Nodular histiocytic/mesothelial hyperplasia: a lesion potentially mistaken for a neoplasm in transbronchial biopsy

Benign Mesothelial Cells in transbronchial biopsy specimens: A potential diagnostic pitfall for lung cancer

Bronchoscopy is a common diagnostic procedure used to identify lung cancer. Specimens acquired through transbronchial biopsy are pivotal in the diagnosis and molecular characterization of this disease. The occurrence of benign mesothelial cells during a transbronchial biopsy (TBB) is relatively rare. Furthermore, these lesions can sometimes be erroneously identified as malignant, potentially resulting in unwarranted or inappropriate treatment for patients with and without lung cancer. In this retrospective analysis, we examined 619 TBB cases at our institute from 2019 to 2021. Benign mesothelial cells were identified via immunohistochemical studies in eight (1.3%) of 619 cases. These cells were classified into three patterns based on their cellular morphology: monolayer, lace, and cobblestone. Recognizing this phenomenon during the procedure is crucial to accurately distinguish benign mesothelial cells from their cancerous counterparts.

Cytomorphology of nodular histiocytic/mesothelial hyperplasia

Nodular histiocytic/mesothelial hyperplasia (NHMH) is a pathologic entity that has not been well characterized in the cytopathology literature. This is unfortunate because if unrecognized, NHMH may be misdiagnosed when encountered in cytology specimens. The aim of this communication is to accordingly alert cytologists about NHMH by means of an illustrative case report.

Cardiac Excrescences of Unusual Origin

Mesothelial/monocytic incidental cardiac excrescences (cardiac MICE) are a rare finding that are most often discovered incidentally either upon echocardiography or invasive cardiovascular procedures. In total, less than 50 known cases have been reported since first being discovered over 30 years ago. They are typically benign lesions; however, there has been a reported case of cardiac MICE being responsible for severe cardiopulmonary compromise and another case of the lesion embolizing leading to cerebral infarctions and ultimately death. Cardiac papillary fibroelastomas are also uncommon lesions found in the heart though they are not as rare as cardiac MICE. They are also benign and are typically attached to valvular surfaces; however, they also can be found as mobile masses. Just as cardiac MICE, they are capable of causing turbulent flow and thrombus formation and have been reported as the cause of ischemic events due to their ability to embolize. We present a case of cardiac MICE and cardiac papillary fibroelastoma in an individual who initially presented with neurologic symptoms concerning for a cerebrovascular accident. The patient was found to have a left ventricular mass composed of both cardiac MICE and cardiac papillary fibroelastomas.

Reactive histiocytic proliferation in the pleural fluid mimicking metastatic signet ring adenocarcinoma

Reactive nodular and diffuse histiocytic proliferations of mesothelial and non-mesothelial lined sites have been sporadically reported in the literature. However, there is no cytologic literature describing this process. We report a case of reactive histiocytic proliferation mimicking a metastatic signet ring adenocarcinoma in pleural fluid from a 33-year-old white male. Ancillary studies such as immunohistochemistry should be used to elucidate the cell of origin and avoid diagnostic errors.

Nodular histiocytic/mesothelial hyperplasia as consequence of chronic mesothelium irritation by subphrenic abscess

PURPOSE

Pleural nodular histiocytic/mesothelial hyperplasia is a nodular histiocytic/mesothelial proliferation, often delimiting cystic cavities, due to irritation by a pulmonary noxa. Case report results: The patient had right pleural parietal and diaphragmatic thickness, with pleural effusion, without lung alterations. He previously underwent left hemicolectomy and liver resection, due to a diverticulitis and a liver histiocytes-rich abscess. Video-assisted thoracoscopy biopsy showed a double population of reactive mesothelial cells and histiocytes.

CONCLUSION

Nodular histiocytic/mesothelial hyperplasia represents a potential pitfall for pathologists. Immunohistochemistry is crucial for the differential diagnosis with some malignancies. We suggest that in our patient, a chronic mesothelium inflammation happened by transdiaphragmatic involvement as a consequence of the liver abscess. Some pathogenetic mechanisms are hypothesized.

A unique case of diffuse histiocytic proliferations mimicking metastatic clear cell carcinoma in the hydrocele sac

Reactive histiocytic proliferations are extremely rare in paratesticular structures. Nodular histiocytic proliferations have been described in mesothelial-lined locations and only at few nonmesothelial sites. Diffuse histiocytic proliferations are described only in the pelvic peritoneum. We report the first case of diffuse histiocytic proliferation in the hydrocele sac of a 45-year-old man. Predominant histiocytes showed clear cytoplasm and signet ring-like change. Mucicarmin stain did not demonstrate mucin in the cytoplasm. Immunohistochemistry (IHC) staining showed nonspecific staining of these cells with carcinoembryonic antigen and negative staining with epithelial membrane antigen, pan-Cytokeratin, calretinin, cytokeratin 7, 20 and prostate-specific antigen. Strong diffuse cytoplasmic positivity for CD68 defined the mononuclear phagocyte nature of these cells. Diffuse histiocytic proliferations can occur in the hydrocele sac. Histochemical and IHC stainings are critical for accurate diagnosis and to avoid unnecessary surgery.

Lesion of aggregated monocytes and mesothelial cells: mesothelial/monocytic incidental cardiac lesion

A 58-year-old woman with a history of childhood acute rheumatic fever and resultant mitral valve stenosis was admitted to our cardiovascular surgery clinic complaining of tachycardia, dyspnea, and chest pain. After clinical and radiological findings were evaluated, mitral valve replacement, tricuspid De Vega annuloplasty and plication, and resection of giant left atrium were performed. Atrial thrombus was removed from the top of the left atrial wall. Operation material considered as thrombus was sent to a pathology laboratory for histopathological examination. It was diagnosed with mesothelial/monocytic incidental cardiac lesion (cardiac MICE). Microscopic sections revealed that morphological features of the lesion were different from thrombus. The lesion was composed of a cluster of histiocytoid cells with abundant cytoplasm and oval shaped nuclei and epithelial-like cells resembling mesothelial cells within a fibrin network. Epithelial-like cells formed a papillary configuration in the focal areas. Mitotic figures were absent. Here we present a case which was incidentally found in a patient who underwent mitral valve replacement surgery, as a thrombotic lesion on the left atrium wall.

Lymphoproliferative conditions of the serosa

CONTEXT

A diverse and complex variety of lymphoproliferative diseases may involve the serosa, with widely differing clinical outcomes encompassing a spectrum of benign and malignant conditions.

OBJECTIVE

To review lymphoproliferative disease involving the serosa and to provide a practical approach to the evaluation of lymphoid and plasma cell infiltrates in the serosa, together with a review of various tumors and tumorlike conditions that may mimic lymphoproliferative disease.

DATA SOURCES

Analysis of published literature.

CONCLUSIONS

All forms of hematologic malignancy may involve the various serosal sites, although this is usually observed as secondary involvement in persons with known lymph nodal, marrow-based, or extranodal disease. Primary pericardial, pleural, and peritoneal lymphomas are rare; many nonneoplastic conditions may mimic lymphoma and a variety of nonhematolymphoid tumors may simulate hematologic malignancies. An understanding of the role of ancillary tests, together with an appreciation of their limitations, will prevent misdiagnosis.

Nodular histiocytic aggregates in the greater omentum of patients with ovarian cancer

Nodular histiocytic aggregate (NHA) of the omentum is a rare benign proliferative process composed predominantly of histiocytes with scattered mesothelial cells. NHA is a differential diagnosis for neoplasms or metastatic tumors in cancer patients. To further clarify this clinical pitfall issue, the authors investigated surgical samples of the greater omentum from 96 patients with gastrointestinal malignancies and 53 patients with gynecologic neoplasms. Visible NHA of greater omentum was identified in 3 patients with ovarian neoplasms (borderline mucinous cystadenoma, low-grade papillary serous cystadenocarcinoma, and juvenile granulosa-cell tumor) but in none of the patients with gastrointestinal malignancies. Similar lesion was also identified on the cell blocks from peritoneal washings in 1 of the 3 patients. Grossly, the lesions formed small yellow-red nodules on the greater omentum, and the NHA lesion was also found diffusely on the surface of the appendix and fallopian tubes in 2 of the 3 patients. Histological study showed that typical NHA changes over an inflammatory background, which may indicate that NHA is a consequence of a chronic inflammatory process of omentum. The predominant infiltration of T lymphocytes in the NHA lesions indicates that the aggregation of histiocytes may be related to the activation of T-cell immunity. This report has first demonstrated visible NHA in the greater omentum of patients with ovarian malignancies, and awareness of this entity should be brought to clinicians to avoid misdiagnosis.

Cardiovascular pseudoneoplasms

CONTEXT

Primary cardiac tumors are rare and the great majority are benign neoplasms. Mass-forming reactive and pseudoneoplastic growths are less common, but recognizing and distinguishing these lesions from the neoplasms they resemble is critical to appropriate patient care.

OBJECTIVE

The general clinical, imaging, gross pathologic, and histologic features of 5 important pseudoneoplasms (inflammatory myofibroblastic tumor, hamartoma of mature cardiac myocytes, mesothelial/monocytic cardiac excrescences, calcified amorphous tumor, and lipomatous hypertrophy of the atrial septum) are discussed, with an emphasis on features differentiating them from other benign and malignant tumors.

DATA SOURCES

Pertinent citations of the literature and observations from the authors' experience are drawn upon.

CONCLUSIONS

While lacking malignant potential, these lesions can be associated with considerable morbidity and occasional mortality. Their recognition is important in guiding patient management, providing both guidance for appropriate therapy and avoidance of inappropriately aggressive and toxic treatments.

Histiocytic disorders of the lung

Histiocytic proliferations involving the lung span a broad spectrum. Some proliferations are primary; others represent a histiocytic response secondary to conditions in which there may be isolated lung involvement or the lung may be involved as part of a systemic process. Primary histiocytic lung disorders, particularly those of uncertain histogenesis are a heterogeneous and intriguing group of disorders. Although they have been the focus of attention by clinicians and pathologists alike, much is unknown about their etiopathogenesis. Owing to this uncertainty, our understanding of these processes is in a state of flux, and is likely to change as more information is brought to light. This review will focus on pulmonary histiocytic proliferations of uncertain histogenesis. Other histiocytic lesions will be dealt with in brief.

Mesothelial cells in transbronchial biopsies: a rare complication with a potential for a diagnostic pitfall

The presence of pleural tissue in transbronchial biopsies (TBs) is an incidental finding that has been rarely reported in the literature. It has the potential for causing wrong histologic diagnoses. Clinically, the significance of unintended pleural sampling by bronchoscopy is unknown. TBs containing mesothelial cells from 6 adult patients were studied using immunohistochemical stains. Clinical information was obtained with emphasis on the immediate postbronchoscopy period. The TBs were performed by 6 different bronchoscopists at 4 institutions because of pulmonary infiltrates in 5 patients and a mass lesion in 1 patient. All samples contained lung parenchyma and bronchial wall. They showed clusters of medium to large size polygonal cells with pink to amphophilic dense cytoplasm, round to oval nuclei, and prominent nucleoli. Some of the cells lined stroma and others were detached forming ribbons. They were initially disregarded, interpreted as carcinoma, judged as mesothelial cells, or interpreted as drug-induced reactive epithelial cells. They were positive for cytokeratin and showed nuclear staining for calretinin. They were negative for TTF-1, S100, CEA, and CD68. However, in 1 case, CD-68 positive histiocytes were admixed with enlarged reactive mesothelial cells corresponding to the so-called nodular histiocytic mesothelial hyperplasia. Chest x-ray films performed the same day after bronchoscopy showed no pneumothorax. Incidental sampling of the pleura may occur during the performance of TB and mesothelial cells may mimic carcinoma, pneumocytes, or macrophages. It is important to be aware of the presence of mesothelial cells in clinically uncomplicated TB to avoid an erroneous diagnosis of malignancy.

Mesothelial/monocytic incidental cardiac excrescences of the heart: case report and literature review

Mesothelial/monocytic incidental cardiac excrescences (MICE) of the heart are rare benign entities that have only been diagnosed incidentally, following cardiac surgical procedure. To date, totally 35 cases have been reported in the English literature. We describe an additional case of cardiac MICE presenting with severe aortic regurgitation for aortic valve replacement in a 20-year-old Chinese male patient. On microscopic examination, the findings initially were confused with true neoplasm. However, the related gross appearance, clinical history and further immunohistochemical staining enabled an accurate diagnosis. We review the relevant literature and found that immunohistochemical staining, especially the anti-cytokeratin antibody (AE1/AE3) and KP1 (CD-68) that were used by most investigators previously, was significant while making the diagnosis, because the two components of the cells show a contrast immunoreactivity to these two makers. The pathologists should always be alert to this entity while diagnose a cardiac surgery specimen.

A Case of Mesothelial/Monocytic Cardiac Excrescence Causing Severe Acute Cardiopulmonary Failure

Mesothelial/monocytic incidental cardiac excrescence (MICE) is a benign lesion composed of a haphazard mixture of mesothelial cells, histiocytes, and fibrin, often found incidentally during cardiac valve replacement. Its pathogenesis is controversial with some authors favoring an artifactually produced amalgam while others espoused a reactive phenomenon. Clinically, this entity is important because of potential misdiagnoses as malignancies. We report a case in a 65-year-old man with severe acute aortic regurgitation. A 2.0-cm mobile aortic valve vegetation was documented by transesophageal echocardiography prior to any cardiac instrumentation. At surgery, the lesion was immediately visualized together with free-floating vegetation in the left ventricular outflow tract. Routine and immunohistochemical examination showed a nodule composed of predominantly histiocytes and mesothelial cells, together with fibrin and scattered neutrophils. To our knowledge, this is the first reported case of a mesothelial/monocytic cardiac excrescence causing acute cardiopulmonary failure. The literature on MICE is reviewed with discussion of its etiology.

Establishment of permanent cell lines purified from human mesothelioma: morphological aspects, new marker expression and karyotypic analysis

This study reports the establishment of three major subtypes of human mesothelioma cells in tissue culture, i.e. the epithelioid, sarcomatoid and biphasic forms, and compares their phenotypic and biological characteristics. Primary cells isolated from biopsies or pleural exudates were subcultured for over 50 passages. We evaluated immunoreactivity using various mesothelial markers related to histological patterns of these cell lines. For epithelioid cells, calretinin and cytokeratin were found to be useful and easily interpretable markers as for control mesothelial cells. The biphasic form was only partially positive and the sarcomatoid type negative. Vimentin was expressed by all cell lines. BerEP4, a specific marker for adenocarcinoma, was negative. Interestingly, while the macrophage marker CD14 was negative, immunoreactivity for a mature macrophage marker (CD68) was expressed by all cell types, suggesting that this marker might constitute an additional tool useful in the differential diagnosis of mesothelioma. At the ultrastructural level, a cell surface rich in microvilli confirmed their mesothelial origin. PCR analysis revealed that none of the cell lines contained SV40 DNA. Karyotypic analyses showed more complex abnormalities in the epithelioid subtype than in the sarcomatoid form. These cell lines may be useful in the study of cellular, molecular and genetic aspects of the disease.

Nodular histiocytic/mesothelial hyperplasia: a potential pitfall

We present five cases of nodular histiocytic/mesothelial hyperplasia (two peritoneal, two pulmonary, and one pericardial) with identical microscopic features. All the lesions were biphasic and composed of cohesive monotonous epithelioid clusters of polygonal or oval cells with round or deeply grooved nuclei in association with darker cuboidal cells. Because of the increased cellularity and monotonous histologic pattern with some degree of cytologic atypia, neoplastic processes were seriously considered in the differential diagnoses. The majority of the cells marked as histiocytes by immunostain. A few scattered individual cells or small epithelial cell clusters were confirmed by calretinin stain to be mesothelial cells. The histologic patterns of the current lesions, irrespective of the location, were identical to nodular histiocytic/mesothelial hyperplasia. Histiocytic proliferations can be erroneously confused with primary mesothelial lesions or neoplasms such as granulosa cell tumor, eosinophilic granuloma, chronic myelogenous leukemia, and carcinoma. The purpose of this article is to describe the clinicopathologic features of nodular histiocytic/mesothelial hyperplasia and help familiarize pathologists with this lesion to prevent an erroneous diagnosis, particularly when it occurs in locations where mesothelial cells are not normally present.

Expression of the histiocytic marker PG-M1 in granuloma annulare and rheumatoid nodules of the skin

BACKGROUND

The expression of PG-M1, the most specific histiocytic marker, has not yet been studied in granuloma annulare (GA) and other palisaded granulomas of the skin. We evaluated the reactivity of PG-M1 with a series of GA and rheumatoid nodules (RN) to establish the sensitivity and potential usefulness of this marker in the diagnosis and characterization of these entities.

METHODS

Histological sections from 30 GA and 15 RN were immunostained with PG-M1. For comparison, additional sections were stained with KP-1 and lysozyme. The stains were recorded as negative, weakly positive (1+) and strongly positive (2+).

RESULTS

PG-M1 stained all cases of GA (100%). KP-1 and lysozyme stained 26 (86%) and 18 (60%) GA cases, respectively. PG-M1 exhibited a significantly stronger staining intensity (1.8 +/- 0.07) when compared with KP-1 (1.4 +/- 0.13) (p = 0.018) and with lysozyme (0.9 +/- 0.15) (p < 0.0001). All RN were stained by PG-M1 (100%). KP-1 and lysozyme stained 14 (93%) and six (40%) RN cases, respectively. PG-M1 staining intensity (1.6 +/- 0.13) was slightly higher than that of KP-1 (1.4 +/- 0.18) (p = 0.27) and significantly higher than that of lysozyme (0.4 +/- 0.13) (p < 0.0001).

CONCLUSIONS

PG-M1 is consistently and strongly expressed by the histiocytic population of GA and RN, being more sensitive and reliable than other histiocytic markers. We recommend its use in difficult cases in which the histiocytic nature of the lesion needs to be confirmed.

The histiocytic marker PG-M1 is helpful in differentiating histiocytes and histiocytic tumors from melanomas

Previous studies have shown that immunohistochemical stains for histiocytes are immunoreactive for melanomas. Accordingly, their value in differentiating histiocytes and histiocytic lesions from melanomas was questioned. PG-M1, the most specific histiocytic marker, was not evaluated in these studies. Our aims were to assess the reactivity of PG-M1 with a series of primary cutaneous and metastatic melanomas and to establish the potential usefulness of this antibody in the differentiation between histiocytes and histiocytic tumors and melanomas. PG-M1 staining was performed in 50 primary cutaneous and metastatic melanomas. For comparison, additional sections were stained with KP-1 and lysozyme (commonly used as histiocytic markers) and with S-100 and HMB-45 (commonly used as melanoma markers). The intensity (1+, 2+) and extent (1+ to 4+) were recorded semiquantitatively. PG-M1 stained weakly (1+) and focally (2+) only four cases of melanoma (8%). In contrast, histiocytes were strongly reactive for PG-M1 in all cases, being readily differentiated from melanoma cells including the positive cases. KP-1 stained melanoma cells in 44 cases (88%), lysozyme in 11 cases (22%), S-100 in 50 cases (100%), and HMB-45 in 48 cases (96%). No changes were found after restaining of selected KP-1 and lysozyme positive melanomas using an endogenous avidin/biotin blocking kit. PG-M1 is helpful in discriminating histiocytes and histiocytic lesions from melanoma cells. We recommend its inclusion in any antibody panel put together to distinguish between them.

Nodular histiocytic hyperplasia of the endometrium

We describe the clinical and pathologic features of four cases of nodular histiocytic proliferation in the endometrium. We have been able to find only one brief reference to this lesion in the literature. The lesion in each case was a detached nodule composed of aggregates of histiocytes within a biopsy or curettage specimen. The constituent cells differed from foamy histiocytes of the endometrium in that they had either lobulated or ovoid, vesicular nuclei, distinctive cytoplasmic margins, and a moderate amount of amphophilic cytoplasm. Mitoses were frequent (up to 11 per 10 high-power fields) in one case but were absent in the remaining cases. On immunohistochemical staining, CD68 and lysozyme were strongly expressed in the cytoplasm. Neither estrogen receptor nor progesterone receptor was expressed in contrast to the background endometrium. The cells were also negative for S-100 and cytokeratin. Each patient's postcurettage course was uneventful. The cause of nodular histiocytic proliferation of the endometrium is currently unknown, although response to intracavitary debris has been suggested. The lesion should not be confused with a variety of reactive, inflammatory, or neoplastic conditions, such as xanthogranulomatous endometritis, malakoplakia, histiocytic granuloma, hormonal changes of the endometrial stroma, Langerhans' cell histiocytosis, morular metaplasia, extravillous trophoblast, or exaggerated placental site reaction.

The role of imaging in malignant pleural mesothelioma

Imaging plays an essential role in the diagnosis, staging, and follow-up of patients with malignant pleural mesothelioma (MPM). The diagnosis is often suggested by a unilateral pleural mass with a moderate to large pleural effusion seen on chest radiographs, but computerized tomography (CT) is the most frequently used technique for evaluation of the lungs in patients with MPM. CT not only suggests pulmonary metastases typically manifested as nodules or masses, but also can demonstrate underlying lung disease often caused by prior asbestos exposure. Magnetic resonance (MR) imaging may be helpful in selected patients with potentially resectable disease to further examine the local extent of tumor. Imaging with positron emission tomography (PET) using the radionuclide imaging agent (18)F fluoro-deoxyglucose (FDG) takes advantage of a basic property of tumor cells, increased glucose metabolism to identify malignant lesions. PET provides not only anatomic information, especially regarding mediastinal node metastasis, but also biochemical information about the lesion. These imaging modalities help triage patients to the most appropriate diagnostic and treatment options. Following patients after therapy usually relies on chest radiographs, although CT can more accurately describe response to therapy. This review will focus on radiologic evaluation in diagnosing, staging, and follow-up patients with MPM.

Cytologic clue of so-called nodular histiocytic hyperplasia of the pleura

So-called "nodular histiocytic hyperplasia" (NHH) is a benign histiocytic lesion caused by mechanical irritation, inflammation, and tumor. Frequently, it has been confused with mesothelial lesions and other malignant neoplasms. The diagnostic clue is proliferating cells in the lesion showing diffuse, strong immunoreactivity against the histiocytic marker, CD68. Recently, we encountered a case of so-called NHH of the pleura and confused it with various malignant neoplasms on histologic examination. An 80-yr-old Korean female presented with ascites, pleural effusions, and nodules on the pleural base. Both ascites and pleural effusion tapping smears displayed moderate cellularity, vaguely nodular cellular aggregates mainly composed of mononuclear cells with bland morphology, entrapped mesothelial cells, and background lymphocytes. Pleural biopsy demonstrated vaguely nodular, compact cellular aggregates of reactive histiocytes which were immunoreactive against CD68. Based on our case, cytologic examination as well as immunohistochemical study should be stressed in the case of so-called NHH. They can provide us more credible morphologic clues to reach a more accurate diagnosis than histologic examination alone, and we can avoid invasive procedures or unnecessary therapies to patients. To our best knowledge, this is the first report describing the cytologic features of so-called NHH in the English-language literature.

Pathology of the pediatric regio inguinalis: mysteries of the hernia sac exposed

Inguinal hernia repair is one of the most common surgeries performed on children. However, the value of routine histologic examination of hernia sac tissues continues to be debated. Although the surgical pathology of herniorrhaphy tissues is usually simple, occasional examples have unexpected findings that potentially lead to inappropriate management or that have added clinical implications. These along with surgical-quality assurance issues need to be considered in cost-benefit arguments. This article reviews basic histology, common potential pitfalls, and significant unexpected conditions encountered in the surgical pathology of the inguinal hernia sac in children.

Mesothelial hyperplasia with reactive atypia: diagnostic pitfalls and role of immunohistochemical studies-a case report

The cytomorphologic features of highly reactive mesothelial cells can be difficult to distinguish from malignant cells. We report on an unusual case of mesothelial hyperplasia in a pericardial effusion. The specimen contained bizarre-shaped cells and large tissue fragments in a patient with a history of lung carcinoma. The atypical cells were negative for CEA and LeuM-1 and positive for cytokeratins (AE1/3) and HBME-1. Strong HBME-1 positivity supported a mesothelial origin of the atypical cells and led to the diagnosis of reactive mesothelium. While HBME-1 cannot be used as the sole marker to establish an mesothelial origin; its use in a immunohistochemistry panel may be useful in individual cases to distinguish reactive mesothelial cells from carcinoma in effusion cytology. Diagn. Cytopathol. 2000;22:113-116.

Lesions described as nodular mesothelial hyperplasia are primarily composed of histiocytes

It is known that, on occasion, histiocytic proliferations can be confused with reactive mesothelial cell hyperplasia or with a malignant process. We report four cases of histiocytic proliferation, two occurring in the pleura in a 23-year-old woman and a 78-year-old woman, respectively, one in a hernial sac of a 2-year-old boy, and one in the lamina propria of the bladder of a 74-year-old man with a noninvasive papillary transitional cell carcinoma. The morphologic features of the pleural lesion of the 23-year-old woman and of the hernial sac lesion of the 2-year-old boy, as well as the bladder lesion, were similar to those reported in cases of the so-called nodular mesothelial hyperplasia. The pleural lesion in the 78-year-old woman consisted of a proliferation of cells with a signet ring-like morphology that was originally interpreted as either an unusual form of mesothelial hyperplasia or a metastatic signet ring cell adenocarcinoma. Because of mitotic activity and some cellular atypia in the bladder lesion, the possibility of invasive transitional cell carcinoma into the lamina propria was considered before immunohistochemical studies were performed. Staining for keratin showed only a few positive cells in the hernial sac and pleural lesions, whereas most cells reacted for the histiocytic marker CD68. Immunohistochemical studies on the bladder lesion also demonstrated strong staining for CD68, but no reactivity for keratin was observed. Based on these results, it is concluded that all of the lesions are primarily reactive histiocytic proliferations and because they may occur in other locations aside from the serosal membranes, the designation "nodular histiocytic hyperplasia" appears to be more appropriate than that of nodular mesothelial hyperplasia. It is important that the reactive nature of these lesions be recognized because on occasion they may present high mitotic activity or may show signet ring-like morphology and thus they can be confused with a malignancy.