imaging of blow out metastases of bone-Pictorial essay

introduction Metastases (Secondaries) literally means “a placing in the midst of neoplasm remote from the primary”. Metastasis to bone spread by extension, through hematogenous and lymphatic routes. In practice, these metastases may resemble primary bone tumors on imaging. Such lesions include giant cell tumor, brown tumor, pseudo tumor of hemophilia, aneurysmal bone cyst and chondromyxoid fibroma. Even primary tumors such as lytic osteosarcoma and plasmacytoma may simulate blow out metastasis [1]. Image guided biopsy most often distinguishes primary from secondary, although occasionally undifferentiated metastases may pose a problem [2, 3].

introduction Metastases (Secondaries) literally means "a placing in the midst of neoplasm remote from the primary". Metastasis to bone spread by extension, through hematogenous and lymphatic routes. In practice, these metastases may resemble primary bone tumors on imaging. Such lesions include giant cell tumor, brown tumor, pseudo tumor of hemophilia, aneurysmal bone cyst and chondromyxoid fibroma. Even primary tumors such as lytic osteosarcoma and plasmacytoma may simulate blow out metastasis [1]. Image guided biopsy most often distinguishes primary from secondary, although occasionally undifferentiated metastases may pose a problem [2,3].

review of the literature
In considering the pathogenesis of the lytic lesion, it is important to look into pathological factors.
Majority of tumor emboli are deposited in the bone marrow. Most of these metastatic bone tumors have been present for sometime before they are recognised as perceptible alteration of bone density. Hematogenous dissemination occurs by the spread through blood vessels mainly the veins which are the path ways for tumor emboli. The three major organs which contain metastasis are lung, liver and bones.
In several large postmortem series, it is estimated the incidence of metastases to the bone to be from 22 to 35% of all patients with malignancies. 80% of all metastases have the primary in the lung, breast, genitourinary and gastrointestinal malignancies. About 70% of metastases in women are from breast and uterine cervix [4,5]. Melanoma also can produce osseous metastases [6,7].

Discussion
It is often noted that during a routine survey of bones, a large lytic lesion is noted without any known malignancy. At this time, it is a challenge to have a cost effective work up to detect the primary tumor. Another major problem is to assess the metastatic disease in the presence of a known malignancy. It is ideal to have bone scintigraphy to have a total body scan to identify/ rule out metastatic disease. However, total body MRI is the best choice but it is costly and not easily available. Metastatic disease may present as a single or multiple foci of lytic, blastic or mixed pattern. Bone metastases constitute 70% of all malignant bone tumors. Metastases to the bone may be lytic which constitute about 75%. Bone metastasis are seen in 7-28% of follicular thyroid carcinoma patients and 1.4-7% of papillary thyroid carcinoma patients. Bone is the second most common site for metastases from thyroid after lung. Thyroid carcinoma metastases are mostly osteolytic [8]. Sclerotic metastases constitute 15%. Mixed type of both lytic and sclerotic lesions constitute about 10%. Metastases occurring distantly from the origin of the primary malignant tumor is due to molecular mechanism. The spread may be direct extension of the tumor, through lymphatic or through hematogenous routes. Occasionally, metastases may be transplanted during surgery by using the same instruments etc., [9].
Why lytic bone metastases occur and form different morphological changes on imaging is not known. They may be permeative, moth eaten, geographic or ballooned out. The latter is generally solitary but may be at multiple sites. These lesions often arise from the trabecular bone. Cortical metastases are also well known, particularly in carcinoma of lung etc., imaging methods

Angiography
Role of CT angiography: Bone window settings show high level of details of bone. High vascularity can be detected in CTA.

Role of MRI in metastases:
Detects 28% more mets than scintigraphy, focal lytic areas show decreased signal on T1 and increased signal on T2.
Pet CT: Detects many more than actual lesions, because any other non neoplastic lesion can be detected. Hence it is non specific but advantageous to detect the primary also.
Angiography: It is not done as a routine except to embolize a highly vascular and potentially bleedable lesion in any case, "early detection is early cure!" The common primary malignancies that produce ballooned out metastatic lesions arise from thyroid, lung, breast, kidney and melanoma.
Bone metastasis are seen in 7-28% of Follicular thyroid carcinoma patients and 1.4-7% of papillary thyroid carcinoma patients. Bone is the second most common site for metastases from thyroid after lung. Thyroid carcinoma metastases are mostly osteolytic (Figure 1a-g). CTA would help the highly vascular nature (Figure 1h,i,j).
Carcinoma of the lung is another malignancy where blow out metastases are common. Plain films are adequate but CT and other imaging methods delineate the nature of matrix (Figure 2a-l).
Bone metastases from carcinoma lung occur in 62.5% (ribs>spine). Sternal metastasis from lung cancer-3.8% of which 95% occur in the body 5% in the manubrium part (Figure 2m,n). Sternal metastasis remain solitary, confined to sternum as it does not have contact with parvertebral venous plexus unlike vertebral metastasis which show multicentric lesions. Isolated sternal metastasis from renal cell carcinoma, hepatocellular carcinoma, and breast carcinoma has been reported in the literature.
Blow out metastases from carcinoma of the breast are less common than thyroid and lung. However, occasionally this type of metastasis may be encountered (Figure 3a-d).
Although, melanoma is not a common malignant tumor amongst Indians but blow out metastasis also may be seen in this entity ( Figure 4).
Literature is filled with case reports of blow out metastases occurring in other malignancies arising from gastrointestinal, head and neck, genitourinary tract and other systems.
MRI depicts the lesion and shows different components of the tumor (Figure 5a,b).

role of radionuclide scanning in metastases
Valuable imaging modality with technetium phosphate compounds. Highly sensitive for lytic lesions. However, it is non specific. A solitary focus of increased radio activity is seen in 10% of metastases [10,11] (Figure 6a,b).

Differential diagnosis
In the differential diagnosis of blow out metastases, the following primary malignant and benign lesions come into consideration. These are listed in Table 1. Table 1: Differential diagnosis of blow out metastases.
• Primary malignancies of the bone, lytic osteogenic sarcoma etc (Figure 7). Benign tumors and tumor like bone lesions with blow out appearance include giant cell tumor, aneurysmal bone cyst, simple bone cyst, brown tumor, pseudo tumor of hemophilia and chondromyxoid fibroma [12].
To differentiate primary malignancies of the bone, certain criteria such as, site, age, aggressive nature, transitional zone, matrix mineralisation and soft tissue swelling should be considered. Metastasis may occur in any part of the bone whereas most of the primary malignancies occur in the metaphyseal zone. Beyond the age of 45 years, plasmacytoma and lymphoma are the only primaries producing lytic changes. Otherwise metastasis should be considered.
The transitional zone in metastasis is usually thin. The matrix is lucent in lytic metastasis. Unless there is pathological fracture soft tissue swelling is usually not present in metastasis.

conclusion
In medical practice, particularly in orthopedic oncology, ballooned out lesions of bone are often encountered. The metastatic blowout lesion is often solitary. These should be distinguished from primary bone tumors as well as tumor like lesions. The radiological criteria for the diagnosis of primary bone tumors are mentioned. A blow out lesion of bone is an indication to search for primary lesion, in the thyroid, lung, breast, kidney etc.