Primary Vascular Tumors of Bone - Imaging and Differential Diagnosis

Introduction:

Primary vascular bone tumors are uncommon cancerous conditions that develop within the bone and involve blood vessels. They are categorized into three types: benign, intermediate, and malignant. While their imaging characteristics may lack specificity, certain features can provide indications of their benign or malignant nature. These distinguishing features are discussed in detail below. The diagnostic process presents challenges, and achieving an accurate diagnosis is crucial for appropriate management.

What Are Primary Vascular Tumors of Bone?

Primary vascular bone tumors are rare neoplastic conditions that occur directly within bone tissue and involve blood vessels. An irregular bone growth pattern and the proliferation of blood vessels characterize them. These tumors are divided into three categories:

• Benign Vascular Tumors: These tumors are slow-growing and do not spread to other body parts. Some examples include hemangioma, which comprises blood vessels, and angiomatosis, such as Gorham-Stout's disease. Although these benign, non-cancerous tumors do not cause life-threatening conditions, they can lead to symptoms and discomfort due to their location and size.

• Intermediate Vascular Tumors: The characteristics of these tumors fall between malignant and benign. They exhibit locally aggressive features but cannot be classified as malignancies. An example is epithelioid hemangioendothelioma, an aggressive tumor that rarely metastasizes.

• Malignant Vascular Tumors: Cancerous tumors can invade surrounding tissues and spread to distant areas. Some examples include angiosarcoma and epithelioid hemangioendothelioma. These tumors grow aggressively and can metastasize to other areas.

What Are the Imaging Features of Primary Vascular Tumors of Bone?

Since vascular bone tumors encompass various types, their imaging findings are not highly specific. However, certain features can indicate whether they are benign or malignant. The imaging features of benign vascular tumors include:

1. Hemangioma: These benign tumors consist of blood vessels and appear as well-defined areas on X-rays. They manifest as hypoechoic (dark) masses. Irregular bone patterns are observed, potentially leading to outer bone layer expansion. A characteristic calcified structure named phleboliths, seen as darker spots, is distinctive. On CT scans, (computed tomography scan) these lesions are well-circumscribed, dark areas with a peripheral enhancement pattern due to increased blood supply. MRI (magnetic resonance imaging) typically reveals bright areas on T2-weighted images due to heightened blood supply. Contrast-enhanced MRI demonstrates centripetal fill-in during the venous phase and peripheral enhancement during the arterial phase. Digital subtraction angiography can offer further insight into the blood supply. Hemangioma's appearance varies based on location and size, necessitating correlation with clinical findings and consideration of biopsy.

2. Gorham-Stout's Disease: This condition involves spontaneous bone resorption, also known as vanishing bone disease. Initial X-ray signs include dark areas or lucent foci within the bone marrow (intramedullary) or below the bone layer (subcortical). As the disease progresses, these lucent foci expand, leading to substantial bone loss. Moreover, osteoblastic activity is diminished, impeding bone repair and formation. There is also a lack of periosteal reaction, wherein the outer periosteum fails to generate new bone.

The imaging findings of intermediate vascular tumors include:

1. Epithelioid Hemangioma: These tumors are predominantly observed in the cutaneous (layers of skin tissue) and subcutaneous layers (layers below the skin). They typically manifest as solitary lesions. During ultrasound, they exhibit varying levels of brightness or echogenicity and appear hypervascular (with an increased number of blood vessels). CT scans show these tumors to possess densities akin to soft tissues, which can be similar or lower in density compared to muscles. Following the administration of a contrast dye, they can exhibit varying degrees of intensity. On MRI, they present a solid, distinct, and well-defined appearance, occasionally with a lobulated appearance. In rare cases, these tumors can lead to cortical bone destruction and expand into the surrounding tissues.

The imaging findings of malignant vascular tumors include:

1. Angiosarcoma: It is one of the most aggressive forms of tumors, accounting for less than 1 percent of all sarcomas (a type of cancer that occurs in bone and soft tissues). On plain radiographs, these are observed as hollow areas, typically located in the outer and middle parts of the bone. Multiple growths on the bone can be present. They have the potential to metastasize to other skeletal regions. CT scans reveal lytic or destructive lesions, sometimes resembling moth-eaten or irregular areas on the bone. They can extend into soft tissues, displaying infiltrative growth patterns. In MRI, the extent of infiltration into soft tissue can be assessed.

2. Epithelioid Hemangioendothelioma: General features of these tumors include lobular structures containing blood vessels, well-defined boundaries, some areas of separation, regions with a tendency to grow, erosion of bone surfaces, and infiltration into soft tissues. On X-rays, they manifest as expanding areas resembling cysts or voids, and bone surfaces appear undulating. CT scans show features akin to X-rays, with no evidence of calcification. On MRI, they appear rounded with surrounding bone marrow and dark spots indicating absent blood flow.

What Is the Differential Diagnosis of Primary Vascular Tumors of Bone?

The differential diagnosis of primary vascular bone tumors includes:

• Aneurysmal Bone Cysts: These are benign bone lesions with local aggressiveness. On plain radiographs, they present as blown-out or ballooned areas.

• Giant Cell Tumors: These are locally aggressive bone tumors with multinucleated giant cells. On radiographs, they display lytic lesions with well-defined margins, often resembling a soap bubble appearance.

• Osteosarcomas: These bone tumors originate from the metaphysis of long bones. Radiographically, they exhibit a sunburst appearance. They are aggressively destructive lesions with ill-defined borders.

• Ewing Sarcoma: This aggressive bone cancer commonly affects young adults and children. Radiographically, they manifest as destructive lesions with poorly defined margins, often showing an onion-skin appearance.

• Chondrosarcomas: Arising from chondrocytes (cartilage-forming cells), these tumors appear as destructive lesions with well-defined borders on radiographs. Calcified areas can also be present.

• Vascular Malformations: These congenital anomalies occur in blood vessels and are non-cancerous. Examples include capillary malformations, venous malformations, and lymphatic malformations.

• Myeloma: This condition affects cancerous white blood cells. It involves abnormal plasma cells in the bone marrow and elevated levels of monoclonal proteins in the blood.

Conclusion:

Vascular bone tumors encompass many disorders, posing a diagnostic challenge. Accurate diagnosis holds significant importance for appropriate treatment planning. Imaging modalities, including radiographs, CT, MRI, PET-CT (positron emission tomography and computed tomography), and angiography, play pivotal roles in assessing these tumors, offering insights into their characteristics, extent, and metabolic activity.