Primary Alveolar Soft Part Sarcoma of Bone: A Rare Occurrence in 8 Patients, Posing a Diagnostic Challenge

A retrospective review of the hospital records from 2011 to 2020 (inclusive) yielded 86 diagnosed cases of ASPS in a 10-year period. After a detailed review of the radiologic and histopathologic details, 8 cases (9.3%) were included as ASPS primarily arising in the bone. Five of the 8 patients were treated at our institution, and 3 were referred for a second consultation. The clinical information (including follow-up), radiologic images, and corresponding reports were available in 7 cases. Occult metastasis from other sites was ruled out in all these cases based on clinical evaluation and positron emission tomography scan. In a single case, we received paraffin blocks for consultation and the corresponding radiology report was available from the referring center.

Sections stained with hematoxylin and eosin were available in all 8 cases. Immunohistochemistry was performed on paraffin-embedded tissue in 7 cases. The Ventana Benchmark XT automated system was used for immunohistochemistry. The various antibodies used included cytokeratin (AE1/AE3), TFE3, desmin, HepPar1, glypican 3, synaptophysin, S-100p, paired box gene 8 (PAX8), α-methylacyl CoA racemase (AMACR), myoblast determination protein 1 (MyoD1), integrase interactor 1 (INI1), CD30, CD163, and CD68. Appropriate positive and negative controls were applied. Table 1 lists antibodies used along with specifications of clones and dilutions.

Five of the patients were female and 3 were male. The age of the patients ranged from 13 to 56 years (median, 33 years). The primary bone involvement was seen in the humerus (n = 1), tibia (n = 1), fibula (n = 3), calcaneus (n = 1), rib (n = 1), and radius (n = 1). Pain was the universal presenting symptom.

The tumors were centered in the diaphyseal and meta-diaphyseal regions in all the involved long bones. Pathologic fracture was found in the patient with tumor in the humerus. Radiographs of the affected bones showed expansile lytic lesions with cortical destruction and a soft tissue component in all patients. Magnetic resonance imaging revealed expansile lytic lesions with a wide zone of transition in the diaphyseal region of affected bones. Cortical break with a large extraosseous soft tissue component could also be demonstrated on radiologic imaging in all patients. Presence of distant metastases could be excluded in 7 of 8 patients by positron emission tomography scan. Metastatic workup was not available in one patient. The commonest sites of metastases were lungs in 6 of 7 patients, followed by liver (n = 1), vertebrae (n = 2), and brain (n = 1). Five of 7 patients had metastases at the time of primary presentation. None of the patients had any abnormality in the kidneys.

All patients underwent diagnostic biopsy, and all except one showed characteristic histopathologic features of ASPS. Final diagnosis was rendered on tumor morphology and positive nuclear expression of TFE3 on immunohistochemistry. On microscopic examination, the tumor cells were arranged in a nested/alveolar pattern separated by thin fibrovascular septa. Tumor cells were large with well-defined cell membranes and contained abundant eosinophilic and granular cytoplasm. Mild nuclear anisocytosis, vesicular chromatin, and prominent nucleoli were evident in the tumor cell nuclei. On immunohistochemistry, a strong positive expression of TFE3 within tumor cell nuclei could be demonstrated in 6 of 8 cases. Figure 1, A through D, shows a compilation of radiography, histopathology, and immunohistochemistry for TFE3 of ASPS of the humerus (case 1). TFE3 expression was lacking in one case and the test could not be performed in another case. Periodic acid-Schiff–diastase stain highlighting intracytoplasmic crystals and granules could be demonstrated in these 2 cases. However, both of these cases exhibited classical histomorphology features of ASPS. Figures 2, A through C, and 3, A through C, show radiography and histopathology images of primary ASPS of the fibula (case 3) and radius (case 8), respectively.

Unlike other cases, the conventional alveolar pattern was less pronounced in the biopsy from the rib tumor (case 5). It showed a malignant tumor with cohesive sheets of large tumor cells with abundant eosinophilic cytoplasm. However, immunohistochemistry for TFE3 was strongly positive in tumor cell nuclei.

Application of immunohistochemistry panel varied in all cases, and was essentially determined by differential diagnoses in each case. Other immunohistochemistry markers included were HepPar1, glypican 3, PAX8, AMACR, AE1/AE3, desmin, myoD1, S-100p, synaptophysin, CD163, CD68, CD30, and INI1. The frequency of immunoreactivity for these markers is mentioned in Table 1.

Four of 8 patients underwent surgical treatment for primary tumor and 2 patients refused surgical as well as medical treatment. Treatment details were not available for the other 2 patients. Local excision of tumor was performed in 3 patients, whereas above-knee amputation had to be undertaken in a single patient. Pulmonary metastasectomy (twice) was performed on 1 patient subsequent to excision of primary tumor. Two of the 4 patients received adjuvant chemotherapy, and 1 patient also received adjuvant radiotherapy. Treatment with interferon α was given to 1 patient.

Follow-up was available for 6 of 8 patients. Two of the 6 patients were alive with disease, and 4 patients succumbed to locally recurrent and progressive metastatic disease. Three of the 4 patients experienced local recurrence after surgical excision. One of the 2 patients who refused any form of treatment died of metastatic disease, and the other was discharged upon counseling about the guarded prognosis because of presence of metastasis. She was lost to further management and follow-up. We have no information about treatment details and follow-up of 1 patient.

Table 2 summarizes clinical presentation, radiology, treatment details, and follow-up of all 8 patients.

ASPS is regarded as a high-grade soft tissue sarcoma with poor prognosis.^1,2  The overall survival rate is 82% at 2 years and 56% at 5 years.^1,2,14  Negative prognostic factors include age, tumor size more than 10 cm, distant metastasis at the time of diagnosis, and primary site in the trunk.^1,14  Contrary to the general finding of affected patients being of the pediatric age group or young adults,^1,15  4 patients in our series were age 30 years or older, the oldest being 56 years, and 4 were age 25 years or younger. Older age can be highlighted as a bad prognostic parameter. Older age also adds to the diagnostic challenges because of stronger possibilities of metastatic carcinomas.

In addition to conventional sites, ASPS can be found in unusual locations like the bladder, stomach, gynecologic tract, and bone.^1,2  The tumor can involve the bones in 3 different ways: (1) by extension of the soft tissue tumor, (2) as part of metastatic disease, or (3) as a primary tumor.¹⁶  Primary intraosseous ASPS is exceedingly rare.⁹  To the best of our knowledge, there are only 26 such cases reported in the world literature.^6–13,16  The present series of 8 cases of primary bone ASPS highlights their clinicopathologic features along with challenges in their management.

Radiologically, the tumor epicenter in the bone was noted in all 8 cases in our study. A common radiologic feature of all cases in our study was that of a destructive lesion with poorly defined tumor margins and epicenter in the bone, indicative of a primary malignant bone tumor.^6,9,17,18  The tumors appeared as large, lytic, and expansile lesions, associated with a cortical break and a significant soft tissue component, indicating the locally destructive nature of the tumors. All the tumors occurring in the extremity location were noted in the diaphysis. On magnetic resonance imaging, increased signal intensity on both T1- and T2-weighted images was seen.¹⁹  Although the diagnosis of the primary malignant bone tumor was established in all patients, none of the 8 patients in our series could be diagnosed with ASPS in radiology. However, the radiology opinion was critical in all patients: it established the primary bone origin of the tumors and ruled out secondary extension or metastasis from soft tissue.

ASPS is histologically characterized by a distinctive morphology in the form of large polygonal cells arranged in a uniform organoid or nested pattern with central discohesion resulting in a characteristic alveolar pattern.^20–23 

ASPS shows a distinctive unbalanced translocation between the X chromosome and chromosome 17: ASPSCR1-TFE3 translocation der(17)t(x;17)(p11; q25), which is considered the initiating event in ASPS.^24–26  Unbalanced translocation results in the creation of an ASPSCR1-TFE3 translocation fusion protein. An antibody directed against the C terminus of TFE3 has been developed and has been demonstrated to be a highly sensitive and specific marker not only for ASPS but also for tumors harboring TFE3 gene fusions. Strong nuclear immunohistochemical positivity for antibody to TFE3 is a robust marker for the presence of this fusion transcript.²⁴  It is a useful adjunct to making the diagnosis of ASPS. In addition to immunohistochemistry for TFE3, ASPS shows immunologic expression of cathepsin K, calretinin, S-100p, and, focally, desmin (in 50% of cases).^1,2,20  ASPS is typically negative for epithelial markers (cytokeratin [CK]/epithelial membrane antigen [EMA]), neuroendocrine markers (synaptophysin, chromogranin A), and melanocytic markers (S-100p, human melanoma black [HMB45], melan-A).²  Classic morphology and strong nuclear positivity for TFE3 on immunohistochemistry were confirmatory diagnostic features of ASPS in 6 of our 8 cases.

The morphologic differential diagnosis of ASPS includes a wide range of tumors displaying a similar alveolar/nested growth pattern, TFE3 expression, or both.²⁰  These include metastatic renal cell carcinoma (RCC), metastatic hepatocellular carcinoma (HCC), and metastatic adrenocortical carcinoma (ACC), which warrant discussion in the context of primary bone ASPS because they are relevant to our report in this study. All these tumors have a striking histologic similarity to ASPS and have strong propensity to metastasize to the bones.^27–29  The clinical, radiologic, and histopathologic dilemma is escalated even further with advancing age of the patients. Four of the 8 patients in our series were younger than 25 years of age, 1 was 30 years old, and the remaining were older than 40 years of age, with the oldest patient being 56 years old. Moreover, it is also common for patients suffering from RCC, HCC, or ACC to present with bone metastasis with occult primary.^27–29  The diagnosis of primary ASPS of bone is likely to be missed if the index of suspicion is low. The impact of misdiagnosis on prognosis and management of the patient is considerable in view of the much worse prognoses of HCC, RCC, and ACC.^27–30 

The diagnostic difficulty becomes even more pronounced if there are lesions in the liver that would most rationally be considered as a primary liver tumor with bone being a metastatic site.^10,20,30  This was particularly exemplified in one of our patients (with ASPS of the humerus, case 1, Table 2), in whom biopsy from the liver showed a striking morphologic similarity to HCC. We had to resort to an extended immunohistochemistry panel of antibodies by inclusion of liver-specific markers, namely HepPar1 and glypican 3, in the primary tumor in the humerus and deposits in the liver and lungs. Biopsies from both sites (humerus and liver) showed features of ASPS. The impact of misdiagnosis on prognosis and management of the patient is considerable.³⁰ 

Differential diagnoses of metastatic RCC and intraosseous paraganglioma were considered in case 4, which warranted use of antibodies to PAX8 and AMACR. Two patients with primary ASPS of the humerus and fibula in our series are still alive after undergoing resection of primary bone tumor and amputation, respectively, despite the presence of liver and lung metastasis, thus highlighting the relative difference in prognosis between HCC and ASPS.

Careful thought concerning the tumor morphology is essential in the above situations, and the application of an appropriate panel of immunohistochemistry is justified. Lack of expression of epithelial markers should raise a strong suspicion of a tumor other than ACC, RCC, and HCC, even though ACC is well known for its paucity of expression of cytokeratins.^10,20,31  None of these 3 tumors would show positive immunostaining for TFE3, though TFE3 can be positively expressed in certain translocation-related RCCs.³² 

ASPS generally replicates its characteristic morphology with little variation from case to case.²⁰  Hence, deviation from its conventional morphology hampers the thought process, leading to incorrect differential diagnoses. Anaplastic-appearing tumor cells with marked variation in nuclear size were described by Evans.²¹  Other atypical features, such as sheetlike architecture, frequent mitotic figures, spindling, xanthomatous change, and coagulative tumor cell necrosis, have also been reported.^20–22  One of our patients (case 5, ASPS of rib, Table 2) posed this diagnostic challenge in the form of loss of conventional alveolar morphology of ASPS. The tumor exhibited sheets of large cells resembling histiocytes. Erdheim-Chester disease was considered as a differential diagnosis in this patient, along with metastatic RCC. Although a positive immunohistochemistry marker for TFE3 with lack of histiocytic markers (CD163, CD68) was useful in forming a correct diagnosis, we were faced with a formidable combination of unusual site and unconventional morphology in this case.

Immunohistochemistry with antibody to TFE3 remains the cornerstone for the diagnosis of ASPS at any site. However, specificity of TFE3 immunohistochemistry gets diluted by the fact that TFE3 positivity is seen in some other tumors, such as translocation-associated RCC (TrRCC), perivascular epithelioid cell tumor, a small proportion of epithelioid hemangioendotheliomas (EHEs), paraganglioma, granular cell tumor, solid pseudopapillary neoplasm of pancreas, and ovarian sclerosing stromal tumor.^32–39  The distinction of ASPS from these tumors is essentially based upon the site of tumor, morphology, and results of relevant immunohistochemical markers.^20,33–36 

TrRCC has a predilection to occur in children and young adults, similar to ASPS. It not only resembles ASPS morphologically but also shares its underlying molecular signature in the form of der(17)t(x;17)(p11;q25) translocation. One of the 2 variants of TrRCC (type 1) is labeled as ASPS-like TrRCC. The presence of a renal mass along with other immunohistochemistry markers, notably positive AE1/AE3, in TrRCC distinguishes these 2 entities.^32,40  Other markers positive in TrRCC are PAX8, HMB45, Melan-A, cathepsin K, and TFE3.³² 

In the context of bone involvement, EHE deserves mention because this tumor can affect the bones primarily. Multifocal osseous involvement is one of the manifestations of this tumor.⁴¹  Although only a small proportion of EHEs show TFE3 immunostaining, the finding can be confounding considering the presence of epithelioid phenotype, which is shared by ASPS and EHE. Vascular markers, namely CD31, CD34, and ERG, are expressed on tumor cells of EHE by immunohistochemistry.⁴¹ 

Intra-osseous paragangliomas have been reported⁴²  that show overlapping morphologic features with ASPS.^20,43  Nevertheless, TFE3 can be positive in paraganglioma. However, unlike ASPS, paragangliomas show diffuse and strong positivity for chromogranin A and synaptophysin.²⁰ Table 3 shows differential diagnoses of ASPS especially in the context of bone involvement.

ASPS is known to recur and metastasize years after surgical treatment for the primary tumor. Lungs and bones are the common sites of metastasis of ASPS.⁴⁴  It is imperative to extract clinical history to rule out a delayed metastasis of ASPS to the bone from other sites before arriving at the exceedingly rare diagnosis of primary bone ASPS.

Although treatment of choice for primary as well as metastatic ASPS remains surgery, the utility of adjuvant chemotherapy is still a challenge.^45,46  ASPS is generally refractory to chemotherapy or radiotherapy. Surgical treatment of ASPS includes radical excision of primary and metastatic lesions combined with radiotherapy, chemotherapy, or both. The role of radiotherapy is suggested when the surgical margin is insufficient.⁴⁴  Metastatic inoperable ASPS is usually resistant to conventional chemotherapy and radiotherapy, and is hence incurable despite prolonged overall survival.⁴⁷  Aggressive behavior defined by local recurrence and distant metastases of ASPS is reemphasized by the clinical course of disease seen in our series. Distant metastases were seen at the time of primary presentation in 5 of 8 patients (62.5%). Four patients succumbed to progressive, treatment-refractory disease, 2 subsequent to surgical excision. One patient who refused any treatment and was later lost to follow-up had widespread metastases (case 4). Although metastasis at presentation is an indication of the tumor's aggressive behavior, 2 patients (cases 1 and 3) are still alive with metastatic disease. One of those underwent pulmonary metastasectomy on 2 occasions.

Recently some therapeutic role of targeted therapy is promising in advanced ASPS. The role of antiangiogenic therapy, namely anti–vascular endothelial growth factor drugs (cediranib), is suggested in advanced ASPS owing to its highly vascular nature. The role of the tyrosine kinase inhibitor sunitinib and related drugs, namely crizotinib, pazopanib, and cabozantinib, has been described.^48,49  In one of our patients with metastatic pulmonary disease (case 2), treatment with interferons resulted in stabilization of the disease for 30 months.

Two of 8 cases in the present study were initially evaluated and treated at general hospitals. One case (case 2) had delay in diagnosis because the patient was initially treated for infection, and in another case (case 6) the lesion was excised incompletely without prior appropriate imaging, leading to local recurrence and rendering the limb nonsalvageable. We emphasize that treatment of bone and soft tissue sarcomas should be done at specialized sarcoma centers, where all the expertise required for their management is accessible.^50,51 

ASPS is a rare tumor usually arising in the soft tissues. Primary bone involvement is very rare. The combined application of clinical, radiologic, histomorphologic, and immunohistochemical features is helpful to arrive at a definite diagnosis of ASPS, including at uncommon locations, such as bone in the present series. Our experience of 8 cases of primary bone ASPS emphasizes diagnostic challenges given the rarity of the site of this tumor as well as its aggressive behavior. It is essential to consider differential diagnoses of metastasis of tumors that closely resemble ASPS, both morphologically and genetically, and also have proclivity for bone metastasis. A thorough clinical and radiologic examination is necessary to exclude metastasis. Primary ASPS of the bone should preferably be managed with complete surgical resection, preferably at a specialized sarcoma treatment center. Currently, adjuvant therapies seem to have a limited role.