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INTRODUCTION
Orbital metastasis from solid tumor is a rare entity usually found in patients with multiple organ metastases. The common primary tumor sites are breast, prostate gland, lung, etc. Typical imaging features of orbital metastasis include bone destruction with contiguous mass, extraocular muscle mass, and diffuse intraconal mass formation (12).
Small cell lung cancer (SCLC) is a well-known malignancy, having an early spread and multi-organ involvement; however, there are only a few reported cases of orbital metastases present as diffuse and large tumor destroying the bony orbit (345). Herein, we describe the image findings of a solitary extraocular muscle metastasis of SCLC, presenting as a discrete extraocular muscle mass with mild homogeneous enhancement.
CASE REPORT
A 74-year-old man presented with right ocular pain radiating to the right temporal area, of 1-month duration. He developed diplopia and aggravation of orbital pain 10 days prior to investigation. Motility examination of the right eye revealed impaired medial gaze. Ophthalmologic examination indicated normal visual acuity in both eyes, with normal anterior and posterior segments. He had no significant medical history. Lab-oratory tests showed the white blood cell count and C-reactive protein levels were within the normal limit. Initial clinical diagnosis included inflammatory pseudotumor and cavernous sinus syndrome, since the patient presented with right ocular pain.
Contrast enhanced orbital computed tomography (CT) obtained 35 seconds after contrast injection, revealed a bulging mass at the right medial rectus muscle belly with mild homogeneous enhancement. Adjacent orbital fat infiltration or bony destruction was absent (Fig. 1A). Magnetic resonance imaging (MRI) of the orbit showed a bulging mass along the medial rectus muscle with superiorly displaced medial rectus muscle. The mass showed homogenous hyperintensity to extraocular muscle on T2-weighted images, and mild rim enhancement on contrast enhanced T1-weighted images. Perilesional fat infiltration was absent (Fig. 1B). Based on the image features, differential diagnosis included primary tumor of the extraocular muscle, such as lymphoma or inflammatory pseudotumor or myositis.
Plain chest radiograph for routine check-up incidentally revealed a left hilar mass. Consecutive CT of the chest showed conglomerated lymphadenopathy in the left hilum. A total body 18F-fluorodeoxy glucose positron emission tomography scan revealed abnormal uptake in only the left hilar region and the right orbit (Fig. 1C). These findings raised a possibility that the orbital mass could be a metastasis from lung cancer.
The patient underwent transbronchial needle aspiration for the left hilar mass, and the histopathologic diagnosis confirmed SCLC. Successive incisional biopsy of the right medial rectus muscle confirmed metastatic carcinoma from SCLC (Fig. 1D). Patient was finally diagnosed as SCLC with solitary orbital metastasis. The patient started chemotherapy with platinum-based regimen. After 6 cycles of chemotherapy, the orbital symptom improved, and the extraocular mass showed marked decrease in size (Fig. 1E).
DISCUSSION
Metastasis to the soft tissues of the orbit is relatively uncommon (2). Hematogenous metastases to the orbit are most commonly the result of breast carcinoma in women, and carcinoma of the lung, kidney, or prostate in men. Imaging appearances of orbital metastasis are various, and may occur in any part of the orbital compartments. Many metastases target the bony wall of the orbit, affecting the orbital tissues as the metastasis expands (6). Shields et al. (2) reported that 41 of 64 orbital metastatic tumors showed a diffuse configuration, and the remaining 23 cases were round to ovoid and well-circumscribed, depending on the tumor origin. Multiplicity of lesions suggests a diagnosis of metastatic disease (6).
Extraocular muscle metastases are rare, and discrete extraocular muscle metastases are even rarer, constituting only 9% of orbital metastases (7). The majority of patients with metastases of the extraocular muscle not only have a prior diagnosis of primary malignancy at presentation, but also the metastasis occurs late in the course of the systemic malignancy. The most frequently affected extraocular muscle is the medial rectus, followed by the lateral rectus, the superior rectus, and the inferior rectus (7). There are few case reports of orbital metastases from SCLC (345), and they usually present as diffuse soft tissue mass involving the extraocular muscle with wall destruction of the bony orbit. However, our case presented with a solitary mass within the extraocular muscle. There are some reports of metastasis confined to the extraocular muscle from other primaries, such as carcinoid tumor or prostate cancer, involving single or multiple extraocular muscles (8). These lesions are well-defined, fusiform, or round masses within the extraocular muscle belly, with heterogeneous and minimal enhancement on the MR images. Our case revealed similar image features.
Differential diagnosis of extraocular muscle lesions includes lymphoma, inflammatory pseudotumor, or myositis. Lymphoma appears more commonly as diffuse ill-defined orbital disease rather than a well-circumscribed, round or oblong mass (9). However, radiological and clinical signs of both lymphoma and extraocular muscle metastasis are non-specific, and include painless muscle enlargement or masses with heterogeneous contrast enhancement (10). Inflammatory myositis often extends anteriorly to involve tendon insertion, and may show a ragged, fluffy border of the involved muscle with infiltration and obliteration of the contiguous fat, particularly in the peripheral surgical space between the periosteum of the orbital wall and the muscle cone (6).
In our case, the patient had no previous history of malignancy. Initial chest radiograph played an important role in the final diagnosis. Char et al. (1) reported that 11 of 31 (25%) patients had no known primary malignancy at the time of diagnosis of orbital metastasis. Therefore, metastasis should be included in the differential diagnosis of patients presenting as a discrete mass of EOM muscle with minimal and heterogeneous enhancement on MR images, even though the patient has no known primary malignancy.
In conclusion, we described here a case of solitary extraocular muscle metastasis from SCLC, presenting as bulging mass of the extraocular muscle. CT and MR showed a discrete mass of the right medial rectus muscle belly, with homogenous signal intensity and mild rim enhancement. Although solitary EOM metastasis is rare, a better understanding of imaging and clinical features would be useful in accurate diagnosis of orbital mass.
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