Riboflavin and mouse hepatic cell structure and function. II. Division of mitochondria during recovery from simple deficiency.

Mice which had been on a riboflavin-free diet for 6-8 wk were given daily intraperitoneal injections of riboflavin. The hepatic mitochondria, which in the deficient animals were greatly enlarged, were restored to normal dimensions within 3 days. Normalization of the mitochondrial population was brought about by division of the giant organelles. Dividing mitochondria were characterized by a membranous partition separating the inner compartment into two distinct chambers. Such organelles showed varying degrees of pinching at the level of the partition. The most common site of partition formation was at the base of a small mitochondrial bud. During the 1st day of recovery, dividing mitochondria were so common that they could be easily found in mitochondrial pellets. Injection of riboflavin into normally nourished mice also produced an apparent increase in the frequency of dividing mitochondria in the liver cells.

Infiltrating myoepithelioma of the breast.

This report describes the histologic and ultrastructural features of a spindle cell myoepithelioma arising in the breast of a 60-year-old woman. By light microscopy, the tumor consisted of interlacing bundles of elongated cells sometimes arranged in a storiform pattern. No intraductal or invasive carcinoma was identified. Ultrastructurally, the neoplasm was composed of activated, differentiated myoepithelial cells showing evidence of squamous metaplasia, i.e., spindle-shaped cells joined by mature desmosomes and encompassed by remnants of basal lamina containing a well-developed rough endoplasmic reticulum, pinocytotic vesicles, prekeratin filaments, and longitudinally oriented bundles of microfilaments with fusiform densities. Numerous cisternae of rough endoplasmic reticulum suggested heightened metabolic activity, whereas the presence of cytoplasmic prekeratin filaments indicated squamous metaplasia. The cells comprising most previously reported "myoepitheliomas" examined by electron microscopy lacked many of the distinctive fine structural features of the normal myoepithelial cell. At present, the diagnosis of myoepithelioma should be based on strict ultrastructural criteria. The clinical behavior of this particular lesion in the breast is unknown. If the tumor follows the course of comparable salivary gland neoplasms, the likelihood of systemic spread is low. Primary therapy for a tumor proven to be an infiltrating myoepithelioma of the breast should be directed at complete excision of the lesion.

Follicular dendritic cell tumor: report of 13 additional cases of a distinctive entity.

Follicular dendritic cell (FDC) tumor is an extremely rare malignant neoplasm with approximately 17 well-documented cases in the literature. We report 13 additional cases of this distinctive neoplasm. There were seven men and six women, with a mean age of 46.5 years (range, 27-62 years). There was involvement of cervical lymph nodes (six cases), mediastinum (three cases), axilla, tonsil, spleen, and peripancreatic soft tissues (one case each). The neoplasms were grey to tan, ranging in size from 1 to 13 cm. They were formed by oval to spindle cells with eosinophilic cytoplasm growing in sheets and fascicles, with a focal storiform pattern and whorls reminiscent of those seen in meningioma. The nuclei were oval or elongated with thin nuclear membranes, inconspicuous or small eosinophilic nucleoli, and clear or dispersed chromatin. Typically, the tumor cells were intimately admixed with small lymphocytes, with a prominent perivascular cuffing. Multinucleated tumor cells were present in seven cases. Necrosis, marked cellular atypia, high mitotic rate, and/or abnormal mitoses were present in seven cases. The tumor cells were positive for CD21 (10 of 11), CD35 (10 of 11), Ki-M4p (seven of eight) Ki-FDRC1p (six of seven), vimentin (five of nine), and S100 protein (five of nine). One case stained with actin. In situ hybridization, done in six cases, did not show Epstein-Barr virus RNA sequences. Ultrastructural examination of eight cases showed long, complex, occasionally interdigitating cytoplasmic processes joined by desmosomes. The behavior of these tumors is more akin to that of a low-grade soft tissue sarcoma than a malignant lymphoma and is characterized by local recurrences and occasional metastases. Two patients died of tumor, two were alive with recurrent or metastatic disease, eight were alive with no disease, and one was lost to follow-up.

Neuroendocrine differentiation is a common feature of thymic carcinoma.

Immunohistochemical evidence of neuroendocrine differentiation in the form of reactivity for synaptophysin, neuron-specific enolase, and/or chromogranin was found in 11 of 19 (58%) thymic carcinomas having the typical morphologic features of that tumor type. Four of these 19 cases were studied ultrastructurally, and neuroendocrine-type cytoplasmic dense-core granules were found in two. In contrast, 84 thymomas were negative for these markers, except for a focal immunoreactivity for neuron-specific enolase in areas of medullary differentiation in half of the lymphocyte-rich tumors. The results of this study show that in the thymus, similar to most other organs, neuroendocrine differentiation is not limited to tumors with an identifiable neuroendocrine appearance in hematoxylin-eosin-stained slides, such as carcinoid tumor and small cell carcinoma, but rather that it represents a common event shared by the major types of malignant epithelial tumors of that organ.

The lipid-rich epithelioid glioblastoma.

The authors add to the literature an account of four aggressive glial neoplasms characterized by diffuse cytoplasmic lipidization and a cohesive architectural disposition in epithelioid nests and sheets. These neoplasms arose in the cerebral hemispheres of adults and tended to a circumscribed neuroradiologic presentation that in two instances prompted an unrewarding preoperative search for an extracranial primary. One represented recurrent disease in a patient being followed for a biopsy-proven low-grade astrocytoma. Three cases were collected by way of consultation from pathologists uncertain as to their primary versus metastatic derivation. The apparent expression of cytokeratins and epithelial membrane antigen further conspired to obscure the glial lineage of these peculiar neoplasms, which are best regarded as tumors of the astrocytic series.

Divergent myosarcomatous differentiation in retroperitoneal liposarcoma.

We describe four patients with retroperitoneal liposarcomas undergoing myosarcomatous differentiation. The patients (two men and two women) were 47, 48, 68, and 72 years of age when first seen. The primary tumors were large retroperitoneal, well-differentiated liposarcomas, one featuring areas of dedifferentiation (without muscle elements). A myosarcomatous component became evident at the first recurrence in three cases and at the second recurrence in one. This component was always within dedifferentiated areas and in three of the cases coincided with the emergence of the latter. The muscle component had exclusively leiomyosarcomatous phenotype (alpha-smooth-muscle actin reactivity) in one case, exclusively rhabdomyosarcomatous phenotype (myoglobin reactivity) in two cases, and combined leiomyosarcomatous and rhabdomyosarcomatous phenotype (alpha-smooth-muscle actin and myoglobin) in one case. Ultrastructural studies of one of the tumors with a rhabdomyosarcomatous component revealed the presence of sarcomeres. Two patients died of extensive retroperitoneal disease, one patient died following the attempted removal of a recurrence, and one patient is alive and free of disease. These cases demonstrate that the dedifferentiated component of liposarcoma may exhibit a myosarcomatous component, a feature analogous to that previously described in dedifferentiated chondrosarcoma.

Diffuse idiopathic pulmonary neuroendocrine cell proliferation presenting as interstitial lung disease.

Pulmonary neuroendocrine cell (PNEC) hyperplasia typically occurs as an adaptive response in persons living at high altitudes and as a reactive response in the setting of lung injury. However, previous studies suggest that PNEC hyperplasia can occur in the absence of preexisting lung disease and may even give rise to airway disease through the development of pulmonary tumorlets and airway fibrosis and perhaps the release of paracrine secretions. We describe a patient with diffuse PNEC proliferation of a probable hyperplastic nature developing in the absence of a chronic pulmonary disorder who presented clinically with an interstitial lung process. Open lung biopsy displayed a florid intraepithelial population of PNEC diffusely involving the distal airways and alveoli with desquamation and filling of alveolar spaces by nests of PNEC. The presence of alveolar thickening was attributable to the intraepithelial proliferation of PNEC associated with interstitial fibrosis and accounted for mild reductions in the pulmonary diffusing capacity. The neuroendocrine differentiation of this proliferation was evident by light microscopic and ultrastructural examination. The absence of airway fibrosis and pulmonary tumorlets was in agreement with the lack of clinical airway disease in this case. The intraepithelial growth and absence of parenchymal invasion in this lesion favor a diffuse, florid PNEC hyperplasia with mild dysplastic features over a pulmonary neuroendocrine neoplasm.

Primary leiomyosarcoma of bone: a clinicopathologic, immunohistochemical, and ultrastructural study of 33 patients and a literature review.

Leiomyosarcoma of bone is a rare tumor in an unusual location. Previous analysis of this entity mostly involved small numbers of cases with limited follow-up. Thirty-three patients with leiomyosarcoma of bone between 1977 and 1996 were studied, and the histologic appearance and grade were correlated with subsequent treatment and clinical behavior. To be included in this study the tumor had to be intraosseous, with other primary sites of origin clinically excluded. Also, most of the sarcomatous tissue (> or =70%) had to be of intramedullary location with only limited extraosseous extension. The patient's age at diagnosis ranged from 13 to 77 years (average 44.4). The gender distribution was equal. The long bones were preferentially affected (64%), with the lower extremity, around the knee joint, predominantly involved. Five patients (15%) developed postradiation leiomyosarcomas. The histologic analysis showed that the osseous leiomyosarcomas are most commonly of the classic type, followed by the epithelioid, myxoid, and pleomorphic variants. Immunoreactivity for smooth muscle markers (smooth muscle actin, common muscle actin, desmin) was positive in all tumors, and ultrastructural confirmation was obtained in 21% of cases. All sarcomas were histologically graded, which accurately reflected the subsequent prognosis. Seventy-five percent of the lesions were high-grade and the rest low-grade. The histologic grade of the tumors correlated with both the recurrence as well as the metastatic rates and together with the clinicopathologic stage of disease represented the cornerstone on which prudent therapy should be based.

Nodal blue nevi. A study of three cases.

Three cases of blue nevi associated within lymph nodes were identified. These nodal blue nevi were all incidentally discovered in relation to axillary lymph nodes removed as part of modified radical mastectomy procedures for carcinoma of the breast. Only one lymph node from each axillary dissection contained the lesion. In one case, gross examination revealed black streaks in the capsule of the lymph node, providing the first reported macroscopic illustration of the blue nevus within the capsule of a lymph node. The lesions each occupied small portions of the lymph node capsules, with penetration into perinodal fibroadipose tissue in two cases and extension along intranodal fibrous trabeculae in the third case. Histologically, heavily pigmented dendritic and bipolar nevus cells were admixed with melanophages. Ultrastructural examination of one case documented the presence of mature melanosomes and possible basement membrane material. None of the patients had notable skin lesions and all were free of disease at last follow-up. Nodal blue nevi are probably an unusual variant of the nevus aggregate associated with lymph nodes. These lesions limited to the nodal capsule and supporting stroma should not be mistaken for malignant melanoma.

Cytokeratin-positive malignant tumors with reticulum cell morphology: a subtype of fibroblastic reticulum cell neoplasm?

Cytokeratin-positive interstitial reticulum cells (CIRCs) have been described as a subset of fibroblastic reticulum cells (FBRCs) normally found in lymph nodes, the spleen, and tonsils. Although tumors derived form other reticulum (dendritic) cells, specifically follicular dendritic cells, interdigitating dendritic cells, and cytokeratin-negative FBRCs, have been well documented and are now accepted, this is not the case for tumors of CIRCs. A possible reason for this failure is the difficulty in distinguishing them from other tumors, particularly carcinoma. We report three cases of cytokeratin-positive malignant tumors with a reticulum cell morphology: two located in the mediastinum and one in the soft tissue in the proximal forearm. All cases coexpressed vimentin, and one case coexpressed smooth muscle actin and desmin, resulting in a phenotype similar to that of some normal CIRCs. Although metastatic carcinoma from an occult or regressed primary tumor cannot be excluded completely, we raise the possibility of a CIRC origin for these cases.

Histogenesis of benign pleomorphic adenoma (mixed tumor) of the major salivary glands. An ultrastructural and immunohistochemical study.

Twenty-two benign pleomorphic adenomas of the major salivary glands were studied by transmission electron microscopy and immunohistochemical techniques (three cases) in order to characterize the cell types comprising the epithelial and so-called mesenchymal regions of the tumors. Light- and electron-microscopic studies showed the tumors to consist of variable mixtures of neoplastic ductular epithelial cells, rare acinar cells, and metaplastic myoepithelial cells. Many of the loosely organized "stromal cells" contained structures indicative of their myoepithelial origin, e.g., perinuclear tonofilaments, ectoplasmic actin microfilaments, and remnants of basement membrane. Polyclonal antikeratin antisera strongly stained ductular epithelial and myoepithelial cells, squamoid cell nests, and periductular myoepithelial cells, whereas myxoid and chondroid cells were less intensely stained. Monoclonal cytokeratin antibody AE1 stained only the ductular epithelial cells in both the normal glands and tumors. In contrast, S-100 protein, which is present only in scattered acinar cells and myoepithelial cells in the normal parotid gland, was found in the ductular and periductular myoepithelial cells, isolated myxoid cells, and chondroid and cartilagenous cells in the tumors. Actin was found in all the cell types of the tumor but staining was strongest in the ducts. Double immunofluorescence staining for cytokeratin and vimentin revealed coexpression of both types of intermediate filaments in occasional normal acinar and intercalated duct myoepithelial cells, and in some cells in the myxoid and chondroid regions of the tumors. In the tumors, vimentin was present in occasional periductular myoepithelial cells, stellate myxoid cells, and especially in chondroid cells and chondrocytes. Our findings indicate that benign pleomorphic adenomas of the major salivary glands are pure epithelial cell tumors. The histologic complexity of these neoplasms is due to the ability of the neoplastic ductular myoepithelial cell to modulate its morphologic appearance and intermediate filament composition, and to produce large amounts of matrix substances. We further postulate that these tumors arise from neoplastically transformed intercalated ducts.

Renal myxoma. A report of two cases and review of the literature.

Renal myxomas are rare neoplasms. Seven cases have been reported, of which only two are convincingly diagnosed as myxoma; the remaining cases exhibit features of sarcoma, fibroepithelial polyp, or myxolipoma. We report two additional cases; one in a 52-year-old man and another in a 68-year-old woman. They were discovered incidentally by radiological examination. The resected kidney in both patients contained a well-demarcated gelatinous intraparenchymal tumor, which consisted of occasional slender spindle cells scattered in an abundant myxoid stroma, closely resembling myxomas of other sites. The tumor cells showed immunoreactivity for vimentin but not for S-100 protein, epithelial membrane antigen (EMA), CAM 5.2, HHF-35, or smooth muscle actin. Ultrastructural features were of fibroblast-like cells with an elaborate network of cytoplasmic processes. The differential diagnosis of myxoid tumors of the kidney includes myxoid variants of renal sarcomas and carcinomas, renomedullary interstitial cell tumors, and fibroepithelial polyps. It is important to recognize the existence of a renal myxoma, to avoid confusing this benign tumor with the malignant neoplasms with secondary myxoid features that may involve the kidney.

The purely epithelioid malignant peripheral nerve sheath tumor.

The purely epithelioid malignant peripheral nerve sheath tumor (PNST) is a rare form of PNT possibly first described by McCormack et al. in 1954. This tumor type is distinguishable from the glandular PNT and PNT with differentiated neuroepithelium (medulloepithelioma and neuroepithelioma) and differs from the ordinary epithelioid PNT because of the absence of a spindle cell component typical of malignant PNSTs. The two examples of purely epithelioid malignant PNT we describe arose in the popliteal fossa from the sciatic and tibial nerves of men with no definite evidence of von Recklinghausen's neurofibromatosis. Both tumors were partly mucinous, and so closely mimicked carcinoma and a few non-neurogenic myxoid sarcomas histologically that their final classification depended upon proof of a neural origin.

Histology, immunohistochemistry, and ultrastructure of the verruga in Carrion's disease.

Twenty-six verruga peruana nodules were studied. The presence of Factor VIII-related antigen and Ulex europaeus lectin binding, and the ultrastructural finding of rudimentary cell junctions and pinocytotic vesicles establish the endothelial character of the proliferating cells in the verruga nodules. Whereas superficial lesions could show an angiomatoid pattern, deep-situated nodules tended to present a compact type of growth. Electron-microscopic studies have shown that Bartonella bacilliformis was found abundantly in the extracellular spaces in the florid lesions and that no organisms were present in the late, resolving subcutaneous nodules. Although no true intracellular "viable" microorganisms were noted, pseudopods of cytoplasm entrapping one or two bacteria and surrounding matrix substance were seen often. The characteristics of cytoplasmic inclusions previously described in verruga cells as "chlamydozoa" were detailed. The ultrastructure of the inclusions corresponded to endothelial phagocytic cells in which complex invaginations of the cell surface had produced a labyrinth of interconnected channels and vacuoles containing degraded bacteria, extracellular matrix components, or both. We conclude that in light microscopy the finding of Rocha-Lima's inclusions is the only definite morphologic evidence of the presence of bartonella in verruga lesions.

Gastrointestinal autonomic nerve tumors. A clinicopathological, immunohistochemical, and ultrastructural study of 12 cases.

The gastrointestinal autonomic nerve tumor (GAN tumor) is an uncommon stromal tumor of the intestinal tract and retroperitoneum first described by Herrera and associates in 1984. Distinction of GAN tumors from other gastrointestinal stromal tumors is based on electron microscopic findings. Thus far there have been 12 reported cases. We present an additional 12 GAN tumors, identified by us during 4 years. There were seven male and five female patients and they ranged in age from 10 to 85 years (mean: 58 years). Sites of the tumors were stomach (three), jejunum (two), ileum (four), mesentery (one), and retroperitoneum (two). Eight of the tumors measured > 10 cm in greatest dimension. Usually well circumscribed, the neoplasms were tan to light pink, sometimes hemorrhagic, and soft. There was a variety of histologic patterns including fascicles, palisades, and whorls. Mitotic activity varied from 0 to 23 mitosis per 10 high-power fields (HPF). Using a panel of 10 immunohistochemical stains, only vimentin was consistently positive. There was neuron-specific enolase reactivity in six and S-100 protein reactivity in two cases. All muscle markers were negative. Ultrastructural studies showed neuron-like cells with long axonic cytoplasmic processes ending in bulbous synapse-like structures containing dense-core neurosecretory granules and clear vesicles. Basement membrane was absent. These features are reminiscent of ganglia of the intestinal autonomic nervous system. The patients were followed for 5-125 months (mean of 26 months). Tumor recurred or metastasized to the liver in seven patients (58%) and four patients died with tumor. There were correlations between tumor size (> 10 cm), mitotic count (at least five per 10 HPF), and aggressive behavior.

Low-grade adenosquamous carcinoma of the breast. A clinocopathologic study of 32 cases with ultrastructural analysis.

Low-grade adenosquamous carcinoma of the breast is an uncommon neoplasm of uncertain pathogenesis, clinical behavior, and malignant potential. This report describes the clinical and pathologic features of 32 cases of low-grade adenosquamous carcinoma. All patients presented with palpable tumors ranging from 0.6 to 8.6 cm (mean, 2.8 cm). Origin from an intraductal papillary tumor was found in 12 cases, including three with adenomyoepitheliomatous features. Electron microscopy disclosed glandular and squamous differentiation; the squamous cells often lined ducts that were structurally analogous to the acrosyringium of the eccrine sweat gland. Treatment consisted of mastectomy (13 patients) or excisional biopsy (19 patients). A single lymph node in one patient with a 3.5-cm primary carcinoma harbored metastatic adenocarcinoma. Axillary dissection revealed no metastases in 11 other patients. Another patient with an 8.0-cm breast tumor had metastatic adenosquamous carcinoma in the lung at initial diagnosis. After follow-up of 12 to 124 months, 20 of 25 patients had no recurrence. Five women treated by excisional biopsy had local recurrences in the breast. In one patient, the local recurrence was ultimately fatal due to invasion of the hemithorax. Estrogen and progesterone receptor studies were negative in 13 of 15 cases studied by biochemical analysis. The two tumors that were hormone-receptor positive were histologically associated with a papilloma and an adenomyoepithelioma, respectively. In the latter case, immunohistochemical studies showed the carcinoma to be hormone-receptor negative. Hormone receptor activity was limited to the adenomyoepitheliomatous component. This study confirms the largely indolent, but locally aggressive, clinical course of low-grade adenosquamous carcinoma of the breast. Although complete limited excision of small lesions may be curative, tumors greater than 3.0 cm may require more aggressive therapy.

Large plaque-type blue nevus with subcutaneous cellular nodules.

Unusual or atypical melanocytic nevi can be confused with malignant melanoma. The authors present two cases of an unusual variant of blue nevus that were misdiagnosed initially as malignancy. Both lesions were asymptomatic and characterized clinically by childhood onset, with slow enlargement during adolescence and subsequent nodule formation. One lesion, which measured 24 cm in greatest dimension, was located on the anterior chest wall of a 53-year-old woman. The other lesion, which measured approximately 15 cm in greatest dimension, was located on the lateral abdominal wall of a 20-year-old man. Both lesions were characterized by a multifocal dermal and subcutaneous proliferation of fusiform and dendritic pigmented melanocytes. The histologic appearance of individual foci ranged from dermal melanocytosis to common blue nevus and cellular blue nevus. The cellular foci were located in the subcutis and involved, in one patient, the stroma of the breast. The cells were immunoreactive for S-100 protein, gp100 (HMB-45), and Melan-A (A103). Ultrastructural analysis revealed melanocytes typical of blue nevus. The woman underwent complete excision of the lesion, and the man underwent only partial excision of the lesion. On clinical follow-up of 32 and 19 months, respectively, both patients are alive and well with no evidence of recurrence or progression. Because the lesions presented clinically as large plaques and were diagnosed histologically as blue nevi with subcutaneous foci of cellular blue nevus, we term this rare variant of blue nevus large plaque-type blue nevus with subcutaneous cellular nodules. Recognition of this lesion enhances our knowledge of the morphologic spectrum of melanocytic tumors and helps to avoid confusion with malignant melanoma.

Presacral carcinoid tumors: report of three cases and review of the literature.

The clinical, microscopic, immunohistochemical, and ultrastructural features of three carcinoid tumors of the presacral region are reviewed. All tumors occurred in young women and did not involve the rectum. The predominant microscopic pattern was trabecular. The differential diagnosis included paraganglioma and myxopapillary ependymoma. Immunohistochemically, neuroendocrine markers and low molecular weight cytokeratins were expressed in all cases. Neurosecretory granules were identified in the single case studied by electron microscopy. One case was associated with a tailgut cyst (retrorectal cystic hamartoma). Two patients were treated with complete local excision and are free of disease 3 and 4 years after surgery. One case metastasized to both breasts and recurred locally after an incomplete excision. This report expands the already long list of sites where carcinoid tumors can arise. The frequent association of these tumors with tailgut cysts and their histologic similarities to rectal carcinoid tumors suggest that the most likely derivation of presacral carcinoid tumors is from hindgut rests.

Neuroendocrine carcinomas of the larynx. A study of two types, one of which mimics thyroid medullary carcinoma.

We studied 13 neuroendocrine carcinomas of the larynx. They constituted 59% of the 22 nonepidermoid carcinomas of the larynx seen at Memorial Hospital during a 45-year period, and for which adequate material was available for review. Four tumors were histologically identical to small cell carcinomas of the lung and were classified as small cell neuroendocrine carcinomas (SCNC). One case represents one of the original descriptions of the laryngeal SCNC. No SCNC was argyrophil, and of the three studied immunohistochemically, all contained neuron-specific enolase, one carcinoembryonic antigen (CEA) and one serotonin. Nine tumors were large cell carcinomas (LCNC). Eight LCNC were argyrophil, and all nine contained neuron-specific enolase, six calcitonin, four CEA, one HCG, two serotonin, and two somatostatin. The laryngeal neuroendocrine carcinomas commonly presented in chronic cigarette smokers with mean ages of 63 (SCNC) and 60 (LCNC), were not associated with other endocrine tumors, and proved highly fatal in spite of radical surgery and radiation therapy. At last follow-up only one patient was alive (after 13 months). Patients dying with SCNC survived a mean of 11 months, and those with LCNC, 36 months. To determine whether the laryngeal LCNC most closely resembles pulmonary neuroendocrine tumors, head and neck paragangliomas, or thyroid medullary carcinoma (TMC), they were histologically, histochemically, and immunohistochemically compared with control cases of each group. Overall, LCNC most closely resembles TMC, and given the frequency with which each presents as a neck mass, misinterpretation of one for the other is very possible. Evidence is provided suggesting that some LCNC have also been mistaken for the laryngeal paraganglioma.

Malignant peripheral nerve sheath tumors arising from ganglioneuromas.

Two typical malignant peripheral nerve sheath tumors (PNST) arising in preexisting ganglioneuromas are described. To the best of our knowledge, this association of tumors has not been reported in detail previously. Neither patient had the stigmata nor family history of Von Recklinghausen's neurofibromatosis. In both cases, the ganglioneuromas evolved from more primitive neuroectodermal tumors (one neuroblastoma, one ganglioneuroblastoma) and both patients developed their malignant PNST at previously irradiated sites. Both patients died within 2 years of the diagnosis of their malignant PNST. The origin of these malignant PNSTs from Schwann cells is supported by an ultrastructural analysis of 27 neuroblastomas, ganglioneuroblastomas, and ganglioneuromas.