Staging Bone and Soft Tissue Tumors

Magnetic Resonance

MR has gained a prominent position in the diagnosis and management of patients with musculoskeletal sarcoma. It directly exhibits the lesion in relationship to surrounding normal structures with exquisite anatomic detail ( Fig. 99-1 ). The MR protocol for bone and soft tissue sarcoma can be identical. We routinely use fast, or turbo, spin-echo for all sequences. A longitudinal T1-weighted sequence is used to exclude or determine intraosseous extent because high contrast between low signal intensity of tumor and relatively high signal intensity of normal fatty or hematopoietic marrow is combined with excellent spatial resolution. It is important to include an anatomic point of reference in the field of view, usually the nearest joint. The coronal plane is preferred around the shoulder, wrist, pelvis, and hip because it best displays tumor and tumor-containing osseous structures relative to the joints. For the same reason, the sagittal plane is the preferred longitudinal plane around the knee. The plane that best displays the tumor in relation to bone is chosen around the elbow and ankle. In the spine, the sagittal plane often is preferred over the coronal plane because it usually shows the relationship between a lesion and the spinal canal to the best advantage. In addition to these high-resolution images of the primary tumor, a large–field-of-view T1-weighted longitudinal sequence is made using the body coil in order to look for skip metastases.

A fat-suppressed, T2-weighted sequence is used for imaging in the axial plane. The axial plane displays the relationship between tumor (cortical), bone, and soft tissues, such as muscle compartments and neurovascular bundles. The axial and longitudinal planes are both used to evaluate the relationship between tumor and the curved joint surfaces. When fat-suppressed, T2-weighted sequences cannot be obtained, for instance because of field inhomogeneity, a short tau inversion recovery (STIR) sequence can be used.

For comparison with the contrast-enhanced images, a T1-weighted sequence is also obtained in the axial plane. This also has the advantage of having a second plane to evaluate intraosseous extent. Depending on location, a coronal plane may be used instead of the axial plane (see Fig. 99-1 ). An alternative to multiplanar image acquisition that has not been tested in oncologic imaging is 3D fast spin-echo (3D-FSE).

A gadolinium-chelate–enhanced T1-weighted image displays additional information because the generated contrast between the various tissue types depends on vascularity, permeability of tumor vessels, volume of interstitial space, and interstitial pressure. Gadolinium is paramagnetic and thus reduces both T1 and T2 relaxation times. The dominant effect is concentration dependent. In normal tissue and in tumor, the T1 effect is dominant, and an increase of signal intensity is observed in enhancing tissue. Signal drop, because of T2 shortening, can be observed only when concentration is high, such as in the artery when the first bolus passes or when the Gd chelate accumulates in the bladder.

The additional information of Gd-chelate–enhanced MR images is therefore best displayed on fat-suppressed T1-weighted sequences, which have an intrinsic high spatial resolution (see Fig. 99-1 ). Gd-chelate–enhanced images are therefore taken in both a longitudinal and axial plane 3 to 5 minutes after bolus injection of 0.2 mL of Gd chelate per kilogram of body weight, administered intravenously with a flow of 2 mL/second. Because of the small volume (a patient weighing 60 kg will receive only 12 mL), a saline flush is administered directly after the administration of the contrast agent. Cellular areas such as in viable tumor will enhance more than normal tissue or liquefied necrotic tumor. This, combined with the high spatial resolution, contributes to identifying small nests of viable tumor in critical areas. In addition to confluent necrotic and liquefied areas, viable tumor with a high interstitial pressure, usually located centrally, will enhance not at all, or very late ( Table 99-1 ).

A similar observation can be made in cartilaginous tumors, where we can appreciate very late enhancement because of diffusion of contrast agent into the tumor matrix. In general, sarcoma with low cellularity and/or large tumor matrix, such as osteoid in osteosarcoma, and well-differentiated cartilage with mucoid in well-differentiated chondrosarcoma will enhance poorly and/or late. On the other hand, angiogenesis in the margin of necrotic areas and the reactive tissue zone will enhance. These differences in enhancement patterns are a function of time, and it is for this reason that dynamic Gd-chelate–enhanced imaging is used to increase the accuracy in identifying viable tumor. As a rule of thumb, tissue that enhances within 6 seconds after arterial enhancement observed close to the tumor represents viable tumor, and tissue that enhances later represents a reactive tissue zone. These differences in the start of enhancement are best appreciated on subtraction images. The subtraction images can also be used to place regions of interest for more quantitative analysis of enhancement curves (see Chapter 100 ). Because of this time frame, it is important to have a temporal resolution of at least 3 seconds. Spatial resolution and number of sections sampled in this time frame should be as high as possible but should not be increased at the cost of the temporal resolution. The temporal resolution and the cutoff value of 6 seconds are based on empiric evidence as well as on theoretic pharmacokinetic modeling. Gd-chelate–enhanced MR angiography may be used as an additional sequence to evaluate involvement of the neurovascular bundle.

Computed Tomography

Multidetector row CT is the optimal modality in identifying, or excluding, pulmonary metastases. The 3D dataset is obtained with small collimation (e.g., 1 mm), and axial reconstructions of 3 to 5 mm are analyzed in cine mode and multiplanar reconstructions on a viewing station ( eFigs. 99-1 and 99-2 ). The thicker reconstructions facilitate differentiation between vessels and nodules. Multiplanar reconstructions may increase accuracy because they allow a second look and because the periphery of the lung, the area where metastases preferentially occur, can be analyzed in planes that are orthogonal on bordering structures, such as the diaphragm. Unless conventional chest radiographs show metastatic disease, CT should be obtained before treatment to diagnose, or exclude, metastatic disease. Because of the high sensitivity of CT, many small benign nodules are also detected; therefore, histologic proof of pulmonary metastases is needed when a few small nodules are found.

The role of CT in local staging is limited to a small number of specific situations. The anatomic detail that CT offers is useful when anatomic information is needed preoperatively in osseous benign tumors and tumor-like conditions. CT is routinely used in planning and executing percutaneous therapy and biopsy procedures.

Bone Scintigraphy

Technetium-99m methylene diphosphonate ( ^99m Tc-MDP) has been the method of choice for screening for osseous metastases because of its capability to image the entire skeleton. However, whole-body MRI using T1-weighted and STIR sequences and a floating table has become available and has the advantage of superior sensitivity and specificity compared with planar bone scintigraphy (see Chapter 97 ).