Uncommon presentation of synovial sarcoma in the tibial nerve: Diagnostic insights and surgical management in a young patient

INTRODUCTION

Synovial sarcomas (SSs) are malignant soft tissue tumors, representing about 10% of all soft tissue sarcomas.^[1] Previously thought to arise from the synovium due to their periarticular location, it is now known that they can develop in various body parts, with <5% in direct contact with the synovium.^[2] The term “synovial sarcoma” is thus somewhat misleading, as these tumors are believed to arise from “multipotent stem cells” capable of differentiating into both “epithelial and mesenchymal cell types.” Primary SSs of the nerve are extremely rare, with only a handful of cases documented in medical literature.^[3]

In this report, we describe an uncommon case of a 15-year-old male patient diagnosed with SS of the tibial nerve in the popliteal region. This case represents the tenth instance documented in the literature involving SS of the tibial nerve, with popliteal fossa involvement in only 4 previous cases and successful limb preservation achieved in only one of those.^[4-8] Our discussion will emphasize the critical radiological features, particularly ultrasonographic, that are essential for considering and confirming this rare diagnosis.

CASE REPORT

A 15-year-old male patient with no significant medical history presented with a concern about swelling in the calf and popliteal region of his left lower limb. The swelling had been present for 6 months, gradually increasing in size. Upon seeking medical attention, a local examination revealed a firm, non-tender swelling approximately 5 × 7 cm in size on the posterior aspect of the left leg, with no overlying skin changes. The patient also experienced difficulty in weight-bearing. Active ankle and toe movements were observed, and the range of motion in the knee joint was normal. The Tinel’s sign was positive. However, the patient did not report any neural symptoms. A conventional radiograph of the knee did not show any significant abnormalities [Figure 1]. However, an ultrasound examination revealed a large, heterogeneous hypoechoic mass with irregular borders in the popliteal region, measuring approximately 10.3 × 3.6 × 5.3 cm. The lesion demonstrated a few hyperechoic foci with distal shadowing, suggestive of calcifications, and increased internal vascularity on Doppler. The mass was separating and splaying the tibial nerve fascicles [Figure 2].

Close

Figure 1:

(a and b) Conventional radiograph of the knee showing no significant abnormality.

Export to PPT

Close

Figure 2:

(a and b)Ultrasound examination revealed a large, heterogeneous hypoechoic mass with irregular borders in the popliteal region in close approximation to the tibial nerve (arrow); (c-f) causing separation and splaying of tibial nerve fascicles (arrows); (g) The lesion demonstrated mildly increased internal vascularity on Doppler; and, (h) Panoramic view showing the tibial nerve entering the lesion.

Export to PPT

Magnetic resonance imaging (MRI) revealed a large, lobulated, oblong-shaped, soft tissue mass with predominantly T2/proton density fast spin echo hyperintense signals, a few linear hypointense bands, and isointense signals giving “triple signal pattern” or “bunch of grapes” appearance. The mass was located in the intramuscular plane of the popliteal fossa, extending along the posterior muscles of the upper third of the leg beneath the gastrocnemius muscle. It caused displacement and compression of the posterior tibial nerve, with the possibility of an eccentric origin from the nerve. A “positron emission tomography-computed tomography (PET-CT) scan,” conducted as part of the metastatic workup, revealed increased metabolic uptake in the soft tissue mass in the popliteal fossa region, with no distant metastasis [Figure 3]. Radiological differentials included soft tissue sarcoma, specifically malignant peripheral nerve sheath tumor (given its relation with the tibial nerve), SS, and undifferentiated pleomorphic sarcoma.

Close

Figure 3:

(a-d) Magnetic resonance imaging of the knee showing a large, lobulated, oblong-shaped soft tissue mass with predominantly T2/PDFS (proton density fat-saturated) hyperintense signals, a few linear hypointense bands, cystic changes showing triple signal pattern/bunch of grapes sign. The mass caused displacement and compression of the posterior tibial nerve, with the possibility of an eccentric origin from the nerve; (e) A positron emission tomography-computed tomography (PET-CT) scan, conducted as part of the metastatic workup, revealed increased metabolic uptake in the soft tissue mass in the popliteal fossa region, with no distant metastasis.

Export to PPT

A biopsy of the lesion revealed a tumor comprising spindle cells typically arranged in long fascicles and a herringbone pattern with diffuse membranous positivity for CD99, transducer-like enhancer of split 1, and weak focal positivity for epithelial membrane antigen and pan cytokeratin. The tumor cells were immune-negative for S100. The histopathological diagnosis was monophasic SS [Figure 4]. The patient was scheduled for wide local excision. During surgery, the mass was found to be arising from the tibial nerve and was encasing the posterior tibial artery. The mass was excised, and the posterior tibial artery was repaired using a short saphenous vein graft, while a cable nerve graft from the sural nerve was used for tibial nerve repair [Figure 5]. The post-operative period was uneventful, and the patient is now on regular follow-up.

Close

Figure 4:

Histopathological examination of the biopsy specimen revealed: (a) Hematoxylin and eosin stained (H&E) section showing tumour comprising of spindle cells, typically arranged in long fascicles and herringbone pattern (H&E ×100); (b) Higher magnification showing fascicles of monotonous spindle cells with a herringbone growth pattern and a little intervening stroma (H&E ×400); (c-f) On immunohistochemistry (IHC), the tumour cells are showing diffuse membranous positivity for CD99 (x100) (c), nuclear (Transducer-Like Enhancer of Split 1) TLE1 positive staining (×400) (d), weak focal (Epithelial membrane antigen) EMA positivity (×200) (e), and weak Pan cytokeratin positivity (×200) (f); (g-h) The tumour cells are immunonegative for S100 (×200) and show Ki67 proliferating activity (×400).

Export to PPT

Close

Figure 5:

(a) Per-operative picture showing the mass arising from the tibial nerve; (b) The excised tumour is shown.

Export to PPT

DISCUSSION

SS, while uncommon, is a highly aggressive malignancy accounting for approximately 10% of soft tissue sarcomas. It predominantly affects younger individuals (males more than females), with the majority of cases occurring in individuals aged 20–50 years. Contrary to the term SS, it typically develops in the peri-articular region rather than directly involving joints. Lower limbs are more commonly affected, with less frequent involvement of other locations in the body. Superficial lesions are detected earlier than deep-seated ones, with lesions manifesting as a slowly growing benign-appearing condition.^[1,2]

Pathologically and using immunohistochemistry, three primary forms of SS can be distinguished: “Monophasic type (spindle-shaped cells), biphasic type (epithelial and spindle cells), and poorly differentiated type (rhabdoid pattern).” This tumor is characterized by “translocation t (X;18) (p11.2;q11.2)” in more than 90% of cases, which can be detected using “fluorescence in situ hybridization” or “reverse transcription-polymerase chain reaction” techniques.^[9]

SS of neural origin is extremely rare, with <50 cases reported to date in the literature. This case represents the tenth instance documented in the literature involving SS of the tibial nerve, with popliteal fossa involvement in only 4 previous cases and successful limb preservation achieved in only one of those.^[3-8] Each peripheral nerve is composed of axons that encircle Schwann cells (where schwannomas, neurofibromas, or malignant peripheral nerve sheath tumors can develop). The initial protective layer, known as the endoneurium, surrounds the Schwann cell and is comprised of reticular and collagen fibers. Multiple bundles of these fibers are then enclosed by a second layer, the perineurium, which is made up of epithelioid cells and serves as the point of origin for conditions such as perineurinomas. The outermost sheath is the epineurium, consisting of a fibrous tissue layer that encases the fiber bundles and contains mesenchymal cells that can potentially give rise to SS.^[10] Here is the summary of previously reported tibial nerve SS cases:

Spielmann et al. described a case of a 43-year-old man with a mass in the left popliteal fossa, which, on radiological investigations, was described as intraneural in origin, arising from the posterior tibial nerve. Their provisional diagnosis was peripheral nerve sheath tumor; however, on incisional biopsy, it turned out to be a monophasic spindle cell SS. The patient was subsequently treated with an above-knee amputation because of unresectable involvement of the tibial nerve and involvement of the common peroneal nerve as well.^[4]

Scheithauer et al. presented a case series of ten histopathologically confirmed SS of neural origin. Their cohort comprised 7 females and 3 males, including 4 pediatric patients. In their cohort, 4 tumors affected the tibial nerve (2 in the popliteal fossa, 1 in the medial ankle, and 1 in the posterior compartment of the leg). Popliteal fossa cases in their cohort underwent above-knee amputation.^[5]

Larque et al. retrospectively studied SS of nerves that were interpreted as benign peripheral nerve sheath tumors on imaging. Their cohort comprised 5 histopathologically confirmed SS of neural origin, out of which 2 tumors affected the tibial nerve (1 in the medial ankle and 1 in the calf).^[6]

Hashimoto et al. described a case of an 11-year-old female patient with a mass in the popliteal fossa, which, on imaging and histopathological examination, turned out to be SS. Their patient was treated with limb preservation by repairing the tibial nerve with a sural nerve graft.^[7]

Hemerková et al. presented a case of SS of the tibial nerve (medial ankle and foot) in an elderly woman. The tumor was surgically excised, followed by external radiotherapy to the tumor bed.^[8]

Our case is different from the previously reported cases as the size of the tumor in our patient was quite large, yet it was successfully excised with limb preservation. None of the previously reported cases highlighted the importance of ultrasound in diagnosing this entity. In our case, ultrasound was the game changer as the primary diagnosis of neural origin tumor was made on this modality. Because of the higher spatial resolution of ultrasound, nerves can be easily traced, and any neoplastic lesion of the nerve can be localized. For surgical planning, cross-sectional imaging is needed, preferably MRI. The tumor gives a characteristic “triple signal pattern” or “bunch of grapes” appearance on T2-weighted images because of the heterogeneous nature of the tumor. PET-CT can be performed to exclude metastases before surgery.

CONCLUSION

SSs are rare, aggressive soft tissue tumors that can arise from various parts of the body, including peripheral nerves, as demonstrated by our case of SS originating from the tibial nerve in a 15-year-old patient. This case underscores the importance of comprehensive imaging, particularly ultrasound, in diagnosing and planning the treatment of such tumors. Despite its large size, successful excision with limb preservation was achieved, highlighting advances in surgical techniques and the critical role of accurate pre-operative diagnosis.