Granulomatous Peritonitis

Both, infectious and noninfectious agents can cause granulomatous peritonitis. Among the former, Mycobacterium tuberculosis is the most common, and, less frequently, fungi and parasites. Granulomas are also induced by foreign material including keratin, by vernix caseosa or by meconium, in the form of necrotic pseudoxanthomatous nodules or as a postcautery reaction.²

Non-Granulomatous Histiocytic Lesions

Histiocytic infiltrates rather than discrete granulomas are occasionally found in the peritoneum.⁹ Melanin-rich histiocytes are sometimes found in cases where an ovarian dermoid cyst has ruptured. The spillage contains melanin, which the peritoneal histiocytes phagocytose. Grossly, the peritoneum may appear to be stained black or display small tumor-like nodules on its surface. Distinction of benign peritoneal melanosis from metastatic malignant melanoma is usually straightforward because of the bland nuclear features of the pigmented histiocytes and the absence of mitoses. Appropriate immunohistochemical stains can further indicate that the cells are histiocytes and not atypical melanocytes.¹⁶

Occasionally, foci of endometriosis may disclose an abundance of histiocytes filled with ceroid, a wax-like, finely granular, and golden to yellow-brown pigment that is a form of lipofuscin, a lipid-containing residue of lysosomal digestion that is considered an aging or ‘wear and tear’ pigment. Ceroid is believed to be the end result of the breakdown of blood products after removal of iron. These histiocytic foci are sometimes called ‘necrotic pseudoxanthomatous nodules.’2,17,18

Fibrosing Lesions

Sclerosing peritonitis is a reactive process in which a thickened fibrous or myofibromatous stroma develops on the peritoneal serosa. It is often idiopathic,19,20 although in some cases the cause is identified, such as prior peritoneal inflammation or a ruptured ovarian dermoid with spillage of the contents (see previous granulomatous reactions),^10,21 chronic dialysis,^22–26 or after surgical procedures.

In some cases, the sclerosing peritonitis has been described as part of a syndrome, often in association with a ‘luteinized thecoma of the ovary.’27–32 Clinically, most of the women are young, usually under 30 years of age. Common presenting signs include abdominal enlargement and sometimes small bowel obstruction. Ascites may be present. Even when the patients have a luteinized thecoma, none has endocrine symptoms. A significant number of patients have been exposed to propranolol-type beta-blocking agents or antiepileptics.

Grossly, opaque to light-brown 1–3 mm granules or nodules appear matted together on the peritoneum or on the serosa of the involved organs. The omentum is usually indurated. Microscopic examination discloses a fibrotic process, with various chronic inflammatory cells (Figure 31.3). There is usually some degree of mesothelial hyperplasia. Deeper tissues are relatively spared. Nodules are composed of moderately cellular fascicles of benign-appearing spindle cells resembling fibroblasts and myofibroblasts that contain occasional mitotic figures. In addition to cytokeratin reactivity, the cells also disclose immunoreactivity for vimentin and smooth muscle actin.^27,30

Rarely, single or multiple fibrous nodules ranging up to 6 cm may occur in the gastrointestinal tract or mesentery in adults.³² Microscopically, the lesions are composed of fibroblasts, collagen, and scattered mononuclear inflammatory cells. The fibroblastic cells show variable immunoreactivity for vimentin, CD117, muscle-specific actin, smooth muscle actin, and desmin, with negative staining for CD34 and ALK-1. These nodules have been designated as ‘fibrous pseudotumors.’³²

Occasionally, sclerosing lesions may be difficult to distinguish from desmoplastic mesothelioma, especially when the biopsy specimen is small. These tumors, however, are very rare in the peritoneal cavity, especially in women. Features that favor a diagnosis of mesothelioma include nuclear atypia, necrosis, organized patterns of collagen deposition (fascicular or storiform), and destructive infiltration into adjacent tissues.

Some patients with sclerosing peritonitis have been successfully treated utilizing antiestrogens and/or GnRH agonists.

Tumor-Like Lesions

Ovarian Remnant Syndrome

This condition exists if a patient who has had a ‘total bilateral oophorectomy’ later develops a palpable mass or experiences pelvic pain or other symptoms referable to ovarian tissue that has been left behind (Figures 31.14 and 31.15). This condition is described more fully in Chapter 24.

Supernumerary or Accessory Ovaries

Supernumerary ovaries are ectopic ovaries located at some distance from the eutopic ovary. It is rare but occasional cases have been reported in the peritoneal cavity.⁴⁶ This condition is described more fully in Chapter 23.

Splenosis

Nodules of splenic tissue, usually less than 1 cm in diameter, are randomly distributed in the peritoneal cavity. The etiology is trauma, most commonly a motor vehicle accident, which has resulted in splenic rupture.⁴⁷ Splenosis is generally asymptomatic but may cause abdominal or pelvic pain simulating endometriosis, or produce intestinal obstruction due to the development of adhesions. Splenosis may be encountered as an incidental finding or mistakenly interpreted as endometriosis, benign or malignant vascular tumors, or metastatic cancer.^48–50

Trophoblastic Implants

Finding disseminated trophoblastic implants in the peritoneum is uncommon (Figure 31.16). They may occur on occasion with peritoneal pregnancy, or following laparoscopic treatment of tubal pregnancy, where the frequency has been estimated at 3.6%.^51,52 Viability is suggested by rising human chorionic gonadotropin concentrations following surgery. The condition is best avoided by meticulous inspection of the abdomen after resection of the tubal pregnancy. Microscopically, the implants may show trophoblastic tissue including chorionic villi. Some implants, however, may resemble a placental site nodule.

Infarcted Appendix Epiploica

Appendices epiploicae are small polypoid processes of adipose tissue that project from the serosa of the large intestine, especially the transverse and sigmoid colon. Occasionally, they undergo torsion, infarction, and later detachment and can be found lying free within the peritoneal cavity.53,54 Typically in these cases, the center contains hyalinized fibrous tissue and often some residual adipose tissue that is mummified. The outer rim and variable portions of the core may calcify, resulting in a hard tumor-like mass (Figure 31.17).

Adenomatoid Tumor

Adenomatoid tumors are benign neoplasms of mesothelial origin, encountered most often in the fallopian tubes where frequently they are sieve like or multicystic. In contrast, they are also found subserosally in the uterine corpus near the fallopian tube, where they more usually simulate leiomyomas. They are seldom encountered elsewhere in the peritoneal cavity (see Chapter 21). Clinically, they are asymptomatic, and rarely recur after adequate excision. Grossly, adenomatoid tumors are usually solitary, less than 2 cm in diameter and have a white-gray appearance. Microscopically, multiple small slit-like or ovoid spaces are lined by a single layer of cells. Nuclear atypia is absent or minimal, and mitotic figures are rarely seen.

Well-Differentiated Papillary Mesothelioma

A rare form of peritoneal mesothelioma is the well-differentiated papillary mesothelioma. Most patients are of reproductive age, although an occasional patient has been postmenopausal. Also encountered in males, less common sites include the tunica vaginalis testis, pericardium, and pleura. These tumors are typically asymptomatic and often found incidentally at operation. Grossly, they are usually multiple, broad-based, wart-like excrescences that are polypoid or slightly nodular. Color and texture are similar to ovarian cortical tissue but sometimes firmer. They are generally small, usually measuring less than 2 cm in diameter.⁵⁵ An occasional tumor is solitary.⁵⁵

On microscopic examination, the neoplasm consists of relatively thick papillae composed of dense fibrous or hyalinized tissue covered by a single layer of cytologically benign, small flattened to cuboidal cells (Figure 31.18). Nuclei are bland, with a low nuclear grade (Figure 31.19). Mitoses are rare, usually under 1, but may be as high as 3, mitotic figures per 10 HPFs. The diagnosis should be made with caution, as malignant mesotheliomas may have foci that, viewed in isolation, resemble this tumor.⁵⁶ These lesions can usually be reliably distinguished from serous epithelial tumors, since the architecture of the latter discloses feathery irregular clusters of cells in which the nuclei are far more atypical and higher grade. Psammoma bodies may be encountered in rare cases. These tumors are nearly always benign, but rare tumors have acted aggressively.^57,58

Diffuse Malignant Mesothelioma

Peritoneal diffuse malignant mesotheliomas are much less common than their pleural counterparts, accounting for about 10% of all malignant mesotheliomas.⁵⁹ Only one-third of these tumors occur in middle-aged or postmenopausal women and they must be distinguished from the more prevalent serous adenocarcinomas, including those arising from the peritoneum itself and those metastatic from an ovarian or fallopian tube primary. The survival rate for women with malignant mesothelioma is worse than that for women with serous adenocarcinoma, and the treatment of the two diseases currently differs.

Clinical manifestations usually are nonspecific and include ascites, abdominal discomfort, digestive disturbances, and weight loss. Ascites is present in most cases, and cytologic examination of the ascitic fluid may be diagnostic in some cases. The diagnosis, however, usually requires laparotomy or laparoscopy and biopsy. While most malignant mesotheliomas are highly aggressive, some peritoneal malignant mesotheliomas pursue a more indolent course.⁶⁰ It is generally stated in the literature that asbestos exposure is uncommon in women with peritoneal mesothelioma. In one (2003) population-based study of peritoneal malignant mesotheliomas, 29% of 96 men had asbestos-related jobs whereas none of 113 women had occupational or environmental risk factors.⁶¹ In fact, men with peritoneal mesotheliomas typically have had a heavier burden and more prolonged exposure to asbestos than men with pleural mesotheliomas. Most males with peritoneal malignant mesotheliomas reported in the literature survived less than 2 years after diagnosis, although there have been occasional long-term survivors. A study of peritoneal malignant mesotheliomas in women,⁶⁰ however, found that 40% of the patients survived longer than 4 years. The histopathologic subtype (see later) is of prognostic significance, as biphasic peritoneal malignant mesotheliomas are associated with a much shorter survival than pure epithelial tumors⁶² and deciduoid mesotheliomas are usually rapidly fatal.^63,64 Increasing nuclear and nucleolar size has been shown to correlate with shorter survival in epithelial tumors.⁶² Also, p16 loss independently correlates with increased risk of death according to one study,⁶⁵ while another failed to identify any morphologic features that differentiated those cases with a highly aggressive course from indolent ones.⁶⁶ Two studies have identified a number of favorable prognostic factors including an age less than 60 years, low nuclear grade, low mitotic index, minimal residual disease after cytoreduction, and lack of deep invasion.^67,68

Intra-Abdominal Desmoplastic, Small Round Cell Tumor

Desmoplastic small, round cell tumor is the descriptive designation for a rare, undifferentiated, and highly aggressive tumor that, with few exceptions, involves the peritoneal serosa⁷⁸ and contiguous organs such as the kidney and ovary.⁷⁹ It usually appears during adolescence and early adulthood, with a mean age of 25 years, but occasionally may be found in older women.^80,81 It is far more common in men than in women. The prognosis is poor.

Grossly, most tumors are bulky abdominal masses that have spread diffusely over the peritoneal surface with prominent involvement of the tunica vaginalis or the ovaries, mimicking a primary testicular or ovarian tumor.⁸² The characteristic microscopic pattern is nests of ‘small, blue cells’ embedded in a desmoplastic fibrous stroma (Figure 31.28). The tumor cells are uniform with scanty cytoplasm and indistinct cell borders (Figure 31.29). About one-third of tumors exhibit a wider range of morphologic features, principally as spindle-shaped cells with epithelioid to focally sarcomatoid arrangements. Mitotic figures are numerous and foci of necrosis are usually present. Invasion of vascular spaces is common but lymph node involvement is rare.

Virtually all tumors are CK positive (CK monoclonal antibodies CAM 5.2, AE1/AE3), but lack CK20 expression, indicative that they are not of large intestinal origin.66,83,84 Roughly four-fifths are also reactive with antibodies to EMA, neuron-specific enolase, desmin (with paranuclear dot-like reactivity) (Figure 31.30), and vimentin, suggestive that there is both epithelial and mesenchymal (divergent) differentiation. Between two-fifths and two-thirds of tumors express Ber-EP4, CD57 (Leu-7), CD15 (Leu-M1), and CA125, suggestive that the tumor is not mesothelial in origin. Wilms’ tumor (WT1) protein is detected immunohistochemically in 90% of cases.⁸⁵ Most other common immunohistochemical stains lack reactivity in most cases. Electron microscopic examination shows the tumor cells have mesenchymal–fibroblastic features.

Coexpression of epithelial and mesenchymal antigens distinguishes the desmoplastic small, round cell tumor from other small round and blue cell tumors occurring in this age group. These antigenic properties have challenged the popular notion that the intra-abdominal desmoplastic small round cell tumor is a ‘blastomatous’ tumor derived exclusively from the primitive mesothelium.

Desmoplastic small round cell tumor exhibits a reciprocal translocation t(11;22)(p13;q12), resulting in fusion of the EWS1 gene on chromosome 22 and the Wilms’ tumor suppressor gene (WT1) on chromosome 11, which appears to be unique for this tumor.⁸⁶ The translocation results in a loss of three specific amino acids⁸⁷ and appears to have an oncogenic effect. Other translocation patterns have also been described.^86,88 The fusion protein that is produced seems to function as a potent activator of transcription, suggesting that the Wilms’ tumor gene gains function as a result of the fusion. Thus, the fusion gene seems to function as a dominant oncogene in this disease.⁸⁹

After initial treatment (aggressive surgical debulking and postoperative chemotherapy, external beam radiotherapy, or both), there may be an initial response, but more than 99% of patients die of tumor progression.

Solitary Fibrous Tumor of Peritoneum (‘Fibrous Mesothelioma’)

Solitary fibrous tumors, previously called ‘fibrous mesotheliomas,’ are primitive tumors composed of fibroblasts and primitive mesenchymal cells that can manifest multidirectional differentiation. They are rare tumors found most often in the pleura,⁹⁰ but do occur occasionally in the peritoneum.^91,92 Most patients remain well after tumor excision, although occasional neoplasms have acted aggressively.

Grossly, the tumors vary in size from 1 cm to over 20 cm in diameter (Figure 31.31). They are usually solitary and appear encapsulated by fibrous tissue. Microscopically, tumors are composed of spindle cells in a markedly collagenized stroma, often with abundant blood vessels, in a hemangiopericytoma-like pattern (Figure 31.32). Tumor cells may have fascicular, cord-like, and irregular arrangements and are found interspersed in strands in between thick collagen bundles. Nuclei are often vesicular and the nucleoli inconspicuous. Mitoses are rare.

Tumor cells are reactive for vimentin but not for CK. CD34, a sialylated transmembrane glycoprotein found initially in endothelial cells and myeloid progenitor cells, is usually demonstrable.⁹⁰ CD31, a platelet endothelial cell adhesion molecule, is not. In contrast, desmoplastic mesotheliomas, tumors in the differential diagnosis, are reactive for CK but not for CD34.⁹³

Other Tumors

A variety of tumors arise rarely in the peritoneum. Their histogenesis is not always certain. Of the less rare tumors, adenosarcomas are often associated with endometriosis.⁹⁷ Carcinosarcomas have also been described,⁹⁸ as have the stromal sarcomas⁹⁹ and rhabdomyosarcomas.¹⁰⁰ Pure epithelial tumors, such as clear cell adenocarcinoma, have been described, also in association with endometriosis.⁹⁷ Any tumor usually associated with an endometrial origin could well have arisen in extrauterine endometriosis. This subject is discussed more fully in Chapter 22.

Pseudomyxoma Peritonei

Pseudomyxoma peritonei is a clinical term that refers to the accumulation of jelly-like mucus in the pelvis or abdominal cavity (‘gelatinous ascites’) resulting from peritoneal spread of a low-grade mucinous tumor, usually of the appendix and less commonly of other intestinal locations.¹⁰¹ Unilateral or bilateral ovarian involvement is common in these cases (Figure 31.33). The ovarian and appendiceal tumors may present simultaneously or metachronously.

Gliomatosis Peritonei

Gliomatosis peritonei is a rare condition in which peritoneal implants composed largely or exclusively of fully mature glial tissue are found in the abdominal cavity, usually in association with a solid ovarian teratoma,120–122 which can be mature or immature.¹²³ Tears in the capsule of the ovarian tumor have been identified, suggesting a mechanism by which the gliomatous tissue leaks into the abdominal cavity (Figure 31.44). Nevertheless, a molecular genetic study has suggested that glial implants may also arise by metaplasia of pluripotent peritoneal stem cells.¹²⁴

At the time of laparotomy, either when the ovarian tumor is discovered or subsequently, implants in the peritoneum are found to be composed of glial tissue only (Figures 31.44 and 31.45). Occasionally, other teratomatous elements are identified. Microscopic implants should be graded separately from the ovarian tumor, and this will determine whether subsequent therapy is needed. Usually, the implants are grade 0 or 1 (Figure 31.45). In some cases, they are grade 2 or 3. Although most patients with this condition do well, recurrences have been recorded¹²⁵ as well as subsequent malignant transformation.¹²⁶ This condition is described more fully in the section on ovarian teratomas in Chapter 29. Gliomatosis peritonei has also been reported in a patient with a ventriculoperitoneal shunt.¹²⁷

Strumosis Peritonei

Strumosis peritonei is a rare condition in which nodules found singly or throughout the omentum are composed largely of well-differentiated thyroid tissue. The lesion most likely represents a metastatic or implanted form of malignant struma ovarii. Most cases occur in association with a solid ovarian teratoma or a struma ovarii. Nodules may be several millimeters to centimeters in size and grossly, on cut section, resemble colloid (Figure 31.46). Microscopically, the thyroid tissue may resemble a macrofollicular adenoma (Figure 31.47). Like gliomatosis peritonei, a defective capsule has been found in most cases with ‘implants’ from the ovary, suggesting a mechanism of spread into the abdominal cavity. While some patients with the condition do well, recurrence is unpredictable and some patients have had a clinically more aggressive course. This condition is described more fully on ovarian teratomas in Chapter 29.

Endometriosis

Endometriosis is a disease principally involving the peritoneal cavity (see Chapter 22).

Peritoneal Serous Lesions

Serous lesions of the peritoneum include those that are non-neoplastic (endosalpingiosis) and neoplastic, which are morphologically analogous to their ovarian counterparts.

Endosalpingiosis

Endosalpingiosis is the presence of benign glands lined by tubal-type epithelium involving the peritoneum, subperitoneal tissues, ovarian surface, and retroperitoneal lymph nodes. This disorder occurs almost exclusively in females, typically during their reproductive years, although occasional cases have been described in postmenopausal women. Endosalpingiosis is almost always an incidental finding at laparotomy and some cases accompany endometriosis in laparoscopic biopsies of women being investigated for infertility.¹²⁸

An origin from the secondary müllerian system is favored by most investigators, but the association of endosalpingiosis with chronic salpingitis implicates implantation of sloughed tubal epithelium as a possible histogenetic mechanism in some cases.¹²⁹ Similarly, its association with serous borderline tumors suggests that some foci of endosalpingiosis may represent tumor implants that have undergone maturation.^130,131

At the time of second-look laparotomy, endosalpingiosis in the absence of residual tumor does not justify additional treatment.¹³²

Endosalpingiosis, if seen macroscopically, may appear as a focal granularity, with a single to few tiny bumps or cysts. Microscopically, it appears as multiple, simple glands, lined by a single layer of tubal-like epithelium (Figure 31.49) exhibiting pale ciliated cells, secretory cells, and intercalated or ‘peg’ cells; i.e., the three cell types of the normal epithelium of the fallopian tube. Nuclei are basally situated, mitotic activity is absent, and there is no nuclear atypia. Psammoma bodies are often present within the glandular lumens or in the surrounding stroma. If the epithelium is destroyed, the psammoma bodies may be found free in the stroma. The glands are surrounded by PAS-positive basement membranes and show PAS-positive, diastase-resistant material in their lumens. Immunoreactions for estrogen and progesterone receptors are usually positive.

The glands of endosalpingiosis may show irregular contours, crowding, and intraluminal stromal papillae. Occasionally, the process may be so extensive as to mimic a neoplasia and the glands may be larger, exhibiting architectural complexity; however, the cells lack significant nuclear atypia (Figure 31.50).

The term atypical endosalpingiosis refers to lesions in which there is cellular stratification, including cellular buds, cribriform patterns, and varying degrees of cellular atypia, occurring in the absence of a serous borderline tumor. Histologically, such lesions merge with peritoneal serous borderline tumors (see Chapter 25).¹³³ Not uncommonly, the serous foci exhibit more epithelial proliferation than would be expected in benign lesions. The criteria used for assessing whether the excessive growth reflects neoplasms of borderline malignancy are identical to those for serous tumors in general and are summarized in Table 31.3. Rare extraovarian borderline and malignant serous tumors have been shown to arise from endosalpingiosis.^65,134

Endosalpingiotic glands should be distinguished from mesonephric remnants, which are commonly found in the region of the fallopian tube. Mesonephric tubules are typically lined by a single layer of nonciliated, low columnar to cuboidal cells.

Serous Tumors (Primary and Metastatic)

Endocervicosis

Benign glands of endocervical type involving the peritoneum have been described as ‘endocervicosis.’ Nearly all reported cases involve the peritoneum overlying the posterior uterine serosa, cul-de-sac, vaginal apex, outer wall of the uterine cervix, and the urinary bladder.143,144 Microscopically, the glands are located predominantly within the smooth muscle of the muscularis propria and mimic invasive well-differentiated adenocarcinoma. The presence of mild epithelial atypia and reactive periglandular stroma contributes to this misdiagnosis. However, the absence of a mucosal-based tumor and severe nuclear atypia facilitate the diagnosis of endocervicosis.^143,145

Peritoneal Endometrioid, Clear Cell, and Transitional Cell Lesions

Occasional peritoneal cysts lined exclusively by endometrioid epithelium may occur, but it is often unclear whether these may represent endometriosis in which the stroma is absent or atrophic. Rare cases with clear cells, usually in the form of clear cell adenocarcinoma, have also been described and most likely represent clear cell adenocarcinomas arising from peritoneal endometriosis. Walthard rests, which are rests of transitional (urothelial-like) epithelium, are common occurrences on the tubal serosa, particularly the superior and posterior surfaces, and throughout the broad ligament.

Deciduosis

Deciduosis, a form of ‘stromal müllerianosis’ disclosing foci of ectopic decidualized stromal cells, is commonly observed in association with pregnancy. Groups of decidual cells, indistinguishable from those found in gestational endometrium, occasionally appear in the pelvic and lower abdominal peritoneum, beneath the ovarian surface epithelium, and in the plicae of the fallopian tubes. It is also rarely seen in lymph nodes. Patients with trophoblastic disease also regularly exhibit deciduosis, mediated, as in a normal pregnancy, by high plasma levels of chorionic gonadotropins. Ectopic pseudodecidual reactions also occur with exogenous progesterone administration. These changes occur rarely in the absence of such histories and have mostly been identified in premenopausal women. Most lesions are incidental findings.

If seen grossly, the nodules are tan to pale brown, slightly gelatinous, and rarely more than several millimeters. Occasionally, the lesions may reach several centimeters in size and consist of multiple, tiny, soft nodules separated by thin, white, rubbery septa.¹⁴⁶

In many locations, such as in fallopian tube plicae or adjacent to a corpus luteum, the decidua-like cells are pure. In the peritoneum, the decidualized cells are often admixed with smooth muscle cells, giving rise to the condition ‘disseminated peritoneal leiomyomatosis,’ also known as ‘leiomyomatosis peritonealis disseminata.’ Although the presence of the decidua in such cases has been used to argue that this condition develops by differentiation of the subcelomic mesenchyme with the formation of multiple tumor-like nodules (see later), there is no consensus that this is the true mechanism.

The decidual reaction most often appears as solid clusters of cohesive, decidualized cells with sharp cell borders (Figure 31.53). Cytoplasm may be abundant and glass like or may show some degrees of degeneration (Figure 31.54). Some cells have a clear vacuolated cytoplasm that superficially resembles the soap bubble-like physaliferous cell of chordoma (Figure 31.54). Some nodules may also show transitions between a more solid and a more myxoid pattern of decidual reaction (Figure 31.55).

Disseminated Peritoneal Leiomyomatosis

Disseminated peritoneal leiomyomatosis is a rare condition characterized by widespread nodules of benign smooth muscle on the peritoneal surface of the pelvis and abdomen in women of reproductive age (see Chapter 19). Most patients have uterine leiomyomas at the time of diagnosis.^147–149 The most common presentation is as an incidental finding at the time of caesarean section. The intraoperative appearance is so alarming that frozen section examination is often requested to rule out peritoneal carcinomatosis. The peritoneal myomatous nodules develop on a background of an altered hormonal milieu, such as with pregnancy, oral contraceptive use,¹⁵⁰ hormonal therapy, or steroid-producing ovarian tumors.¹⁵¹ While this lesion is often classified as a tumor-like condition, since it commonly regresses, the tumors in some cases persist, suggesting a neoplastic process. On rare occasions, some have shown malignant transformation.^152–154

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