CHAPTER 15 Soft tissues

Clinical aspects

The use of FNB and cytodiagnosis in the primary evaluation of a mass lesion in the soft tissues has become an accepted diagnostic modality using the cytodiagnosis as the morphological basis for a discussion of therapeutic options. A pretreatment cytological diagnosis offers several advantages. Patient anxiety can be relieved by providing an instant diagnostic suggestion followed by a discussion of therapeutic options based on the cytodiagnosis in combination with clinical and, at times, radiological data. FNB has a minimal risk of complications and the risk of tumor spread is negligible. FNB also allows easier collection of material from different parts of large tumors than open and core needle biopsy, important in case of tumor heterogeneity. Furthermore, FNB is less traumatic to the tissues than open or core needle biopsy.

Due to the relative rarity of primary tumors of soft tissue and to the extraordinary range of different types of tumors that can arise in these sites, a case can be made for centralising the diagnostic work-up of soft tissue tumors to a multidisciplinary clinic in an orthopedic oncology center. A specialised clinic facilitates the accumulation of invaluable experience based on the consistent correlation of clinical, radiological, cytological and histopathological findings at the time of diagnosis, ancillary laboratory tests, the results of treatment and the long-term outcome. It is hardly possible for the individual pathologist to gain a comparable level of experience over his/her professional lifetime.

However, to be efficient, highly specialised clinics of this kind need to draw material from a large population and consequently are only established in large academic centers. It is not possible to refer all patients with a soft tissue tumor or mass to a multidisciplinary center, particularly since most of these lesions are benign (benign lipomatous tumors). There is still a need for an easily accessible diagnostic service to be available at the community level in order to facilitate the referral of patients with suspected soft tissue tumors to an orthopedic oncology clinic. Guidelines for referral have been proposed. The recommendations of the Orthopedic Tumour Centre at the Lund University Hospital, based on epidemiologic data and on the difficulties experienced in the management of sarcomas, especially deep-seated sarcomas which were initially biopsied or surgically treated outside specialist centers, may serve as an example. The recommendations, suggested in 1997 and still valid, are that all patients with a deep-seated soft tissue tumor irrespective of size, or with subcutaneous tumors larger than 5 cm should be referred untouched. An FNB may be performed but the smears should be reexamined at the center, if possible before the patients first visit.¹

The place of FNAC in the investigative sequence

The diagnosis and classification of soft tissue tumors is one of the most difficult areas in surgical pathology. The relative absence of recognisable tissue architectural patterns in cytological preparation makes diagnosis by FNB even more difficult. However, open biopsy or inadequate surgical excision of a primary sarcoma may compromise fascial planes and necessitate more extensive (mutilating) surgery than if the tumor had been left intact.

The current use of FNB as the primary and, in a majority of cases, the definitive diagnosis of soft tissue tumors before treatment is based on the following points:

• In most centers, the treatment of soft tissue tumors is primary, radical surgery. At surgery, knowledge of the specific histogenetic type is of less importance in planning the operation than the size and site of the tumor and its relationship to vessels, nerve bundles and bone.

• The essential task for the cytopathologist is to accurately identify the tumor/mass as a soft tissue tumor, benign or malignant, and to exclude a reactive process or other malignancies. Specific tumor typing and malignancy grading are, of course, of value if at all possible.

• Clinical management is based on the combined evaluation of clinical data, radiographic findings and cytological examination. (The approach is similar to that used for breast lesions.) If there is a discrepancy between the three investigation modalities, reexamination of data is necessary and repeat FNB or supplementary biopsy (core needle or surgical biopsy) must be considered. If the treatment involves neoadjuvant, preoperative therapy the cytodiagnosis must be equivalent to a histological tissue diagnosis, i.e. providing histogenetic tumor type and malignancy grading.

• Close cooperation between the cytopathologist and the orthopedic surgeon is important. For optimal results, the patients should be referred to a specialist center with the tumor untouched, according to the recommended guidelines. The cytopathologist and the surgeon together should decide the insertion point of the needle. Diagnostic accuracy is optimal when the same person performs the needling and examines the smears.

A broad categorisation of a soft tissue tumor as of primary soft tissue origin, benign, low-or high-grade malignant, is possible in the majority of cases if the multidisciplinary approach suggested above is taken. Although many pathologists are reluctant to attempt cytological subclassification of primary soft tissue tumors and tumor-like lesions, a basic knowledge of the cytology of the most common tumors is important. A wide variety of benign tumors/lesions as well as sarcomas have been characterised cytologically by comparative studies of aspirates from series of cases of the same tumor type and by correlative studies of FNB smears and histological sections. The tumors/lesions are listed in Table 15.1.

Table 15.1 Soft tissue tumors/lesions which have been cytologically characterised in smears

Accuracy of diagnosis

The results of FNA cytology of soft tissue tumors are good overall if patients with tumors suspected of malignancy are referred to centers specialising in this field. Åkerman et al.² reported their 20-year-experience of FNAC of soft tissue tumors as a preoperative tool in 1994. In their series of 517 cases, the aspirated material was insufficient for diagnosis in 6% and an erroneous cytological diagnosis was rendered in 5% of adequate smears, 85% of the benign tumors were reported as benign and 89% of the sarcomas were classified as malignant soft tissue tumors. Other centers have reported similar results. Many reports highlight the difficulty to correctly subtype sarcomas histogenetically and to differentiate between low-grade sarcoma and benign soft tissue tumors.³^–⁵

Technical considerations

The biopsy technique is as for any palpable lesion. Local anesthesia is rarely needed. Multiple passes are usually necessary to obtain sufficient and representative material. Alcohol-fixed H&E- or Pap-stained smears and air-dried smears stained with MGG or Diff-Quik should be used in parallel. Wet fixation gives excellent nuclear detail, particularly of cells in microscopic solid tissue fragments, as is often the case with several types of sarcoma. Air-dried smears (MGG; Diff-Quik), however, provide more information about cytoplasm and stromal components.

The site of the biopsy should, if possible, be chosen in consultation with the surgeon responsible for the treatment. A single point of entry and tattooing the insertation point on the skin is helpful if the needle track is to be included in the surgical excision. Ultrasonography (US), computed tomography (CT) or magnetic resonance imaging (MRI) can be helpful in finding viable tissue and avoiding cystic and necrotic areas in extensively necrotic or cystic tumors. US guidance is also of great value in the biopsy of small, deep-seated (intra- or intermuscular) tumors. If sufficient cellular material can be sampled, immunocytochemical studies and chromosomal or molecular genetic analysis may give valuable diagnostic information.

The parallel use of FNB and core needle biopsy (CNB) in selected cases, both sampling methods performed by the cytopathologist, combines the advantages of both techniques. In our experience, a core needle with an outer diameter of 1.2 mm is sufficient and well tolerated by patients. FNB allows wider sampling of large tumors and an immediate assessment based on air-dried smears (especially Diff-Quik-stained), while the CNB samples facilitates evaluation of microarchitecture and in many cases provide more material for ancillary studies.⁶ The risk of tumor cell dissemination in the needle track is not greater for CNB than for FNB; it is negligible for both.

Cytological findings

Primary soft tissue tumors in general

Common characteristics

• A variable yield in both benign tumors and sarcomas, smears from tumors rich in intercellular collagen or heavily hyalinised are generally poor in cells, tumors rich in vessels may yield only blood or blood mixed with a few scattered cells,

• A mixture of single cells and small tissue fragments (‘microbiopsies’); the proportion of single cells to fragments differs in different tumor types,

• Myxoid ground substance and/or fragments of intercellular collagen,

• Spindle-shaped nuclei,

• Multinucleated cells, giant cells and bizarre nuclei,

• Shape of cells highly variable; cytoplasmic projections, indistinct cell borders,

• Malignant nuclear features may be absent in high-grade malignant sarcomas,

• Benign tumor-like lesions or tumors may show prominent cellular pleomorphism,

• Mitoses rare in benign tumors and low-grade malignant sarcomas.

Fine needle biopsy samples from high-grade malignant sarcomas are usually abundant, whereas the yield from benign tumors and low-grade malignant sarcomas depends on the tumor type. Collagen-rich intercellular stroma, areas of hyalinisation, fibrosis and prominent vascularity influence the yield. Infrequently, single cells dominate the smears whether from benign tumors or sarcomas. Aggregates, bundles of cohesive cells or ‘microbiopsies’ are commonly present in smears, especially from sarcomas of various types. The cells may not form any discernible tissue architectural pattern or they may be oriented in fascicles or streams. Spindle cells which seem to radiate out from a central basement membrane core suggest a vasoformative tumor, solitary fibrous tumor, hemangiopericytoma or hemangiopericytoma-like synovial sarcoma. Bundles of parallel spindle cells are common in smooth muscle tumors, occasionally in tumors of Schwann cell origin, and occur at times in fibroblastic tumors. Palisading of nuclei can be seen in Schwann cell tumors and smooth muscle tumors and rosette-like figures (at times with a center of finely fibrillar material) in neuroblastoma and in Ewing’s family tumors.

Myxoid ground substance may have a more or less fibrillar structure and stains brightly violet/red/purple in MGG (epithelial mucus is homogenous blue/violet). It is less conspicuous and may be almost invisible in wet-fixed H&E- and Pap-stained smears. Collagen is also obvious in MGG as strands of red/violet dense material between cells or isolated in the background. Capillary vessels are prominent in tissue fragments of myxoid liposarcoma and fragments of vessels are often observed in the myxoid ground substance of myxofibrosarcoma. Inflammatory cells (nodular fasciitis) and mast cells (synovial sarcoma) should be looked for. Necrosis is suggestive of high-grade malignant sarcoma.

The cytoplasm of mesenchymal tumor cells is generally not as fragile as that of many epithelial tumor cells and lymphoid cells with the exception of the small round cell malignancies such as alveolar rhabdomyosarcoma, neuroblastoma and the Ewing’s family tumors, granular cell tumor and alveolar soft tissue sarcoma. Cell borders are usually indistinct and the cytoplasm may be drawn out into elongated processes or thin strands which anastomose with those of adjacent cells. Aggregates of cells have a syncytial cytoplasm. Nuclei tend to be eccentric in rounded cells. Spindle-shaped nuclei are common in smears from soft tissue tumors in general and are often present also in highly pleomorphic tumors. In synovial sarcoma and cellular myxoid liposarcoma (round cell liposarcoma) nuclei may be quite plump or rounded. Predominantly rounded nuclei are characteristic of granular cell tumor, alveolar soft tissue sarcoma, alveolar rhabdomyosarcoma and Ewing’s family tumors. In general nuclear pleomorphism is proportional to the grade of malignancy.⁷ Important exceptions occur, however. Anisokaryosis, marked variation in nuclear shape and multinucleation are often prominent in the various types of benign pseudosarcomatous soft tissue lesions as nodular fasciitis, proliferative fasciitis and myositis and pseudomalignant myositis ossificans. In such lesions, nuclear chromatin is bland and evenly distributed, but nucleoli may be large and prominent. Nuclear pleomorphism and chromatin clumping may be minimal or absent in some low-grade malignant sarcomas and even in some high-grade malignant sarcomas such as synovial sarcoma. The malignant potential in such tumors is mainly suggested by high cellularity, nuclear crowding, mitotic figures and occasionally by the presence of necrosis. Bizarre nuclei with lobulation, budding and nuclear satellites are common in high-grade tumors. Mitoses are mainly found in high-grade malignant sarcomas; nodular fasciitis is an important exception. Abnormal mitotic figures indicate high-grade sarcoma.

Comprehensive descriptions of the cytological patterns of primary soft tissue tumors can be found in two recent publications by Åkerman and Domanski and Gonzáles-Cámpora.^8,⁹

Soft tissue tumors are usually classified histogenetically and in this section we use the classification proposed in the WHO fascicle Pathology and Genetics of Tumours of Soft tissue and Bone 2002 ¹⁰ and Weiss SW, Goldblum JR, Soft tissue tumours, ed V.¹¹

Fibroblastic/myofibroblastic tumors and fibrohistiocytic tumors

Benign tumors

The pseudosarcomatous lesions are an important target for needling in this histogenetic group of tumors/lesions. Nodular fasciitis is among the commonest and the most frequently needled. Dahl and Åkerman reported 13 cases with cytology 1981;¹² at present, our material comprises more than 70 cases, all with remarkably similar cytomorphology. In more recent investigations of the cytological features of nodular fasciitis, the results are similar to those of Åkerman and Dahl.^13,¹⁴ The most important feature is the pleomorphism of the proliferating fibroblasts/myofibroblasts. Nuclei are predominantly spindly, but a proportion of cells have plump, ovoid or kidney-shaped nuclei. Bi-and/or multinucleated forms are always present and, if looked for carefully, ganglion cell-like binucleate cells with triangular shape and eccentrically placed nuclei are found (Fig. 15.1). A high cell content, nuclear pleomorphism, prominent nucleoli and the presence of mitoses may suggest malignancy, but the pale, bland nuclear chromatin is a clear indication of the benign nature of the lesion (Fig. 15.2). The correct diagnosis depends on the clinical presentation and the anatomical site (a rapidly, often tender subcutaneous nodule most frequently appearing in the upper extremity, trunk, head and neck) combined with such cytologic features as a myxoid background, actively proliferating fibroblasts/myofibroblasts and the presence of inflammatory cells.

Fig. 15.1 Nodular fasciitis

Proliferating fibroblasts embedded in a myxoid background; note binucleate cell with abundant cytoplasm and eccentric nuclei (ganglion cell-like) (MGG, HP).

Fig. 15.2 Nodular fasciitis

Poorly cohesive, proliferating fibroblasts; plump, oval, irregular nuclei, small nucleoli, bland chromatin; myxoid matrix not discernible in H&E-stained wet-fixed material (H&E, HP).

Proliferative myositis and fasciitis are less frequent pseudosarcomatous processes but are important with regard to the rapid growth (common to all these lesions) and their occurrence in children. Their cytomorphology is similar to that of nodular fasciitis although the myxoid matrix is less prominent and the ganglion cell-like cells are often numerous with very prominent nucleoli (Fig. 15.3). In proliferative myositis regenerating multinucleated muscle fibers are commonly present (Fig. 15.4). An important clinical sign is that these lesions, especially nodular fasciitis, can disappear spontaneously or diminish substantially in size within 3–4 weeks after needling.^15,¹⁶ Another lesion which cytologically demonstrates prominent reactive cellular changes is pseudomalignant myositis ossificans (PMO). PMO is a rapidly growing lesion (intramuscular or subcutaneous), smears showing a mixture of proliferating fibroblasts/myofibroblasts, osteoblasts with prominent reactive changes and multinucleated giant cells of osteoclastic type (Fig. 15.5). In PMO an ossification in a zonal pattern is a typical find within 3–4 weeks and, according to our experience, spontaneous resolution is common. The cytomorphology in our series of five cases¹⁷ is similar to that described by Dodd et al.¹³

Fig. 15.3 Proliferative fasciitis

Ganglion cell-like cells are often numerous with very prominent nucleoli (H&E, HP).

Fig. 15.4 Proliferative myositis

Regenerating muscle fibers are commonly present (MGG, HP).

Fig. 15.5 Myositis ossificans

A mixture of proliferating fibroblasts/myofibroblasts, osteoblasts and multinucleated giant cells of osteoclastic type (MGG, LP).

Fibromatoses such as desmoid fibromatosis and palmar and plantar fibromatosis may partly resemble the pseudosarcomatous lesions, especially nodular fasciitis, in FNB smears. However, the nuclei are more consistently spindle and the marked nuclear pleomorphism in those lesions is not seen. Collagen fragments are common and myxoid ground substance unusual, as are inflammatory cells. Strands and clusters of spindle cells showing moderate anisokaryosis, and more or less acellular fragments of collagen are characteristic of desmoid fibromatosis, abdominal as well as extra-abdominal, in FNB smears (Fig. 15.6). When the tumor infiltrates striated muscle, muscle fragments and regenerating multinucleated muscle fibers are commonly seen (Fig. 15.7). The most important differential diagnoses are the rare low-grade malignant fibrosarcoma and monophasic fibrous synovial sarcoma. Smears from palmar and plantar fibromatoses (Dupuytren) can be surprisingly cellular, but the cells are uniformly spindled with bland spindled nuclei (Fig. 15.8). Recent summaries of the cytologic characteristic features of desmoid fibromatosis have been published 2003⁸ and 2006.¹⁸

Fig. 15.6 Desmoid fibromatosis

The two typical components are shown here: a cluster of loosely cohesive ovoid or spindle fibroblasts and a fragment of collagenous stroma (MGG, HP).

Fig. 15.7 Desmoid fibromatosis

Muscle fragments and regenerating multinucleated muscle fibers are commonly seen (H&E, HP).

Fig. 15.8 Palmar fibromatosis (Dupuytren)

Relatively cellular smear of poorly cohesive spindle cells; bland spindle nuclei; no microarchitectural pattern; no obvious stromal material (MGG, HP).

Other benign fibroblastic/myofibroblastic and fibrohistiocytic tumors are solitary fibrous tumor (SFT) and hemangiopericytoma. According to recent investigations of these tumors, they are considered to be closely related, most probably representing two variants of the same entity.¹⁰ SFT most commonly occurrs in the pleura. SFT of the soft tissues is most often seen in adults as a deep-seated mass. In tissue sections the tumors are variably cellular and collagenous bands and hyalinisation are common findings. The fibroblastic spindle cells have no specific features and there is generally insignificant cellular atypia. A hemangiopericytoma-like vascular pattern is often present. The cytomorphology of SFT has been described in a few cases.^19,²⁰ Bland spindle cells arranged in tight cell clusters associated with ropy collagen, as well as dispersed cells often with stripped nuclei, seem to be the typical appearance in FNB smears (Fig. 15.9A). The tumor cells stain positively for CD34 (Fig. 15.9B) and occasionally for CD99 and bcl-2.

Fig. 15.9 Solitary fibrous tumour

(A) Bland, plump spindle fibroblastic cells arranged in tight cell clusters as well as dispersed cells associated with strands of collagenous stroma (H&E, IP); (B) Spindle cells staining positively for CD34 (cell block, immunoperoxidase, LP).

Hemangiopericytoma, earlier thought to be an example of a pericytic neoplasm, is in current classifications considered as a fibroblastic tumor. Hemangiopericytoma is most common in deep soft tissue. At present, it is considered as a diagnosis of exclusion since some other tumors, notably monophasic synovial sarcoma and mesenchymal chondrosarcoma, may exhibit the same vascular pattern of branching vessels (staghorn pattern) as hemangiopericytoma.¹⁰ The cytologic findings of hemangiopericytoma include branching vessel fragments; the tumor cells have bland ovoid or rounded nuclei and small cytoplasmic processes. Like SFT, hemangiopericytoma most often stains for CD34 and CD99. It is often difficult, sometimes impossible, to give a type-specific diagnosis of this tumor in FNB smears.

Elastofibroma is a slowly growing tumor, typically sited on the back near the scapula. The characteristic feature of elastofibroma is signs of faulty elastin fibrillogenesis. It is a hypocellular lesion composed of benign fibroblasts/myofibroblasts in a collagenous matrix with degenerated elastic fibers (Fig. 15.10). The typical cytologic findings in elastofibroma have been described in a small series of five cases.²¹

Fig. 15.10 Elastofibroma dorsi

Degenerated elastic fibers are presented in smears (H&E, HP).

Two fibroblastic/myofibroblastic tumors of infancy, infantile fibromatosis colli (torticollis) and fibrous hamartoma of infancy, are occasionally needled. The typical smear of infantile fibromatosis colli, clinically often mistaken for malignancy, shows a mixture of spindle fibroblasts and regenerating mono- or multinucleated muscle fibers These regenerating fibers may be pleomorphic with prominent nucleoli, not to be mistaken for malignant cells (Fig. 15.11).^22,²³

Fig. 15.11 Infantile fibromatosis colli

Single fibroblasts and a regenerating multinucleated muscle fiber (MGG, HP).

The rare fibrous hamartoma of infancy is a subcutaneous tumor-like mass in the upper arms, shoulder and axillary region. It is composed of a mixture of mature fat, strings of fibrous tissue and primitive mesenchymal cells. The cytologic findings in the few cases published ²⁴ are fragments of normal fat mixed with clusters or runs of bland spindle cells. We have had the opportunity to study the smears of two cases (Fig. 15.12).

Fig. 15.12 Fibrous hamartoma of infancy

Normal fat cells mixed with clusters or runs of bland spindle cells (MGG, HP).

Tenosynovial giant cell tumor is a relatively common benign fibrohistiocytic tumor related to the tendon sheaths of the fingers and hands. Although the clinical presentation is quite characteristic, confirmation by FNB is often requested. Smears are relatively cellular and consist of mainly dispersed plump fibrohistiocytic cells with oval pale nuclei and bland chromatin. There is a variable number of scattered osteoclast-like multinucleated giant cells, which may be numerous (Fig. 15.13).²⁵

Fig. 15.13 Tenosynovial giant cell tumor

A cellular smear of mainly single plump spindle cells with oval bland nuclei; Numerous scattered multinucleated giant cells (H&E, IP).

Malignant tumors

Malignant fibrous histiocytoma (MFH) was recognised in 1963 as a specific entity of probable fibrohistiocytic origin and was considered the commonest primary sarcoma of soft tissue. A number of subtypes were described, among which pleomorphic malignant fibrous histiocytoma was the most common. MFH is at present challenged as a specific histotype and the consensus is that MFH shows no evidence of histiocytic differentiation. On reexamination with extended immunohistochemistry and electron microscopy, many sarcomas initially diagnosed as pleomorphic MFH have been reclassified as dedifferentiated leiomyosarcoma, pleomorphic liposarcoma, pleomorphic rhabdomyosarcoma, soft tissue osteosarcoma or as high-grade malignant tumors of non-mesenchymal origin.^26,²⁷ Thus in current textbooks such as the WHO fascicle on soft tissue and bone tumors, the diagnosis pleomorphic MFH is reserved for a small group of pleomorphic sarcomas, which after thorough investigation show no specific histotype. The typical cytology of pleomorphic sarcoma of MFH type is described in two series.^28,²⁹ Most often, atypical spindle cells dominate the smears, but large atypical cells with more or less abundant vacuolated or foamy cytoplasm are usually also present and may be numerous, as well as multinucleated tumor giant cells (Fig. 15.14). Smears from both deep and subcutaneous tumors may contain small amounts of myxoid matrix. The diagnosis of giant cell malignant fibrous histiocytoma, formerly considered as a subtype of MFH, is currently reserved for undifferentiated pleomorphic sarcoma with numerous osteoclast-like multinucleated giant cells.¹⁰

Fig. 15.14 Pleomorphic sarcoma of the MFH type

Moderately cohesive pleomorphic cells; nuclei pleomorphic with malignant chromatin, mainly oval, a few spindled forms; abundant pale syncytial cytoplasm (MGG, HP).

Myxofibrosarcoma (MFS), formerly regarded as a myxoid subtype of malignant fibrous histiocytoma, is at present considered as a specific entity, a malignant fibroblastic tumor with a variable, most often abundant amount of myxoid stroma. MFS is a common subcutaneous sarcoma in elderly patients, although it is also found in skeletal muscle and in the retroperitoneum. MFS generally contains abundant myxoid ground substance in FNB smears (Fig. 15.15). The low-grade malignant tumors are dominated by spindle-shaped cells with slight to moderate nuclear atypia. They may resemble intramuscular cellular myxoma or paucicellular myxoid liposarcoma, but typical lipoblasts are not present and the typical characteristic vascular component of thin-walled branching vessels in the aspirated tumor fragments, as seen in myxoid liposarcoma, are not present. MFS typically displays fragments of coarse, often curved vessels in the myxoid back ground (Fig. 15.16), a feature not seen in FNB smears from intramuscular myxoma (see Fig. 15.54). In high-grade malignant MFS, the cellular pleomorphism and nuclear atypia is as marked as in pleomorphic sarcoma of MFH type. The cellular features of myxofibrosarcoma have been amply described.³⁰^–³² Low grade fibromyxoid sarcoma (LGFMS) is a variant of fibrosarcoma, first described in 1987. Two series have been published 1993 and 1995.^33,³⁴ This type of sarcoma displays alternating areas of myxoid and collagenous stroma. The tumor cells are spindle-shaped with bland nuclei. As stromal vessels of the same type as in myxofibrosarcoma may be present in the myxoid areas, these low-grade malignant sarcomas may be difficult to distinguish from each other. LGFMS and cellular intramuscular myxoma may be deceptively like. The cytology of LGFMS has been thoroughly described in a report of eight cases (Figs 15.17 and 15.18).³⁵