Histiocyte-rich rhabdomyoblastic tumor: rhabdomyosarcoma, rhabdomyoma, or rhabdomyoblastic tumor of uncertain malignant potential? A histologically distinctive rhabdomyoblastic tumor in search of a place in the classification of skeletal muscle neoplasms

Fetal rhabdomyoma in a Thoroughbred filly

A newborn Thoroughbred filly presented with a large mass located on the ventro-rostral aspect of the mandible. The mass was surgically removed under general anaesthesia. Upon gross examination, the mass was well circumscribed with a heterogeneous lobulated appearance. The results of histopathology and immunohistochemistry were consistent with a diagnosis of congenital fetal rhabdomyoma. On follow-up at six months post-surgical excision, no abnormalities were noted on clinical examination.

Inflammatory rhabdomyoblastic tumor, pheochromocytoma, and pulmonary hamartoma in a patient with neurofibromatosis type 1: a case report

BACKGROUND

Inflammatory rhabdomyoblastic tumors are relatively recently recognized soft tissue tumors with a low malignant potential. Here, we present a case of concurrent inflammatory rhabdomyoblastic tumor (IRMT), adrenal pheochromocytoma, and pulmonary hamartoma in a patient with neurofibromatosis type 1 (NF1). To our knowledge, this is the first time that this constellation of tumors has been described in the literature.

CASE PRESENTATION

A female patient in her late 20s with known NF1 was diagnosed with an inflammatory rhabdomyoblastic tumor, pheochromocytoma, and pulmonary hamartoma in a short succession. IRMT was found to harbor a near-haploid genome and displayed a typical immunohistochemical profile as well as a focal aberrant p53 expression pattern.

CONCLUSIONS

This case report strengthens the theory that defects in the tumor suppressor NF1 play a central role in the pathogenesis of inflammatory rhabdomyoblastic tumors and that IRMT may be part of the spectrum of neurofibromatosis type 1 related tumors.

Myxoid Pleomorphic Liposarcoma

Myxoid pleomorphic liposarcoma (MPLPS) shows a strong predilection for the mediastinum and can affect a wide age range. Clinically, MPLPS exhibits aggressive behavior and demonstrates a worse overall and progression-free survival than myxoid/round cell liposarcoma (MRLPS) and pleomorphic liposarcoma (PLPS). Histologically, MPLPS is characterized by hybrid morphologic features of MRLPS and PLPS, including myxoid stroma, chicken wire-like vasculature, univacuolated and multivacuolated lipoblasts, and high-grade pleomorphic sarcomatous components. In terms of molecular features, MPLPS is distinct from other lipomatous tumors as it harbors genome-wide loss of heterozygosity.

Inflammatory Rhabdomyoblastic Tumor: From a Nebulous Smooth Muscle Neoplasm to a Novel Skeletal Muscle Tumor Subtype

Inflammatory rhabdomyoblastic tumor is a recently introduced name for neoplasms currently included in the World Health Organization classification of soft tissue tumors under the rubric inflammatory leiomyosarcoma. Inflammatory rhabdomyoblastic tumor is an excellent example of how surgical pathologists working in conjunction with tumor biologists can greatly improve tumor classification to the benefit of patients. Over the last 28 years, understanding of this entity has undergone a fascinating evolution. This review serves as a summary of the latest findings in inflammatory rhabdomyoblastic tumor research and a diagnostic manual for the practicing surgical pathologist.

Inflammatory Rhabdomyoblastic Tumor of the Posterior Pharyngeal Wall

This case report describes a healthy man in his 40s who presented with a 1-year history of snoring, sleep apnea, dysphonia, and dysphagia owing to a large mass of the posterior pharynx and was diagnosed with an inflammatory rhabdomyoblastic tumor.

Inflammatory Rhabdomyoblastic Tumor: Clinicopathologic and Molecular Analysis of 13 Cases

Inflammatory rhabdomyoblastic tumors (IRMTs) are newly recognized skeletal muscle tumors with uncertain malignant potential. We investigated 13 IRMTs using clinicopathologic, genetic, and epigenetic methods. The cohort included 7 men and 6 women, aged 23 to 80 years (median, 50 years), of whom 2 had neurofibromatosis type 1. Most tumors occurred in the deep soft tissues of the lower limbs, head/neck, trunk wall, and retroperitoneum/pelvis. Two tumors involved the hypopharyngeal submucosa as polypoid masses. Eight tumors showed conventional histology of predominantly spindled cells with nuclear atypia, low mitotic activity, and massive inflammatory infiltrates. Three tumors showed atypical histology, including uniform epithelioid or plump cells and mitotically active histiocytes. The remaining 2 tumors demonstrated malignant progression to rhabdomyosarcoma; one had additional IRMT histology and the other was a pure sarcoma. All 11 IRMTs without malignant progression exhibited indolent behavior at a median follow-up of 43 months. One of the 2 patients with IRMTs with malignant progression died of lung metastases. All IRMTs were positive for desmin and PAX7, whereas myogenin and MyoD1 were expressed in a subset of cases. Targeted next-generation sequencing identified pathogenic mutations in NF1 (5/8) and TP53 (4/8). All TP53 mutations co-occurred with NF1 mutations. TP53 variant allele frequency was much lower than that of NF1 in 2 cases. These tumors showed geographic (subclonal) strong p53 immunoreactivity, suggesting the secondary emergence of a TP53-mutant clone. DNA methylation-based copy number analysis conducted in 11 tumors revealed characteristic flat patterns with relative gains, including chromosomes 5, 18, 20, 21, and/or 22 in most cases. Widespread loss of heterozygosity with retained biparental copies of these chromosomes was confirmed in 4 tumors analyzed via allele-specific profiling. Based on unsupervised DNA methylation analysis, none of the 11 tumors tested clustered with existing reference entities but formed a coherent group, although its specificity warrants further study.

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.

Pulmonary inflammatory leiomyosarcoma represents a potential diagnostic pitfall of DNA methylation-based classification of sarcomas: a case report

BACKGROUND

Pulmonary inflammatory leiomyosarcoma (PILMS) is a rare type of myogenic tumor with prominent lymphohistiocytic infiltration. Despite their histological similarities, PILMS is immunohistochemically and genetically distinct from soft tissue inflammatory leiomyosarcoma, and its clinicopathological picture including DNA methylome data remains still unknown.

CASE PRESENTATION

Here we present a case of PILMS in an 18-year-old male who underwent lobectomy. As reported previously, the current case demonstrated spindle myoid cell proliferation with smooth muscle differentiation within a prominent lymphohistiocytic infiltration and a diploid genome with a MUC3A gene alteration. DNA methylation analysis predicted this case to be an "inflammatory myofibroblastic tumor" (IMT) according to the Deutsches Krebsforschungszentrum (DKFZ) classifier. The data was analyzed by t-distributed stochastic neighbor embedding, which plotted the case tumor in the vicinity of IMT, however, there were no IMT histological features. These discordant results could be due to background non-neoplastic inflammatory cells.

CONCLUSIONS

As the DNA methylation classification of PILMS might be a potential diagnostic pitfall, an integrative histological and genetic approach is required for its accurate diagnosis.

Lipoblastoma-Like Tumor and Fibrosarcoma-Like Lipomatous Neoplasm Represent the Same Entity: A Clinicopathologic and Molecular Genetic Study of 23 Cases Occurring in Both Men and Women at Diverse Locations

Lipoblastoma-like tumor (LLT) is a benign soft tissue tumor demonstrating mixed morphologic features of lipoblastoma, myxoid liposarcoma, and spindle cell lipoma but lacking genetic alterations associated with those tumors. LLT was originally thought to be specific to the vulva but has since been reported in the paratesticular region. The morphologic features of LLT overlap with those of "fibrosarcoma-like lipomatous neoplasm" (FLLN), a rare, indolent adipocytic neoplasm considered by some to form part of the spectrum of atypical spindle cell and pleomorphic lipomatous tumor. We compared the morphologic, immunohistochemical, and genetic features of 23 tumors previously classified as LLT (n = 17) and FLLN (n = 6). The 23 tumors occurred in 13 women and 10 men (mean age, 42 years; range, 17 to 80 years). Eighteen (78%) cases arose in the inguinogenital region, whereas 5 tumors (22%) involved noninguinogenital soft tissue, including the flank (n = 1), shoulder (n = 1), foot (n = 1), forearm (n = 1), and chest wall (n = 1). Microscopically, the tumors were lobulated and septated, with variably collagenized fibromyxoid stroma, prominent thin-walled vessels, scattered univacuolated or bivacuolated lipoblasts, and a minor component of mature adipose tissue. Using immunohistochemistry, 5 tumors (42%) showed complete RB1 loss, with partial loss in 7 cases (58%). RNA sequencing, chromosomal microarray, and DNA next-generation sequencing study results were negative for significant alterations. There were no clinical, morphologic, immunohistochemical, or molecular genetic differences between cases previously classified as LLT or FLLN. Clinical follow-up (11 patients [48%]; range, 2-276 months; mean, 48.2 months) showed all patients were alive without disease, and only one patient had experienced a single local recurrence. We conclude that LLT and FLLN represent the same entity, for which "LLT" seems most appropriate. LLT may occur in either sex and any superficial soft tissue location. Careful morphologic study and appropriate ancillary testing should allow for the distinction of LLT from its potential mimics.

Rhabdomyosarcoma Arising in Inflammatory Rhabdomyoblastic Tumor: A Genetically Distinctive Subtype of Rhabdomyosarcoma

"Inflammatory rhabdomyoblastic tumor" (IRMT) is a recently coined name for a distinctive soft tissue neoplasm characterized by slow growth, a dense histiocytic infiltrate, scattered, bizarre-appearing tumor cells with morphologic and immunohistochemical evidence of skeletal muscle differentiation, a near-haploid karyotype with retained biparental disomy of chromosomes 5 and 22, and usually indolent behavior. There are 2 reports of rhabdomyosarcoma (RMS) arising in IRMT. We studied the clinicopathologic and cytogenomic features of 6 cases of IRMT with progression to RMS. Tumors occurred in the extremities of 5 men and 1 woman (median patient age, 50 years; median tumor size, 6.5 cm). Clinical follow-up (6 patients: median, 11 months; range 4-163 months) documented local recurrence and distant metastases in 1 and 5 of 6 patients, respectively. Therapy included complete surgical resection (4 patients) and adjuvant/neoadjuvant chemo/radiotherapy (6 patients). One patient died of disease, 4 were alive with metastatic disease, and one was without evidence of disease. All primary tumors contained conventional IRMT. Progression to RMS appeared as follows: (1) overgrowth of monomorphic rhabdomyoblasts with diminished histiocytes, (2) monomorphic spindle cell morphology with variably pleomorphic rhabdomyoblasts and low mitotic activity, or (3) morphologically undifferentiated spindle cell and epithelioid sarcoma. All but one were diffusely desmin-positive, with more limited MyoD1/myogenin expression. All RMS arising in IRMT, either primary or metastatic, demonstrated widespread loss of heterozygosity with retained heterozygosity of chromosomes 5 and 20, and all but one displayed additional gains and losses involving loci containing oncogenes/ tumor suppressor genes, most often CDKN2A and CDKN2B. RMS arising in IRMT have unique clinicopathologic and cytogenomic features, warranting classification as a distinct, potentially aggressive RMS subtype. It should be distinguished from other RMSs, particularly fusion-driven spindle cell RMS and pleomorphic RMS.

Unusual case of inflammatory rhabdomyoblastic tumor in a pediatric patient

Inflammatory rhabdomyoblastic tumor (IRMT) is a rare, recently described skeletal muscle neoplasm of uncertain malignant potential. We report an unusual tumor in the right arm of a 5-year-old boy, which is the first case of a pediatric IRMT. Immunohistochemically, most cells in the tumor were positive for CD163 and CD68 staining. The neoplastic cells themselves showed a skeletal muscle phenotype with a diffuse expression of desmin and a focal expression of myoD1. Mitotic activity was low (1/10 HPF), and no necrosis was observed.

Fine-needle aspiration biopsy diagnosis of histiocyte-rich rhabdomyoblastic tumor (inflammatory leiomyosarcoma): A case report

Histiocyte-rich rhabdomyoblastic tumor (HRRMT) is an exceedingly rare soft tissue tumor of primitive myogenic differentiation. We report herein a case involving the soft tissue in the left lateral peri-scapular region in a 68-year-old female with a 2-month history of a non-painful soft tissue mass. Ultrasound revealed a solid, ovoid subcutaneous mass lesion that lacked significant internal vascularity. Percutaneous fine-needle aspiration (FNA) biopsy with concomitant core needle biopsy was performed, and a diagnosis of HRRMT was rendered. Cytologic smears were hypercellular, composed of a mixture of foamy histiocytes and a variably cohesive population of epithelioid and plasmacytoid to vaguely spindled cells in cohesive clusters and singly dispersed. Histologic material showed sheets of epithelioid and plasmacytoid to spindled cells with admixed foamy histiocytes with distended, vacuolated cytoplasm. To our knowledge, this is the first reported example of HRRMT evaluated by FNA biopsy.

[Changes in the WHO classification (2020) of soft tissue tumors]

The article provides an overview of the main changes in the current (2020) WHO classification of soft tissue tumors, as well as selected updates that have occurred since the release of the classification.

Xanthogranulomatous epithelial tumors and keratin-positive giant cell-rich soft tissue tumors: two aspects of a single entity with frequent HMGA2-NCOR2 fusions

Xanthogranulomatous epithelial tumor (XGET) and keratin-positive giant cell-rich soft tissue tumor with HMGA2-NCOR2 fusion (KPGCT) are two recently described neoplasms with both distinct and overlapping clinical and histopathologic features. We hypothesized that XGET and KPGCT may be related and represent a histologic spectrum of a single entity. To test this, we sought to characterize the clinical, radiographic, immunohistochemical, ultrastructural and molecular features of additional tumors with features of XGET and/or KPGCT, which we refer to descriptively as keratin-positive xanthogranulomatous/giant cell-rich tumors (KPXG/GCT). The archives were searched for potential cases of KPXG/GCT. Clinical and imaging features were noted. Slides were assessed for histologic and immunohistochemical findings. Ultrastructural and next generation RNA sequencing-based analysis were also performed. Nine cases were identified arising in seven women and two men [median age of 33 years (range: 12-87)]. Median tumor size was 4 cm (range: 2.4-14.0 cm) and tumors presented in the thigh (2), buttock (1), forearm (2), groin (1), cranial fossa (1), ilium (1), and tibia (1). Morphologically, tumors were most frequently characterized by a fibrous capsule, with associated lymphoid reaction, enclosing a polymorphous proliferation of histiocytes, giant cells (Touton and osteoclast-types), mixed inflammatory infiltrate, hemorrhage and hemosiderin deposition, which imparted a variably xanthogranulomatous to giant cell tumor-like appearance. One case clearly showed mononuclear cells with eosinophilic cytoplasm characteristic of XGET. All cases expressed keratin and 7 of 9 were found to harbor HMGA2-NCOR2 fusions including cases with xanthogranulomatous appearance. One patient developed local recurrence and multifocal pulmonary lesions, which were radiographically suspicious for metastases. Shared clinical, histologic and immunohistochemical features, and the shared presence of HMGA2-NCOR2 fusions supports interpretation of KPXG/GCT as a single entity which includes XGET and KPGCT. Given limited clinical follow-up to date and rare cases with apparently aggressive findings, we provisionally regard these tumors as having uncertain biologic potential.

Inflammatory leiomyosarcoma/rhabdomyoblastic tumor: a report of two cases with novel genetic findings

Inflammatory leiomyosarcoma (ILMS) is a malignant neoplasm showing smooth muscle differentiation, a prominent inflammatory infiltrate, and near‐haploidization. These tumors have significant pathologic and genetic overlap with the recently described “inflammatory rhabdomyoblastic tumor (IRT),” suggesting that ILMS and IRT may belong to one entity. Herein, we describe two cases of ILMS/IRT with attention to new cytogenetic and sequencing findings. The tumors were composed of sheets and fascicles of variably pleomorphic tumor cells showing spindled and epithelioid to rhabdoid morphology and a prominent histiocyte‐rich inflammatory infiltrate typical of ILMS/IRT. In case 1, chromosomal microarray analysis showed a near‐haploid pattern with loss of heterozygosity resulting from loss of one copy of all autosomes except for chromosomes 5, 20, 21, and 22. Case 2 showed areas with high‐grade rhabdomyosarcomatous transformation. In this case, the low‐grade tumor component revealed a hyper‐diploid pattern with loss of heterozygosity for most of autosomes but with a normal diploid copy number state except for chromosomes 5, 20, and 22, which showed a relative gain. The high‐grade tumor component showed a similar pattern of copy‐neutral loss of heterozygosity with additional abnormalities, including mosaic segmental gains at 1p, 5p, 8q, 9p, 20q, and segmental loss at 8p. Next‐generation sequencing identified sequence variants in NF1, TP53, SMARCA4, KRAS, and MSH6. MSH6 variant was confirmed as germline, consistent with the diagnosis of hereditary nonpolyposis colorectal cancer (HNPCC) syndrome in one of our study patients and suggestive that ILMS/IRT might be part of the HNPCC cancer spectrum.

EWSR1-WT1 gene fusions in neoplasms other than desmoplastic small round cell tumor: a report of three unusual tumors involving the female genital tract and review of the literature

Desmoplastic small round cell tumor (DSRCT) is a high-grade round cell sarcoma that typically arises in the abdominopelvic cavity of young males, co-expresses keratins and desmin, and carries a pathognomonic EWSR1-WT1 gene fusion. The EWSR1-WT1 gene fusion is generally considered specific for DSRCT, although there are two reports of this fusion in tumors otherwise lacking features of DSRCT. We report three female genital tract tumors with EWSR1-WT1 fusions but showing morphologic and immunohistochemical features incompatible with DSRCT. The tumors occurred in the uterine cervix, uterine corpus/ovaries, and vagina, respectively, of 46, 30, and 20-year-old women. Two tumors consisted of a sheet-like to fascicular proliferation of relatively uniform spindled to occasionally more epithelioid cells arrayed about thick-walled, hyalinized, and capillary-sized vessels, with distinctive areas of pseudovascular change, and absence of desmoplastic stroma. The third tumor resembled a monomorphic spindle cell sarcoma with necrosis. All had diffuse desmin and variable but more limited keratin expression, two of three expressed smooth muscle actin, and all were negative for h-caldesmon, CD10, estrogen receptor, myogenin, N-terminus WT-1, and S100 protein. One patient received neoadjuvant chemotherapy and radiation therapy followed by resection and is disease-free 42 months after diagnosis. Another patient was managed by resection only and is disease-free 9 months after initial diagnosis. The remaining patient recently underwent resection of multifocal pelvic disease. Comprehensive differential gene expression analysis on two tumors compared to two classic DSRCTs with known EWSR1-WT1 fusions resulted in 1726 genes that were differentially expressed (log2 fold change >2 or < -2) and statistically significant (FDR < 5%). In combination with previous reports, our findings suggest pleiotropy of the EWSR1-WT1 fusion is possible and not limited to DSRCT. Subsets of non-DSRCT EWSR1-WT1 positive tumors may represent discrete entities, but further study is necessary.

Clinicopathological analysis and genomic profiling of a rare histiocyte-rich rhabdomyoblastic tumor: A case report

RATIONALE

Skeletal muscle tumors are traditionally classified as rhabdomyomas or rhabdomyosarcomas. However, some soft tissue tumors cannot easily be identified as benign or malignant. We report a case of a histiocyte-rich rhabdomyoblastic tumor, with pathologic characteristics distinct from either rhabdomyoma or rhabdomyosarcoma. In contrast to rhabdomyosarcomas, the tumor cells exhibited low mitotic activity, lacking obvious morphologic atypia. Clinically, the tumor followed a very indolent course. Overall, the tumor did not fit classification criteria for either benign or malignant.

PATIENT CONCERNS

A 58-year-old Chinese man was admitted to Qilu Hospital on September 8, 2018, with a >20 year history of a mass in the middle of the left thigh. A few months prior to admission, he had experienced the pain from the mass extending to the distal left lower extremity. He had no prior history of significant disease or relevant family history.

DIAGNOSES

Microscopically, numerous histiocytes and foamy cells covered the actual tumor cells that were positive for desmin, MyoD1, and myogenin, suggesting striated skeletal muscle cell differentiation. However, cross-striations were not detected in the tumor cells. The tumor was characterized by a non-infiltrative growth pattern and a low level of Ki67. A diagnosis of histiocyte-rich rhabdomyoblastic tumor was suggested.

INTERVENTIONS

The thigh mass was surgically resected September 12, 2018.

OUTCOMES

The patient recovered well postoperatively, and was free of tumor recurrence or metastasis, followed to September 12, 2020 (23 months).

LESSONS

Histiocyte-rich rhabdomyoblastic tumor cells have minor atypia, indicating possible malignant potential. However, the tumor behavior was quit indolent. Due to the conflicting clinical and pathologic aspects of the tumor, to label it as rhabdomyosarcoma seemed inaccurate, potentially prompting over treatment. Interestingly, mutations were detected in NF1, AXIN2, CHEK2, DNMT3A, KMT2D, and RB1 through next-generation sequencing. These mutations suggest disruptions in Ras signaling, the Wnt pathway, methyltransferases, and the cell cyclepotentially influencing the development of this histiocyte-rich rhabdomyoblastic tumor. This unusual tumor should be incorporated into the WHO Classification of Soft Tissue Tumors owing to its unique characteristics.

"Inflammatory Leiomyosarcoma" and "Histiocyte-rich Rhabdomyoblastic Tumor": a clinicopathological, immunohistochemical and genetic study of 13 cases, with a proposal for reclassification as "Inflammatory Rhabdomyoblastic Tumor"

Inflammatory leiomyosarcoma (ILMS), defined as "a malignant neoplasm showing smooth muscle differentiation, a prominent inflammatory infiltrate, and near-haploidization", is a very rare soft tissue tumor with a generally favorable prognosis. The morphologic features of "histiocyte-rich rhabdomyoblastic tumor" (HRRMT) are similar to those of ILMS, although this lesion shows by definition a skeletal muscle phenotype. Recent gene expression profiling and immunohistochemical studies have also suggested that ILMS and HRRMT may be related. We studied the clinicopathologic, immunohistochemical and genetic features of four cases previously classified as ILMS and nine classified as HRRMT. Tumors from both groups tended to occur in the deep soft tissues of the extremities of young to middle-aged males and exhibited indolent behavior. Morphologically, all were well-circumscribed, often encapsulated, and showed a striking histiocyte-rich inflammatory infiltrate admixed with variably pleomorphic tumor cells showing spindled and epithelioid to rhabdoid morphology, eosinophilic cytoplasm, and prominent nucleoli, but few, if any, mitotic figures. Immunohistochemically, the tumor cells expressed desmin, alpha-smooth muscle actin, and the rhabdomyoblastic markers PAX7, MyoD1, and myogenin. H-caldesmon expression was absent in all cases, using the specific h-CD antibody. Karyotypic study (1 HRRMT) and genome-wide copy number analysis (7 HRRMT, OncoScan SNP assay), revealed near-haploidization in four cases, with subsequent genome doubling in one, an identical phenotype to that seen in ILMS. We propose reclassification of ILMS and HRRMT as "inflammatory rhabdomyoblastic tumor", a name which accurately describes the salient morphologic and immunohistochemical features of this distinctive tumor, as well as its intermediate (rarely metastasizing) clinical behavior.

Hiding in plain sight: Gene panel and genetic markers reveal 26-year undiagnosed tumor-induced osteomalacia of the rib concurrently misdiagnosed as X-linked hypophosphatemia

Tumor-induced osteomalacia (TIO), caused by phosphaturic mesenchymal tumors (PMTs), is a rare paraneoplastic syndrome characterized by frequent bone fractures, bone pain, muscle weakness, and affected gait. These tumors typically secrete high levels of Fibroblastic Growth Factor 23 (FGF23), a hormone which acts on the kidney to cause hypophosphatemia, ultimately impairing bone mineralization. In this case report, we present a 41-year-old female with FGF23-mediated hypophosphatemia with a 26-year delay in TIO diagnosis and a concurrent misdiagnosis of X-linked hypophosphatemic rickets (XLH). Given an absence of family history of hypophosphatemia, a 13-gene hypophosphatemia panel including XLH (PHEX gene) was performed and came back negative prompting a diagnostic search for a PMT causing TIO. A ⁶⁸Ga-DOTATATE PET/CT scan revealed the presence of a 9th right rib lesion, for which she underwent rib resection. The patient's laboratory values (notably serum phosphorus, calcium, and vitamin D) normalized, with FGF23 decreasing immediately after surgery, and symptoms resolving over the next three months. Chromogenic in situ hybridization (CISH) and RNA-sequencing of the tumor were positive for FGF23 (CISH) and the transcriptional marker FN1-FGFR1, a novel fusion gene between fibronectin (FN1) and Fibroblast Growth Factor Receptor 1 (FGFR1), previously determined to be present in the majority of TIO-associated tumors. This case demonstrates the notion that rare and diagnostically challenging disorders like TIO can be undiagnosed and/or misdiagnosed for many years, even by experienced clinicians and routine lab testing. It also underscores the power of novel tools available to clinicians such as gene panels, CISH, and RNA sequencing, and their ability to characterize TIO and its related tumors in the context of several phenotypically similar diseases.

SRF-FOXO1 and SRF-NCOA1 Fusion Genes Delineate a Distinctive Subset of Well-differentiated Rhabdomyosarcoma

Rhabdomyosarcoma (RMS) encompasses a heterogenous collection of tumors in which new groups have recently been identified that improved the World Health Organization (WHO) classification. While performing RNA-sequencing in our routine practice, we identified 3 cases of well-differentiated RMS harboring new fusion genes. We also analyzed these tumors through array-comparative genomic hybridization. Clinically, these tumors were deep paraspinal tumors, occurring in neo-nat and young children. The patients underwent resection and adjuvant therapy. At the time of last follow-up (ranging from 12 to 108 mo), they were alive without disease. Histologically, these tumors consisted of well-differentiated rhabdomyoblastic proliferations with nuclear atypia, infiltrative borders, and a specific growth pattern. These tumors harbored new fusion genes involving SRF and either FOXO1 or NCOA1. We compared the expression profiles of these 3 tumors to the expression data of a series of 33 skeletal muscle tumors including embryonal RMSs, alveolar rhandomyosarcomas, RMSs with VGLL2 fusions, RMSs with the myoD1 mutation, EWSR1/FUS-TFCP2 epithelioid and spindle cell RMSs of the bone, and rhabdomyomas with PTCH1 loss. According to clustering analyses, the 3 SRF-fused tumors formed a distinct group with a specific expression profile different from that of the other types of skeletal muscle tumors. Array-comparative genomic hybridization showed a recurrent gain of chromosome 11. These 3 tumors define a new group of RMS associated with a fusion of the SRF gene. FOXO1 rearrangements, usually used to confirm the diagnosis of alveolar RMS and identify poor-outcome RMSs, were identified in a nonalveolar RMS for the first time.

Histiocyte-rich rhabdomyoblastic tumor: a report of two cases and a review of the differential diagnoses

Histiocyte-rich rhabdomyoblastic tumor is a recently described skeletal muscle neoplasm of uncertain malignant potential, characterized by slow growth, a fibrous capsule containing peripheral lymphoid aggregates, spindle-to-epithelioid cells with a rhabdomyoblastic immunophenotype, and a dense histiocytic infiltrate. It most commonly arises within the muscles of the lower legs and trunk in young-to-middle-aged men, and initial reports suggest indolent behavior. In this paper, we present two additional cases of histiocyte-rich rhabdomyoblastic tumor with similar clinicopathologic features and discuss the differential diagnosis including its overlap with inflammatory leiomyosarcoma.

Inflammatory leiomyosarcoma shows frequent co-expression of smooth and skeletal muscle markers supporting a primitive myogenic phenotype: a report of 9 cases with a proposal for reclassification as low-grade inflammatory myogenic tumor

Inflammatory leiomyosarcoma (ILMS) is a very rare soft tissue tumor that usually follows an indolent clinical course, but long-term follow-up studies are lacking. Recent publications primarily focused on its genetic profile characterized by a near haploid genome. One study also showed these tumors to have upregulation of genes known to be crucial for skeletal muscle differentiation. Nevertheless, immunohistochemical expression of skeletal muscle markers, as well as markers that would help to distinguish ILMS from a long list of relevant differential diagnostic entities, has not been extensively studied. Nine cases of ILMS were collected and stained by a broad IHC panel which, besides others, contained MyoD1, myogenin, and PAX-7. A subset of cases was also analyzed by 2 different NGS assays and by MDM2 fluorescence in situ hybridization. Five male and 4 female patients ranged in age from 25 to 54 years (mean, 36 years). The tumors showed a predilection for intramuscular sites of the lower limbs (n = 4) and back (n = 2), whereas the remaining 3 cases affected an unspecified skeletal muscle, lung, and omentum. Follow-up with an average length of 10.6 years (range 0.5-22) was available for 8 patients. The omental tumor spread locally within the abdominal cavity, but the patient has been free of disease 7 years after treatment. None of the 5 patients with somatic soft tissue tumors (and follow-up longer than 1.5 years) had either recurrence or metastasis. Immunohistochemical studies revealed a substantial expression of skeletal muscle markers in almost all cases. This phenotype coupled with a highly characteristic genotype and significantly more indolent clinical behavior as compared with conventional leiomyosarcoma of deep soft tissue offers a strong rationale to change the current nomenclature. Based on the clinicopathological features and gene expression profile, we propose the name low-grade inflammatory myogenic tumor.

The current landscape of rhabdomyosarcomas: an update

Rhabdomyosarcomas are malignancies associated with a rhabdomyoblastic phenotype which can be demonstrated morphologically or by immunohistochemistry for MYOD1 and myogenin. Rhabdomyosarcomas are currently subdivided into 4 types in the 2013 WHO classification of tumors of soft tissue and bone, including embryonal rhabdomyosarcoma, alveolar rhabdomyosarcoma, spindle cell/sclerosing rhabdomyosarcoma, and pleomorphic rhabdomyosarcoma. Recent studies have significantly impacted this classification with the emergence of three distinct new subtypes of rhabdomyosarcomas, namely rhabdomyosarcoma with MYOD1 mutations, rhabdomyosarcoma with TFCP2 fusions, and rhabdomyosarcoma with VGLL2/NCOA2 fusions. Although all these tumors share the terminology "rhabdomyosarcoma," their morphology, clinical behavior, and underlying molecular alterations are dramatically different. Finally, the presence of a rhabdomyoblastic phenotype within a tumor is by no means a diagnostic of a rhabdomyosarcoma, as this may be seen in many other mesenchymal malignancies, such as mesenchymal chondrosarcomas, malignant peripheral nerve sheaths tumors, and biphenotypic sinonasal sarcomas. In this review, we present the main clinical, morphological, and molecular features of these tumors and discuss the evolution of the current classification.