Terrien's Marginal Degeneration: This condition is characterized by peripheral corneal thinning, often with distinct borders and circular or elliptical lesions, typically located in the superior or inferior cornea near the limbus. It is more common in males but can occur in females, and the presentation matches the description given.

Peripheral Ulcerative Keratitis (PUK): This condition involves inflammation of the cornea and can lead to thinning, especially in the peripheral areas. It may be associated with systemic diseases such as rheumatoid arthritis or lupus. The circular lesions could be a manifestation of the ulcerative process.
Mooren's Ulcer: A rare, painful ulceration of the cornea, often starting peripherally and progressing centrally. It can present with distinct borders and may have a circular appearance, although it's typically more irregular.
Lipoid Degeneration: A condition where lipid deposits accumulate in the cornea, potentially leading to thinning, especially in the peripheral areas. However, it's less commonly associated with distinct circular lesions.

Scleritis: Inflammation of the sclera that can lead to thinning and may mimic corneal thinning. It's crucial to differentiate as scleritis can be associated with severe systemic diseases and requires prompt treatment.
Infectious Keratitis: Bacterial, fungal, or parasitic infections can cause corneal ulcers and thinning, sometimes with distinct borders. Missing an infectious cause could lead to severe consequences, including vision loss.

Ellis-Van Creveld Syndrome: A rare genetic disorder that can include corneal abnormalities, among other systemic features. While it's an unlikely cause for the specific presentation of peripheral corneal thinning with circular lesions, it's a consideration in the differential diagnosis of rare corneal conditions.
Corneal Dystrophies: Certain dystrophies, like the peripheral form of lattice corneal dystrophy, can cause peripheral corneal changes. However, these conditions are rare and typically have distinctive features that may not exactly match the described presentation.