Kaposiform Hemangioendothelioma |
| Kaposiform
hemangioendothelioma (KHE) is a rare, locally aggressive vascular tumor
that primarily affects infants and young children, though it can
occasionally present in adults. First distinguished as a separate
entity in 1993, KHE occupies an intermediate position in the spectrum
of vascular neoplasms — neither fully benign nor overtly
malignant — and is characterized by its capacity for locally
invasive and infiltrative growth. Its estimated annual incidence is
approximately 0.071 per 100,000 children, though the true prevalence is
likely higher, as small or atypical lesions may be misclassified as
other vascular anomalies. A slight male predominance has been noted in
larger case series. The underlying cause of KHE remains incompletely understood. Most cases arise sporadically without an identifiable trigger, though rare reports describe worsening after trauma, surgery, infection, or vaccination, suggesting that inflammatory stimuli may aggravate the disease. At the molecular level, somatic mutations — including activating variants in the GNA14 gene — have been identified in a subset of specimens, implicating the MAPK/ERK signaling pathway. Whether these mutations are causative or secondary is still debated. Dysregulation of both angiogenesis and lymphangiogenesis are central to KHE's pathology. Elevated vascular endothelial growth factor-C and its receptor (VEGFR-3), along with high angiopoietin-2 levels, have been implicated in driving abnormal vessel formation and contributing to the consumptive coagulopathy that defines the disease's most dangerous complication. On histopathology, KHE is recognized by infiltrating, rounded, confluent nodules composed of spindle-shaped endothelial cells that form malformed lymphatic channels and slit-like vascular lumina. Microthrombi, hemosiderin deposits, and eosinophilic hyaline bodies are frequently observed. Immunohistochemical staining shows positivity for vascular markers CD31 and CD34, and lymphatic markers D2-40, VEGFR-3, and Prox-1, while staining is negative for GLUT-1 — a feature that distinguishes KHE from infantile hemangioma — and for HHV-8. KHE exhibits considerable clinical heterogeneity. Lesions are classified by depth of involvement into three morphological types: superficial (confined to skin and subcutaneous tissue), mixed (involving both superficial and deep structures including muscle, bone, or joint), and deep (no cutaneous involvement, affecting internal organs, retroperitoneum, mediastinum, or bone). A further clinically important subgroup is intracavitary KHE — tumors arising primarily within body cavities such as the thoracic or abdominal cavity, retroperitoneum, or pelvic space, where the bulk of the lesion is contained within the cavity and may involve serosal surfaces, mesentery, or cavity-specific structures. The mixed type is the most common overall, while deep and intracavitary lesions account for a smaller proportion of cases but carry the greatest diagnostic challenge and the most severe clinical burden. Cutaneous KHE typically presents as erythematous, violaceous, or darkened patches, plaques, or firm nodules. When associated with the Kasabach-Merritt phenomenon (KMP), lesions become tense, engorged, purpuric, hot to the touch, and painful. Deep lesions without skin involvement may present with nonspecific symptoms such as abdominal distension, jaundice, respiratory distress, or unexplained thrombocytopenia and coagulopathy, often resulting in significant diagnostic delay. Intracavitary lesions in particular may compress adjacent organs or critical structures as they enlarge, leading to pleural effusion, pericardial effusion, peritoneal effusion, obstructive jaundice, gastrointestinal bleeding, or airway compromise. Approximately 90% of cases become evident within the first year of life, and about half of cutaneous lesions are detectable at birth. Unlike infantile hemangioma, KHE does not undergo spontaneous involution and tends to progress without treatment. The most feared complication of KHE is the Kasabach-Merritt phenomenon, which occurs in 42–71% of patients. KMP is defined by profound thrombocytopenia — with median platelet counts around 21 × 10?/L at presentation — combined with consumptive coagulopathy and hypofibrinogenemia. It is critical to understand that KMP does not occur with infantile hemangioma; it is exclusively associated with KHE and the related tufted angioma. KMP carries a mortality rate estimated between 10–30%, and deaths are most commonly attributable to hemorrhage in vital organs. Intracavitary KHE carries a disproportionately high KMP burden — studies have documented KMP rates approaching 80% in this subgroup, compared to roughly 30% in non-intracavitary cases — a disparity that may reflect the anatomical microenvironment of body cavities, which lack the soft tissue buffering present in superficial or extremity lesions, facilitating early compression of vascular structures, local blood stasis, and amplification of platelet trapping. The rich chylous pools and subserosal lymphatic networks within the thoracic and abdominal cavities may further intensify the local accumulation of coagulant substances and increase susceptibility to KMP. Risk factors for KMP include young age at presentation, large tumor size (particularly greater than 8 cm), and anatomic location — intrathoracic and retroperitoneal lesions are disproportionately associated with KMP. Patients with KMP tend to have larger tumors and present at a younger age than those without. Beyond KMP, KHE causes significant morbidity through musculoskeletal involvement. The tumor's infiltrative and destructive growth can erode bone, invade joints, and alter the mechanical properties of surrounding connective tissue, leading to decreased range of motion, chronic pain, joint contractures, and progressive scoliosis — particularly with thoracic or retroperitoneal lesions. Tumor stimulation, inflammatory cell infiltration, and fibrin deposition near vertebral structures can promote local fibrosis, and during skeletal development, contractures may mechanically pull on bone, eventually driving progressive thoracic deformity. These musculoskeletal complications are more common in older children and can persist even after hematologic parameters normalize. Lymphedema is a recognized long-term sequela, especially with lesions involving the proximal extremities near lymph node basins. Intracavitary KHE is additionally associated with a higher burden of organ dysfunction — including jaundice and abnormal liver function from bile duct or portal vein compression, gastrointestinal hemorrhage from intestinal wall infiltration, and structural deformities such as scoliosis from paraspinal involvement — complications that are far less common in non-intracavitary disease and that can in severe cases impair respiratory and circulatory function and contribute to death. Diagnosis of KHE requires integration of clinical, laboratory, imaging, and pathological data. Ultrasound is useful for superficial or small lesions, typically revealing heterogeneous, ill-defined, hypervascular masses. However, MRI is the preferred modality and provides the most comprehensive assessment of disease extent. On MRI, KHE characteristically demonstrates iso-intensity relative to adjacent muscle on T1-weighted imaging and heterogeneous hyperintensity on T2-weighted imaging, with intense, heterogeneous gadolinium enhancement. Lesions commonly show ill-defined margins, multiplanar involvement, adjacent fat stranding, and bone or joint changes including cortical destruction or remodeling. CT can complement MRI in evaluating bony involvement and is more practical in critically ill patients. Three morphological patterns have been described: well-defined solid mass, solid central mass with surrounding infiltrative regions, and purely infiltrative lesion without a distinct solid core. Biopsy remains the gold standard for diagnosis and should be obtained when clinically safe. In patients with classic KMP and a compatible vascular mass on imaging, histologic confirmation may not always be required before initiating treatment. No FDA-approved therapy exists specifically for KHE. Management must be individualized based on disease severity, the presence or absence of KMP, lesion location, and patient age. For patients with KMP, aggressive combination therapy is required — monotherapy is generally insufficient. The mTOR inhibitor sirolimus, combined with short-term corticosteroids, has emerged as the preferred first-line regimen for KHE with KMP. Sirolimus works by inhibiting the PI3K/AKT/mTOR pathway, which mediates downstream effects of both VEGF-C/VEGFR3 and angiopoietin-2/Tie-2 signaling, and may additionally suppress the proliferation of KHE endothelial cells through effects on autophagy-related pathways. Response rates with sirolimus are high, and angiopoietin-2 levels have been shown to fall significantly with treatment. For patients without KMP, sirolimus monotherapy is effective, and randomized trial evidence demonstrates that low-dose sirolimus (targeting trough concentrations of 5–8 ng/mL) is non-inferior to high-dose sirolimus (10–15 ng/mL) at one year of treatment, with fewer respiratory, skin, and mucosal adverse events. Low-dose sirolimus therefore represents a safer long-term option for non-KMP patients requiring prolonged therapy, though high-dose regimens retain a role for severe cases and those complicated by KMP. Intracavitary KHE, even when treated with sirolimus-based regimens, tends to show lower sustained response rates at 6 and 12 months compared to non-intracavitary disease, underscoring the need for vigilant systemic monitoring and active complication management alongside tumor-directed treatment; outcomes at 24 months, however, appear more comparable between the two groups. Vincristine — alone or combined with corticosteroids or antiplatelet agents such as ticlopidine — remains an alternative first-line option for KMP, with an overall response rate of approximately 72%. Corticosteroids alone yield lower sustained response rates and carry significant long-term side effects. Propranolol and interferon-alpha have been used with variable and generally less reliable results; interferon-alpha in particular carries a risk of serious neurologic complications and is contraindicated in children under one year of age. Topical sirolimus and tacrolimus ointments have shown efficacy for superficial lesions, offering a way to avoid systemic drug exposure in selected patients. Surgical resection is reserved for cases where complete and safe excision is achievable, as the infiltrative nature of most KHE makes curative surgery impractical for the majority. Elective resection during active KMP is generally discouraged due to the risk of worsening coagulopathy and hemorrhage. Arterial embolization can serve as an adjunct in cases with extensive, unresectable lesions or refractory KMP, providing rapid reduction in tumor blood flow. Platelet transfusions should be avoided unless the patient is actively bleeding or being prepared for an invasive procedure. KHE significantly impairs health-related quality of life in affected children and their families. Both physical functioning — particularly in children with activity-limiting musculoskeletal complications — and psychosocial functioning are measurably reduced compared to healthy peers. KMP and activity dysfunction are the strongest risk factors for poor quality of life. Parents of children with KHE consistently report high levels of worry, emotional burden, and disruption to daily activities regardless of lesion location or parental education level. Complete tumor resolution is uncommon. Residual lesions after treatment often persist as vascular staining, telangiectasia, soft tissue fibrosis, or subcutaneous infiltrates. Long-term surveillance is warranted, as untreated residual KHE can continue to infiltrate surrounding tissue and cause progressive fibrosis and joint destruction over time. Early and accurate diagnosis, multidisciplinary management, and individualized treatment remain the cornerstones of improving outcomes in this challenging disease. References: 1- Ryu YJ, Choi YH, Cheon JE, Kim WS, Kim IO, Park JE, Kim YJ. Imaging findings of Kaposiform Hemangioendothelioma in children. Eur J Radiol. 86:198-205, 2017 2- Schmid I, Klenk AK, Sparber-Sauer M, Koscielniak E, Maxwell R, Häberle B. Kaposiform hemangioendothelioma in children: a benign vascular tumor with multiple treatment options. World J Pediatr. 14(4):322-329, 2018 3- Ji Y, Chen S, Li L, Yang K, Xia C, Li L, Yang G, Kong F, Lu G, Liu X. Kaposiform hemangioendothelioma without cutaneous involvement. J Cancer Res Clin Oncol. 144(12):2475-2484, 2018 4- Dai S, Yang K, Qiu T, Zhou J, Zhang X, Chen S, Li L, Ji Y. Health-Related Quality of Life in Children With Kaposiform Hemangioendothelioma. Front Pediatr. 10:720611, 2022 5- Huo J, Chen S, Li J, Liu C. Retroperitoneal kaposiform hemangioendothelioma with kasabach-merritt phenomenon in children: A case report and review of the literature. Front Pediatr. 11:1138689, 2023 6- Zhou J, Lan Y, Qiu T, Zhang Z, Gong X, Zhang X, Yang C, Zhou Z, Zhang Y, Yang M, Fu J, He C, Peng Q, Hu F, Xia C, Kong F, Chen S, Ji Y. Efficacy and safety of high-vs low-dose sirolimus in patients with kaposiform hemangioendothelioma: A randomized clinical trial. J Am Acad Dermatol. 93(1):124-131, 2025 7- Zhou J, Ji Y. Kaposiform hemangioendothelioma. J Am Acad Dermatol. 12:S0190-9622(26)00396-8, 2026 |
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