The patient was a 61-year-old man Who presented with a chief complaint of right thigh pain and numbness for approximately 1 year, accompanied by restricted mobility. He had previously been treated for lumbar disc herniation at an outside hospital, receiving oral ibuprofen, acupuncture, and physical therapy, all of which were ineffective. His symptoms progressively worsened over the last 2 months, with the development of mild skin numbness. The patient had a history of gastric cancer surgery. Physical examination revealed tenderness in the right thigh, and deep palpation identified a firm mass with indistinct borders. No café-au-lait spots, axillary freckling, and iris hamartomas were observed. The patient denied any family history of NF1; however, but both his father and brother had died from leukemia.

A deep-seated mass was palpable in the middle segment of the posterior right thigh. The mass had a moderately firm texture, indistinct borders, slight longitudinal mobility along the course of the nerve trunk, and poor transverse mobility. Sensory abnormalities of the right lower limb included numbness over the anterolateral aspect of the lower leg, dorsum of the foot, and first toe; decreased sensation over the posterior aspect of the lower leg, lateral border of the foot, and sole; and mild dysesthesia involving the posterior thigh and posterior lower leg. Tapping over the projection of the swelling elicited a radiating numbness to the dorsum of the foot (positive Tinel sign). Manual muscle testing (MMT) was performed to assess motor function, with results showing grade 4/5 strength in ankle dorsiflexion, ankle plantarflexion, and great toe dorsiflexion. The Lasegue sign was positive, Patrick’s test and reverse Patrick’s test yielded mixed positive and negative findings. Preoperative manifestations of the mass on the body surface are shown in Figures 1A,B.

As detailed in Table 1.

Magnetic resonance imaging (MRI) revealed a mass with indistinct borders along the course of the sciatic nerve in the posterosuperior segment of the right thigh. The soft tissue mass was located posteromedial to the proximal-to-mid segment of the right femur and was suggestive of a tumor (Figures 2A–C). The mass was irregular and lobulated with indistinct borders, measuring approximately 8.7 cm×4.3 cm×21.2 cm, without significant signal change. The mass demonstrated slightly prolonged signals on T1-weighted images and mixed high-/low-intensity signals on T2-weighted images.

After admission, preoperative preparation was completed. The patient underwent excision of the right femoral mass under general anesthesia. The incision was made along the posterior thigh. A figure-of-eight skin incision was made over the mass, extending approximately 18 cm in length (Figure 3A). The skin, subcutaneous tissue, and deep fascia were incised to expose the intermuscular space. The tumor was located within the semimembranosus and semitendinosus muscles and extended medially toward the anterior thigh, with ill-defined margins. The lesion lacked a capsule and compressed the sciatic nerve (Figure 3B). The sciatic nerve was carefully retracted and protected (Figure 3C). Proximal exploration revealed normal muscle and soft tissue encapsulating the tumor. The tumor measured approximately 10 cm×5.6 cm×23 cm and was irregular in shape. It contained multiple tumor nodules and lacked a capsule. The extent of tumor invasion was identified by dissecting the tumor at the attachment of the gluteus maximus. The muscle was reflected to define the exact tumor margins. The tumor was completely removed by cutting the involved muscle attachments at the pelvic insertion point, outside the compartment. The specimen was sent for histopathological examination (Figure 3D). Postoperatively, the patient reported significant improvement in pain, compression, and numbness compared to the preoperative state. Intraoperative photographs are displayed in the figures.

The gross specimen was a dark red, spindle-shaped mass, without a capsule, measuring 10 cm×5.6 cm×23 cm. The cut surface was hard and displayed a whorled pattern. On microscopic examination (H&E staining), the mass was composed of spindle cells arranged in a relatively dense, phyllodes-like pattern. Fibrous structures were seen in the stroma. The tumor cells were relatively uniform (Figure 4).

Postoperatively, the patient's radiating pain and numbness in the right thigh were significantly relieved compared to the preoperative state. The patient's motor strength recovered to grade V. At 1-, 3-, 6-, and 10-month follow-up visits after surgery, Tinel's sign was negative. Functional recovery of the affected limb was satisfactory. Follow-up MR, showed no evidence of tumor recurrence.

First, in terms of clinical features, previous studies have shown that deep-seated solitary spindle cell tumors mainly occur in young and middle-aged adults, with no obvious gender predilection (3). Clinical symptoms are closely related to the location and size of the tumor and mainly reflect progressive nerve compression, presenting as local pain, distal numbness, sensory disorder, and muscle weakness. These findings are consistent with the chronic pain, sensory abnormalities, and limited mobility presented by the patient in this case, which reflect the typical clinical manifestations of such tumors (2, 4).

Second, regarding imaging diagnosis, MRI is widely recognized as the preferred preoperative method for examining spindle cell tumors (5). Most studies report that such tumors often appear as fusiform masses along the course of the nerve trunk, demonstrating isointense or slightly hypointense signals on T1-weighted images (T1WIs) and hyperintense signals on T2-weighted images (T2WIs). Although the tumor in this case did not show the typical “target sign,” its location along the sciatic nerve course, ill-defined borders, and signal characteristics are consistent with the imaging features of most neurogenic spindle cell tumors reported in the literature. These findings support the value of MRI for preoperative localization and qualitative assessment (6–8).

Third, in terms of pathology and immunophenotype, previous studies have shown that spindle cell tumors are histologically composed of spindle-shaped cells arranged within a myxoid or collagenous stroma. Immunohistochemical positivity for vimentin supports a mesenchymal origin, and a low Ki-67 proliferation index indicates benign biological behavior (9). The pathological findings in this case, including spindle cells arranged in a phyllodes-like pattern, a fibrous stroma, vimentin positivity, and a Ki-67 proliferation index of only 1%–2%, are highly consistent with the characteristics of benign spindle cell tumors reported in previous studies (10).

Finally, in terms of treatment and prognosis, most studies agree that complete surgical resection is the first-line treatment for symptomatic or progressively enlarging solitary spindle cell tumors. Patients with benign histopathology and negative surgical margins have a good prognosis and a low recurrence rate (11). The successful surgical resection and favorable 10-month postoperative follow-up results in this patient further verify these findings (12).

First, in terms of disease incidence and origin, most previous studies have reported that solitary spindle cell tumors more commonly arise in the skin or superficial soft tissues, and some cases are associated with NF1 or present as multiple lesions. In contrast, the tumor in this case is a sporadic solitary lesion without NF1-related manifestations, originating from the deep semimembranosus and semitendinosus muscles of the thigh and directly compressing the sciatic nerve (8, 13). Such deep-seated, nerve-compressing, NF1-unrelated solitary spindle cell tumors are clinically rare and differ from the more common origins and associated backgrounds of spindle cell tumors reported in the literature.

Second, in terms of imaging manifestations, the “target sign” (a low signal center with a high-signal periphery on T2WI) is considered a suggestive imaging feature of some neurogenic spindle cell tumors (such as neurofibromas). However, in this case, the tumor showed irregular, slightly prolonged signals on T1-weighted images and mixed high-/low-intensity signals on T2-weighted images without the typical “target sign,” which is consistent with the imaging performance of a small number of atypical spindle cell tumors reported in the literature but differs from the typical manifestations described in most studies. Such atypical imaging findings increase the difficulty of preoperative differential diagnosis and require careful distinction from schwannoma, aggressive fibromatosis, and other lesions (6, 7).

Third, in terms of immunophenotype, previous studies have shown that some neurogenic spindle cell tumors (such as schwannomas) often exhibit specific immunophenotypic characteristics, such as S-100 positivity, in combination with certain myogenic marker expressions. In this case, the immunophenotype was characterized by strong S-100 positivity (suggesting Schwann cell differentiation), focal CD34 positivity, and negative β-catenin, SMA, and desmin, thereby excluding aggressive fibromatosis and myogenic tumors. This immunophenotypic profile differs slightly from those reported for some neurogenic spindle cell tumors in previous studies and enriches the immunophenotypic data on sporadic, deep-seated benign spindle cell tumors.

In conclusion, this case supplements the clinical, imaging, and pathological data on rare, sporadic, deep-seated solitary spindle cell tumors of the thigh. Its consistency with previous studies confirms the universality of the clinical characteristics, diagnostic methods, and treatment principles for benign spindle cell tumors, while the differences remind clinicians to pay attention to atypical manifestations in clinical practice, improve the accuracy of preoperative diagnosis through comprehensive analysis of multiple examinations, and ensure the effectiveness of surgical treatment and a good prognosis for patients.