Unveiling the uncommon: Alveolar soft part sarcoma- A case series highlighting diagnostic challenges

Case 1

A 35-year-old male presented with complaints of swelling in the throat and dysphagia for 3 months. On contrast-enhanced magnetic resonance imaging (MRI), there was a 3.9 × 2.9 × 4.5 cm heterogenous signal intensity mass lesion epicentered in the left parapharyngeal space [Figure 1a and b]. The mass involved the left lateral pharyngeal wall and left lateral glossoepiglottic fold, with loss of fat planes with the medial pterygoid muscle. It was extending into the left submandibular space and the left parotid gland’s deep part posterolaterally. Subcentrimetric bilateral enhancing cervical lymph nodes were also seen. He underwent wide local excision involving left segmental mandibulectomy with en bloc resection of the tumor with modified radical neck dissection of lymph nodes, along with pectoralis major myocutaneous flap reconstruction with elective tracheostomy. On histopathological examination, the tumor was unencapsulated and showed an infiltrative growth pattern. Tumor cells had a nested arrangement, with cells exhibiting central dyscohesion and alveolar architecture [Figure 1c]. There was lymphovascular invasion seen at the periphery of the tumor, as highlighted in Figure 1d. All margins were free of tumor. IHC revealed transcription factor E3 (TFE3) nuclear positivity and focal desmin expression. PanCK and vimentin were also positive. CD34 was negative in tumor cells and highlighted a rich sinusoidal capillary network. Other IHC markers, such as myogenin, Myo D1, HMB45, and S-100, were negative. Periodic acid–Schiff (PAS) stain highlighted rod-shaped cytoplasmic crystals. Overall features were of ASPS. The patient received 60Gy/30 # by intensity-modulated radiation therapy (IMRT) to the head and neck. On follow-up, contrast-enhanced computed tomography of the face, neck, and chest was performed, showing no residual/recurrent disease. The patient is under close observation since the disease is prone to metastasis, also as lymphovascular invasion was seen on microscopic examination, which was also a risk factor.

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Figure 1: (a-b) Contrast-enhanced magnetic resonance imaging (CEMRI) axial T1 image shows a heterogeneous signal intensity mass lesion epicentered in the left parapharyngeal space, as marked with a red arrow. CEMRI T2 coronal images show a mass, marked with a red arrow involving the left lateral pharyngeal wall and left lateral glossoepiglottic fold, with loss of fat planes with the medial pterygoid muscle and extending into the left submandibular space. (c) [Hematoxylin and eosin (H&E, 200×)] depicts tumour cells with a nested arrangement, as marked with a red arrow and exhibiting central dyscohesion. (d) (H&E, 400×) depicts lymphovascular invasion as marked with a red arrow at the periphery of the tumor.

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Case 2

A 35-year-old female had a progressively enlarging mass in the left elbow for 6 months. She underwent a wide local excision. Grossly, the tumor appeared as a solid, gray-white mass [Figure 2a]. Microscopic examination revealed polygonal tumor cells arranged in a pseud-alveolar pattern with fibrovascular septa [Figure 2b]. No lymphovascular invasion was seen. All margins of excised specimens were free of tumor. IHC showed diffuse TFE3 nuclear positivity [Figure 2c] and negative staining for other markers such as myogenin, myoD1, S-100, and PanCK. PAS stain highlighted the presence of cytoplasmic crystals [Figure 2d].

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Figure 2: (a) Grossly, the tumour appeared as a solid, gray-white mass, as marked with a red arrow. (b) [Hematoxylin and eosin (H&E, 400×)]: Microscopic examination revealed polygonal tumour cells arranged in a pseud-alveolar pattern with fibrovascular septa. (c) (IHC, 400×): Diffuse and strong nuclear expression of transcription factor E3 IHC seen in tumor cells. (d) (1000×): Depicts periodic acid-Schiff stain highlighting the cytoplasmic crystals as marked in red circle.

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Case 3

A 22-year-old female presented with a right breast lump for 8 months, which was gradually increasing in size. She was clinically diagnosed as a fibroadenoma/phyllodes tumor at Civil Hospital with a breast imaging-reporting and data system (BIRADS) category of 4 a. Biopsy was done at an outside hospital and was opined as lobular carcinoma along with features of lobular carcinoma in situ. We received paraffin blocks for review. On microscopic examination, an alveolar pattern of tumor cells was seen along with a few cells infiltrating into the surrounding stroma, resembling lobular carcinoma were also noted. There was the loss of cellular cohesion [Figure 3a]. IHC showed diffuse TFE3 nuclear positivity [Figure 3b] and negative staining for Family of transcription factor: GATA-binding protein 3 (GATA3) [Figure 3c], myogenin, myod1, and SMA. E-cadherin was retained [Figure 3d]. PAS stain highlighted cytoplasmic crystals.

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Figure 3: (a) [Hematoxylin and eosin (H&E, 400×)]: Depicts the alveolar pattern of tumor cells with loss of cellular cohesion, as marked with a red arrow. (b) (IHC, 400×): IHC showed diffuse transcription factor E3 nuclear positivity in the tumor cells. (c) (IHC, 400×): Tumor cells are negative for GATA3. (d) (IHC, 400×): Tumor cells show retained expression for E-cadherin.

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Case 4

A 30-year-old female had a swelling over the left mandibular angle for 10 months. The patient underwent a biopsy, which showed stratified squamous epithelium with no features of dysplasia. Underlying sub-epithelium showed tumor cells which appeared acantholytic, dyscohesive, and had a rhabdoid appearance [Figure 4a]. Biopsy was opined as poorly differentiated carcinoma as tumor cells were scanty and were focally positive for panCK. No classic alveolar pattern was seen in the biopsy. The patient underwent wide local excision with segmental mandibulectomy and modified radical neck dissection of lymph nodes. Gross examination showed a gray white solid mass [Figure 4b]. Microscopic examination revealed a pseudoalveolar pattern with polygonal, rhabdoid-appearing dyscohesive tumor cells and the presence of fibrovascular septa [Figure 4c]. There was no lymphovascular invasion. All margins were free of tumor. IHC showed diffuse TFE3 nuclear positivity and negative staining for other markers. PAS stain highlighted cytoplasmic crystals [Figure 4d].

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Figure 4: (a) [Hematoxylin and eosin (H&E, 400×)]: Depicts stratified squamous epithelium with no features of dysplasia, marked with a red arrow, and the underlying subepithelium shows tumour cells which appear acantholytic, dyschoesive, and had rhabdoid appearance, marked with a blue arrow. (b) Gross examination shows a grey-white solid mass, marked with red arrow (c) (H&E, 400×): Microscopic examination revealed a pseudoalveolar pattern with polygonal, rhabdoid appearing dyschoesive tumour cells and the presence of fibrovascular septa, marked with red arrow, (d) (1000×): Periodic acid-Schiff stain highlighted cytoplasmic crystals, marked with a red circle.

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Case 5

A 40-year-old male patient presented with thigh swelling for one year. There was a sudden increase in size over the last 1 month. He underwent MRI, which on post-contrast T1 axial showed an enhancing mass lesion in the anteromedial compartment of the thigh abutting the tibia of the right leg [Figure 5a and b]. Biopsy was attempted, which showed tumor cells arranged in a pseudoalveolar pattern along with the presence of tumor cells that were plasmacytoid/rhabdoid appearing. The patient underwent a wide local excision. Grossly, the tumor appeared as a fleshy, shiny, yellowish gray-brown mass with few cystic spaces [Figure 5c]. All gross margins were well away from the tumor. On microscopic examination, tumor cells were seen, which were of similar morphology as described in the biopsy. Cells had a pseudoalveolar pattern along with the presence of tumor cells, which were dyscohesive and were surrounded by fibrovascular septae [Figure 5d]. IHC showed diffuse TFE3 nuclear positivity and negative staining for other markers. PAS demonstrated the presence of characteristic crystals.

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Figure 5: (a and b) Magnetic resonance imaging on post-contrast T1 axial showed an enhancing mass lesion in the anteromedial compartment of the thigh abutting the tibia of the right leg, marked with a red arrow. (c) Grossly, the tumour appeared as a fleshy, shiny, yellowish-grey-brown mass with few cystic spaces, marked with red arrow (d). [Hematoxylin and eosin (H&E), 400×]: section. Tumour cells show a pseudoalveolar pattern, with tumour cells appearing dyschoesive and surrounded by fibrovascular septae, marked with red arrow.

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Histopathology and immunohistochemistry

ASPS exhibits distinctive histopathological features, including an organoid or nest-like growth pattern, often with central cell dropout resulting in pseudoalveolar spaces. These nests are separated by numerous sinusoidal capillaries, occasionally mimicking a hemangiopericytomatous pattern. Cytologically, the tumor is composed of large polygonal cells with abundant eosinophilic granular cytoplasm, vesicular nuclei, and prominent nucleoli. Other architectural variants may also be seen, including pseudoglandular changes, cystic areas with myxoid stroma, and occasional rhabdoid morphology.^[1,3,5]

Less common features include nuclear pleomorphism, mitotic activity, necrosis, calcification, and xanthomatous changes. Small-cell morphology, inconspicuous vasculature, and non-alveolar architecture may be particularly deceptive, especially in lingual tumors. Vascular invasion is common and was observed in one of our cases (parapharyngeal mass), suggesting a higher risk for lymph node metastasis. Therefore, a thorough nodal evaluation is essential.^[1,5]

On IHC, ASPS typically shows strong nuclear staining for TFE3, indicative of the alveolar soft part sarcoma chromosome region candidate 1 (ASPSCR1)-TFE3 gene fusion. Other markers frequently expressed include Cathepsin K (100%), calretinin (~46%), focal desmin (~50%), and sarcomeric actin. Some cases may show expression of S100 and neuron-specific enolase, although these are nonspecific. ASPS generally lack expression of epithelial markers (cytokeratins, epithelial membrane antigen [EMA]), melanocytic markers (human melanoma black-45 [HMB-45], Melan-A), neuroendocrine markers (synaptophysin), and glial markers (glial fibrillary acidic protein [GFAP], Neurofilaments [NF]).^[3,5]

Importantly, TFE3 nuclear positivity is not exclusive to ASPS and may also be seen in other tumors such as translocation-associated renal cell carcinoma (RCC) and perivascular epithelioid cell tumors (PEComas). In RCC, markers such as Paired box gene 8 and CD10 are positive, whereas they are negative in ASPS. PEComas, on the other hand, typically express both melanocytic markers, such as HMB-45, melan-A, and MITF, and muscle markers, such as SMA, desmin, and caldesmon, in addition to TFE3, while ASPS remains negative for these markers except TFE3.^[3,5]

Molecular pathology

ASPS is defined by the unbalanced translocation der(17) t(X;17)(p11.2;q25), resulting in the ASPSCR1-TFE3 fusion gene. ASPSCR1-TFE3 translocation is an instigating genetic event in cases of ASPS. The ASPSCR1-TFE3 fusion protein localizes to the nucleus, where it functions as an aberrant transcription factor, causing c-Met overexpression and activation of c-Met signaling. This translocation leads to overexpression of TFE3, which drives tumorigenesis. However, the TFE3 gene fusion has been found in RCC, PEComas, and a few other tumors. Furthermore, immunoreactivity is not specific to ASPS and may be seen in RCC (Xp11), epithelioid hemangioendothelioma (Yes-associated protein [YAP]-TFE3 subtype), perivascular epithelioid cell tumors (TFE3-rearranged PEComas), granular cell tumors, etc.^[9,10]

Therefore, careful interpretation of TFE3 is critical. Among soft tissue sarcomas, ASPSCR1-TFE3 fusion appears to be sensitive and specific and can be detected by either reverse transcription polymerase chain reaction (RT-PCR) or fluorescence in situ hybridization (FISH) studies.^[3] For IHC, technical factors (e.g., antibody dilution, antigen retrieval) may influence staining outcomes. Internal controls and correlation with histomorphology are necessary. While molecular confirmation through FISH, RT-PCR, or next-generation sequencing is ideal, in resource-limited settings, IHC with TFE3 combined with morphology remains a reliable diagnostic surrogate. CD147 and monocarboxylate transporter 1 are also frequently expressed in ASPS but lack specificity.^[3,5,9,10] In our cases, only IHC and special stain with PAS and PAS-D were carried out. The combined approach helped in ruling out other differential diagnoses; hence, molecular/next-generation sequencing was not carried out in our cases.

Differential diagnoses at uncommon sites

The diagnosis becomes particularly challenging when ASPS occurs at rare locations. In our study, one case involved the breast, a highly unusual site. Before considering rare sarcomas, common breast tumors must be ruled out. Differential diagnoses include invasive lobular carcinoma with apocrine differentiation, which may show discohesive cells with apocrine features, resembling ASPS. However, pseudoalveolar architecture and fibrovascular septa seen in ASPS are lacking. Loss of E-cadherin is typical in lobular carcinoma; in our case, E-cadherin was retained.^[11,12] Carcinomas with histiocytoid/apocrine-like cells and apocrine carcinoma may be confused with ASPS. On morphology, cells have abundant vacuolated cytoplasm with histiocytic and apocrine morphology, which is generally not seen in ASPS. Apocrine carcinoma cells have abundant vacuolated or foamy cytoplasm, which is generally confused with ASPS. The diagnosis requires a comprehensive analysis of clinical history, histological morphology, and IHC. ASPS is negative for gross cystic disease fluid protein-15 (GCDFP-15) and androgen receptor, which are typically positive in apocrine carcinomas. The tumors with histiocytoid cells express CD68, unlike ASPS.^[12] In the parapharyngeal region, differential diagnoses include squamous carcinoma, sarcomatoid carcinoma, paraganglioma, and granular cell tumor. However, these have distinct histologic and immunophenotypic profiles. Other important differentials include RCC (especially metastatic), alveolar rhabdomyosarcoma, paraganglioma, malignant melanoma, and granular cell tumor.

While architecture and cytology overlap (nested/alveolar patterns, eosinophilic cytoplasm), IHC and with special stain for PAS and PAS becomes definitive as depicted in tables [Table 2a and b]. PAS-D staining reveals rhomboid or rod-shaped crystals that are diastase-resistant and diagnostic of ASPS, as cited in Figures 2d and 4d.^[3,5,13]. These vary in quantity and may be absent in some cases.

We hence recommend that, as there are challenges in the diagnosis of ASPS, both clinically and microscopically, a combined approach should be initiated to reach a final diagnosis for better management of the patient.

Treatment and prognosis

Diagnosis of ASPS necessitates a multimodal approach. The mainstay of treatment is surgical excision with wide margins. Metastasectomy may be performed where feasible. Radiotherapy [IMRT]) reduces local recurrence risk. Chemotherapy has limited efficacy; however, targeted therapies such as crizotinib (c-MET inhibitor), tyrosine kinase inhibitors (Vascular endothelial growth [VEGF]/Mesenchymal-epithelial transition factor [MET]), and immunotherapies (Programmed cell death protein [PD-1]/Programmed death-ligand 1 [PD-L1] checkpoint inhibitors) have shown promise.^[11,14]

ASPS are not graded but are regarded as high-grade by definition.^[5] Despite slow growth, ASPS has a high metastatic potential and can be fatal. The 5-year overall survival rate for ASPS ranges from 47% to 68%, with rates of 73-100% for localized cases and 20-62% for distant metastases.^[15] Poor prognostic factors^[5] include: Distant metastases at diagnosis, tumors located on the trunk, tumor size >10 cm, and older age. Conversely, early-stage, resectable tumors (e.g., lingual ASPS) in children often have a better prognosis.^[5]

This case series highlights the diagnostic challenges in identifying ASPS, especially when it occurs in rare or atypical locations such as the breast. The tumor’s histological resemblance to other neoplasms, such as lobular carcinoma, rhabdomyosarcoma, and metastatic RCC, emphasizes the need for a comprehensive IHC panel along with PAS staining. While TFE3 nuclear positivity is an important IHC marker for ASPS, it is not entirely specific; therefore, a panel of IHC markers is essential to rule out other TFE3-positive tumors. The presence of PAS-positive, diastase-resistant crystals provides additional diagnostic support. With emerging evidence suggesting a potential role for immunotherapy in advanced ASPS, future directions may focus on molecular profiling and clinical trials involving immune checkpoint inhibitors.