Pediatric Pathology

PEComa

Rita Alaggio


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Clinical History A 2 year-old female was referred to the local hospital for failure to thrive. Family and personal health history were negative. Ultrasound performed for suspected urinary infection showed a voluminous abdominal mass. CT scan confirmed a solid lesion with necrotic areas in the right abdomen, extending to the left and right iliac fossa, adherent to the liver, right kidney, vena cava and abdominal wall, measuring 12x60 cm. An open biopsy was done and intraoperative examination confirmed a malignant neoplasm. Histology was reviewed at the University of Padova, and the child received 2 courses of chemotherapy (IVA2). Due to poor response to chemotherapy, the child underwent surgery and the mass was resected with part of the omentum. Nodules on the anterior peritoneum, right iliac fossa, pelvis and sigmoid colon were removed and gastric, and periduodenal and mesenteric lymph-nodes were biopsied. After surgery a new chemotherapy regimen (Ifosfamide/Adriamicine) was given. Six months later, the tumor recurred but a debulking operation was not considered possible. The child, who did not receive any other treatment, is alive with progressive disease and multiple masses in the abdomen after 2 years. The seminar slide originates from the primary resection.


Slide 1
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Figure 1
The lesion is well circumscribed and focally surrounded by a thin fibrous pseudocapsule.
Figure 2
This highly cellular neoplasm is composed of spindle cells with a vaguely fascicular pattern. There is a complex vascular network, formed by small capillaries, mostly arranged longitudinally.
Figure 3
Foci of necrosis are present.

Figure 4
The cells are elongated with oval nuclei and clear or eosinophilic cytoplasm.
Figure 5
Mitoses are present.

Figure 6
Areas with epithelioid cells, arranged in a nested pattern with a typical chicken-wire vascular pattern, are also seen.
Figure 7
At higher power the epithelioid cells are characterized by clear cytoplasm, and nuclei show mild variation in shape and size with frequent nucleoli.

Macroscopy
The initial incisional biopsy was a tiny specimen of about 1.5 cm. The mass that was further resected measured 13 cm in maximum diameter: it was multinodular, covered by peritoneum, with a glistening surface and a prominent vascular network. The tumor had a fleshy appearance, with solid areas exhibiting a whitish-pink color mixed with hemorrhagic and necrotic foci. The majority of the secondary nodules were necrotic.

Microscopy
On microscopic examination, both the initial biopsy and the resected tumor consisted of spindle cells with clear, elongated cytoplasm and distinct cell borders arranged in fascicles accompanied by a complex vascular network. In some areas the cells exhibited a vaguely epithelioid morphology, had clear to eosinophilic cytoplasm and were aggregated in nests, surrounded by thin blood vessels. Nuclei varied in size and shape from oval to round with finely-dispersed chromatin, sometimes clumped, and inconspicuous nucleoli, that occasionally became prominent. Neither adipocytes nor thick-walled blood vessels were observed. Foci of necrosis and hemorrhagic areas were frequent, with focal hemosiderin pigment. Mitoses were present, occasionally atypical. Mitotic rate was 15/50HPF. There were no features of angiolymphatic invasion. At the periphery, the neoplasia appeared well circumscribed and surrounded by a thin fibrous pseudo-capsule. Secondary nodules were mostly characterized by collections of macrophages surrounding necrotic foci. One nodule showed a neoplasm with the same features as the main mass.

Immunohistochemically the tumor cells were strongly positive for HMB45, SMA and H-caldesmon. S100, cytokeratins, EMA, and p53 were negative. Cyclin D1 was positive in more than 70% of nuclei.

Diagnosis
Malignant PEComa, with features of m alignant clear cell myomelanocytic tumor of the falciform ligament/ligamentum teres

Discussion
Clear cell myomelanocytic tumor of the falciform ligament/ligamentum teres (CCMT) was described by Folpe et al in 2000 as a clinicopathologic entity, typically arising in the abdomen of young females, with predilection for the ligamentum teres and falciform ligament of the liver [1]. It shows a distinctive morphology, characterized by spindle cells with clear or eosinophilic cytoplasm arranged in fascicles or nests, and is part of the large family of PEComa [1, 2, 3, 4]. The term "PEComa" was first coined by Zamboni et al in 1996 to identify a group of mesenchymal neoplasms originating from the perivascular epithelioid cell (PEC) [5]. The PEC is a newly identified cell, with a not yet recognized normal counterpart, characterized by morphologic variability, an intimate relationship with blood vessels and a typical co-expression of muscular and melanocytic markers [6, 7]. The PEComa family includes angiomyolipomas, lymphangio-leiomyomatosis, clear cell "sugar" tumor of the lung and a group of entities variously reported in the literature as "clear cell myomelanocytic tumor (CCMT) of the falciform ligament/ligamentum teres" and "abdominopelvic sarcoma of perivascular-epithelioid cells" [3, 4, 8].

The current case represents an example of malignant PEComa, with typical features of CCMT [1, 9]. The tumor showed multiple peritoneal nodules. The main mass was poorly responsive to chemotherapy and exhibited a rather monotonous morphology, characterized by elongated cells with clear cytoplasm and only focal epithelioid features. Most of the secondary nodules had almost completely responded to therapy and showed a collection of foamy macrophages.

PEComa family tumors typically occur in young woman and adults. To our knowledge, less than 20 cases have been reported so far in childhood, mostly as single cases, and only one large series by Folpe included 6 pediatric CCMT [1, 9, 10, 11, 12, 13, 14, 15, 16].

Pediatric PEComa occur more frequently in females. Only one case has been reported in a male, as a second neoplasm following neuroblastoma [13]. The most frequent site is the abdomen (falciform ligament, omentum, bowel wall}, followed by the pelvis and kidney [1, 9, 10, 11, 12, 13, 14, 15, 16]. The prognosis in PEComa is not easily predictable and there are not strict histologic criteria for malignancy. Malignant PEComa, even with typical features of CCMT, are being increasingly reported [17, 18, 19, 20, 21]. According to the WHO classification, tumors showing infiltrative growth, hypercellularity, nuclear enlargement and hyperchromasia, high mitotic rate, atypical mitoses and coagulative necrosis are prone to malignant behavior [8]. Folpe et al have proposed a stratification of PEComas in three groups: malignant, for tumors showing at least two unfavorable morphologic markers (size >5cm, infiltrative growth pattern, high nuclear grade and cellularity, mitotic rate more than 1/50HPF, necrosis, vascular invasion); benign, when tumors are devoid of any unfavorable marker; and uncertain malignant potential, when only one unfavorable marker is found [9]. According to these criteria, malignant PEComas represent about 20% of the cases in adult patients [9]. Only few clinically malignant PEComas have been reported in childhood, mostly characterized by epithelioid morphology, all in the abdomen, and with evidence of unfavorable prognostic features [1, 9, 10, 11, 12, 13, 14, 15, 16]. The CCMT had been originally described as a lesion with a favorable clinical course, but the low number of cases and short follow-up were inconclusive [1]. Among the 6 cases with CCMT morphology in the recent study by Folpe, only 1 had aggressive behavior [1]. The case reported here displayed more than two morphologically unfavorable markers and shared many features with a CCMT of the broad ligament, with malignant histology, described by Kim et al in a 12 year old girl [10]. Both cases had multiple peritoneal nodules at initial diagnosis. The term "PEComatosis" was used by Fadare et al [22] for a case of PEComa of the uterine cervix associated with multicentric peritoneal nodules, in a female patient who had tuberous sclerosis (TBS). PEComatosis has been described in few patients with TBS, but it has never been reported in sporadic PEComas [23]. A "field effect", probably related to the tuberous sclerosis, has been suggested as a possible pathogenetic mechanism responsible of the propensity to develop multiple nodules in the abdomen [22]. Our patient had no underlying conditions and the nodules were regarded as metastatic spread from the main mass.

The CCMT and PEComas in general may be confused with a variety of both benign and malignant tumors. In a child with an abdominal mass adherent to the kidney, the differential diagnosis includes clear cell sarcoma of the kidney [11, 24] or a renal carcinoma associated with Xp11.2 translocation/TFE3 gene fusion [24]. CCSK is positive for vimentin and negative for melanocytic markers and SMA [24]. Differential diagnosis from TFE3 renal cell carcinoma may be more difficult, especially in PEComas that exhibit an epithelioid morphology. Some Xp11.2 renal carcinomas, such as those associated with t(6;11)(p21.1;q12) or t(X;17)(p11.2;q23), may show a clear cell morphology with a nested architecture and an immunohistochemical profile identical to PEComa, with scant or negative expression of epithelial markers, and positive melanocytic markers such as melan-A and HMB-45 [25, 26]. Moreover, some PEComas may show TFE3 immunostaining [9]. In both tumors it has been hypothesized that TFE3 is involved in the inappropriate activation of some targets of MiTF, resulting in the expression of melanocytic markers [9, 25]. In fact PEComas and renal carcinomas expressing TFE3 are generally negative for MiTF, suggesting that the TFE3 induces the melanocytic phenotype in those tumors that are MiTF negative. The differential diagnosis between these two lesions is made possible by the different intensity of HMB45, only focal in carcinoma, and negative SMA [25, 26]. Moreover, at light microscopy, some carcinomas show pink hyaline globules corresponding to aggregates of basement membrane material [25].

The positive staining for melanocytic markers (HMB45, MiTF) may suggest the diagnosis of clear cell sarcoma of soft parts (CCSSP) (the so-called melanoma of soft parts). Histological features may be deceptive because CCSSP and PEComa share the same epithelioid or spindle cell morphology with clear cytoplasm and nuclei with prominent nucleoli. Melanin and multinucleated cells may be present in both. The evidence of positive SMA immunostaining, associated with negative S100, is an important diagnostic feature, even if occasional PEComas may be SMA negative. Moreover CCSSP shows a typical translocation t(12;22)(q13,13)(EWS-ATF1) [27].

Although in adult patients other soft tissue neoplasms have to be considered in the differential diagnostic spectrum, such as gastrointestinal stromal tumors and leiomyosarcomas, these are very rare in children and can be excluded by an adequate immunohistochemical panel, since GIST and leiomyosarcomas are negative for melanocytic markers. Moreover GISTs are positive for CD117. Nevertheless, occasional CD117 reactivity in a PEComa may be misleading [14].

The nested architecture of PEComa may also simulate the typical zellballen pattern of paraganglioma, a tumor which may also be pigmented. In paraganglioma, S-100 decorates sustentacular cells, and tumor cell are positive for chromogranin, Synaptophysin, and neuron-specific enolase. The distinction between alveolar soft part sarcoma and PEComa is aided by detection of PAS-positive diastase-resistant crystalline structures in the cytoplasm. Also, the same t(X;17) translocation as in alveolar soft part sarcoma, but unbalanced, has been found in a subgroup of TFE3 renal carcinomas [28].

The cytogenetic features of PEComa have not been extensively investigated. Loss of heterozygosity for the TSC2 gene, on 16p13, which is the alteration most frequently found in both sporadic and tuberous sclerosis-associated angiomyolipomas (AML), has been detected in PEComas, suggesting a relationship to classic AML [29, 30, 31, 32]. It is possible that the histogenesis of PEComas might be related to the role of TSC in the negative regulation of the Wnt-beta catenin pathway that controls cell proliferation and differentiation, but further investigations are required [33]. A chromosomal loss of 17p, the region corresponding to the TP53 oncosuppressor gene, has also been identified by CGH [32]. Nevertheless, PCR studies have failed to show mutations or allelic losses in TP53 gene, confirmed also by negative immunostains for p53. Other genetic alterations reported include deletion of 1p, deletions on chromosome 19, and chromosomal gain on 12q, 2q, 3q, and 5 [32]. The possible role of cyclin D1 in the pathogenesis of PEComas has also been investigated [21]. Cyclin D1 is only transiently expressed in normal cells, but may be constitutively expressed in some tumors such as melanomas. Soucek et al [34] have demonstrated an association of cyclin D1 abnormalities with loss of the TSC gene, and positive immunostaining has been reported in some PEComas. Interestingly, Weinreb et al demonstrated negative staining for cyclin D1 in an aggregate of cells distant from the main mass and with the same histological features, while the main mass was strongly positive [21]. They have suggested that these small nodules, called "PECosis", may represent the precursor lesion of PEComa. The present case was immunoreactive for cyclin D1, but FISH analysis failed to show abnormalities in number of copies or translocations of the gene.

In conclusion, PEComas are very rare tumors that occasionally occur in children. As in adults, some cases may have malignant behavior. The recent prognostic classification based on morphologic criteria proposed by Folpe et al. [9] helps to predict the clinical course. The challenge of the future is the development of new and effective treatments. At this time, the treatment is based on complete surgical excision, because radiotherapy and chemotherapy are not effective. Rapamicin is a promising drug. It is an inhibitor of m-TOR, which is activated in sporadic AML and extrarenal PEComas. M-TOR activates the protein kinase p70S6K and its isoform p85S6K, both detected by immunohistochemistry with cytoplasmic and nuclear staining respectively. Perhaps in the future this will provide a basis for a targeted therapy in aggressive unresectable tumors [4, 35].

Table: Differential Diagnosis of PEComa

PEComa Melanoma CCSSP CCSK ASPS TFE3-RC Paraganglioma
Histology
Spindle/Epithelioid cells +/+ +/+ +/- +/- +/- +/- +/-
Clear cytoplasms + + + + + + +
Prominent nucleoli + + + + + + +
Multinucleated cells + + + + + + +
Nested pattern + + + -/+ + + +
Cytoplasmic inclusions - - - - + - +
Melanin + + + - - - +/-
Immunophenotype
Vim + + + + + + +
CK - - - - - 50%+ -
EMA - - - - - 50%+ -
S-100 -/rare+ + + - - - + *
HMB45 + + + - - + -
MiTF 50% + + - - - -
SMA + - - - - - -
Cromogranin, Syn - - - - - - +
TFE3 30% - - - - + -
Molecular Characterization - - t(12;22) - t(X;17) t(X;17)

Legend:

CCSSP= Clear cell sarcoma of soft parts; CCSK= Clear cell sarcoma of the kidney; ASPS= Alveolar soft part sarcoma; TFE3-RC= Renal carcinoma associated with Xp11.2 translocation/TFE3 gene fusion. Vim= vimentin; CK= cytokeratins; MiTF= microphthalmia transcription factor; SMA= smooth muscle actin

*in sustentacular cells

Take Home Messages
  • Beware of tiny biopsies: lesions with unusual morphological features may be misleading

  • The prognostic categorization of some mesenchymal lesions remains controversial and is an area for future investigation

  • When in doubt about the interpretation of lesions unexpected for the patient's age, remember: tumors do not read medicine books.

References
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