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Skeletal muscles represent 50% of total body mass and receive a large percentage of total cardiac output. Despite this, hematogenous metastases to skeletal muscles have been reported to be rare. It is believed that muscle motion, muscle pH, and the muscle's ability to remove tumor-produced lactic acid contribute to the resistance of skeletal muscles to metastatic disease [1, 2].

Despite factors mitigating against metastases, there have been multiple reports of skeletal muscle metastases from lung, breast, colonic, renal, ovarian, gastric, and sarcoma primary malignancies [1, 3,4,5,6,7,8]. There have been a few reports in the literature of MR imaging and CT appearances of skeletal muscle metastases. To our knowledge, there have been no previous reports indicating the MR appearance of skeletal muscle metastases occurring in sites of previously documented trauma. We report eight biopsy-proven cases of skeletal metastases occurring in sites of previously documented skeletal muscle trauma.

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We retrospectively reviewed MR imaging performed at a large orthopedic oncology service from January 1994 to December 2000 for biopsy-proven metastases to skeletal muscles. Two thousand sixty-eight patients were seen during this period. Therefore, 28 patients with biopsy-proven skeletal muscle metastases constituted 1.35% of the patients seen during this period. Our retrospective review found that all 28 patients had known primary malignancies and had undergone biopsy of the skeletal muscle mass to document new metastatic spread. These 28 patients were derived from a retrospective database search of skeletal muscle biopsies performed on patients referred to an orthopedic oncologic surgery service.

All 28 patients underwent MR imaging with either a 1.0-T scanner (Impact; Siemens Medical Systems, Erlangen, Germany) or a 1.5-T scanner (Signa; General Electric Medical Systems, Milwaukee, WI). Several different scanning protocols were used depending on the scanner and the site of the metastasis. All 28 patients' records and imaging studies were retrospectively reviewed. The patient age, sex, type of primary malignancy, previous type of therapy, and time period between diagnosis of primary malignancy and detection of muscle metastasis were noted. The retrospective review was performed by the oncology nurse who was aware of the purpose of the study.

Of these 28 patients, eight had a documented clinical history of previous trauma at the site in which a skeletal metastasis developed. In these eight patients, a note was made of the time interval between trauma and detection of skeletal metastasis, whether any imaging studies were performed at the time of trauma, and whether the patient was actively being treated for the primary malignancy at the time of trauma or at the time of detection of skeletal metastasis.

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Of the 28 patients with biopsy-proven skeletal metastases, the average age was 62 years (range, 38-79 years). Thirteen of the 28 patients were women. Of these 28, eight patients had a documented clinical history of previous trauma at the site of skeletal metastasis. Five of these eight patients had MR imaging performed before the development of a metastasis. MR imaging documented a hematoma in three of these five patients and a partial muscle tear in two. The hematomas and partial muscle tears were in the exact same skeletal muscle location in which metastatic disease subsequently developed. Metastatic disease was documented by MR imaging and subsequent biopsy. The three patients who did not have MR imaging at the time of injury had documentation in their patient charts of significant impaction injuries with resultant hematomas correlating with the degree of injuries. In each of these patients, the site and size of the hematomas were documented in the clinical charts on the basis of a physical examination. One patient had sonographic correlation documenting the size of a calf hematoma at the time of injury. Clinical follow-up was performed, and the described hematomas in each of these patients resolved at the follow-up physical examination.

The eight patients with documented metastases at sites of previously documented trauma all had known malignancies at the time of their trauma. The primary malignancies were as follows: four patients with lung carcinoma, two with breast carcinoma, one with renal cell carcinoma, and one with sarcoma as a primary malignancy. All eight patients had advanced disease at the time of diagnosis, requiring chemotherapy. All were actively receiving chemotherapy at the time of trauma.

All eight patients had significant trauma documented in their medical records at the time of injury. The resultant skeletal muscle injury was thought to be consistent with the mechanism of injury. There was no spontaneous hemorrhage or hemorrhage from minimal trauma. The clinical notes describing the mechanism of injury and the treatment plan by the oncologists treating the patients were reviewed. In each case, it was noted that the trauma, mechanism of injury, and hematoma or partial muscle tear correlated with the injury. The three patients with hematomas experienced significant impaction injuries that resulted in hematoma formation. Medical records documented that the two patients with partial muscle tears had impaction injuries with simultaneous twisting motion by the patients, resulting in partial tears of the affected muscles. In each case, the documented injury was treated with observation and rest with clinical resolution in the interim between the trauma and onset of metastasis in the area.

The discovery of metastasis occurred on average 28 months after the trauma (range, 16 months-6 years posttrauma). At the time of discovery of skeletal muscle metastases, none of the patients were receiving chemotherapy. The site of skeletal muscle metastasis was the only imaging finding of residual or recurrent disease in each of the eight patients. Each patient had a CT examination of the chest, abdomen, and pelvis, and a whole-body nuclear medicine bone scan was also performed. A biopsy of each patient was necessary because it affected clinical management.

All skeletal muscle metastases had an MR appearance of a mass with isointense signal or dark signal as compared with muscle on T1-weighted images, enhancement on gadolinium-enhanced T1-weighted images, and increased signal on T2-weighted or short tau inversion recovery images. All metastases had mass effect on the adjoining soft-tissue structures. Figure 1A,1B reveals the appearance of a calf hematoma on sonography (Fig. 1A). This hematoma subsequently resolved at follow-up clinical examination. Twenty months later, the patient developed a soft-tissue mass in the area of the previous trauma. This was seen on a sagittal T2-weighted MR image (Fig. 1B). Figure 2A,2B shows the appearance of a hematoma in the right rectus femoris muscle resulting from an impaction injury sustained in this area (Fig. 2A). This hematoma resolved at clinical examination. The patient developed a soft-tissue mass in the area of a previous trauma 18 months later. This was revealed on a coronal proton density—weighted MR image (Fig. 2B). Figure 3 shows skeletal muscle metastases to the supraspinatus and infraspinatus muscles in the same area of trauma documented 22 months earlier.

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Fig. 1A. —52-year-old man with known lung carcinoma. Right posterior calf sonogram shows hematoma (arrow) that occurred after trauma.

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Fig. 1B. —52-year-old man with known lung carcinoma. Sagittal T2-weighted MR image obtained 20 months after sonogram shows interval development of soft-tissue mass (arrow) in area of previous trauma. Mass was proven at biopsy to be skeletal metastasis from primary lung carcinoma.

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Fig. 2A. —47-year-old man with known lung carcinoma. Axial T1-weighted MR image shows hematoma in right rectus femoris muscle (arrow) that occurred after trauma.

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Fig. 2B. —47-year-old man with known lung carcinoma. Coronal proton density-weighted MR image obtained 18 months after original MR examination shows interval development of soft-tissue mass (arrow) in area of previous trauma. Mass was proven at biopsy to be skeletal metastasis from primary lung carcinoma.

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Fig. 3. —63-year-old man with known lung carcinoma. Coronal T2-weighted MR image shows skeletal muscle metastases (arrow) to supraspinatus and infraspinatus muscles in same area of trauma documented 22 months earlier. Mass was proven at biopsy to be skeletal metastasis from primary lung carcinoma.

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Skeletal muscle is resistant to both primary and metastatic carcinoma. Previous reports have cited factors such as contractile activity, pH changes, the accumulation of metabolites, intramuscular blood pressure, and local temperature as reasons for muscle resistance to malignancy [9,10,11].

Weiss [12] found that cancer cells survive best in denervated muscle compared with electrically stimulated muscle. This finding lends support to the belief that most cancer cells die soon after hematogenous spread to muscle because of an inhospitable mechanical, pH, and metabolic environment in normally functioning muscles. Muscles that are injured may have a different mechanical, pH, or metabolic environment that is more favorable to the survival of metastatic cancer cells.

Skeletal muscle injury may alter muscle physiology and result in increased susceptibility to development of metastatic disease at such sites. Each of the patients had documented significant trauma with resultant skeletal muscle injury thought to be consistent with the mechanism of injury. All patients were receiving chemotherapy at the time of injury. Each of the patients had solitary, clinically evident metastases at the exact site of a previously documented skeletal trauma. These metastases all occurred after chemotherapy had been completed.

Skeletal muscles can remove tumor-produced lactic acid. This may decrease the ability of malignant cells to induce tumor neovascularity. Muscle motion itself may result in mechanical tumor destruction [1, 13]. Muscle injury results in decreased use of the injured muscle and may alter muscle physiology. Muscle injury may decrease the ability of muscle to remove lactic acid. There is resultant lactic acid buildup that may produce a more hospitable environment for tumor cells to metastasize.

Skeletal muscle injury does result in focal hyperemia that increases blood flow to the area. This may also result in increased susceptibility to metastatic seeding by circulating malignant cells. However, normally functioning skeletal muscles receive a large percentage of cardiac output. Despite this fact, normally functioning skeletal muscle is resistant to both primary and metastatic carcinoma.

All patients with documented trauma and hematoma or partial muscle tears were on chemotherapy at the time of injury. It is possible that micrometastases were present at the area of injury. Factors mitigating against this are that the hematomas or muscle tears were considered concordant with the mechanism of injury as documented in the patient records at the time of injury. Also, all hematomas or muscle tears resolved clinically without any apparent short-term sequelae. The earliest that a metastasis was found in any of the patients was 16 months after the injury. All eight patients completed their chemotherapy within 4 months after the documented trauma. None of the muscle injuries were considered suspicious for sequelae of metastases, as documented by oncologists caring for the patients.

In conclusion, skeletal muscle injury may alter muscle physiology and result in increased susceptibility to the development of metastatic disease at such sites.