EP1040 - A rare case of adult anal sphincter complex rhabdomyosarcoma poses complex onco-surgical challenges in treatment

Purpose/Background: Rhabdomyosarcoma of the anal sphincter complex is a rare, malignant soft tissue tumor arising from mesenchymal cells with skeletal muscle differentiation, often affecting children but also seen in adults. Clinically, it often mimics benign conditions such as perianal abscesses or condyloma, leading to diagnostic delays. Imaging shows a mass involving the sphincter, with alveolar and embryonal types most common. Diagnosis is often at a late stage with nodal or distant spread.

Management requires a multimodal approach: neoadjuvant chemotherapy, surgical resection (often abdominoperineal excision when sphincter preservation is not feasible), and adjuvant radiotherapy. Prognosis is poor, especially with advanced disease or alveolar subtype.

Methods/Interventions: To our knowledge, one adult case of anorectal rhabdomyosarcoma has been described in literature. We report a similar case of a female in late her 30s.

Results/Outcomes: A woman in her late 30s with no significant medical history presented with a gradually enlarging, painless perianal mass without bleeding, drainage, pain or fecal incontinence. Initially suspected to be pilonidal abscess disease, she was referred to community general surgeon where CT scan revealed a complex, >5 cm echogenic mass (Image 4.1,4.2). She was referred to colorectal surgery, where examination showed the mass protruded with the skin posterolateral to anal orifice and was firm in consistency and non-tender. Digital rectal examination was negative for anal canal or rectal involvement. In addition, digital rectal examination did not illicit tenderness and anoscopy was normal. Biopsy via interventional radiology revealed tumor cells concerning for rhabdomyosarcoma. Multiple pathology opinions were obtained including Tempus xR next generation sequencing. They confirmed alveolar rhabdomyosarcoma with a PAX3:FOXO1 translocation. Immunohistochemistry was positive for desmin, myogenin, and MYOD1. MRI suggested the mass likely emanating from external anal sphincter muscle (See image 5.1,5.2). The case was reviewed in GI and pediatric oncology tumor boards. Staged as T1aNxMx (stage 3, group IIIa), she was categorized as intermediate-risk. PET scan showed an FDG-avid right external iliac node (Image 6). She was elected to proceed with VAC chemotherapy followed by likely abdominoperineal resection aggressive resection based on the response from combination chemotherapy approach.

Conclusion/Discussion: Rhabdomyosarcoma is a malignant soft tissue tumor arising from mesenchymal skeletal muscle cells, more common in children and rare in adults. It has two main histologic subtypes: embryonal (more common, better prognosis) and alveolar (20% of cases), the latter often marked by PAX3/7:FOXO1 gene fusions and linked to a more aggressive course.

While commonly found in the head, neck, extremities, or genitourinary tract, anorectal involvement is rare. Perianal rhabdomyosarcoma is difficult to diagnose early due to symptom overlap with more common conditions like anorectal abscesses or pilonidal disease. It often presents with tenderness, erythema, and swelling. In this case, diagnosis was delayed by 4 months after an initial asymptomatic mass. MRI is crucial to assess tumor relationship to sphincter anatomy; here, it showed close proximity to the external anal sphincter. Reported cases of anorectal rhabdomyosarcoma, when the patient is a surgical candidate, the preferred surgical approach is abdominal perineal resection with excision of the sphincter complex to achieve negative margins.

Pathology
Image 1.1, 1.2- Tumor consists of nests of primitive small to intermediate monomorphic round cells and rhabdomyoblasts with central loss of cellular cohesion. The nests are separated by fibrovascular septa.

Image 2.1, 2.2- (400x) - This field contains rhabdomyoblasts and cells with clear, glycogenated cytoplasm.

Image 3.1 Desmin Immunohistochemistry, 3.2 Myogenin Immunohistochemistry, 3.3 - MYO-D1 immunohistochemistry
Radiology Findings
Image 4.1, 4.2: CT Imaging - Orange arrow showing heterogenous, irregular shape mass. Blue arrow demonstrates mass protruding between gluteal cleft out through skin.

Image 5- MRI - Green arrow shows mass near circumscribing external anal orifice and emanating external anal sphincter complex.

Image-6.1, 6.2 - FDG Avid activity of posterior perianal tumor extending trans-gluteal cleft. Blue arrow demonstrates FDG avid right external iliac lymph node.