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malignant fibrous histiocytoma, is a term which has been used loosely to encom- pass a wide variety of neoplasms which have in common the presence of a thin- walled branching vascular pattern (described below) {294, 676, 1535}. As such, HPC is difficult to define at this time as a discrete entity, although lesions showing pericytic differentiation un- doubtedly exist and were included in Stout’s original descriptions {2036, 2040}.

The prototypical pericytic neoplasm is myopericytoma {825} (see page 138) and sinonasal HPC (see Tumours of the Upper Respiratory Tract) also appears to be pericytic in nature.

Epidemiology

The discrete subset of lesions remaining as HPC is rare. In light of the hetero- geneity of lesions classified as HPC, there are no meaningful estimates of inci- dence. Myopericytoma appears sub- stantially more common than the other discrete subset of lesions known as HPC which cannot currently be otherwise classified.

The discrete subset of soft tissue lesions known as HPC which currently justify retention of this nomenclature occur most often in middle-aged adults with an apparent female predominance. Lesions formerly known as infantile HPC fall within the spectrum of infantile myofi- bromatosis {1412} (see respective sec- tion on page 59).

Haemangiopericytoma

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Fig. 2.62 Malignant extrapleural solitary fibrous tumours. A Hypercellularity and marked cytological atypia. Note the atypical mitosis. B Moderately cellular area with brisk mitotic activity.C Hypercellularity, marked cytological atypia, and areas of tumour necrosis (left).

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Fig. 2.63 Haemangiopericytoma. A This 12 cm pelvic mass was associated with hypoglycemia. Note the uniform, spongy cut surface. B,C,D Note the evenly distributed cellularity (in contrast to usual SFT) and the prominent branching vascular pattern. E Even in the more solid areas, tumour cells are arranged around numerous thin-walled vessels. Tumour cells are small with monomorphic nuclei and eosinophilic cytoplasm. F Diffuse positivity for CD34 is shared by the spectrum of lesions known as hae- mangiopericytoma and solitary fibrous tumour.

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Sites of involvement

The subset of soft tissue lesions which, for the time being, are still named HPC, arise most often in deep soft tissue, par- ticularly pelvic retroperitoneum. A small- er proportion of cases arise in the proxi- mal limbs or limb girdles. Histologically comparable lesions also occur in the meninges (see WHO Blue Book on Tumours of the Nervous System).

Clinical features

Most tumours present as a slowly grow ing mass which, in the abdomen, may cause intestinal or urinary symptoms. Occasional cases, similar to SFT, are associated with hypoglycemia due to secretion of insulin-like growth factor {1671}.

Macroscopy

Convincing examples of so-called HPC in soft tissue tend to be well-circum- scribed masses with a yellowish or tan cut surface and a fleshy or spongy con- sistency. Large vessels may be evident on the cut surface. Haemorrhage is com- mon but necrosis is infrequent. Tumour size is variable but most cases are 5-15 cm in maximum diameter.

Histopathology

The discrete residual subset of so-called HPC closely resembles the cellular areas of SFT, albeit with the consistent pres- ence of numerous, variably ectatic or compressed, thin-walled branching ves- sels often having a staghorn configura- tion. Tumour cells are usually closely packed, spindle-shaped to round, of uni- form size, with small amounts of pale or eosinophilic cytoplasm with indistinct margins and small, bland often vesicular nuclei. Cytological pleomorphism is gen- erally not a feature. In contrast to SFT, stromal hyalinization and varying cellu- larity are not usual features. The mitotic rate is highly variable. Some cases con- tain a prominent adipocytic component (such cases are known as lipomatous HPC – see below). These lesions also often show varying cellularity and are increasingly regarded as a variant of SFT. Tumours which very often were classified as HPC in the past include (among oth- ers) solitary fibrous tumour (p. 86), monophasic synovial sarcoma (p. 200), infantile myofibromatosis (p. 59), myopericytoma (p. 138), infantile fibrosarcoma (p. 98), deep fibrous histio-

cytoma (p. 114) and mesenchymal chon- drosarcoma (p. 255).

Immunohistochemistry

The discrete subset of so-called HPC, comparable to SFT, shows fairly consis- tent positivity for CD34 and CD99, both of which are widely expressed in fibrob- lastic tumours. Endothelial markers are negative, as also (in most cases) are actin and desmin.

Ultrastructure

Most of the lesions reported as HPC have shown only undifferentiated spindle cell

or fibroblastic features. Convincing evi- dence of true pericytic differentiation is not seen.

Genetics

Few cytogenetically investigated HPCs, located in the lung, tongue, brain, cere- bellum, soft tissues, and intrabdominally, have been reported {1477}. The vast majority of cases have had near- or pseu- dodiploid karyotypes with the number of aberrations ranging from one to more than 20. The chromosome aberrations are quite disparate, but breakpoints in 12q13-15 and 19q13 have been identi-

Fig. 2.64 Lipomatous haemangiopericytoma. A Many such lesions show features of solitary fibrous tumour in addition to containing numerous mature adipocytes. Note haemangiopericytoma-like branching vessels B An intimate admixture of bland spindle cells and mature adipocytes.

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fied in almost half of the cases and one- fourth of the cases, respectively. In two cases, there was a balanced t(12;19)(q13;q13), in one case as the sole anomaly. Among the genomic imbal- ances, losses are predominating. Recurrent imbalances include loss of segments in 3p, 12q, 13q, 17p, 17q, 19q, and the entire chromosome 10, and gain of 5q sequences.

Prognostic factors

At least 70% (probably more) of HPCs pursue a benign clinical course, while the remainder are malignant. Histological criteria for malignancy are imprecise and prognostication in HPC has long been regarded as difficult. There have been no recent prognostic studies confined to the

discrete subset of lesions which might nowadays be termed HPC. However, older data from major centres {598} sug- gest parameters similar to those used currently for SFT – specifically, 4 or more mitotic figures per 10 high power fields is the single feature most worrisome for malignancy. The presence of necrosis or nuclear pleomorphism, particularly in the context of a tumour >5 cm in diameter may also portend malignant behaviour.