The Voronoi theory of the normal liver lobular architecture and its applicability in hepatic zonation.

The precise characterization of the lobular architecture of the liver has been subject of investigation since the earliest historical publications, but an accurate model to describe the hepatic lobular microanatomy is yet to be proposed. Our aim was to evaluate whether Voronoi diagrams can be used to describe the classic liver lobular architecture. We examined the histology of normal porcine and human livers and analyzed the geometric relationships of various microanatomic structures utilizing digital tools. The Voronoi diagram model described the organization of the hepatic classic lobules with overall accuracy nearly 90% based on known histologic landmarks. We have also designed a Voronoi-based algorithm of hepatic zonation, which also showed an overall zonal accuracy of nearly 90%. Therefore, we have presented evidence that Voronoi diagrams represent the basis of the two-dimensional organization of the normal liver and that this concept may have wide applicability in liver pathology and research.

Activation of the Met receptor by cell attachment induces and sustains hepatocellular carcinomas in transgenic mice.

Overexpression is the most common abnormality of receptor tyrosine kinases (RTKs) in human tumors. It is presumed that overexpression leads to constitutive activation of RTKs, but the mechanism of that activation has been uncertain. Here we show that overexpression of the Met RTK allows activation of the receptor by cell attachment and that this form of activation can be tumorigenic. Transgenic mice that overexpressed Met in hepatocytes developed hepatocellular carcinoma (HCC), one of the human tumors in which Met has been implicated previously. The tumorigenic Met was activated by cell attachment rather than by ligand. Inactivation of the transgene led to regression of even highly advanced tumors, apparently mediated by apoptosis and cessation of cellular proliferation. These results reveal a previously unappreciated mechanism by which the tumorigenic action of RTKs can be mediated, provide evidence that Met may play a role in both the genesis and maintenance of HCC, and suggest that Met may be a beneficial therapeutic target in tumors that overexpress the receptor.

Glutathione monoethyl ester ameliorates caerulein-induced pancreatitis in the mouse.

Studies in animal models suggest that oxygen radicals may be important in the pathogenesis of acute pancreatitis. Because glutathione is an essential component of the defense against radical-mediated cellular injury, we investigated whether pancreatic glutathione content is influenced by inducing acute pancreatitis and whether augmenting the intracellular supply of glutathione would alter the course of pancreatitis. Caerulein, a decapeptide cholecystokinin analogue, induces acute necrotizing pancreatitis in mice when given in high doses (50 micrograms/kg per h) over a period of 6 h. The pancreatic glutathione content (total, GSH + GSSG) in mice treated with high-dose caerulein fell to 17% of normal within 4 h of beginning caerulein and recovered toward normal after discontinuing caerulein treatment. Mice treated with glutathione monoethyl ester (20 mmol/kg 1 h before caerulein, 10 mmol/kg 3 and 7 h after starting caerulein) were found to have blunted depletion of pancreatic glutathione, diminished histologic evidence of pancreatitis (necrosis, inflammation, and vacuolization), and lower serum amylase values compared with mice treated with caerulein alone. These findings suggest that the profound depletion of pancreatic glutathione caused by hyperstimulation of the pancreas with caerulein is critically important in the pathogenesis of acute caerulein-induced pancreatitis.

Beneficial effects of cholecystokinin-receptor blockade and inhibition of proteolytic enzyme activity in experimental acute hemorrhagic pancreatitis in mice. Evidence for cholecystokinin as a major factor in the development of acute pancreatitis.

The effects of the cholecystokinin (CCK)-receptor antagonist proglumide, the protease inhibitor gabexate, and the hormones secretin and cholecystokinin-octapeptide (CCK-8) were studied in a model of acute hemorrhagic pancreatitis induced by feeding mice a choline-deficient, ethionine-supplemented (CDE) diet. Injections of gabexate and proglumide from initiation of CDE diet (before induction of pancreatitis) increased survival from 37% (diet alone) to 85 and 75%, respectively, and also ameliorated histological alterations and increases in serum amylase concentration and pancreatic activated trypsin. Secretin had no major beneficial effect. When proglumide or gabexate were given after induction of pancreatitis, proglumide still increased survival to 75%, whereas gabexate no longer did. Injection of nontoxic doses of CCK-8 before proglumide or gabexate injections completely abolished all beneficial effects and also increased the severity of pancreatitis due to CDE diet alone. Blockade of CCK receptors and early inhibition of protease activity may be beneficial in severe acute pancreatitis. Cholecystokinin appears to play a contributory role in the development of pancreatitis.

Expression of LewisX and sialylated LewisX antigens in human colorectal polyps.

The LewisX (LeX) antigen [characterized by trisaccharide Gal beta 1----4 (Fuc alpha 1----3)N-acetylglucosamine] is an oncodevelopmental antigen in the human colon. Monoclonal antibodies (MoAbs), anti-SSEA-1 and AH8-183, which recognize LeX antigen either on short oligosaccharide side chains or as a terminal immunodeterminant on longer carbohydrate side chains of glycoconjugates, bind to most colon cancer tissues but also to some normal colon mucosae. However, the monoclonal antibodies FH1, FH4, FH6, and IB9, which recognize extended difucosylated and trifucosylated LeX structures or their sialylated derivatives, are more cancer-associated because they rarely bind to normal colon mucosa. In the present study, these MoAbs were used to compare the expression of various LeX-related antigens in premalignant (adenomatous) and nonpremalignant (hyperplastic) colorectal polyps. Antigen expression in polyps was also compared to antigen expressions of normal colon mucosa and colon cancer tissues. The four MoAbs recognizing extended LeX antigens bound to adenomatous polyps (APs) significantly more than to hyperplastic polyps (HPs). In contrast, anti-SSEA-1 and AH8-183 recognizing monofucosyl LeX were less able to distinguish between APs and HPs. In APs, staining with the four MoAbs recognizing extended LeX antigens correlated with the premalignant parameters of larger polyp size, more severe dysplasia, and increased villose component. However, staining with AH8-183 correlated only with polyp size, and anti-SSEA-1 correlated only with polyp size and degree of dysplasia. In general, the staining frequency of HPs was similar to that of normal colon mucosa, although FH6, which did not stain any specimens of normal mucosa, stained a few HPs. The staining frequency of APs was less than that of colon cancer tissues, but these differences were generally not statistically significant. In conclusion, extended LeX antigens and their sialylated derivatives are cancer-associated antigens that are expressed preferentially in premalignant colon polyps, that tend to correlate with malignant potential in these polyps, and that may eventually help to define mechanisms involved in the polyp-to-cancer sequence.

Cancer-associated alterations of blood group antigen expression in the human pancreas.

Certain alterations of blood group substance (BGS) expression have been observed in gastrointestinal cancer tissues. However, in the pancreas little is known about BGS expression by normal or malignant tissue. The present immunohistochemical study analyzed simultaneously the expression of A, B, H, Lewisa (Lea), and Lewisb (Leb) antigens in specimens of normal pancreas, chronic pancreatitis, and pancreatic carcinoma (primary and metastatic). In normal pancreas all five antigens were expressed in ducts, ductules, and acini, but not in islets. Acinar cells expressed A, B, H, and Leb in supranuclear cytoplasm, whereas Lea was found mainly on centroacinar cells. Only BGSs that were appropriate for the host's blood type were expressed, except for one case of Lea deletion. BGS expression by chronic pancreatitis tissue closely resembled that by normal tissue. In primary pancreatic cancer two cancer-associated alterations were noted that were not found in either normal pancreas or chronic pancreatitis. Deletion of an expected A, B, H, or Leb antigen occurred in approximately 25% of cases, particularly in more poorly differentiated cancers. Incompatible expression of an unexpected A or B antigen occurred in 33% of cases, regardless of degree of differentiation. Metastatic pancreatic cancers also exhibited BGS deletion and incompatibility. In both primary and metastatic cancers the incidence of incompatible A or B expression was higher in cancers from the United States than in cancers from Japan, but the incidence of BGS deletion was similar between the two countries. It was concluded that deletion of A, B, H, or Leb antigens and incompatible expression of A or B antigens are cancer-associated events in the pancreas.

Cancer-associated alterations of blood group antigen expression in human colorectal polyps.

Human colorectal carcinoma tissues may exhibit several patterns of altered blood group substance (BGS) expression: reappearance of A, B, H, or Lewisb antigens in distal colon; deletion of BGS in the proximal colon with or without precursor substance accumulation; and incompatible BGS expression in proximal or distal colon. The present study evaluated these cancer-associated alterations in colorectal polyps with different malignant potential. With respect to ABH antigens, hyperplastic polyps (HPs), considered to have no malignant potential, did not exhibit incompatibility and only a few cases demonstrated BGS reappearance or deletion. Adenomatous polyps (APs) however, frequently reexpressed ABH antigens or expressed incompatible BG-A or B in 27% of polyps; one specimen demonstrated BG-B deletion. Precursor expression was not found in HPs but was frequently observed in APs. Reappearance of ABH in distal polyps was significantly correlated with increasing grade of dysplasia, but was not significantly correlated with polyp size or histological type. With respect to Lewis antigen expression, Lewisb reappearance occurred in almost every distal polyp, and Lewisa-Lewisb coexpression was also quite common. Lea deletion was frequently noted, especially in HP, but the significance of this finding is unclear. This study indicates that several antigenic alterations that occur in colorectal cancer tissues also appear in premalignant polyps, and often in early stages of the neoplastic process. The observation that incompatible expression of BG-A or B occurs only in AP and cancer tissues (as well as mucosa adjacent to cancer) but not in fetal colonic mucosa, adult normal colonic mucosa, or HP, suggests that this may be a cancer-specific phenomenon.

Hepatocellular carcinoma arising in a focus of multilobular adenoma. A case report.

This report presents a case of hepatocellular carcinoma arising in a large multilobulated adenoma in the noncirrhotic liver of a 29-year-old woman. This unusual case demonstrates the potential for a benign proliferative process in the liver to evolve into a malignant neoplasm.

Plastic-embedded endoscopic biopsies. Diagnostic advantages.

As the use of endoscopic biopsy has increased in recent years, the pathologists whose job it is to interpret these small specimens have been asked to give more specific diagnoses. Plastic embedding has proved to be a useful diagnostic tool because it provides better morphology than routine paraffin embedding and because enzyme histochemical and immunohistochemical markers can be used. We applied these techniques to endoscopic biopsies hoping to increase the diagnostic yield. Biopsies from 75 patients were fixed in paraformaldehyde, embedded in methacrylate, and sectioned at 2 mu. These sections were then compared with routine sections from the same patient. Additional special stains were used and enzyme histochemistry, or immunohistochemistry was performed on the plastic- or paraffin-embedded tissue as needed. We found that in 26.7% of the cases, plastic sections resulted in more specific diagnoses than was possible with paraffin sections. When distinguishing lymphomas from poorly differentiated carcinomas, this method provided much better morphologic differentiation and better demonstration of leukocyte common antigen than keratin staining. Identification of B- or T-cell antigens was possible on plastics but not on paraffin. Furthermore, lesions such as histiocytosis X and cryptosporidiosis were more accurately identified. Thus, we found that the plastic-embedded tissue provided all the information yielded by routine paraffin embedding and also improved the diagnostic yield on certain types of neoplastic or infectious processes.

Lipopeliosis. An immunohistochemical and clinicopathologic study of five cases.

Lipopeliosis is an unusual liver lesion in which sinusoids become engorged by fat globules. It arises in newly transplanted donor livers that display varying degrees of fatty change. If, after transplantation, hepatocyte necrosis secondary to ischemic or preservation injury occurs, the fat escaping the hepatocytes becomes sequestered in the sinusoidal spaces. We previously described this lesion in a case report; we now describe four more cases to define better the incidence, immunohistochemical features, and clinical spectrum. Of 101 transplanted livers, the lesion was noted in five of 28 (18%), with both preservation injury and mild to moderate fatty change present 1 week following transplantation. Factor VIII-related antigen, collagen IV immunoperoxidase, and oil red O stains confirmed the engorgement of sinusoids by fat droplets, and a stain for CD68-positive cells identified a macrophage reaction around the fat droplets. The patient in our original report developed severe graft dysfunction with residual scarring of the centrilobular zone. Two of the four additional cases had no residual side effects from the lipopeliosis; however, two cases were associated with loss of graft. We conclude that lipopeliosis may be fairly common (5% of transplants). Its clinical outcome can vary greatly and most probably depends on the extent of hepatocellular necrosis.

Proposal for standardized criteria for the diagnosis of benign, borderline, and malignant hepatocellular lesions arising in chronic advanced liver disease.

Although current literature contains many cases of putative premalignant hepatocellular proliferations and small hepatocellular carcinomas, no consistent nomenclature and diagnostic criteria have been put forward to describe them. These nodules, which are being detected by radiographic techniques in cirrhotic livers and removed during transplantation procedures, represent a new and challenging histologic spectrum of liver pathology. In this study, a multinational panel of five liver pathologists reviewed 23 such nodules and were able to reach a consensus on the diagnostic criteria and to devise a standard nomenclature to describe the histologic lesions. We recommend that benign nodules showing little histologic difference from cirrhotic nodules be classified as regenerative or macroregenerative, and nodules with atypical features not diagnostic of carcinoma be classified as borderline. Such standardization should facilitate further study of the pathologic features and clinical behavior of these lesions.

Enzyme histochemistry of hepatocellular neoplasms.

We examined a series of hepatocellular neoplasms, including 4 adenomas, 7 hepatoblastomas, and 18 hepatocellular carcinomas (HCC) with enzyme histochemistry in plastic-embedded sections. Our most striking observation was that there was a distinct difference in the staining pattern for alkaline phosphatase (Alk0) in benign and malignant tumors. Non-neoplastic controls (normal liver, reactive lesions) and benign neoplasms showed a distinctive canalicular pattern of staining with Alk0. Malignant neoplasms, however, showed a virtual absence of Alk0 staining; 6 of 7 hepatoblastomas and 17 of 18 HCCs were practically devoid of staining, while the two positive cases showed a pattern easily discernible from normal. The sensitivity of the observed Alk0 staining pattern in detecting malignant hepatocellular neoplasms was 92% and the specificity was 100%. Cytoplasmic gamma-glutamyl-transferase (GGT) was present in a minority of HCCs, but faint staining was also seen in normal liver or in adenomas. It appears that these nonmorphologic techniques may aid the surgical pathologist in the differential diagnosis of primary hepatocellular neoplasms.

Hepatic angiomyolipoma: a clinicopathologic study of 30 cases and delineation of unusual morphologic variants.

Hepatic angiomyolipoma (AML) is frequently misdiagnosed. HMB-45 is a promising immunomarker for this tumor that leads to recognition of some AMLs with unusual morphology. The purpose of this collaborative study is to better define the morphologic variations of AML. Thirty AMLs were examined, including four biopsy specimens and two fine-needle aspirates. The diagnosis was confirmed by the presence of HMB-45-positive myoid cells. Almost half the cases were originally misdiagnosed as carcinomas or sarcomas. There was marked female predominance (25:5), and the mean age was 48.7 years (range 29-68). Three patients (10%) had evidence of tuberous sclerosis and all had renal AML. According to the line of differentiation and predominance of tissue components, the tumors was subcategorized into mixed, lipomatous (> or = 70% fat), myomatous (< or = 10% fat), and angiomatous type. The mixed type was the most common (11 resected cases), comprising sheets of epithelioid muscle cells admixed with islands of adipocytes, abnormal vessels, and frequently, hematopoietic cells. Six tumors (including three from biopsy specimens) were heavily fatty and showed predominantly adipocytes with epithelioid and short spindle myoid cells webbed between fat cells. Of 10 myomatous AMLs, five tumors showed a pure sinusoidal trabecular pattern and comprised mainly epithelioid cells. Typically, mature adipocytes were absent or scanty, but fat was seen as fine droplets within cytoplasm or as occasional large globules in sinusoids. Pelioid and inflammatory pseudotumor-like patterns were identified focally. Regarding cellular features of the myoid cells, most of the epithelioid cells were either eosinophilic or clear with spiderweb cell morphology. Three AMLs showed an almost purely oncocytic appearance with scanty fat. Large pleomorphic epithelioid cells existed as small foci. Spindle cells arranged in long fascicles were uncommon. D-PAS-positive globules were common around pelioid areas. Brown pigments with staining characteristics of hemosiderin and/or melanin were noted. In conclusion, we propose HMB-45-positive myoid cells as the defining criterion of hepatic AML, which is a tumor capable of dual myomatous and lipomatous differentiation and melanogenesis. Because of its protean morphologic appearance, recognition of the various variant patterns and cell types is important for a correct diagnosis, assisted by immunohistochemical confirmation with HMB-45. Trabecular and oncocytic cell tumors appear to stand out as distinctive subtypes.

Intragraft cytokine profile during human liver allograft rejection.

Forty-three human liver allograft biopsies and normal liver were directly analyzed for inflammatory and immunoregulatory cytokine gene expression by polymerase chain reaction (PCR). IL-5 gene expression was predominantly present in biopsies from liver allografts with histopathological evidence of acute rejection. IL-2 gene expression was rarely observed in rejecting allografts or allografts without evidence of rejection. In contrast, IL-4 message was readily detectable in the majority of liver allografts regardless of clinical status. The inflammatory mediators IL-1 beta, TNF-alpha, and IL-6 were detected with similar frequency in rejecting allografts and allografts without evidence of rejection. These findings suggest that inflammatory and immunoregulatory cytokines are produced within the allograft. Moreover, IL-5 may play a role in the local mechanisms of liver allograft rejection.

Fibrosing cholestatic hepatitis in renal transplant recipients.

Fibrosing cholestatic hepatitis in a specific histologic manifestation of hepatitis B virus infection consisting of periportal fibrosis, hepatocyte ballooning, cholestasis, a relatively scant inflammatory infiltrate, and marked overexpression of hepatitis B viral antigens in hepatocytes. Until recently, fibrosing cholestatic hepatitis had been reported only in recipients of liver allografts. In this report, we present two patient in whom this lesion developed following renal transplantation. Both patients had previous liver biopsies showing relatively mild histologic changes. In one patient, the lesion developed early after retransplantation, during the period of maximal immunosuppression. However, in the second patient this lesion developed after withdrawal of immunosuppression. In both cases, death occurred within a few months because of progressive liver disease. Since this lesion can develop in "relatively healthy" hepatitis B carriers following transplantation of organs other than liver, these patients should have careful monitoring of their liver disease. Moreover, since the disease may progress despite withdrawal of immunosuppression, these patients would clearly benefit from the development of more effective therapies for posttransplant hepatitis B.

Hepatic granulomas following liver transplantation. Clinicopathologic features in 42 patients.

Liver granulomas have long been known to pose diagnostic problems for pathologists; however, their prevalence and associated etiologic factors have not been studied in liver transplant patients. We reviewed 3632 liver biopsy specimens from 563 patients at two institutions and identified 42 patients with posttransplant granulomas. A possible or probable etiologic factor was identified in 30 (71%) cases. Most were epithelioid granulomas and microgranulomas located in the parenchyma associated with hepatocyte necrosis (21 cases, 50%). Portal-based granulomas were associated with recurrent primary biliary cirrhosis (5 cases, 12%), acute cellular rejection (2 cases, 4.8%), and a foreign body-type reaction (1 case, 2.4%). One case was associated with tuberculosis (2.4%), 4 cases occurred in a fatty liver (9.5%), and 8 patients had liver granulomas but no other significant abnormality. The granulomas were most frequent in the first 7 months after transplantation when the patients were biopsied more often and underwent episodes of rejection or acute hepatitis. Portal-based granulomas in this period were usually associated with acute cellular rejection. After 7 months, the frequency of granulomas as well as the number of biopsies decreased and portal-based granulomas associated with recurrent primary biliary cirrhosis were most common (5 cases, 12%). Rare, late-appearing parenchymal granulomas were also seen (3 cases) and consisted of 1 lipogranuloma and 2 cases of epithelioid granuloma. The latter were thought, in 1 patient, to be associated with parenchymal hepatocyte necrosis; the others were of unknown etiology.

Failure patterns of cryopreserved vein grafts in liver transplantation.

Reports of early success with cryopreserved saphenous veins (CSV) as arterial conduits led us to develop cryopreserved iliac veins (CIV) as interposition grafts for portal vein reconstruction in living-related liver transplantation (LRLT). Despite encouraging short-term results, retrospective analysis of long-term cryopreserved vein graft performance in LRLT at our institution has revealed a high rate of late graft failures. Between July 1992 and JUly 1994, interposition grafts (CIV for portal vein interposition n=4, CSV for portal vein interposition n=3, and CSV for hepatic artery interposition n=2) were utilized in 7 LRLT. (Two transplanted organs had both CIV and CSV grafts.) Recipients included 5 children and two small adults (median: 3.5 years, range: 0.5--59 years). Posttransplant follow-up in excess of 36 months revealed portal vein (PV) and hepatic artery (HA) complications of cryopreserved grafts in each patient. PV complications included aneurysm (n=4) diagnosed at 28, 24, 18, and 1.5 mo, stricture (n=1) diagnosed at 11 mo, and thrombosis (n=1) diagnosed at 18 mo posttransplantation. All portal vein complications have been managed without retransplantation, but one (PV thrombosis) necessitated surgical shunt therapy. Each CSV hepatic artery interposition graft has been complicated by thrombosis (diagnosed at 11 days and 24 mo posttransplant) necessitating retransplantation. Based on these observations, we have adopted alternative strategies for HA and PV reconstruction. At present, 11 LRLT have been performed without cryopreserved vein conduits over 17 mo with no vascular complications. While this study does not permit statistical analysis, these results discourage the use cryopreserved iliac veins for portal interposition and cryopreserved saphenous veins for arterial interposition in liver transplantation.

Chronic nonspecific inflammatory bowel disease of the cecum and proximal colon in children with grossly normal-appearing colonic mucosa: diagnosis by colonoscopic biopsies.

The diagnosis of inflammatory bowel disease rests on radiologic, endoscopic, and histologic criteria. Five patients, 2 to 17 years of age, sought medical attention because of chronic abdominal pain, diarrhea, and heme-positive stools. Rectal biopsies, visual inspection of colonic mucosa through the colonoscope, and contrast radiographs of the large and small intestine yielded nonspecific results. Serial endoscopic biopsies demonstrated a gradient of inflammatory changes diminishing in severity distally from the ileocecal valve and cecum. The disease process was most evident in specimens from the cecum, whereas biopsies distal to the transverse colon had a normal histologic appearance in all five patients. Biopsies from the proximal colon may provide evidence of inflammatory bowel disease not detectable using standard techniques. The combination of chronic abdominal pain, diarrhea, and heme-positive stools associated with inflammatory changes in biopsy specimens obtained from the proximal colon, but normal findings on radiologic, colonoscopic, and rectal biopsy examinations, may represent an early stage in the evolution of chronic nonspecific inflammatory bowel disease, including ulcerative colitis or regional enteritis (Crohn disease).

Disseminated intravascular meconium in a newborn with meconium peritonitis.

A 3-day-old premature infant with meconium peritonitis, periventricular leukomalacia, and pulmonary hypertension died with respiratory insufficiency. An autopsy disclosed intravascular squamous cells in the lungs, brain, liver, pancreas, and kidneys. Numerous pulmonary capillaries and arterioles were occluded by squamous cells, accounting for pulmonary hypertension. Brain parenchyma surrounding occluded cerebral vessels showed infarct and gliosis. A mediastinal lymph node filled with squamous cells alluded to the mechanism by which these cells from the peritoneal cavity likely entered the bloodstream--namely, via diaphragmatic pores connecting with lymphatics. Thus, disseminated intravascular meconium rarely may complicate meconium peritonitis and have devastating consequences.

Chromosomal alterations in ulcerative colitis-related and sporadic colorectal cancers by comparative genomic hybridization.

Both ulcerative colitis (UC)-related and sporadic colorectal cancers are thought to evolve through a multistep process of genomic instability, accumulation of genomic alterations, and clonal expansion. This process may involve different genomic changes in UC-related cancers than in sporadic cancers because of the origin of UC-related cancers in an inflammatory field. This study was designed to define the specific genomic events occurring in UC-related cancers. Comparative genomic hybridization (CGH) was performed on 32 UC-related and 42 stage-matched sporadic colorectal cancers. The mean number of chromosomal alterations per case was similar in the UC-related and sporadic tumor groups (8.6 in UC, 8.1 in sporadic). The 2 tumor groups shared many chromosomal alterations: losses on 18q (78% UC v69% sporadic), 8p (53% v50%), 17p (44% v57%), and gains on 8q (63% v45%), 20q (44% UC v67%), and 13q (44% UC v38%). However, differences in the frequency and timing of specific alterations were observed. Chromosome 5q was lost in 56% of UC-related but in only 26% of sporadic cancers. Alterations of chromosome 8 were associated with stage progression in UC-related, but not in sporadic cancers. In contrast, 18q loss was associated with stage progression in sporadic cancers only. Thus, differences in the frequency and timing of individual chromosomal alterations suggest that genetic progression in these 2 tumor groups may follow multiple pathways.