Cancer can spread to skeletal muscle, but this event is exceedingly rare. This process, known as skeletal muscle metastasis, occurs when cancer cells from a primary tumor travel through the body and successfully colonize muscle tissue. Although skeletal muscle makes up about 40% of the body’s total mass, it is an uncommon site for secondary tumor growth compared to organs like the liver, lungs, or bone. Cancers most often associated with this rare spread include those originating in the lung, the gastrointestinal tract, the urinary system, and malignant melanoma.
Understanding the Pathways of Metastasis
Metastasis is the process where cancer cells leave their original site and form a secondary tumor elsewhere in the body. For cancer to spread to a distant location, cells must first break away from the primary tumor and enter the circulatory system. The two main pathways for this long-distance travel are the bloodstream and the lymphatic system.
The most frequent route for cancer cells to reach skeletal muscle is through hematogenous spread, meaning they travel via the bloodstream. Once in circulation, these circulating tumor cells (CTCs) must survive the immune system and the physical stresses of blood flow. They then adhere to the lining of the blood vessel walls, exit the vessel, and attempt to establish a new colony within the muscle tissue.
Why Skeletal Muscle Resists Cancer Colonization
Skeletal muscle tissue possesses unique characteristics that make it a highly hostile environment for most metastatic cancer cells. These factors explain why metastasis to muscle is so rare.
Physical Barrier
The dense organization of muscle fibers provides a physical barrier that actively impedes the invasion and migration of foreign cells. The tightly packed, parallel arrangement of myofibers physically restricts the space cancer cells need to proliferate and organize into a mass.
High Blood Flow
The muscle’s high vascularity and constant blood flow also play a significant role in resistance. Skeletal muscle is highly perfused, meaning it receives a large volume of blood, which may quickly flush out any circulating tumor cells before they can firmly adhere to the vessel walls and exit into the tissue. This constant movement of blood creates a high shear stress that can physically damage or dislodge tumor cell clusters.
Inhibitory Metabolism
The metabolic environment of muscle tissue is generally inhibitory to tumor growth. Skeletal muscle metabolism produces lactic acid and maintains a lower pH level compared to other tissues. This acidic, low-pH microenvironment is unfavorable for the proliferation and survival of many types of metastatic cancer cells, which typically thrive in different metabolic conditions.
Mechanical Stress
The constant contraction and relaxation associated with physical activity create mechanical stress within the tissue. This mechanical disruption can inhibit the adhesion of tumor cells to the extracellular matrix, a necessary step for colonization. Movement also physically crushes or destroys tumor emboli before they can develop the new blood vessels, a process called angiogenesis, which is necessary for a tumor to grow beyond a microscopic size.
Distinguishing Primary Muscle Tumors from Metastasis
It is important to understand the difference between a tumor that starts in the muscle and one that spreads to it. A primary muscle tumor, such as a rhabdomyosarcoma or leiomyosarcoma, originates from the muscle or connective tissue cells themselves. These primary malignant tumors are a type of sarcoma, which is a cancer arising from mesenchymal tissue.
Metastatic muscle tumors, in contrast, are composed of cells that originated elsewhere, such as in the lung or kidney. When a biopsy is performed, the cancer cells found in the muscle will microscopically resemble the cells of the primary tumor, such as a lung carcinoma or a melanoma.
The distinction is significant because the diagnosis and treatment pathway differ substantially between primary sarcomas and metastatic carcinomas. A painful mass in the muscle is often initially suspected to be a primary soft tissue sarcoma, due to the rarity of true metastasis. Proper classification is necessary to determine the correct systemic treatment plan.
Identifying and Diagnosing Muscle Metastases
Skeletal muscle metastasis often presents with non-specific symptoms, which can make initial identification challenging. The most common clinical sign is a localized, painful mass that can be felt under the skin, sometimes accompanied by swelling or tenderness. In some cases, the metastasis is only discovered incidentally during imaging scans for other reasons, rather than due to specific symptoms.
Diagnostic imaging typically begins with Magnetic Resonance Imaging (MRI), the preferred method for evaluating soft tissue masses. MRI provides detailed images that help determine the exact size and location of the lesion and its relationship to surrounding structures. Computed Tomography (CT) and Positron Emission Tomography (PET) scans also assist in staging the cancer and assessing the metabolic activity of the mass.
Radiological findings are not specific enough to confirm the diagnosis, as many benign and malignant masses can look similar on a scan. A definitive diagnosis requires a biopsy, where a small sample of the tissue is removed and examined under a microscope. The pathologist confirms that the cells in the muscle mass match the known primary tumor, which is the only way to confirm a skeletal muscle metastasis.