in AP lambs (0.012 ±â€¯0.003) than early (0.003 ±â€¯0.0005) and late (0.004 ±â€¯0.002) MV controls. CONCLUSIONS: Lung development continues and lungs are protected from injury during AP support relative to mechanical ventilation. LEVEL OF EVIDENCE: n/a (basic/translational science).