Many imaging techniques are now at the disposal of the oncologist, not only for diagnosing musculoskeletal tumors but also for evaluating the response to radiation or chemotherapy and for posttreatment follow-up. The choice of modality will reflect the cell of origin for the diagnosis and the type of treatment undertaken for long-term surveillance.
Whether a bone or soft tissue neoplasia is the concern, the initial evaluation of a mass should be performed with plain radiography.27 For bone lesions, plain films will show the aggressiveness of the tumor, and these remain the most specific modality for developing a differential diagnosis. For soft tissue masses, plain films can narrow the differential diagnoses by displaying or excluding the presence of mineralization in or around the lesion and detect changes in adjacent bone.28
When evaluating plain films, it is important to search for periosteal new bone and, if present, characterize it (Figure 32.5). This feature, which is often overlooked, can be extremely helpful in determining the behavior of a lesion. Periosteal new bone that is unilaminar or multilaminar but uninterrupted, is associated with benign processes, including low-grade neoplasia, trauma, or indolent infection. Periosteal new bone that is interrupted signifies an aggressive process, which may be either neoplastic or inflammatory.
Following plain films, CT is indicated for imaging primary tumors of bone and soft tissue lesions that are mineralized. For bone lesions, CT is superior to MRI in characterizing changes, including subtle areas of cortical thinning or destruction, and in providing information that may assist in planning an approach for biopsy.29,30 CT easily displays the relationship of soft tissue masses to neurovascular structures and joints, assisting in determining the choice of surgical procedure (resection versus amputation) and the surgical approach.
CT is excellent for showing architectural detail in small bones, including ribs. An area where CT remains underutilized is in evaluation of bone scan abnormalities in cases where plain films fail to demonstrate the pathology; this is especially true for focal tracer uptake in ribs in the patient with a known primary malignancy. Thin-section scans targeted to the area of concern can almost always differentiate between a traumatic etiology (often forgotten by the patient) and metastatic disease.
CT guidance is commonly used for percutaneous biopsy.31 With skin markers over the area of concern, the exact area of interest can be accurately localized with respect to depth and proximity to vital structures. This method is especially useful for core biopsies where precise needle localization might be essential.32
One argument for using MRI rather than CT has been the multiplanar imaging capability of MR imaging, allowing scan acquisition in the sagittal, coronal, or oblique plane. This situation is no longer the case. The advent and popularity of multidetector row CT has placed this technique again in the forefront of diagnostic imaging.33 Especially for bone tumors, rapid acquisition of slices at submillimeter increments and the ability to reformat in any plane displays anatomy and pathology to an advantage never before possible with CT.34,35
Not uncommonly, when a person complains of a new mass or bone pain, an MRI is the first study ordered. Although this is frequently diagnostic for traumatic lesions, the appearance of malignancy with MRI is generally nonspecific.36 Plain films are far superior for characterizing bone lesions. MRI may overestimate the extent of a lesion if it is fractured and will almost uniformly overlook foci of mineralization. The role of MRI is thus primarily one of staging rather than diagnosis.29,37,38
Although the radiology literature initially advocated the use of intravenous gadolinium for imaging musculoskeletal tumors, subsequent work has shown that the contrast agent generally adds nothing to the diagnosis or local staging of musculoskeletal neoplasia. Attempts within the past decade to add specificity to MRI with use of dynamic contrast imaging have been popular overseas but have not added diagnostic information at a clinically significant level.39 The advantage of contrast enhancement, as originated in imaging of the central nervous system, reflects the destruction of the blood-brain barrier by a pathologic process. There is no corollary for such a barrier in the musculoskeletal system. Contrast enhancement thus reflects local vascularity and the size of the extracellular fluid compartment. As these features significantly overlap for benign and malignant processes, the technique adds nothing to tumor conspicuity, diagnosis, or staging.40
Technetium 99m medronate (99mTc-MDP) remains the mainstay for detection of bone metastatic disease. This may, however, not remain the case. Research using total-body MRI has suggested that this modality may not only be more sensitive than bone scintigraphy but also allows simultaneous imaging of solid organs, including the brain and liver.41 One caveat is that evaluation of ribs with MRI is limited because of both the small size of the bone and respiratory motion. Another possible problem with whole-body MRI is the common detection of incidental lesions in both bone and soft tissues throughout the body. Studies with whole-body CT screening have shown that significant financial resources go into workup of these findings, often with invasive procedures, for confirmation of a benign diagnosis.
There is a recent trend for utilizing molecular imaging in conjunction with cross-sectional imaging, specifically MRI. Recent published research appears very promising, likely opening new chapters specifically in musculoskeletal tumor
Aside from its role in routine screening and detection of metastatic disease, the 99mTc-MDP bone scan can serve other purposes.44,45 Not uncommonly, metastatic disease may present as a solitary symptomatic lesion in an area that is not readily amenable to biopsy for technical reasons.46 In this instance, a bone scan may show that the process is actually multifocal and plain film correlation might display another focus that is more readily approachable. Radionuclide bone scans are also useful for evaluation of primary bone lesions that might be multifocal, such as brown tumors from hyper-parathyroidism.47 Bone lesions are often incidentally discovered on imaging studies obtained for unrelated reasons. In cases where the diagnosis is in question, a radionuclide scan will show the activity of the lesion. If tracer uptake is normal, the lesion can generally be disregarded. If the area is either photopenic or shows increased activity, further workup is warranted.
Thallium (201Tl chloride) single photon emission computed tomography (SPECT) scanning has been used for determining a tumor's response to therapy as well as searching for metastatic disease.48 The concept is quite attractive, because CT and MRI show only morphology, not tumor metabolism. Unfortunately, thallium uptake is nonspecific, with activity seen not only in malignancy but also in benign neoplasia and traumatic and inflammatory disorders. Its use has thus declined, being replaced by positron emission tomography (PET).49
PET imaging is addressed in depth in Chapter 33. Suffice to say here that trials are now under way to define the exact role of PET in diagnosis and screening for a variety of tumors, including various types of sarcoma.50 Although relatively recent, the advent of PET/CT has had a great impact on musculoskeletal oncology, as foci of abnormal activity can be precisely localized on the CT image and evaluated for their significance.51
Ultrasound plays little role in the routine diagnosis, staging, and follow-up of sarcomas. Although it can be used for image guidance for biopsy,52 this is usually more easily accomplished with CT. Ultrasound may be useful for detecting suspected tumor recurrence in the patient with implanted hardware, where artifact precludes other means of cross-sectional imaging.53,54
Occasionally ultrasound may be used for the diagnosis of vascular lesions by using color Doppler imaging mode.55 In addition, ultrasound is helpful in differentiating fluid collections from solid masses.
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