Infiltration of the medial rectus muscle as the first manifestation in a primary orbital lymphoma

A 70-year-old woman presented with a 3-month history of persistent unilateral headache with orbital pain irradiation. Multiple analgesic treatments were used with no pain relief.

On ocular examination, she had no-restricted ocular motility but gradually progressive pain on adduction of the right eye. No proptosis or loss of vision were found. The left eye and ocular adnexa were normal. Previous history of breast cancer without metastases diagnosed 3 years ago was reported. Currently the patient is disease-free and without any ongoing treatment. Her family history was noncontributory.

The contrast-enhanced CT scan of the orbits showed a marked thickening of the internal rectus muscle (IRM) of the right orbit (Fig. 1a-b). The MRI with and without ivc showed a thickened muscle with isointense signal in both T1, T2 and STIR sequences with homogeneous contrast enhancement and restricted diffusion (Fig. 2a-g). Otherwise, normal findings. The study was completed with an FDG/PET-CT, which confirmed the high metabolism in the topography of the IRM (Fig. 2e).

Lymphoid masses in the orbit, conjunctiva, lacrimal gland and eyelids are usually low-grade tumours, representing 10-15% of all orbital tumours according to different series and up to 55% of the orbital malignancies.
It can be a primary lymphoma or a systemic infiltration of it [2, 3]. Usually, primary lymphoma affects lacrimal glands and less frequently it may present like a diffuse infiltrative form of the extraocular muscles [3]. Normally it appears slightly hyperdense on CT and iso- or hypointense on MRI T1 and T2 sequences.
The breast cancer history pointed to a metastatic lesion since, apart from the orbital fat, the ocular muscles are the most frequent localisation [2, 3]. Metastatic or other malignant lesions are usually ill-defined and have high signal on T2 and STIR sequences or lower ADC values [4].
The clinical presentation, with sub-acute orbital pain and the muscle thickening, were in favour of a pseudotumour diagnosis. On MR images, inflammatory pseudotumours are usually isointense to hypointense relative to muscle on T2-weighted images and have an increased attenuation into dynamic late phase.
The less probable disease was thyroid orbitopathy. Typically, it produces painless bilateral proptosis with bilateral enlargement of the extra-ocular muscles spearing the muscle tendons insertion [5].
To help distinguish lymphoma from the other diagnoses we can use the ADC map. It was demonstrated that it has a significantly lower ADC value than the other orbital diseases [6].
The imaging studies are helpful in order to define the extent and the accurate location of the ocular disease, to determinate if it is accessible for a biopsy and, most importantly, they are narrowing down the possibilities for the differential diagnosis. Very often, a biopsy is necessary to make the precise diagnosis and facilitate an appropriate treatment, like in this case. Administrating chemotherapy or radiotherapy for a primary ocular lymphoma in most cases has a good response and a high cure rate [4, 5], although it was described that 30% of the patients are at risk of developing a systemic lymphoma after 10 years from the initial diagnosis [3].
After performing a biopsy for our patient, the results revealed a low-grade B-cell lymphoma infiltrating the IRM. The bone marrow aspiration was normal, discarding the possibility of systemic lymphoma. Radiotherapy was administered.
Our patient had a great response to the radiotherapy and currently she is disease-free with normal nuclear and imaging studies (Fig. 3).

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