What is Omental Tumors?
The omentum is an organ inside your abdomen, or belly, that’s connected to your spleen, stomach, pancreas, and colon. It gets its blood supply from two arteries. The omentum is made up of a type of fat tissue that’s sandwiched between layers of connective tissue which contains different types of cells, including those involved in healing, immunity, and blood vessel formation. The omentum helps maintain the balance in your abdominal area, including fluid movements, healing, forming new blood vessels, controlling infections, fat storage, and also acts as a reservoir for a type of cells known as stem cells.
Some immune cells within the omentum can move through small openings called fenestrations. Areas known as milky spots contain these immune cells arranged around blood vessels near the fenestrations. These milky spots become more active during infections and help absorb fluid, and produce certain chemicals associated with the body’s response to stress, including dopamine, epinephrine, norepinephrine, and choline acetyltransferase. Lymph from these milky spots drains to lymph nodes located near the stomach and spleen.
Stem cells within the omentum can promote the growth of blood vessels by producing substances such as vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF). Inflammation and low oxygen levels can increase the production of VEGF; these substances can help the migration of nerve stem cells to the injury site. The omentum also plays a role in stroke recovery, treatment for Alzheimer’s Disease, and increasing blood flow to arms and legs with inadequate blood supply.
The omentum can sometimes become a site for the spread of cancer. Some theories suggest that immune cells within the milky spots can control inflammation and also contribute to the spread of cancer. However, the omentum environment, including the milky spots, fat cells, and other tissue-forming cells can encourage the spread of cancer. Cancer spreading to the omentum often results in a worse outcome, as the omentum may contribute to resistance to chemotherapy and further growth of the tumor.
Rarely, cancer can originate in the omentum itself, causing a primary omental tumor. These include liposarcoma, hemangiopericytoma, fibrosarcoma, leiomyosarcoma, mesothelioma, and malignant fibrous histiocytoma. More commonly, the omentum can become the site of cancer spreading from other places, most commonly uterine, ovarian, colorectal, and stomach cancers. Sometimes, a gastrointestinal tumor can originate in the omentum or be located within the omentum as spread disease. The omentum can also become a site for inflammatory processes that may be hard to distinguish from cancers.
What Causes Omental Tumors?
It’s fairly common for cancer to spread to the omentum, a layer of fatty tissue that covers and supports the stomach and intestines. However, it’s much rarer for a tumor to start in the omentum. The most commonly reported tumor that begins in the omentum is called a hemangiopericytoma, which comes from particular cells in the blood vessels.
Other tumors known to occasionally start in the omentum include rhabdomyosarcoma (a type of muscle cancer), small cell carcinoma (a type of lung cancer), malignant solitary fibrous tumor, malignant fibrous histiocytoma (a type of soft tissue tumor), lipomas (benign fat tissue tumors), yolk sac tumor (a type of testicular cancer), and angiofibrolipoma (a type of benign tumor in fat and blood vessels).
Additionally, there is a type of cancer called mesothelioma that can sometimes start in the omentum. This is often related to exposure to asbestos. Gastrointestinal stromal tumors, another type of cancer, usually spread to the omentum from elsewhere but can sometimes start there too.
According to the 2020 classification of soft tissue tumors by the World Health Organization, most tumors that originate in the omentum are classified as soft tissue tumors. Soft tissue tumors start from cells in tissues like fat, muscle, and connective tissue. Liposarcoma (fat cell cancer) and leiomyosarcoma (smooth muscle cell cancer) are the most common types of these tumors in the abdomen, but it’s rare for them to start in the omentum.
There is also a type of tumor called a desmoplastic round cell tumor that can start in the omentum. This is a type of mesenchymal tumor which starts from cells in the connective tissue and it’s caused by a mix-up in genetic material from two specific genes.
It’s common for cancer to spread to the omentum, usually from cancers in the abdomen and pelvis. This used to be thought to be directly related to how close the omentum is to the original tumor, however, recent studies suggest that it could also be due to cancer cells traveling in the blood (a process called hematogenous spread).
Risk Factors and Frequency for Omental Tumors
Smooth muscle tumors make up about 37% of primary omental tumors, or tumors in the tissue that surrounds the abdominal organs. These types of tumors have been most frequently reported in men around the age of 50. Liposarcoma, a subtype of these tumors, had 19 cases reported between 1936 and 2019. Leiomyosarcoma, another subtype, had 23 cases reported up until October 2016. These typically developed in people in their fifth decade of life and were slightly more common in men.
The World Health Organization (WHO) includes several types of tumors, such as hemangiopericytoma, lipomatous hemangiopericytoma, and giant cell angiofibroma, under a category called extra-pleural solitary fibrous tumors (SFTs). These types of tumors make up about 5% of all sarcomas. While rare, 14 cases have been reported of SFTs developing from the omentum. Around 78% to 88% of these are benign, meaning they are not cancerous. They primarily occur between the fifth and seventh decade of life, with equal occurrence in males and females.
Most primary omental tumors are a smaller occurrence of their corresponding extra-omental metastatic tumors. Fifty-four cases of omental extra gastrointestinal stromal tumors have been documented, usually diagnosed at around 65 years of age. Omental lipomas make up around 7% to 9% of all lipomas and omental mesothelioma represents 10% of all cases of mesothelioma.
Omental metastatic disease, where the disease has spread from other parts of the body to the omentum, is more common than primary omental tumors. The most common origins of omental metastatic disease are from the ovaries, uterus, stomach, or colon. It’s also worth noting that about 10% of patients with stomach cancer have omental metastases and this often shortens the length of time they are disease-free.
Signs and Symptoms of Omental Tumors
Primary omental tumors are masses that grow in the fatty tissue covering the abdominal organs. Patients with these types of tumors often don’t notice anything is wrong at first. But as the tumor grows, some common symptoms might appear, like:
- Feeling discomfort in the stomach area
- Sudden increase in waist size
- Feeling a lump in the belly
- Nausea
- Feeling full quickly when eating
- Weight loss
However, in some rare cases, the tumor might break open unexpectedly and trigger an inflammatory response in the body. During a physical examination, a doctor might be able to feel the tumor in various parts of the belly. Sometimes, fluid called ascites might also build up in the belly. Large primary omental tumors may not show any signs or symptoms.
When cancer spreads to the tissue that covers the stomach, intestines, and other organs in the abdomen (omentum), it generally means the disease has progressed. The symptoms are similar to primary omental tumors, but the patient might also show physical signs of advanced cancer, like an abdominal mass, ascites, and tenderness upon touch. The most frequent signs are discomfort in the belly (in 56% of patients) and a feeling of a mass (35%). The symptoms aren’t tied specifically to the original tumor, but rather, represent the late stages of cancer. Sometimes, these metastasized tumors in the omentum are discovered unexpectedly during imaging tests.
Testing for Omental Tumors
Detecting an omental tumor, which is a tumor in the fatty layer that covers the organs in your belly, can be tricky with just clinical and radiographic findings. Usually, a definitive diagnosis can be made through an examination of the tumor under a microscope.
Just to walk you through the process, a type of tumor known as a well-differentiated liposarcoma can be tough to pick out from regular fat or other abdominal tumors on a CT scan or an MRI. Also, if the mass is sticking to other organs, it makes the evaluation more difficult.
The preferred methods for diagnosing an omental tumor involve using contrast-enhanced CT or MRI of your abdomen and pelvis. With these methods, doctors might be able to spot the tumor appearing as flat, multi-lobed mass with different densities. Angiography, which is an X-ray of the blood vessels, could also help by revealing the blood supply to the tumor. If the tumor has a lot of blood vessels, it’s more likely to be a cancerous growth.
Omental solitary fibrous tumors (SFTs) are identified as a growth of fibrous tissue within the omentum, the fatty tissue that covers your abdominal organs. These SFTs are well supplied with blood and are often accompanied by a low-density cystic component, denoting tissue death and calcification. But the challenge is that imaging tests usually can’t distinguish an SFT from other similar tumors.
The appearance of a lipomatous lesion, which is a benign tumor composed of fat cells, is usually well-defined and uniform or may have fibrous partitions and calcification. Indications of a cancerous growth include thicker septations, irregular shape, invasion into surrounding structures, and associated lymph gland inflammation. A Positron Emission Tomography (PET) scan might help to differentiate a lipoma, a benign growth of fat cells, from a well-differentiated liposarcoma, a type of cancer that arises in fat cells.
A couple of other types of omental tumors, such as hemangiopericytoma and desmoplastic small round cell tumors have non-specific features on imaging. This means it’s not always easy to tell them apart from other omental masses just by looking at them on a scan. A variety of tests, including angiography, Color Doppler ultrasound, MRI, and PET scan can help clue doctors in on the specifics of these tumors.
Back to diagnosing the omental tumors, Needle aspirations of omental tumors are typically not very helpful. It’s because they often don’t provide a good enough tissue sample. Instead, the tissue from the tumor needs to be closely examined under a microscope using techniques like immunohistochemistry, to make a definitive diagnosis. In simpler terms, it’s like checking for specific markers on the cells of the tumor to tell them apart from other types of cancer.
Apart from these imaging techniques, doctors might also use a guided needle biopsy or laparoscopy to obtain tissue for further examination. Fluorescence in situ hybridization (FISH) can help to pinpoint chromosomal abnormalities and is particularly useful when diagnosing desmoplastic small round cell tumor. A blood test for cancer antigen 125 (CA-125) could also guide diagnosis, as some omental tumors may lead to increased levels of this protein. Although this is not specific, it can be a hint towards the prognosis and also an aid in monitoring how well you are responding to the treatment.
Treatment Options for Omental Tumors
For detailed guidelines on treating different types of cancer, you can visit the National Comprehensive Cancer Network’s website at nccn.org/guidelines.
Typically, omental tumors, tumors found in a layer of the abdomen, are treated through resection, a surgical procedure to remove the tumor. This treatment may involve reducing the tumor’s size or removing as much of the tumor as possible, especially when the disease is advanced. In many cases, combining surgery with chemotherapy and radiation therapy has been found to extend a patient’s life.
Treatment recommendations for primary omental tumors are mostly based on case studies and expert opinions. Favorable outcomes are usually seen when liposarcoma and leiomyosarcoma, types of omental tumors, are completely removed. Chemotherapy, radiation, and immunotherapy might offer varying results when the disease cannot be surgically removed. Unfortunately, the disease often comes back after treatment for an advanced disease, and the long-term survival rate remains low.
Most omental SFTs, another type of tumor, are benign or non-cancerous. However, even these can return years after surgery. Whether additional therapy is beneficial is unclear. Still, some reports suggest that adding radiation therapy to the treatment might slow down the disease. If these tumors are malignant or cancerous, medication such as imatinib and sunitinib, which are also used for a digestive tract cancer called Gastrointestinal stromal tumors (GISTs), are often used. For omental GISTs, a surgery paired with imatinib therapy has been found to prevent the disease from returning and prolonging a patient’s life.
Another type of omental tumor, mesothelioma, has been treated successfully in clinical trials using surgery combined with heated chemotherapy delivered directly to the abdomen during surgery. This approach has shown to extend the average survival time. Similarly to pleural mesothelioma, a type of lung cancer, a combination drug treatment of pemetrexed and cisplatin has been employed for omental mesothelioma that’s inoperable. Immunotherapy and radiation therapy are also options.
For more advanced-stage cancers that frequently spread to the omentum, such as uterine, stomach, ovarian, and colorectal cancer, the standard care includes removing the omentum during surgery. The evidence regarding the benefit of this procedure concerning survival times is not entirely clear. Nevertheless, experts suggest that reducing the size of the tumor through surgery improves overall survival and provides important information about the stage and prognosis of the disease. Despite the increased resistance to chemotherapy seen in metastatic disease within the omentum, surgery still remains the most effective treatment.
What else can Omental Tumors be?
Omental tumors are often confused with ovarian tumors, some malignant tumors that begin in the pelvic region, and widespread diseases such as lymphoma and tuberculosis. There are many kinds of primary and metastatic omental tumors that a patient could possibly have. These include the following:
- Fibrosarcoma
- Spindle cell sarcoma
- Liposarcoma
- Leiomyoma
- Hemangiopericytoma
- Lipoma
- Desmoid tumor
- Fibroma
- Mesothelioma
- Leiomyosarcoma
- Myosarcoma
- Rhabdomyosarcoma
- Leiomyoblastoma
- Endothelioma
- Desmoid small round cell tumor
- Endometrial adenocarcinoma
- Ovarian cancer
- Gastric cancer
- Colorectal cancer
- Primary peritoneal cancer
- Neuroblastoma
- Primitive neuroectodermal tumors
Surgical Treatment of Omental Tumors
In cases of advanced stage (III or IV) tumors within the abdomen, doctors often recommend a procedure known as an omentectomy and lymphadenectomy. These operations involve surgical removal of parts of the body like the omentum (a large fatty structure hanging off the middle of your abdomen), and lymph nodes around the area of the tumor. The primary goal of these operations is to thoroughly examine and evaluate the extent of the cancer.
Research suggests that removing the omentum might also help to improve the outcome for cancer patients. The omentum has been found to create an environment that assists tumor growth, so removing it can disrupt a tumor’s ability to flourish. This is because the omentum contains cells that can increase the activity of the cancer.
When it’s possible, doctors typically recommend surgical removal for all primary omental tumors, meaning those that originate in the omentum. The idea is to cut off the tumor’s “lifeline” and to increase the chances of successful cancer treatment.
What to expect with Omental Tumors
The fate or outcome (prognosis) of individuals with tumors (abnormal cell growth) in their omentum (a large layer of fat that covers the abdominal organs) differs widely. This depends on factors such as the original source of the tumor, the biology of the tumor (how it behaves and grows), the size of the tumor, and how fast it was diagnosed. Tumors that start in the omentum generally don’t show symptoms until later in their development, when they’re larger and harder to remove completely. This also means there’s a higher chance they might come back.
Liposarcoma in the omentum, a specific type of cancer made of fat cells, is more dangerous than the same type of cancer in the arms or legs. In one study, people with liposarcoma in the body or trunk had an average 5-year survival rate (the percentage of patients who live at least five years after their diagnosis) of only 41.9%, compared to 66.7% for those with liposarcoma in the arms or legs. The 5-year survival without the disease coming back for omental liposarcoma depended on the type of cancer cells. High-grade tumors (which grow and spread more aggressively) had a worse outlook than low-grade tumors.
EGIST, which is a rare cancer of the cells in the omentum, has an uncertain prognosis, but it is generally viewed as more aggressive with a higher number of dividing cells, larger size and more likely to spread to distant parts of the body than its counterpart, GIST. This may be because EGISTs don’t show symptoms until later in their development.
Malignant mesothelioma, a type of cancer found in the omentum, has a bleak prognosis and most patients unfortunately die within a year after diagnosis, even with intense treatment to reduce symptoms. However, one group of patients who received a specific therapy called hyperthermic intraperitoneal chemotherapy survived around five years.
Small cell carcinoma or cancer, regardless of where it’s located, also has a poor prognosis, with less than 15% surviving for 5 years and an average survival of less than 5 months, based on the extent and localization of the disease. Solitary fibrous tumors over 20 cm and with more than 4 dividing cells under a microscope also have a poor prognosis.
Studies have shown that individuals with cancer spread to the omentum generally have a worse prognosis due to increased resistance to chemotherapy and different tumor biology. For instance, in patients with ovarian cancer that spread to the omentum, there was a direct relationship between how well the omental tumors respond to chemotherapy and the overall survival. In contrast, for gastric cancer that spreads to the omentum, there is no significant change in the survival rate.
Possible Complications When Diagnosed with Omental Tumors
When a tumor in the omentum (the fatty layer that covers and protects your stomach and intestines) breaks open inside the abdomen, it can cause several problems. Some of these include torsion (twisting) with tissue damage due to lack of blood supply, bleeding inside the tumor, and blockages in the digestive tract.
Potential complications:
- Intraabdominal rupture of the tumor
- Torsion with infarction (twisting leading to tissue damage)
- Bleeding within the tumor
- Blockages in the digestive tract
Preventing Omental Tumors
Primary omental tumors, which are unusual and rare cancers that occur in the omentum (a large fatty layer that covers the organs in your belly), can often come back even after diagnosis and treatment. Because of this significant risk of recurrence, it is vital for these patients to have regular and ongoing checks to keep an eye on their health condition.
In cases where the disease has spread to the omentum from another part of the body, this is called metastatic disease. Patients with such conditions receive assistance from health navigators and are given education tailored to their specific type of primary tumor (the original site where the cancer started). Health navigators are professionals who help guide patients through the healthcare system, coordinating care and offering support throughout the treatment process.
