Updates in non-neoplastic orthopaedic pathology: what you don't know can hurt you!

Even though the average surgical pathologist reviews far more non-neoplastic orthopaedic pathology on a daily basis, most current research focuses on rare tumours and their even less frequent molecular events. Our experiences among consults and focused conferences strongly suggest that there remains a practice gap regarding knowledge and diagnosing specific non-neoplastic orthopaedic conditions. One of the most frequent intraoperative consultations performed in the USA, among both academic and private institutions, relates to revision arthroplasty and the determination of infection in periprosthetic joints. Pathologists play a critical role in this algorithm, helping determine intraoperatively whether patients require antibiotic spacers prior to reimplantation. Many pathology departments have abandoned the examination of arthroplasty specimens because they (and their surgeons) mistakenly believe there is little clinically relevant information to be gained by thorough pathological examination. However, recent literature has challenged this concept, emphasising the importance of distinguishing avascular necrosis (from osteoarthritis/degenerative joint disease with secondary osteonecrosis), subchondral insufficiency fracture, septic arthritis (from so-called 'sterile' osteomyelitis/pseudoabscesses), underlying crystalline diseases and incidental/occult neoplasia. Histological evaluation of historically insignificant orthopaedic specimens, such as tenosynovium from carpal tunnel syndrome/trigger finger, is now seen as valuable in early diagnosis of cardiac amyloidosis. Not infrequently, orthopaedic conditions like haemosiderotic synovitis, osteocartilaginous loose bodies or rheumatoid nodules, may histologically mimic bona fide neoplasms, notably diffuse tenosynovial giant cell tumour, synovial chondromatosis and epithelioid sarcoma, respectively. Here is a review of the more common non-neoplastic orthopaedic conditions, those likely to be examined by the practising surgical pathologist, with updates and guidelines for establishing clinically relevant diagnoses.

DICER1-sarcomas of GYN tract: Expanding on an emerging entity.

Tumors with pathogenic DICER1 mutation are rare and encompass sporadic or hereditary benign, intermediate and malignant tumors. DICER1-associated sarcomas are heterogeneous; however, the prototypical ones in the GYN-tract include embryonal rhabdomyosarcoma, adenosarcoma and moderately to poorly differentiated Sertoli-Leydig tumor. In this report, we present three unique uterine sarcomas with DICER1 mutation and remarkable diffuse round/spindle cell morphology. The tumors occurred in cervix (n = 1), and uterine corpus (n = 2). The patient ages were 30, 37 and 59 years with tumor size of 8.8, 10 and 8.6 cm, respectively. Morphologically all three tumors were characterized by distinct spindle/round cell morphology and various amounts of neuroectodermal differentiation (yolk sac-like tubules, blastomatous areas and rosette formation). Other morphologic features of DICER1-sarcoma reported in the literature including cambium layer, focal or diffuse anaplasia, solid and cystic architecture, and chondroid/osteoid areas were absent. All three sarcomas were positive for SALL4 and had variable neuroendocrine marker expression. Whole genome methylation analysis was performed on one of the uterine sarcomas, which clustered the tumor with embryonal tumor with multilayered rosettes. Follow up information was available on all three cases. Two patients were alive with no evidence of disease 13 and 14 months post operation, while one patient had imaging evidence of local recurrence 4 months post operation. In summary, we describe three unique DICER1-sarcomas and expand the phenotypic spectrum of this emerging entity, particularly with GYN-tract origin.

Back to the future! Selected bone and soft tissue neoplasms with shared genetic alterations but differing morphological and immunohistochemical phenotypes.

Bone and soft tissue tumors (BST) are a highly heterogeneous group largely classified by their line of differentiation, based on their resemblance to their normal counterpart in adult tissue. Yet, rendering a specific diagnosis can be challenging, primarily due to their rarity and overlapping histopathologic features or clinical presentations. Over the past few decades, seemingly histogenetic-specific gene fusions/translocations and amplifications have been discovered, aiding in a more nuanced classification, leading to well-established objective diagnostic criteria and the development of specific surrogate ancillary tests targeting these genetic aberrations (e.g., immunohistochemistry). Ironically, the same research also has revealed that some specific tumor subtypes may be the result of differing and often multiple gene fusions/translocations, but, more interestingly, identical gene fusions may be present in more than one phenotypically and biologically distinct neoplasm, sometimes with entirely different clinical behavior. Prime examples include, EWSR1::ATF1 and, less commonly, EWSR1::CREB1 gene fusions present in both clear cell sarcoma, a malignant high-grade tumor with melanocytic differentiation, and angiomatoid fibrous histiocytoma, a mesenchymal neoplasm of intermediate malignancy with a generally indolent course. Similarly, MDM2 amplification, once deemed to be pathognomonic for atypical lipomatous tumor/well differentiated and dedifferentiated liposarcoma, has been documented in a range of additional distinct tumors, including low grade osteosarcomas (e.g. low grade central and surface parosteal) and high-grade intimal sarcomas, amongst others. Such findings reinforce the importance of careful attention to morphological and clinicoradiological features and correlation with molecular testing before rendering a specific diagnosis. Future classification systems in BST neoplasms cannot be solely based on molecular events and ideally will balance morphologic features with molecular analysis. Herein, we provide a narrative literature review of the more common BST neoplasms with shared genetic events but differing demographics, morphology, immunophenotype, and clinical behavior, re-emphasizing the importance of the hematoxylin and eosin slide and the "eye" of the practicing pathologist.

Vascular Neoplasms With NFATC1/C2 Gene Alterations : Expanding the Clinicopathologic and Molecular Characteristics of a Distinct Entity.

Despite significant advances in their molecular pathogenesis, skeletal vascular tumors remain diagnostically challenging due to their aggressive radiologic appearance and significant morphologic overlap. Within the epithelioid category and at the benign end of the spectrum, recurrent FOS/FOSB fusions have defined most epithelioid hemangiomas, distinguishing them from epithelioid hemangioendothelioma and angiosarcoma. More recently, the presence of EWSR1/FUS :: NFATC1/2 fusions emerged as the genetic hallmark of a novel group of unusual vascular proliferations, often displaying epithelioid morphology, with alternating vasoformative and solid growth, variable atypia, reminiscent of composite hemangioendothelioma. In this study, we further our understanding and morphologic spectrum of NFATC -fusion positive vascular neoplasms by describing 9 new cases, including soft tissue locations and novel fusion partners. Combining with the initial cohort of 5 cases, a total of 14 patients were analyzed, showing slight female predilection and an age range of 10 to 66 (mean 42 y). Twelve patients had solitary lesions, while 2 had multifocal polyostotic (pelvic bones) disease. Overall, 12 lesions were intra-osseous and 2 in soft tissue. By targeted RNA Fusion panels or FISH, there were 6 cases of EWSR1::NFATC1 , 4 EWSR1::NFATC2 , 2 FUS::NFATC2 , 1 EWSR1 rearrangement, and 1 with a novel FABP4::NFATC2 fusion. Follow-up was available in 4 patients. One patient experienced 2 local recurrences, 11 and 15 years postdiagnosis, and one patient experienced progressive disease despite multimodality treatment (curettings, embolization, radiation) over 3 years. In summary, our extended investigation confirms that NFATC -related fusions define a distinct group of vascular neoplasms with variable architecture, epithelioid phenotype, and cytologic atypia, commonly located in the bone, occasionally multifocal and with potential for local recurrence and aggressive behavior but no metastatic potential. Molecular analysis is recommended in diagnostically challenging cases with atypical histology to exclude malignancy.

Malignant female genital tract smooth muscle tumors with adipocytic differentiation: A morphologic, immunohistochemical, MDM2 fluorescence in situ hybridization and molecular genetic study of 6 lipoleiomyosarcomas.

Leiomyosarcoma with adipocytic differentiation or lipoleiomyosarcoma is an uncommon sarcoma of the female genital tract with only a few individual reports in the literature. We therefore performed a morphologic, immunohistochemical, MDM2 gene amplification and RNA and DNA sequencing analysis of a series of gynecologic lipoleiomyosarcoma to better define the clinicopathologic spectrum. Six tumors from 6 patients were identified and classified as spindled lipoleiomyosarcoma (n = 2), mixed spindled and myxoid lipoleiomyosarcoma (n = 1), epithelioid lipoleiomyosarcoma with focal myxoid features (n = 1) and mixed spindled and epithelioid lipoleiomyosarcoma (n = 2). Patient age ranged from 41 to 64 years (mean: 49; median: 50). Primary location included uterine corpus (3), uterine corpus/cervix (2) and broad ligament (1). Tumor size ranged from 4.5 to 22 cm (mean: 11.2; median: 9.8). Four patients had metastasis at presentation or subsequently developed recurrent or distant disease. Patient status was known for 5: 2 dead of disease, 2 alive with disease and 1 alive without evidence of disease. Immunohistochemical expression of smooth muscle markers, ER, PR and WT-1 showed patterns similar to non-adipocytic gynecologic leiomyosarcomas. MDM2 amplification fluorescence in situ hybridization performed on 2 tumors was negative in 1 and equivocal in 1. Sequencing studies performed on 3 tumors found TP53 mutations in 3, with 1 tumor also having an ATRX alteration. No gene fusions were identified. Although lipoleiomyosarcomas have a diverse morphologic spectrum, our findings suggest the smooth muscle component shares morphologic and immunohistochemical features with female genital tract non-adipocytic leiomyosarcomas. Lipoleiomyosarcomas also have genetic alterations associated with non-adipocytic gynecologic leiomyosarcomas.

MDM2 Amplification Status in a Cohort of Well-Characterized Myxofibrosarcoma: A Clinicopathologic Analysis of 22 Tumors.

Myxofibrosarcomas (MFS) present as slowly enlarging superficial masses in elderly patients. Even though these tumors fail to exhibit a distinct immunophenotype, diagnosis is straightforward when they present in subcutaneous tissue. Intramuscular MFS, however, are more challenging to diagnose as the differential also includes dedifferentiated liposarcoma with myxoid features. The vast majority of dedifferentiated liposarcomas show MDM2 amplification, whereas limited data exists as to the MDM2 status of MFS. We sought to explore the rate of MDM2 amplification in cases of classic MFS. Our archives were searched for MFS; only subcutaneous well-sampled resections were included. FISH for MDM2 amplification was performed on each tumor. A cohort of myxoid dedifferentiated liposarcoma resections was studied for comparison. Twenty-two MFS arose in patients aged 44 to 85 years. All tumors contained an infiltrative population of atypical cells embedded in a myxoid stroma with curvilinear blood vessels. MDM2 amplification by FISH was identified in 3 (of 22; 14%) tumors. Available follow up on 17 patients (range 1-96 months; median 13 months) revealed 6 patients with local recurrence and 1 with distant metastasis. Of 3 patients with MDM2- amplified MFS, 1 experienced recurrence and died of unrelated causes, while the second was alive without disease 12 months after diagnosis. Even though the rate of MDM2 amplification by FISH in MFS appears to be low, a subset of cases may show this genetic alteration, which pathologists should be aware of to avoid misclassification as myxoid dedifferentiated liposarcomas. Further studies are necessary to determine if amplification status adds prognostic value.

Prognostic factors for pleomorphic dermal sarcoma: analysis of 1911 cases from the SEER database.

Prognostic factors for pleomorphic dermal sarcoma, a rare undifferentiated neoplasm of the skin, are poorly defined, and typical staging systems do not appear to be appropriate for these neoplasms. We; therefore, sought to identify prognostic factors for disease-specific survival and predictors of metastasis.Pleomorphic dermal sarcomas were identified in the Surveillance, Epidemiology and End Results database (N=1911). Multiple imputation was used to overcome inherent limitations in this dataset to assess prognostic factors using multivariable Cox proportional hazard stratified by (neo)adjuvant radiotherapy and logistic regression for presentation with metastasis.Age, tumour size and metastasis were independent prognostic factors for cutaneous sarcoma-specific survival. Only tumour size was associated with increased odds of presentation with metastasis, with tumours >4 cm at highest risk. Metastasis is the most important factor in determining outcomes, with age and size as lesser factors. Only tumour size is predictive of metastasis, with larger tumours at highest risk.

Plexiform fibrohistiocytic tumor: a clinicopathological and immunohistochemical study of 39 tumors, with evidence for a CSF1-producing "null cell" population.

Plexiform fibrohistiocytic tumor (PFHT) is a mesenchymal tumor of intermediate malignancy, typically occurring in the superficial soft tissues of young patients and displaying a biphasic pattern, with nodules of histiocytoid cells surrounded by fascicles of myofibroblastic spindled cells. The pathogenesis of PHFT is unknown. We comprehensively studied 39 PFHT, occurring in 25 females (66%) and 13 males (34%), ranging from 2 to 55 years of age (median 21 years). The tumors most often occurred in the upper extremity (n = 16, 41%) and ranged from 0.4 to 6.1 cm in size (median 1.5 cm). One patient with known neurofibromatosis type 1 presented with metachronous tumors of the finger and back. Clinical follow-up (29 patients; range 5-168 months; median 60 months) showed 3 tumors to have recurred locally; none was metastasized. One patient died of an unrelated cause; all others were alive without disease at the time of last follow-up. Immunohistochemistry showed the histiocytoid nodules of all cases to contain CD163/CD11c-positive histiocytes and cells negative for both markers ("null cells"). CSF1 expression was present in "null cells" in 7/10 cases (RNAscope chromogenic in situ hybridization). The Ki-67 labeling index was very low (< 5%); Ki-67-positive cells within histiocytoid nodules appeared to represent "null cells." All tested cases were negative for significant mutations or fusion events (TruSight Mutation Panel, TruSight Fusion Panel, Mayo Clinic Melanoma Targeted Gene Panel). We conclude that PHFT may be even more indolent than has been appreciated, although classification as an "intermediate" tumor is correct. We hypothesize that the CSF1-producing "null cells" of PHFT may represent the neoplastic element, with the bulk of the tumor masses comprising recruited and reactive cell populations.

A novel WWTR1::AFF2 fusion in an intra-abdominal soft tissue sarcoma with associated endometriosis.

Application of molecular testing in clinical practice has led to significant advances in the classification of soft tissue sarcomas. Despite remarkable progress, there are still challenging cases that remain unclassified. In this study, we present an unusual spindle cell sarcoma arising in the abdominal cavity of a 37-year-old female. An extensive panel of immunostains was nonspecific for a line of differentiation and the tumor was subjected to targeted RNA sequencing for further classification. The findings showed a novel WWTR1::AFF2 fusion, which was further confirmed by break-apart FISH analysis for WWTR1 gene rearrangement. The tumor was attached to the wall of sigmoid colon and showed a highly cellular proliferation of plump spindle to epithelioid cells arranged in intersecting fascicles. Areas of extensive endometriosis were identified adjacent to the tumor. The immunoprofile was significant for reactivity with desmin, calponin, WT-1, ER, and PR, while negative for CD10, SMA, caldesmon, pan-keratin, ALK, CD117, and S100. The patient is alive and well after 11 months of follow-up. The exact histogenesis of this sarcoma remains unclear, however, the presence of adjacent endometriosis and coexpression of WT1/ER/PR raises the possibility of an unusual endometrioid stromal sarcoma, occurring outside the GYN tract. Additional cases are needed to establish the recurrent potential of this fusion event and to better define its pathogenesis and clinical behavior.

A unique epithelioid vascular neoplasm of bone characterized by EWSR1/FUS-NFATC1/2 fusions.

An increasing number of epithelioid vascular lesions, in particular tumors from the benign and low-grade end of the spectrum, have been characterized by recurrent gene fusions. As a result, the detection of these molecular markers have improved the classification of diagnostically challenging cases. However, despite the significant progress, there are occasional lesions that do not fit in known histologic or molecular groups. Herein, we present five such unclassified epithelioid vascular lesions, which occurred in the bone and showed a distinct morphology composed of alternating vasoformative and solid growth and mild to moderate nuclear pleomorphism. The variegated morphologic appearance resembled that of composite hemangioendothelioma, being distinct from both epithelioid hemangioma and epithelioid hemangioendothelioma, and consistently showed cytologic atypia. Due to their unusual morphologic appearance and negative molecular work-up, targeted transcriptome sequencing was performed in two cases showing the presence of NFATC2 fusions with either EWSR1 or FUS genes. Three additional bone tumors with EWSR1 gene rearrangements were identified by FISH screening of a large cohort of 45 fusion-negative epithelioid vascular neoplasms, one fused to NFATC2 while two others to NFATC1. There were three females and two males, with a wide age range at presentation, mean of 44 years. The lesions occurred in the pelvis, maxillary sinus, and humerus. Two patients presented with polyostotic disease, both located in the pelvic bones. Two patients had available follow-up, one developed two local recurrences in the humerus over a 15-year period, while the other showed no recurrence 4 years subsequent to an en-bloc resection. Tumors were positive for CD31 and ERG, while negative for EMA, CK, synaptophysin, and chromogranin. FISH confirmed this abnormality in all cases, none of them being associated with gene amplifications. Further studies are needed to establish the pathogenetic relationship of this rare molecular subset with other epithelioid vascular tumors and to determine its clinical behavior.

Risk Assessment of Visceral Sarcomas: A Comparative Study of 2698 Cases from the SEER Database.

Soft tissue sarcomas arising in visceral organs are rare and lack validated tumor-staging protocols. Clinicopathologic features and clinical outcomes of 2698 visceral sarcomas identified in the Surveillance, Epidemiology, and End Results Program (SEER) database were compared with sarcomas arising in the extremities/trunk (n = 10,237) or retroperitoneum (n = 1067) using standard statistical techniques. Important prognostic criteria for visceral sarcomas, as in other anatomic sites, included tumor size, histologic grade, and presence of metastatic disease. After adjustment for pertinent confounding factors, visceral sarcomas showed cancer-specific survival rates similar to those arising in the retroperitoneum but had worse outcomes than sarcomas in the extremities/trunk. Therefore, the prognostic performance of two different staging algorithms for retroperitoneal sarcomas was evaluated for their use in staging sarcomas of visceral organs. The current AJCC 8th edition and the recently derived Vanderbilt system for staging retroperitoneal sarcoma both showed adequate discrimination, as assessed by multiple clinical concordance indices, and no evidence of miscalibration. Therefore, the authors concluded that previously validated staging systems for retroperitoneal sarcomas based on conventional prognostic factors (histologic grade, tumor size, and presence of metastatic disease) are applicable to visceral sarcomas and should be incorporated into the next edition of the AJCC Cancer Staging Manual.

A Proposed Staging System for Improved Prognostication of MDM2-amplified Liposarcoma.

Despite the release of anatomic site-specific staging systems for soft tissue sarcomas in the eighth edition of the American Joint Committee on Cancer (AJCC) Cancer Staging Manual, the algorithms for sarcomas arising in the extremities/trunk and retroperitoneum differ only in the staging of lymph node metastasis. The retroperitoneum not only provides a larger potential space for tumor growth before the clinical presentation, but its anatomic complexities complicate surgical resection and adversely affect disease-free survival. Here, we propose a new staging system for MDM2-amplified liposarcomas (well-differentiated and dedifferentiated subtypes) that properly emphasizes retroperitoneal localization, degree of differentiation (histologic subtype), and presence of distant metastasis. A retrospective cohort of 4146 adult patients with surgically resected liposarcoma was extracted from the SEER database to compare the natural history of MDM2-amplified liposarcomas arising in the extremities/trunk or retroperitoneum. Separate training and validation datasets were created, and Cox proportional hazard regression, multivariable nonlinear regression, and nomographic analyses determined the most significant parameters in predicting sarcoma-specific death. A new staging system was derived and its predictive accuracy was compared with the AJCC, eighth edition system using areas under receiver operating characteristic curves and multiple concordance indices. Multivariable analysis showed that dedifferentiation (hazard ratio [HR]=3.7±0.5; P<0.0005), retroperitoneal location (HR=3.2±0.5; P<0.0005), and distant metastasis (HR=2.4±0.6; P=0.002), but not categorized tumor size (pT category), had the largest effects on sarcoma-specific survival. A new staging system based on these predictive factors demonstrated better discrimination between tumor stages, higher concordance with clinical outcomes, and greater predictive accuracy than the AJCC eighth edition staging system (86±1% vs. 83±2%; P=0.005). Statistical analysis of a large national cohort failed to confirm that categorized tumor size is a useful criterion by which to stage MDM2-amplified liposarcoma. A simplified staging system based on anatomic location and dedifferentiation outperforms the current AJCC staging system. Anatomic localization and histologic grade, and not tumor size, should be included in any future liposarcoma-specific staging system.

Diagnoses and Difficulties in Mesenteric Pathology.

Mesenteric diseases are broadly separated into 2 groups: non-neoplastic and neoplastic. Common non-neoplastic mesenteric diseases include those involving the mesenteric vasculature and those of inflammatory processes. Mesenteric inflammatory processes can mimic a neoplastic process. Neoplastic diseases of the mesentery are rare. Generally, the morphology, behavior and diagnostic criteria for mesenteric tumors are similar to their soft tissue or organ-specific counterparts. Their recognition can be challenging because they sometimes are overlooked in differential diagnoses.

Leiomyoma with KAT6B-KANSL1 fusion: case report of a rapidly enlarging uterine mass in a postmenopausal woman.

BACKGROUND: Uterine leiomyomas, in contrast to sarcomas, tend to cease growth following menopause. In the setting of a rapidly enlarging uterine mass in a postmenopausal patient, clinical distinction of uterine leiomyoma from sarcoma is difficult and requires pathologic examination. CASE PRESENTATION: A 74-year-old woman presented with postmenopausal bleeding and acute blood loss requiring transfusion. She was found to have a rapidly enlarging uterine mass clinically suspicious for sarcoma. An abdominal hysterectomy and bilateral salpingo-oophorectomy were performed. A 15.5 cm partially necrotic intramural mass was identified in the uterine corpus. The tumor was classified as a cellular leiomyoma. RNA sequencing identified a KAT6B-KANSL1 fusion that was confirmed by RT-PCR and Sanger sequencing. After 6 months of follow-up, the patient remains asymptomatic without evidence of disease. CONCLUSION: Prior studies of uterine leiomyomas have identified KAT6B (previously MORF) rearrangements in uterine leiomyomas, but this case is the first to identify a KAT6B-KANSL1 gene fusion in a uterine leiomyoma. While alterations of MED12 and HMGA2 are most common in uterine leiomyomas, a range of other genetic pathways have been described. Our case contributes to the evolving molecular landscape of uterine leiomyomas.

Perinephric myxoid pseudotumor of fat: a distinctive pseudoneoplasm most often associated with non-neoplastic renal disease.

In 2009, Tanas et al reported unusual changes in the perinephric fat, mimicking well-differentiated liposarcoma. We report 11 perinephric masses showing similar changes but chiefly arising in patients with non-neoplastic renal disease. Tissue from 11 perinephric masses was retrieved, and immunohistochemistry for IgG/IgG4 and fluorescence in situ hybridization (FISH) for MDM2 amplification was performed. Clinical information was obtained. Cases occurred in 10 males and 1 female (43-84 years of age; median, 63.5 years). Ten patients presented with perinephric masses (size range, 2-28 cm), and one was an incidental finding. Four patients had bilateral or multiple masses. Underlying renal disease included diabetes mellitus (n = 3), end-stage kidney (n = 2), diabetes and end-stage kidney disease (n = 1), chronic pyelonephritis (n = 1), and non-invasive high-grade papillary urothelial carcinoma of the renal pelvis (n = 1). Three patients were not known to have renal disease. Most tumors were submitted as "well-differentiated liposarcoma." The masses consisted of mature fat, myxoid stroma, moderately variable spindled to stellate cells and a mixed inflammatory cell infiltrate. Enlarged, hyperchromatic stromal cells were absent. IgG4-positive plasma cells and MDM2 amplification were absent in all tested cases. Clinical follow-up (11 patients; range, 1-120 months; median, 24 months) showed absent or stable disease in 9 patients; 2 died of unrelated causes. This distinctive pseudoneoplasm usually occurs in association with non-neoplastic renal disease, although similar changes may be identified in the perinephric fat of patients with renal carcinoma. Morphologic evaluation and FISH for MDM2 amplification should allow its distinction from liposarcoma and other mimics.

Genetic and molecular reappraisal of spindle cell adamantinoma of bone reveals a small subset of misclassified intraosseous synovial sarcoma.

Adamantinoma represents a distinct group of bone tumors showing both mesenchymal and epithelial differentiation most commonly involving the tibial diaphysis. Most adamantinomas contain a fibro-osseous component and an epithelial component consisting of squamous or basaloid cells. Adamantinomas are considered malignant neoplasms requiring en bloc excision that frequently recur locally and can rarely metastasize. Rare adamantinomas show an epithelial component consisting predominantly of monomorphic spindle cells, which, combined with an epithelial immunophenotype, can mimic monophasic synovial sarcoma. Synovial sarcoma is very rare in bone. It is considered a high-grade sarcoma that typically necessitates chemotherapy. However, the relationship between spindle cell adamantinoma and intraosseous synovial sarcoma has not been investigated. The current study was prompted by identification of a presumed spindle cell adamantinoma of the tibia with diffuse keratin expression that harbored a SS18 gene region rearrangement. FISH of eight additional bone tumors initially classified as spindle cell adamantinoma based on clinicoradiopathologic findings revealed one additional case with SS18 rearrangement. Histologically, both intraosseous synovial sarcoma and spindle cell adamantinoma demonstrated uniform fusiform nuclei with scant cytoplasm, short fascicles and low mitotic activity. The adamantinomas, but not the synovial sarcomas, were more likely to show overt epithelial differentiation in the form of pseudoglands or squamous nests. Immunohistochemistry of all cases, irrespective of SS18 status, showed diffuse keratin positivity in the spindle cell component, and less consistent EMA positivity. Clinical follow-up was available in both intraosseous synovial sarcomas, one of which recurred and the other metastasized. Two of the six spindle cell adamantinomas with follow-up metastasized. The above findings highlight the morphologic and immunophenotypic overlap between spindle cell adamantinoma and intraosseous synovial sarcoma of the tibia. Investigation of SS18 status to exclude synovial sarcoma is suggested prior to rendering a diagnosis of spindle cell adamantinoma.

High-grade squamous cell carcinoma arising in a tibial adamantinoma.

Adamantinoma of the long bones is a rare, typically low-grade malignant tumor that frequently involves the tibia. Radiographically, adamantinoma is characteristically a lytic, intracortical, and expansile lesion with variable margins. Histologically, adamantinoma is a bimorphic neoplasm, composed of epithelial and osteofibrous elements. Herein, we describe a 72-year-old man with a long-standing tibial mass that, on imaging, rapidly developed cortical destruction with soft tissue extension. Imaging revealed no evidence of a distant site of origin. Needle core biopsy demonstrated high-grade squamous cell carcinoma, and metastasis was initially favored. However, the combined clinicoradiologic and pathologic features were most compatible with a high-grade squamous cell carcinoma arising in adamantinoma. The diagnosis was confirmed in the resection specimen. Both the age at presentation and histologic features make this case unusual and highlight a potential for misdiagnosis in the evaluation of squamous cell carcinoma-containing lesions of the tibia, reinforcing the importance of clinicoradiologic correlation in bone pathology.