Rhabdomyosarcoma: Updates on classification and the necessity of molecular testing beyond immunohistochemistry

Primary Cutaneous Neoplasm With Rhabdomyosarcomatous Differentiation and a Melanoma-Like Mutational Landscape

Malignant melanoma (MM) is notorious for its wide range of morphologic variability. Rarely, MM may lose all melanocytic markers and adopt the morphologic and immunophenotypic characteristics of a different neoplasm in a process known as trans-differentiation (TMM). Distinguishing TMM from primary cutaneous neoplasms may be challenging and is often dependent on the identification of an adjacent conventional melanoma. In particularly difficult cases, molecular analysis may be helpful; TMMs are known to exhibit highly similar mutational landscapes to conventional melanomas (e.g., mutations in NF1, NRAS; variable BRAF V600E). Herein, we present an exceedingly rare case of likely TMM with rhabdomyosarcomatous differentiation in which high tumor mutational burden (TMB) was an important clue to the diagnosis. An 83-year-old woman presented with an 8.2 cm fungating mass on the upper arm. Biopsy revealed a sheet-like proliferation of mitotically active pleomorphic cells which were positive for myogenin/MyoD1 and negative for S100/SOX10. A diagnosis of epithelioid rhabdomyosarcoma was rendered. Subsequent axillary lymph node metastasis prompted whole exome sequencing, which revealed a molecular signature more indicative of MM, including: high TMB (19 mutations/Mb); ultraviolet mutational signature (i.e., preponderance of C>T base changes); TERT promoter mutation; and ARID2 mutation. After discussion at the interdisciplinary tumor board, a diagnosis of TMM was considered most likely, and the patient was initiated on pembrolizumab. Morphologic features more typical of MM than cutaneous sarcomas, such as tumor-infiltrating lymphocytes, junctional epidermal tumor nests, and satellitosis, may provide further clues to the accurate diagnosis of TMM, which has important prognostic and therapeutic implications for the patient.

Spindle cell rhabdomyosarcoma in a cockatiel (Nymphicus hollandicus)

A 2-year-old cockatiel (Nymphicus hollandicus) was presented with a large nodule on the ventral aspect of the right wing. Due to an accidental cutaneous trauma, the patient developed haemorrhage and subsequently died. The main necropsy finding was a pedunculated nodule that extended from the right pronator superficialis and biceps brachii muscles. Cytology of the nodule revealed neoplastic spindle cells with cytoplasmic cross-striations and multinucleate strap cells. Histologically, the nodule was composed of neoplastic spindle cells arranged in fascicles with a herringbone pattern. Neoplastic cells had multifocal cytoplasmic immunolabelling for vimentin and desmin, multifocal nuclear and cytoplasmic immunolabelling for myoblast determination protein 1 and rare cytoplasmic immunolabelling for α-smooth muscle actin. Neoplastic cells were negative for pancytokeratin. The final diagnosis was spindle cell rhabdomyosarcoma, a subtype rare in animals.

Developmental Heterogeneity of Rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is a pediatric embryonal solid tumor and the most common pediatric soft tissue sarcoma. The histology and transcriptome of RMS resemble skeletal muscle progenitor cells that have failed to terminally differentiate. Thus, RMS is typically thought to arise from corrupted skeletal muscle progenitor cells during development. However, RMS can occur in body regions devoid of skeletal muscle, suggesting the potential for nonmyogenic cells of origin. Here, we discuss the interplay between RMS driver mutations and cell(s) of origin with an emphasis on driving location specificity. Additionally, we discuss the mechanisms governing RMS transformation events and tumor heterogeneity through the lens of transcriptional networks and epigenetic control. Finally, we reimagine Waddington's developmental landscape to include a plane of transformation connecting distinct lineage landscapes to more accurately reflect the phenomena observed in pediatric cancers.

Alveolar Rhabdomyosarcoma of Nasopharynx and Paranasal Sinuses in Children Diagnosis and Treatment-Review of the Literature and Case Report

Alveolar rhabdomyosarcoma (aRMS) is a rare pediatric malignant tumor with a poor prognosis, particularly when located in the rhinopharynx and sphenoidal floor, which complicates diagnosis and increases the risk of misclassification as benign growths. The specific genotype of aRMS is associated with a worse clinical outcome. In young children, especially those aged 4 to 12 years, rhinopharyngeal masses are often attributed to chronic adenoiditis; however, other benign (e.g., angiofibroma in boys) and malignant tumors may also be present. Initial symptoms frequently include nasal obstruction, muco-purulent nasal discharge, serous otitis media, sinusitis, and epistaxis. Rhabdomyosarcoma is the second most common ENT neoplasm in children, following lymphoma, with an incidence of approximately 6 cases per 1,000,000 annually. This report presents the case of an 8-year-old boy diagnosed with aRMS, accompanied by a literature review. Alveolar rhabdomyosarcoma should be suspected in children presenting with a vegetative tumor in the rhinopharynx or paranasal sinuses. Combined treatment approaches (surgery, radiotherapy, and chemotherapy) should be tailored to tumor characteristics. Neuronavigation-guided functional endoscopic sinus surgery (FESS) is an effective option for achieving complete tumor excision, depending on tumor size and extent. The prognosis remains reserved and is contingent upon accurate evaluation and timely intervention. Rigorous follow-up, including endoscopic and imagistic investigation, is crucial for early detection of recurrences, thereby improving treatment outcomes.

Primary cutaneous rhabdomyosarcoma with EWSR1/FUS::TFCP2 fusion: four new cases with distinctive morphology, immunophenotypic, and genetic profile

EWSR1/FUS::TFCP2-rearranged rhabdomyosarcoma (RMS) is a rare tumor with an aggressive clinical course, a predilection for craniofacial bones, spindled and/or epithelioid histomorphology, and positive immunohistochemistry (IHC) for epithelial and myogenic markers, along with variable ALK expression. Herein, we present four additional cases of primary cutaneous TFCP2-rearranged RMS. Notably, one tumor (case 1) displayed a varied pathological spectrum, initially presenting as a low-grade spindle cell neoplasm, but progressed into a high-grade spindle/epithelioid tumor. Another case (case 2) exhibited a predominant high-grade epithelioid/rhabdoid morphology. The third case (case 3) demonstrated a biphasic appearance of spindle and epithelioid cell proliferation, presenting with a low-grade morphology, and the last case (case 4) showed a predominant epithelioid morphology. All cases showed myogenic differentiation associated with keratins and ALK immunoreactivity. Interestingly, the two cases with high-grade and epithelioid morphology demonstrated CD30 immunoexpression. RNAseq or FISH revealed EWSR1 or FUS::TFCP2 gene fusion, and two cases with aggressive evolution showed ALK cluster-amplification as well, a finding that has not been previously reported. Two cases displayed aggressive behavior, with case 1 experiencing local recurrences and undergoing transformation into a high-grade epithelioid tumor, whereas case 2 initially presented as an epithelioid high-grade neoplasm, subsequently developing lymph node metastases and shortly thereafter distant metastases. In contrast, patients 3 and 4 are alive with no evidence of disease. The distinctive morphology and immunoprofile of this neoplasm may pose challenges in the differential diagnosis with cutaneous neoplasms showing keratins, ALK, and CD30 immunoreactivity. Nonetheless, ALK and CD30 overexpression may offer avenues for targeted therapy.

Rhabdomyosarcoma of the Maxilla

Rhabdomyosarcoma is the most common soft tissue sarcoma in children but is less frequent in adults, with the head and neck region as primary site. Magnetic resonance imaging (MRI) is the preferred diagnostic imaging tool, though its imaging characteristics are relatively non‑specific and overlap with other soft tissue sarcomas. The prognosis of rhabdomyosarcoma depends on the primary tumour site and size, with parameningeal head and neck localisations having a less favourable prognosis due to the higher risk of spread. Therefore, further imaging including brain and spinal MRI is recommended. Teaching point: The prognosis of rhabdomyosarcoma depends on the primary tumour site and size, with parameningeal head and neck localisations having a less favourable prognosis due to the higher risk of spread.

Localized incompletely resected standard risk rhabdomyosarcoma in children and adolescents: Results from the European Paediatric Soft Tissue Sarcoma Study Group RMS 2005 trial

BACKGROUND

The authors report the prospective evaluation of reduced dose alkylator chemotherapy combined with radiotherapy for European Pediatric Soft Tissue Sarcoma Study Group (EpSSG) standard risk nonalveolar rhabdomyosarcoma (NA-RMS).

PATIENTS AND METHODS

Localized node negative Intergroup Rhabdomyosarcoma Study (IRS) II/III NA-RMS at favorable sites (subgroup C), <25 years old, received five cycles of ifosfamide, vincristine, and dactinomycin (IVA) chemotherapy (30 g/m2 ifosfamide) and four cycles of vincristine and dactinomycin (if receiving radiotherapy), or nine cycles of IVA (54 g/m2 ifosfamide) ± radiotherapy. Delayed primary tumor excision was considered for IRS III tumors. The primary end points were event-free survival (EFS) and overall survival (OS).

RESULTS

From October 2005 to December 2016, 359 evaluable patients were recruited: orbit, 164 (45.7%); head and neck nonparameningeal, 77 (21.4%); and genitourinary non-bladder/prostate, 118 (32.9%). EFS and OS were 77.4% (95% confidence interval [CI], 72.5-81.6) and 93.5% (95% CI, 90.1-95.8), respectively. Lower dose alkylator chemotherapy and radiotherapy achieved 5-year OS of 93.7% but the difference with higher dose alkylator chemotherapy +/- radiotherapy was not significant (p = 0.8003). Adjuvant radiotherapy improved EFS with 5-year estimates of 84.7% versus 65.2% for nonirradiated (p < .0001), but not OS (p = .9298). Omitting radiotherapy for orbital tumors reduced OS (5-year was 87.1% vs. 97.3% for irradiated, p = .0257). Following R0 resection (n = 60), radiotherapy did not significantly improve EFS or OS.

CONCLUSIONS

Radiotherapy for local tumor control allows for reduction of cumulative dose of alkylators in EpSSG standard risk subgroup C RMS patients. The omission of radiotherapy did not affect OS in all patients except those with orbital RMS and was associated with inferior EFS.

Targeting DNA Methylation Machinery in Pediatric Solid Tumors

DNA methylation is a key epigenetic regulatory mechanism that plays a critical role in a variety of cellular processes, including the regulation of cell fate during development, maintenance of cell identity, and genome stability. DNA methylation is tightly regulated by enzymatic reactions and its deregulation plays an important role in the development of cancer. Specific DNA methylation alterations have been found in pediatric solid tumors, providing new insights into the development of these tumors. In addition, DNA methylation profiles have greatly contributed to tune the diagnosis of pediatric solid tumors and to define subgroups of patients with different risks of progression, leading to the reduction in unwanted toxicity and the improvement of treatment efficacy. This review highlights the dysregulated DNA methylome in pediatric solid tumors and how this information provides promising targets for epigenetic therapies, particularly inhibitors of DNMT enzymes (DNMTis). Opportunities and limitations are considered, including the ability of DNMTis to induce viral mimicry and immune signaling by tumors. Besides intrinsic action against cancer cells, DNMTis have the potential to sensitize immune-cold tumors to immunotherapies and may represent a remarkable option to improve the treatment of challenging pediatric solid tumors.

Keeping it real: Merging traditional and contemporary practices in musculoskeletal pathology: A special issue of neoplastic and non-neoplastic bone and soft tissue pathology

There is no shortage of comprehensive review articles on bone and soft tissue pathology, almost always representing a regurgitation of the literature with little to no guidance on personal "best practices," recommended applications of ancillary testing, and alternative points of view. This special issue of Human Pathology uniquely unites evidence-based medicine, where appropriate, with the collective personal experiences of a wide range of accomplished pathologists from varying institutions and backgrounds, addressing problematic areas, updated and sometimes imperfect classification systems, and their personal preferences for cost-effectively incorporating ancillary testing. For the preponderance of general pathologists (and specialists), whether academic or non-academic, non-neoplastic musculoskeletal diseases represent a far higher percentage of their practice than bone and soft tissue neoplasia. One of the most common frozen sections performed at many hospitals throughout the USA is revision arthroplasty, relying on the pathologist to help determine the presence (or absence) of periprosthetic joint infection, largely based on the hematoxylin & eosin (H&E) slide. Not every institution has access to the latest molecular techniques; fortunately, many of the current immunohistochemical antibodies serve as reliable surrogate markers of genetic mutations, allowing for cheaper but accurate diagnoses, when deemed necessary. Furthermore, molecular testing is often not necessary to establish a specific diagnosis, even among neoplasms with known underlying genetic abnormalities. It must be remembered that most bone and soft tissue tumors were recognized and classified correctly, before we uncovered and understood, among a subset, their underlying and unique molecular aberrations. Perhaps not surprisingly, in some cases, more than one molecular pathway may lead to the same histologic tumor subtype. Less commonly, an identical genetic driver/fusion may result in immunophenotypically and biologically distinct neoplasms, sometimes with entirely different clinical behaviors. "Dedifferentiation," a concept recognized among a variety of bone and soft tissue neoplasms, including but not limited to chondrosarcoma, parosteal osteosarcoma, and liposarcoma, needs to be objectively reassessed, particularly for liposarcoma. The following reviews attempt to address the above concepts, re-emphasizing the important role the practicing pathologist continues to (and must) play in the differential diagnoses of neoplastic and non-neoplastic musculoskeletal diseases.