Kyoko Haradome, MD

Orbital lymphoproliferative disorders (OLPDs) frequently appear as an orbital mass lesion (24–49%) and comprise a wide spectrum of diseases ranging from benign to malignant conditions. The majority of malignant OLPDs are orbital lymphomas, particularly mucosa-associated lymphoid tissue (MALT) lymphoma. Other OLPDs comprise several benign, noninfectious, chronic inflammatory diseases, including IgG4-related ophthalmic disease, reactive lymphoid hyperplasia, and idiopathic orbital inflammation. Among them, IgG4-related ophthalmic disease is increasingly recognized and accounts for approximately half of benign OLPDs, on the basis of recent data. Clinically, the discrimination of orbital lymphoma from benign OLPDs such as IgG4-related ophthalmic disease is crucial for treatment planning; the former is amenable to low-dosage radiotherapy, whereas the latter are expected to show a good response to corticosteroid therapy.

Despite the expectation of histologic characterization of the OLPDs by imaging modality, the utility of conventional MR or CT imaging for the differentiation between these 2 groups of entities is disappointing, unfortunately, and their main roles, currently, involve only lesion detection and determination of lesion extent.

DWI is sensitive to the random (Brownian) motion of water molecules, which may reflect changes in tissue cellularity, nucleus-cytoplasm ratio, biomolecular crowding, and the integrity of cellular membranes. Quantitative DWI with apparent diffusion coefficient (ADC) measurements may potentially be useful for discriminating benign from malignant tumors. The ADC of lymphoma has been reported to be relatively low compared to other tumors, presumably because the former has an increased

monoclonal cellularity. However, previous researchers did not focus on the DWI-/ADC-based differentiation between orbital lymphoma versus benign OLPDs. In addition, the value of contrast-enhanced MR imaging in this setting still has not been clarified.

In addition, some conventional MR imaging features, including the presence of a signal void from a vessel in the lesion, which could be influenced by lesion vascularity or findings indicative of sinusitis due to extension of inflammatory changes to the mucosa of the nasal and paranasal cavities, particularly in IgG4-related disease, may also be helpful for the discrimination. However, the value of the associated MR imaging features has never been investigated.

Our data suggested that some characteristic conventional MR imaging features and quantitative evaluations with DWI and contrast-enhanced MRI (particularly ADC measurements) are useful for discriminating orbital lymphoma from benign OLPDs, and that these combined MR findings favorably contribute to a noninvasive tissue characterization of the OLPDs. For future application, ADC measurements may be helpful to assess therapeutic effectiveness of OLPD lesions because their histologic changes, such as tissue cellularity or decreasing edema induced by the therapy, could affect the ADC value.

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