Calcified deep somatic soft tissue leiomyoma of the leg: A case report

Abstract

Deep somatic soft tissue leiomyoma is a rare benign smooth muscle tumor, with around 60 cases reported thus far in the literature. Herein, we report a huge leiomyoma in deep somatic soft tissue with massive calcification in the peroneus longus muscle. A 29-year-old male patient presented with a hard, spindle-shaped, 15-cm-diameter mass of the left lower leg. Plain radiography showed marked calcification within the lesion, and magnetic resonance imaging revealed a well-circumscribed lesion with strong contrast enhancement within the peroneus longus. A core needle biopsy was performed to rule out a soft tissue sarcoma, and deep somatic soft tissue leiomyoma was preoperatively diagnosed. A marginal excision was performed, and the patient recovered without event and has been recurrence-free for 16 months. As far as we know, the present soft tissue leiomyoma was one of the largest to be documented, the previously reported cases being up to 14 cm in maximum diameter. Leiomyomas of deep somatic soft tissue should be included in the differential diagnosis of a huge, deep-seated tumor with marked calcification as well as sarcomas, especially in younger patients.

Introduction

Leiomyomas are a benign soft tissue tumor formed by spindle cells resembling normal smooth muscle cells. ¹ Soft tissue leiomyomas commonly occur in the dermis and subcutaneous tissue, but tumors located deeply in soft somatic tissue are extremely rare. ² In a previous report, leiomyomas accounted for approximately 4% of 1331 cases of benign soft tissue tumors, but no tumors of deep soft tissue origin were included. ³ To date, there have been approximately 60 case reports describing deep somatic soft tissue leiomyomas, mainly in children and elderly patients.^{4 –9} The average tumor diameter of these leiomyomas is 5 cm, with the previously reported maximum size being 14 cm.^2,4 Herein, we report a rare case of a huge, calcified, deep somatic soft tissue leiomyoma arising in the peroneus longus muscle of a young, adult, male patient.

Case

A 29-year-old male patient presented to an orthopedic clinic with the complaint of tingling and swelling in the lateral, left lower leg apparently caused by a mass that had been slowly growing over the previous 2 years. He had no medical history or apparent history of trauma, but might have had minor trauma on a daily basis because he was an amateur rugby player during his school years. Magnetic resonance imaging (MRI) diagnosed a soft tissue tumor, and the patient was referred to our hospital for further treatment. Palpation revealed a hard, spindle-shaped, 15-cm-diameter mass with poor mobility near the fibula on the lateral aspect of the left lower leg. Tenderness, motor paralysis, and sensory disturbance were denied. A blood test returned negative. Plain radiography demonstrated a slightly lobulated soft tissue lesion with clear margins, smooth borders, and marked heterogeneous calcification with a calcified rim (Figure 1). The lesion consisted mostly of dense aggregated or extensive speckled calcification. Cortical destruction and periosteal reaction of the underlying fibula were denied. MRI demonstrated a well-defined lesion with a fat cap sign on the distal aspect within the peroneus longus muscle. The lesion showed isointensity with skeletal muscle on T1-weighted imaging and slightly high intensity on T2-weighted imaging but low signal intensity in the highly calcified areas. The lesion also demonstrated strong and uniform contrast enhancement except in the calcified areas.

Figure 1.

Imaging studies. Anteroposterior (a) and lateral (b) plain radiographs of the left leg showed a markedly calcified lesion. T1-weighted (c) and T2-weighted (d) axial magnetic resonance imaging demonstrating a well-circumscribed lesion within the peroneus longus muscle. (e) T1-weighted axial imaging with fat suppression revealed that the lesion was strongly enhanced after gadolinium injection.

The tentative differential diagnosis based on the imaging studies included ectopic calcification, tumoral calcinosis, and soft tissue sarcoma with calcification. A core needle biopsy performed preoperatively to assess for malignancy revealed spindle cells resembling normal smooth muscle cells densely proliferating in bundle-like arrangements (Figure 2). Their nucleus was oval or cigar-shaped, but no nuclear atypia was observed. Degenerative hyalinization and calcification were also observed. Immunohistochemically, the tumor was diffusely positive for smooth muscle markers, such as desmin, α-smooth muscle action, muscle specific action (HHF35), and h-caldesmon, and was negative for estrogen receptor. Marginal excision was performed after leiomyoma of deep somatic soft tissue origin was preoperatively diagnosed. The tumor was well defined and localized within the peroneal muscle. There was no adhesion of the tumor to the peroneal nerve or underlying fibula. We found numerous vessels on the surface of the mass during operation. The feeder vessels of the mass were branches of the anterior tibial artery. We ligated and stripped them to leave them on the surface of the mass. There was no impairment of blood flow in the lower extremities before or after surgery.

Figure 2.

Pathological findings of the tumor. (a) Histological appearance of the core needle biopsy specimen showed cellular and disarrayed pattern of spindle-shaped tumor cells resembling normal smooth muscle cells with foci of calcification (H&E staining, ×100). (b) A large, hard, encapsulated tumor with gyriform lobulation. (c) The cut surface of the tumor was finely lobulated with yellowish, calcific areas spread throughout. (d) Microscopy demonstrating spindle-shaped tumor cells with oval or cigar-shaped nuclei (H&E staining, ×400). (e) Concentric calcification was observed throughout the tumor (H&E staining, ×100).

Macroscopic examination of the excised tumor demonstrated a huge, hard tumor measuring 15.0 × 6.0 × 4.5 cm in size with expansive growth within a thin, fibrous capsule and numerous feeder vessels on its surface (Figure 2). The cut surface demonstrated that most of the tumor contained massive calcification with degenerative changes and stromal hyalinization. Histologically, the spindle tumor cells with eosinophilic cytoplasm showed dense proliferation in a bundle-like pattern and intermingling with the calcified areas. Hyalinization and concentric dystrophic calcification were observed throughout the tumor, but no tumor necrosis was found. Mitosis and nuclear pleomorphism were denied. The results of immunohistochemical analysis were identical to the needle biopsy findings. Based on the histopathological and imaging findings, somatic deep soft tissue leiomyoma with prominent dystrophic calcification was diagnosed. The patient recovered from his surgery without event and has been recurrence-free for 16 months.

Discussion

Leiomyomas are benign smooth muscle tumors. There are three types: those arising in the abdomen and retroperitoneum, those arising in the cutis or subcutis, and those arising in deep somatic soft tissues.^1,10 Leiomyomas most frequently occur in the abdomen and retroperitoneum. Abdominal and retroperitoneal cases tend to occur in menopausal women, often concurrently with a uterine leiomyoma. ¹ Retroperitoneal leiomyomas may produce multiple lesions and are estrogen receptor-positive. They are relatively large, with the largest, documented instance to date being 37 cm in diameter. ² Skin and subcutaneous soft tissue are also common sites of occurrence. The cutaneous leiomyoma is more common in males than in females, and about half the cases develop multiple, small nodules or papules usually no larger than 2 cm. The tumor also occasionally recurs after resection. ¹¹ Immunohistochemical analysis demonstrates diffuse positivity for smooth muscle actin but negativity for estrogen receptor. ¹² On the other hand, leiomyomas rarely occur in the deep somatic soft tissue of the extremities. Most deep soft tissue leiomyomas are found in elderly individuals and children, and there is no sexual predominance. ² The deep somatic-type leiomyoma usually occurs as a solitary tumor that tends to be larger than other subtypes and shows frequent degenerative and regressive changes, including calcification. It is also characterized by negativity for estrogen receptor expression. Deep somatic soft tissue leiomyomas rarely metastasize or recur after marginal resection.^1,2 Previously reported cases were up to 14 cm in diameter, and massive cases occurred on the thighs, buttocks, and trunk.^2,4 The tumor in the present case was 15 cm in diameter, even though it occurred in the lower leg, making it the largest somatic leiomyoma to be reported thus far in the literature. ⁴ The three groups of leiomyoma mentioned above have different characteristics and, therefore, require different methods of treatment. ²

Calcifying, deep somatic soft tissue leiomyomas have nonspecific features on radiological imaging studies, rendering their preoperative diagnosis difficult. Moreover, leiomyomas are difficult to distinguish from soft tissue sarcomas, such as mesenchymal chondrosarcomas or extraskeletal osteosarcomas, because all these conditions share the feature of hypervascularity. The differential diagnosis of a nonsarcomatous tumor with radiological findings of calcification should include soft tissue chondroma, tumoral calcinosis, ancient hematoma, myositis ossificans, and calcific myonecrosis of skeletal muscle.^5,6 There are no reports of cases of calcified soft tissue leiomyoma with a history of trauma. ⁵ On MRI, leiomyomas show low- to isosignal intensity on T1-weighted imaging and iso- to relatively high signal intensity on T2-weighted imaging because they contain dense smooth muscle fibers. ⁷ Signal-void areas on T2-weighted imaging indicate foci of dystrophic calcification. Marked contrast enhancement is observed in the noncalcified, hypervascular areas. Although none of these imaging features is very specific, their combination may be useful in distinguishing leiomyomas from sarcomas on a preoperative, radiological examination.

Histologically, leiomyomas of deep somatic soft tissue are benign and consist of fully differentiated smooth muscle cells resembling normal smooth muscle cells but containing eosinophilic cytoplasm and cigar-shaped nuclei. The leiomyosarcoma, the malignant counterpart of the leiomyoma, is far more commonly found in deep soft tissue. Therefore, it is important to rule out the leiomyosarcoma histologically in accordance with the established diagnostic criteria. ² In general, the features indicating malignancy, such as atypia, mitosis, and necrosis, are absent in somatic leiomyomas. However, atypical leiomyomas, which are benign but exhibit nuclear atypia, add an element of confusion. ¹³ Moreover, tumors that cannot be classified as either benign or malignant, called smooth muscle tumors of uncertain malignant potential, may develop distant metastases. ¹⁴ This potentially life-threatening tumor requires wide resection and intensive follow-up. Although intratumoral calcification is sometimes observed in soft tissue leiomyomas as a form of secondary degeneration, the tumor in the present case was quite exceptional in that the calcification was extensive and permeated the entire tumor. The underlying mechanism for this phenomenon remains unknown.

Soft tissue leiomyomas can be treated with marginal resection and do not recur even if the resection margins are positive, but malignant soft tissue sarcomas require wide resection. ² Furthermore, soft tissue tumors with a maximum diameter >5 cm and deep-seated tumors are more likely to be malignant. ¹⁵ Thus, pathological analysis of biopsy specimens is essential for preoperative diagnosis.

In the present case, the extremely large soft tissue leiomyoma with massive calcification on the patient’s lower leg was unable to be diagnosed based on preoperative imaging alone and required histological analysis of a needle biopsy specimen. To the best of the authors’ knowledge, to date, the present case has been the first and largest instance of a calcified intramuscular leiomyoma occurring in the peroneus longus muscle. Based on the findings of this report, deep somatic soft tissue leiomyomas should be included in the differential diagnosis of a huge, deep tissue tumor with marked calcification.

Conclusions

The present report described the first case of calcified intramuscular leiomyoma to occur in the peroneus longus muscle. Leiomyomas originating in deep somatic soft tissue are very rare and difficult to diagnose preoperatively by radiological means; the present case required a preoperative needle biopsy to assess for possibility malignancy. The present leiomyoma was the largest reported to date, but marginal resection resulted in complete healing without recurrence.