CT is not an ideal method for characterization of soft tissue metastases. Furthermore, soft tissue metastases cannot be reliably distinguished from the primary soft tissue sarcoma [51].

CT appearance of soft tissue metastases is widely variable: “abscess-like” lesion with rim enhancement and central hypo-attenuation, focal masses with homogeneous contrast enhancement, diffuse metastatic infiltration manifested as soft tissue swelling and diffuse heterogeneous contrast enhancement, multifocal calcification, and intramuscular bleeding with muscle enlargement and area of high attenuation [45]. The most common appearance of soft tissue metastases on contrast-enhanced CT is that of a rim-enhancing mass with central hypo-attenuation (Figs. 24.1, 24.2, 24.3, 24.4, and 24.5) [29].

CT may guide lesion biopsy and direct appropriate local radiation.

MR imaging is the preferred technique for muscle and soft tissue assessment, even though MRI appearances of soft tissue metastasis are not specific [13, 52].

On MRI, soft tissue metastases are of low or intermediate signal intensity compared to normal muscle on T1-weighted sequences and high signal intensity on T2-weighted sequences [17, 22, 42].

However, high signal intensity of metastases on T1-weighted images has been also described, particularly in malignant melanoma metastases because they contain melanin and may often bleed [54]. High signal intensity has also been reported in metastases of renal cell carcinoma, but its cause is still unknown [20, 35].

After administration of contrast medium, most muscle metastases show marked heterogeneous enhancement related to tumor necrosis (Fig. 24.4). In addition, extensive peritumoral enhancement has been reported as one of the characteristic features of IM [13, 48].

Edema of surrounding soft tissue is common [6]. Erosion of the adjacent bone might rarely be observed on MRI [17].

Diffusion-weighted imaging (DWI) is considered as a useful method in the assessment of tumor cellularity in soft tissue sarcomas and can be used as a noninvasive tool to monitor treatment responses [36, 49]. In soft tissue metastases, Surov described low or moderate ADC values and moderate to high signal intensity on diffusion images [44].

Technetium-99 m-labeled nuclear scan can show increased uptake within the soft tissue mass [48]. ¹⁸F-FDG PET/CT was demonstrated to have a higher sensitivity compared with MRI in detecting soft tissue metastases [10 31]. However, there were false positives in few cases, which should be taken into consideration. PET/CT, which performs anatomical and metabolic assessment, is very sensitive in the detection of soft tissue metastases and provides more information for clinical tumor staging. Compared to CT, PET is more sensitive because it detects soft tissue lesions before the density and morphology of the soft tissues are altered, providing a clear view of highly metabolic nodules within the soft tissue. ¹⁸F-FDG PET/CT imaging may reveal the primary tumors of the soft tissue metastases, which is helpful for differential diagnosis.

After appropriate imaging, early percutaneous biopsy is recommended. It is advocated to biopsy in case of solitary or late metastases and in case of unknown primary tumor. Biopsy is not recommended in case of a disseminated disease.

Several benign tumors and soft tissue sarcoma cannot be differentiated radiologically from metastases [5, 16, 28]. Psoas involvement by metastatic disease is frequently confused with psoas abscess or hematoma, both clinically and radiographically. Therefore, confirmation of soft tissue metastases can only be established by histopathology.

Calcification within soft tissue metastases has been confused with myositis ossificans [6]. In addition, ischiogluteal bursitis, which occurs frequently in patients with cancer, can also mimic muscle metastasis [18].

The presence of soft tissue metastasis influences the primary tumor treatment. Reported studies show that the survival of patients with soft tissue metastases ranges from less than 9 months to not more than 3 years after soft tissue metastases diagnosis, although survival up to 5 years has been reported in some cases [24]. The presence of a skin metastasis in a lung cancer patient indicates a poor prognosis, with a median survival of 2–4 months [10, 25, 38].

The treatment of soft tissue metastases depends on their localization and clinical presentation as well as on the prognosis related to the primary tumor. Therapeutic options include observation, radiotherapy, chemotherapy, and surgery. For painful masses in the context of a widespread metastatic disease, radiotherapy, chemotherapy, or both may be indicated based on the primary tumor, the organ involved, the extent of involvement, symptoms attributable to the various sites, the patient’s age, and health status [6].