Soft tissue sarcomas are rare and a heterogeneous group of neoplasms. Over the years imaging has played a crucial role in the initial staging, monitoring response to chemotherapy, and surveillance for recurrence. In the era of rapidly evolving individualized treatment due to evolving chemotherapies, increasing demand has been placed on the role of imaging in the management of this heterogeneous group. Soft tissue arises from the mesenchyme, which differentiates during development to become fat, skeletal muscle, peripheral nerves, blood vessels, and fibrous tissue. Soft-tissue tumors are histologically classified on the basis of the soft-tissue component that comprises the lesion, but this does not imply that the tumor arises from that tissue.
The rapidly evolving technologies in imaging are trying to keep up with the challenge, with development of newer strategies and hybrid imaging. However, many of these newer technologies are either not available or too costly, even in advanced countries, and hence their exact role remains uncertain. Due to this, and the vast diversity of sarcomas, the imaging appearance and management strategies vary and there is no universally accepted algorithm for the role of individual imaging technologies.
Imaging establishes the presence and location of the soft tissue mass, identifies a definitely benign lesion and separates them from indeterminate or possibly malignant lesions, narrows the differential based on imaging characteristics of the tumor, and identifies the local extent (local staging) of the lesion and its relation to the compartment anatomy, vessels, nerves, bone and joint. Its role in biopsy has already been discussed. Once a diagnosis of soft tissue sarcoma is established by biopsy, imaging is used to identify distant metastasis and stage the tumor. Imaging helps monitor response to neoadjuvant chemotherapy and once definitive surgery is performed, is used for monitoring for local recurrence and distant metastasis.
The evaluation of all suspected soft tissue sarcomas should begin with plain radiographs, with at least two views orthogonal to each other. It is low cost, universally available, and involves minimal radiation but is limited by its contrast resolution. Although its usefulness in the evaluation of soft tissue sarcoma is limited, it can provide important diagnostic information. It can sometimes detect the presence of a fat-containing lesion, but more often it is used to detect the presence of and pattern of mineralization that helps establish a diagnosis or narrow the differential. Typical examples of this utility is to distinguish myositis ossificans with its peripheral calcification with central lucent zone reflecting zonal pattern of mineralization, and tumoral calcinosis with its dependent layering of calcium, from other soft tissue masses. Several soft tissue sarcomas may have mineralization and typical examples include extraskeletal mesenchymal chondrosarcoma, extraskeletal osteosarcoma, liposarcoma, and synovial sarcoma. Chondrosarcomas tend to have chondroid matrix resembling rings and arcs, osteosarcomas dense cloud like mineralization, liposarcomas large and coarse, while the synovial sarcomas can have spicules, stippled sand-like, and/or coarse calcifications.
The current utility of CT in evaluation of soft tissue sarcoma is limited, due to concern for radiation, and less contrast resolution compared to magnetic resonance imaging (MRI) . However, in the extremities, radiation is less of a concern compared to the trunk. Based on their ability to attenuate X-rays, the tissues can be broadly divided into fat, fluid, soft tissue, or calcification/ossification density. CT easily identifies fat in fat-containing lesions such as a simple lipoma. However, unless the lesion is composed entirely of homogeneous fat density with or without a few thin delicate septa that show no or mild enhancement, CT cannot reliably distinguish a lipoma/lipoma variant from well differentiated liposarcoma.
The latter commonly will contain nodular or globular non-fatty tissue and/or thick septations which often show enhancement. It is better than plain radiographs and other modalities in detection of soft tissue mineralization and differentiating calcification from ossification. One particular utility of this is to determine the pattern of mineralization which distinguishes myositis ossificans (zonal phenomena with dense peripheral rim of calcification) from a soft tissue sarcoma which lacks that pattern. CT is better than plain radiographs in evaluation of cortical invasion but its accuracy in evaluating intramedullary invasion is less than that of MRI. The majority of sarcomas have CT attenuation slightly less or similar to that of muscles. Contrast material may be used to detect lesions when they are isodense on a non-contrast study, distinguish a cystic from solid lesion, and evaluate internal vascularity and vascular invasion. CT with contrast is the modality of choice for staging if MRI is contraindicated.
The main role of ultrasound is as an initial imaging study for evaluation of superficial soft tissue mass. Most superficial extremity soft tissue masses are benign with lipoma being the most common. US confirms the presence of the mass, measures its dimensions and depth, evaluates its internal echotexture and vascularity, and can differentiate a few typical benign tumors and other non-neoplastic lesions such as ganglion cysts from others which need further evaluation. Ultrasound is widely available and cheap and does not have the side effects of radiation. Color or power Doppler is essential to evaluate the presence of internal vascularity, and differentiate between a cyst and a cystic appearing solid mass. Combination of gray-scale and color Doppler studies can divide the tumors into cystic, solid, or mixed solid and cystic (complex). Simple cystic masses such as ganglion and synovial cysts are well defined, anechoic with posterior acoustic enhancement, and no internal vascularity on Doppler studies.
MRI is imaging modality of choice for evaluation of soft tissue sarcoma, particularly to determine the local extent of the lesion. Lack of ionizing radiation, multiplanar imaging capability, and excellent tissue contrast makes it particularly suitable for evaluation of characteristics and extent of the lesion. MRI can not only distinguish between simple cystic lesions and solid masses but can further help narrow the differential and point towards sarcoma as the potential etiology. MRI can evaluate the size and extent of the tumor and identify invasion of the compartments and important structures such as nerves, vessels and joints. A combination of fat sensitive (T1 weighted) and fluid sensitive (proton density weighted/T2 weighted) sequences without and with frequency selective fat saturation are utilized for imaging. Short tau inversion-recovery (STIR) sequence may replace fat saturation fluid sensitive sequences, and has the advantage of more homogenous fat suppression but has lower signal-to-noise ratio and is susceptible to motion artifact. The T1 weighted sequences show fat, melanin and methemoglobin as high signal intensity.
Tc-99m-methyl diphosphonate bone scan has limited role in the initial evaluation of soft tissue sarcoma. The potential use is to evaluate for bone metastasis. Since bone metastasis from soft tissue sarcoma is rare, unless there is already metastasis to lungs, bone scintigraphy is only used for evaluation of symptomatic patients.
Soft-tissue tumors and tumorlike lesions are encountered often in daily radiologic practice. The vast array of benign and malignant entities can make lesion diagnosis overwhelming for the radiologist. By systematically using clinical history, lesion location, mineralization on radiographs, and SI characteristics on MR images, the radiologist can develop a short and appropriate differential diagnosis. MR images can be particularly useful for characterizing benign lesions that do not require imaging follow-up or biopsy, such as lipomas and ganglia. In cases where a soft-tissue lesion is indeterminate on the basis of clinical and imaging features, biopsy should be considered.