Synovial sarcoma is a primitive mesenchymal malignant tumor with a poor prognosis that frequently occurs in adolescents and young adults [3]. It generally arises near joints, but it can develop in unexpected locations such as the head and neck, heart, kidney, lung, and abdomen, which can result in a preoperative misdiagnosis [4–7]. In the head and neck region, it most commonly occurs in the hypopharynx, but it can also occur in the prevertebral, parapharyngeal, laryngeal, and maxillofacial areas [8, 9]. In addition, patients with synovial sarcoma involving the head and neck tend to be younger than those with synovial sarcoma of the extremities [8, 9]. In such cases, various differential diagnoses should be considered, such as lymphoma, Ewing’s sarcoma, fibrosarcoma, hemangiopericytoma, and malignant peripheral nerve sheath tumor [10, 11].
Although preoperative CT and MRI are necessary to ascertain tumor location, extent, and metastasis [11, 12], the imaging characteristics of PPSS have not been determined, presumably due to its rarity. In particular, MRI is superior to CT in assessing head and neck pathology because it provides a higher spatial resolution and contrast for soft tissue, and determining the MRI characteristics of PPSS will aid preoperative diagnosis.
Here, we report the detailed conventional and advanced MRI characteristics of PPSS. Our patient’s tumor showed low T1- and high T2-weighted signal intensities with homogeneous contrast-enhancement. These findings are consistent with those of a previous report [10]. Notably, the T2-weighted signal intensity, which reflects intratumoral cellularity and water content, appeared hyperintense when compared with the adjacent muscles, thus ruling out lymphoma or Ewing sarcoma, which have low to intermediate signal intensity [13, 14]. Although PPSS, lymphoma, and Ewing sarcoma are all hypercellular tumors, biphasic synovial sarcoma has distinct epithelial and spindle cell components with prominent stromal matrices [2], while lymphoma and Ewing sarcoma have compact cellularity with less stromal prominence [13, 14]. Consequently, lymphomas and Ewing sarcomas can have lower T2-weighted signal intensities than PPSS. Therefore, the T2-weighted signal intensity of the tumor can be used to differentiate PPSS from lymphoma or Ewing sarcoma.
We verified this assumption regarding tumor cellularity using quantitative DWI. DWI can be used to measure the differences in the random displacement of water molecules in tissues [15]. This difference in water mobility is quantified by the ADC value, which is inversely correlated with tissue cellularity [15]. ADC values have been known to be lower in malignant head and neck cancers than in benign tumors, with cut-off values of approximately 1.25 × 10− 3 mm2/sec. Furthermore, due to the compact cellularity of lymphomas, ADC values are lower in lymphomas than in squamous cell carcinomas, with a range of 0.64–0.66 × 10− 3 mm2/sec [16]. Therefore, the ADC value of 0.76×10− 3 mm2/s seen in our patient confirmed that the tumor had higher cellularity than a benign tumor, but lower cellularity than a lymphoma, which was a significant diagnostic clue.
Previous studies that have used DCE-MRI to assess head and neck tumors, adopted different vendors, scan protocols, and software, thus, the generalizability of the results could not be confirmed [17]. However, we observed low values of Ktrans and Vp, both of which have been found to correlate with intratumoral MVD and vascular endothelial growth factor expression in pathologic studies [18]. Interestingly, we observed low MVD on CD34 immunostaining and low intratumoral vascular parameters on DCE-MRI, indicating that DCE-MRI parameters can be used to demonstrate the hypovascularity of PPSS. This is consistent with the findings of a study of synovial sarcoma arising from the kidney [19]. This might aid the differentiation of PPSS from other hypervascular tumors such as hemangiopericytoma. However, further research on the DCE-MRI parameters and their pathologic correlations is warranted.
Ours is the first study to report a high APTw-signal in a patient with PPSS. APTw-MRI is a recently developed molecular imaging technique that detects amide proton constituents in tumors based on chemical exchange saturation transfer between free water and mobile proteins/peptides [20]. A few studies have used APTw-MRI to differentiate between benign and malignant head and neck tumors, and they found that a cutoff APTw-signal of approximately 2% showed a good performance in differentiating benign and malignant tumors [21]. We observed a high APTw-signal (2.6%), which may be a characteristic finding of malignant tumors. However, no studies have compared APTw-signal values between lymphomatous and non-lymphomatous tumors. Further studies evaluating the importance of APTw-MRI for this differentiation are necessary.
In conclusion, PPSS is a rare tumor that is often challenging to diagnose and treat and requires multidisciplinary management. Our patient demonstrated the characteristic findings of PPSS, including a homogeneously enhancing mass with a high signal intensity on T2-weighted conventional MRI, low ADC value on DWI, low Ktrans and Vp on DCE-MRI, and high APTw-signal on APTw-MRI. Considering these imaging findings in addition to the clinical features of PPSS could improve preoperative diagnosis and enable appropriate surgical planning and early treatment.