Approach to the Diagnosis of Soft Tissue Tumors

Clinical Information

The diagnosis of a soft tissue lesion requires a modicum of clinical information and adequate, well-processed tissue. At a minimum, the pathologist should be apprised of patient age, tumor location, and its growth characteristics. In some cases, the results of imaging studies, particularly magnetic resonance imaging (MRI), enhance one’s understanding of the clinical extent of the lesion and its relationship to normal structures (see Chapter 3 ).

Although age rarely, if ever, suggests a particular diagnosis, it is important to know whether the patient is a child. In general, there is little overlap between soft tissue tumors occurring in children and those in adults. Therefore, this critical piece of information essentially presents the pathologist with two groups of tumors from which a differential diagnosis can be constructed. For example, undifferentiated pleomorphic sarcoma (formerly known as malignant fibrous histiocytoma, or MFH) is essentially unheard of during childhood, so one should consider other diagnoses for a pleomorphic tumor in a child. On the other hand, neuroblastoma virtually never occurs after childhood, and such diagnoses should always be made cautiously in adults.

Location also often provides direction in the differential diagnosis. Sarcomas, for the most part, develop as deeply located masses and infrequently present as superficial lesions. Exceptions do occur, however, and include lesions such as dermatofibrosarcoma protuberans (DFSP), epithelioid sarcoma, Kaposi sarcoma, and angiosarcoma ( Box 5.1 ). It is also useful to recall that when carcinomas or melanomas metastasize to soft tissue, they do so usually as small, superficial nodules rather than as large, deeply situated masses. The most common carcinomas that present as soft tissue metastases are pulmonary and renal carcinomas, the former usually appearing as a subcutaneous mass on the chest wall and the latter as a soft tissue mass in almost any location.

Unfortunately, there is a great overlap in the clinical presentation of benign and malignant soft tissue masses, so this information may be least helpful to the pathologist. Most soft tissue sarcomas of the extremities are detected by the patient as a slowly growing mass that has been present for 6 months or more at the time of diagnosis. The duration of benign lesions may be similar, although such lesions are generally described as static or slowly growing. An exception is the rapid development of some cases of nodular fasciitis and related lesions. These superficial, pseudosarcomatous, benign lesions may develop rapidly over 1 to 3 weeks, and we have even encountered some that evolved in a few days, a pattern of growth that seldom, if ever, is encountered with a sarcoma. Therefore, an astute general surgeon can sometimes suggest the diagnosis of fasciitis for a rapidly evolving superficial lesion of the extremity.

Biopsy Diagnosis

In the past, the choice of biopsy technique for soft tissue masses was dictated by the size and location of the lesion (see Chapter 2 ). An incisional biopsy was considered the gold standard for large, deeply situated masses and provided ample material for diagnosis and ancillary studies. Its principal disadvantages included spillage of tumor into adjacent compartments as a result of poor hemostasis or faulty biopsy placement, complications of wound infection, and the usual requirement for hospitalization of the patient. Excisional biopsy, although more expedient and providing the entire lesion for examination, was performed on only small, superficial lesions amenable to complete resection. The current reliance on minimally invasive techniques to procure tissue has significantly changed the biopsy paradigm in the direction of core-needle biopsy (CNB). Use of CNB increased from less than 10% to almost 80% of tumors during the early 1990s and now is performed in our hospitals on essentially all deep soft tissue masses. Consequently, the amount of material available to type and grade sarcomas has significantly decreased, and this trend is likely to continue unabated because of the emphasis on less costly outpatient care. Therefore, it is important to be aware of the limitations and pitfalls of CNB and to keep in mind a few basic principles.

First, the pathologist should be aware of the expectation of the clinician. In some cases the goal of a CNB may be simply to establish that a soft tissue mass is a mesenchymal neoplasm as opposed to a lymphoma or metastatic lesion, a distinction that can usually be made in the majority of cases either morphologically or with judicious use of immunohistochemistry. If definitive surgery will be performed following the needle biopsy, then the most important priority is to determine whether the lesion is a sarcoma. If, however, the intention is to provide preoperative (neoadjuvant) radiotherapy or chemotherapy, every attempt should be made not only to make the diagnosis of sarcoma, but also to specifically classify and grade the lesion. However, as discussed in Chapter 1 , it is not always possible to grade a sarcoma reliably on the basis of a CNB. In particular, it is difficult to discriminate a grade 2 from a grade 3 lesion. Pathologists may find that the best assessment that they can give is the designation low grade or high grade, recognizing that high grade will encompass both grade 2 and 3 lesions. Information is inevitably lost when collapsing a three-tiered system into a two-tiered one; however, a two-tiered system still performs reasonably well and is consistent with therapeutic considerations.

In grading CNBs, one can usually accept the presence of high-grade areas in several needle cores as diagnostic of a high-grade sarcoma, because of the improbability that additional material will result in downgrading the lesion. At the same time, one should also be unwilling to accept a lesion as low grade if the number of core biopsies is small, if the lesion has not been adequately sampled, or if imaging studies suggest features of a high-grade sarcoma (i.e., necrosis). CNBs containing necrosis usually imply a high-grade sarcoma, again because of the improbability that limited material captures a solitary or limited focus of necrosis. However, pathologists must be certain that necrosis is of the coagulative and not the hyaline type and is not reflective of prior therapy or surgical intervention. The corollary to the latter portion of this statement is that as soon as radiation or chemotherapy has been administered, grading becomes unreliable because of alterations in nuclear features, mitotic activity, cellularity, and interstitial hyalinization. Therapy also induces necrosis, although it is not possible to discriminate spontaneous from therapy-induced necrosis. Most important, interpreting CNBs necessitates a close dialogue with clinicians to resolve any inconsistencies between clinical and pathologic diagnoses.