Large autopsy studies report metastasis from lung cancer in nearly every organ system of the body.
Frequent intrathoracic sites of lung cancer spread include hilar and mediastinal lymph nodes, pleura, diaphragm, chest wall, and pericardium, while common extrathoracic sites are liver, adrenal glands, bone, bone marrow, kidney and central nervous system.
Less common sites of metastasis include the gastrointestinal tract, pancreas, thyroid, spleen, pituitary, abdominal lymph nodes, skin and oral cavity.
Haematogeneous skeletal muscle metastasis from lung cancer is an extremely rare finding, while direct invasion of skeletal muscle in the chest wall and trunk is common in lung and other cancers. It must be remembered that primary tumours in skeletal muscle are more common than secondary tumours (1).
Metastatic patterns are not random and appear to be dependent on properties unique to the tumour cell and certain organs (2).
These patterns have been postulated to result either from variations in the vascular anatomy of certain organs or from specific chemometabolic characteristics favouring the arrest and growth of tumour cells.
Several factors critical to the establishment of metastases have been identified, and include: detachment of tumour cells, arrest of circulating tumour cells in vascular endothelium at specific sites, escape into the tissues and angiogenesis to permit the growth of a metastatic implant.
The reason for the rarity of metastatic tumours in skeletal muscle is unclear, but could be related to factors such as blood flow, metabolism, and high tissue pressure.
Irrespective of blood flow, skeletal muscle may be a poor milieu for tumour cells and this may be related to lactic acid metabolism.
Haematogeneous skeletal muscle metastases should be suspected in cancer patients with pain in the location of larger skeletal muscles with negative radiological or radionuclide evaluations for osseous metastasis.
Painless subcutaneous metastases are very common, and are easily recognised by physical examination to be superficial; they may be better felt when the underlying muscles are actively contracted.
In contrast, skeletal muscle metastases are uncommon, deep in location, and painful and it may be not possible to distinguish them clinically in some patients.
This unusual site of metastasis can be localised with CT or ultrasonography in selected patients and confirmed by bedside thin needle aspiration.
Identification of malignant cells by cytology using standard criteria will exclude more common causes of pain and swelling, such as infection, haematoma, or a ruptured muscle.
The optimal treatment of skeletal muscle metastasis is unknown as the prognosis is poor and there are few reports in the medical literature of surgical excision or radiation therapy of the involved muscles (3).
