Primary pulmonary artery synovial sarcoma: a case report

Introduction

Background

Primary cardiac synovial sarcoma (PCSS) is an exceptionally rare malignant tumor, accounting for less than 5% of all primary cardiac neoplasms (1,2). It most commonly arises in the right heart chambers and can occur in the pulmonary artery, although such cases are extremely uncommon. Due to its rarity and nonspecific clinical manifestations, PCSS is frequently misdiagnosed preoperatively as the more common cardiac myxoma or other benign conditions (3,4).

Rationale and knowledge gap

The clinical and imaging features of PCSS often overlap with those of myxoma, particularly on transthoracic echocardiography (TTE), where both may present as mobile, solid intracardiac masses. However, malignant tumors like PCSS may exhibit distinct features such as broad base, infiltrative growth, and atypical perfusion patterns on contrast-enhanced imaging. These subtle differences are often overlooked, leading to delayed diagnosis and suboptimal treatment. A clear understanding of these distinguishing characteristics is critical for timely diagnosis.

Objective

This case report aims to describe a rare case of primary pulmonary artery synovial sarcoma (PPASS) initially misdiagnosed as myxoma, to analyze the reasons for misdiagnosis, and to highlight key imaging clues that may facilitate accurate preoperative identification. We present this article in accordance with the CARE reporting checklist (available at https://acr.amegroups.com/article/view/10.21037/acr-2026-0091/rc).

Case presentation

Patient information

A 35-year-old male presented with a chief complaint of “chest pain after activity for 1 month”. No significant past medical history of hypertension or diabetes was reported. The patient reported no significant past medical history, family history of cardiac tumors, or relevant psychosocial history. All procedures performed in this study were in accordance with the ethical standards of the institutional and/or national research committee(s) and with the Declaration of Helsinki and its subsequent amendments. Written informed consent was obtained from the patient for the publication of this case report and accompanying images. A copy of the written consent is available for review by the editorial office of this journal.

Clinical findings

Physical examination revealed a body temperature of 36.5 ℃, respiratory rate of 20 breaths/min, blood pressure of 121/86 mmHg, and heart rate of 75 beats/min with regular rhythm. No significant pathological murmurs were auscultated in any valve area, and the remainder of the physical examination was unremarkable.

Timeline

The key events are summarized as follows: (I) 1 month prior to admission: onset of exertional chest pain; (II) admission: initial clinical and laboratory assessment, TTE performed; (III) subsequent days: contrast-enhanced echocardiography, cardiac magnetic resonance imaging (MRI), and positron emission tomography/computed tomography (PET/CT) conducted; (IV) hospital day 5 (post-diagnostic workup): surgical resection performed; (V) postoperative day 10: follow-up TTE; (VI) postoperative day 15: patient discharged; (VII) postoperative month 6: imaging revealed recurrence and metastases; initiation of chemotherapy.

Diagnostic assessment

Diagnostic methods

Laboratory findings: immunofixation electrophoresis results were all negative; urinary light chain κ was 0.07 g/L, and urinary light chain λ was 0.08 g/L; coagulation function and D-dimer were within normal limits; N-terminal pro-B-type natriuretic peptide (NT-proBNP) was 2,298 pg/mL. Tumor markers were not tested.

TTE revealed an irregular solid mass at the pulmonary valve orifice, measuring approximately 46 mm × 18 mm. The mass appeared to be attached to the septal side of the right ventricular outflow tract beneath the pulmonary valve, with a broad base and unclear border with the pulmonary valve. It exhibited significant mobility (Figure 1).

Figure 1 Preoperative transthoracic echocardiography showing an irregular solid mass at the pulmonary orifice (arrow).

Contrast-enhanced echocardiography showed mild contrast perfusion within the solid mass, with perfusion intensity lower than that of the adjacent myocardium (Figure 2).

Figure 2 Preoperative contrast echocardiography showing mild contrast perfusion within the solid mass (arrows).

Cardiac MRI demonstrated a well-defined filling defect within the main pulmonary artery, showing expansive growth and measuring 47 mm × 34 mm. The lesion had ill-defined borders with the tunica media of the pulmonary artery, which was atypical for a typical myxoma (Figure 3).

Figure 3 Preoperative MRI showing a filling defect within the main pulmonary artery (arrows). MRI, magnetic resonance imaging.

PET/CT revealed an irregular, mildly hypodense lesion within the main pulmonary artery, with unclear borders and mildly increased fluorodeoxyglucose (FDG) uptake [maximum standardized uptake value (SUV) value 5.4] (Figures 4,5).

Figure 4 Transverse CT scan showing an irregular, mildly hypodense lesion in the main pulmonary artery (arrow). CT, computed tomography.

Figure 5 Transverse fused PET/CT image showing mildly increased FDG uptake in the lesion (arrow). FDG, fluorodeoxyglucose; PET/CT, positron emission tomography/computed tomography.

Intraoperative findings: after median sternotomy and establishment of cardiopulmonary bypass, the pulmonary artery was opened. No tumor emboli were found in the bilateral pulmonary arteries. Two distinct tumors were identified: (I) a 20 mm × 30 mm mass attached to the pulmonary valve leaflet by a 1-cm pedicle; (II) a 30 mm × 40 mm mass attached to the right ventricular outflow tract by a 1.5-cm pedicle. Both were excised en bloc with partial resection of the pulmonary valve leaflet. The valve defect was reconstructed using a bovine pericardial patch. The tricuspid valve was repaired with a 28-mm MC3 annuloplasty ring.

Diagnostic challenges and reasoning

The initial imaging diagnosis was a cardiac mass of undetermined nature, with a leading suspicion for myxoma due to its mobility and solid appearance. However, the atypical features of a broad base, ill-defined border, and only mild (rather than brisk) contrast perfusion created diagnostic uncertainty and raised suspicion for a malignant tumor such as synovial sarcoma.

Final diagnosis and prognostic characteristics

The definitive diagnosis was established postoperatively. Pathology revealed a spindle cell tumor. Immunohistochemistry showed partial expression of CK, EMA, Vim, SMA, and FLI1 in tumor cells. Molecular testing confirmed SS18 gene rearrangement [fluorescence in situ hybridization (FISH) positive]. The final diagnosis was biphasic synovial sarcoma of the pulmonary valve. The Ki-67 index was approximately 60%, indicating high proliferative activity and a potentially aggressive clinical course (2,5,6) (Figure 6A-6C).

Figure 6 Histopathological and molecular features of biphasic synovial sarcoma. (A) H&E staining (×100) showing spindle cell proliferation. (B) Immunohistochemistry showing positivity for CK (×200). (C) FISH analysis using SS18 break-apart probes showing split signals (arrows), confirming gene rearrangement. Staining method: fluorescence; magnification: ×400. CK, cytokeratin; FISH, fluorescence in situ hybridization; H&E, hematoxylin and eosin.

Therapeutic intervention

The patient underwent surgical resection of the cardiac tumor, pulmonary valvuloplasty, and tricuspid valvuloplasty, achieving complete tumor resection. No changes to the intervention were required during the procedure.

Follow-up and outcomes

Ten days postoperatively, follow-up TTE showed complete resolution of the pulmonary artery mass, with no residual abnormalities in the main pulmonary artery or right ventricular outflow tract, and unobstructed blood flow (Figure 7). The patient recovered well postoperatively and was discharged on postoperative day 15. However, 6 months post-surgery, imaging revealed disease progression: nodular lesions around the pulmonary artery (local recurrence) and multiple bilateral pulmonary nodules (lung metastases). Given the aggressive nature, systemic chemotherapy with ifosfamide, pirarubicin, and mesna was initiated. The patient completed two cycles with tolerable toxicity. Long-term outcomes are pending.

Figure 7 Postoperative echocardiography showing complete resolution of the pulmonary artery mass.

Discussion

Strengths and limitations

The strength of this report lies in the detailed multimodal imaging characterization and the integration of pathological and molecular diagnostics. Limitations include the lack of some clinical data and the absence of long-term follow-up information on adjuvant therapy and patient outcomes.

Relevant medical literature and imaging differentiation

Consistent with prior reports, this case demonstrates that PCSS is frequently misdiagnosed as myxoma due to overlapping imaging features on TTE. However, the presence of atypical features served as red flags for malignancy. Specifically, unlike typical cardiac myxomas which usually present as pedunculated masses with clear borders and brisk contrast enhancement, this tumor exhibited a broad base and ill-defined borders. Furthermore, the mild contrast perfusion within the mass was significantly lower than that of the adjacent myocardium, which is atypical for highly vascularized myxomas. These imaging clues should prompt clinicians to consider sarcoma. Furthermore, unlike pulmonary embolism (PE), PPASS typically presents as a progressively enlarging intraluminal mass with internal vascularity rather than acute cardiorespiratory collapse. While PE usually shows complete filling defects with no intrinsic perfusion, this case showed mild heterogeneous perfusion, supporting a differential diagnosis of a solid tumor (7).

Rationale for conclusions and pathological confirmation

The definitive diagnosis relied on the integration of histopathology and molecular genetics. While hematoxylin and eosin (H&E) suggested a spindle cell tumor, immunohistochemistry revealed partial positivity for CK and EMA. Crucially, molecular testing via FISH confirmed SS18 gene rearrangement, which is the gold standard for diagnosing synovial sarcoma and ruling out histological mimics such as leiomyosarcoma (Desmin-negative) or solitary fibrous tumor (CD34-negative).The mild contrast perfusion likely reflects the heterogeneous vascularity of synovial sarcoma, contrasting with the brisk enhancement typical of myxoma. The broad base and unclear border suggest infiltrative growth. The high Ki-67 index (60%) is consistent with an aggressive tumor biology, which was evidenced by the early local recurrence and distant metastasis despite complete surgical resection.

Conclusions

PPASS is a rare and aggressive malignancy that is frequently misdiagnosed as cardiac myxoma due to overlapping imaging features. This case highlights that a broad-based attachment, ill-defined borders, and atypical mild perfusion on contrast-enhanced echocardiography are critical red flags for malignancy. While histopathology provides preliminary evidence, definitive diagnosis relies on molecular detection of the SS18 gene rearrangement. Complete surgical resection remains the cornerstone of management; however, given the high proliferative potential indicated by a high Ki-67 index, immediate postoperative adjuvant therapy should be considered to mitigate the risk of early recurrence and metastasis.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the CARE reporting checklist. Available at https://acr.amegroups.com/article/view/10.21037/acr-2026-0091/rc

Peer Review File: Available at https://acr.amegroups.com/article/view/10.21037/acr-2026-0091/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://acr.amegroups.com/article/view/10.21037/acr-2026-0091/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All procedures performed in this study were in accordance with the ethical standards of the institutional and/or national research committee(s) and with the Declaration of Helsinki and its subsequent amendments. Written informed consent was obtained from the patient for the publication of this case report and accompanying images. A copy of the written consent is available for review by the editorial office of this journal.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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