Feline Diffuse Iris Melanoma: New Data, More Confidence

One of the more common questions I get from new graduates and experienced GPs alike, is some version of: When I’ve got a cat in the exam room with a pigmented iris lesion, how worried should I be and when should I recommend enucleation?

It’s a fair question. The reported metastatic rate for feline diffuse iris melanoma (FDIM) ranges anywhere from 19 to 63 percent—a wide enough window to make clinical decision-making genuinely hard. And historically, a lot of the guidance we had was subjective, without much quantitative data behind it.

A paper out of Germany is trying to change that. At our Spring Journal Club, we walked through Assessment of Diagnostic Accuracy of Clinical Signs in Feline Diffuse Iris Melanoma. This is worth putting on your radar, not because it reinvents the owner conversation, but because it gives clinicians something they desperately need—objective guidance.

What the Study Did

This was a retrospective case series of 67 cats thatunderwent enucleation at a single ophthalmology practice in Berlin, Germany. Researchers correlated preoperative clinical signs with histopathology results to identify which indicators most reliably predict FDIM versus benign iris melanosis. Variables examined included iris thickening, dyscoria, reduced pupillary movement, IOP, and presence of glaucoma.

Worth noting: While a retrospective single-center study with 67 cats is a reasonable for veterinary ophthalmology, a multicenter prospective study would carry more weight. Keep that in mind as you apply these findings to your day-to-day practice.

Key Findings

Iris thickening was the single most strongly associated clinical sign with FDIM, carrying a 6.3-fold higher probability of a neoplastic process. This is the finding that should be top of mind at every ophthalmic exam when you’re looking at a cat with iris pigmentation.

What does iris thickening look like? It can be tricky to describe how iris thickening appears on ophthalmic examination, but the term “velvety” is occasionally used. It is also important to note that many of these lesions begin as seemingly benign freckles that progressively change over time (i.e. spreading across the iris surface or becoming nodular in appearance). So, for the astute clinician, monitoring changes in iridial lesions over time will help guide you in making your clinical decision.

Other significant findings:

• Pigment dispersion in the anterior chamber had a roughly 1.5x higher probability of FDIM. On the anterior lens capsule, that jumped to nearly 4x. Caveat: posterior synechia can also cause ALC pigment deposition, so ensure to interpret this finding in full clinical context.
• Elevated IOP was a strong signal. Iris melanosis cats averaged ~18 mmHg; the FDIM group averaged ~37 mmHg, with each additional mmHg increasing FDIM probability by a factor of 1.077.
• Glaucoma correlated strongly. Not a single cat with confirmed iris melanosis had glaucoma on histopathology. All glaucoma cases fell within the FDIM group. If you’re seeing secondary glaucoma in a cat with iris changes, it is important that this is not overlooked.
• Each additional clinical finding increased the probability of FDIM by a factor of 1.44 — reinforcing the value of a thorough, systematic exam.

What Surprised Us: Dyscoria Is Not the Marker We Thought

Here’s the finding that generated the most discussion in Journal Club: dyscoria—or abnormal pupil shape—showed no correlation with the diagnosis of FDIM in this study.

Why is this surprising? A recent iris biopsy study (Beetz et al., 2025) reported dyscoria in over 76% of FDIM cases. This study found it in fewer than 20% and associated it with a lower probability of malignancy.

Journal Club attendees agreed that we are not ready to dismiss dyscoria as irrelevant, but this paper adds real uncertainty to how much diagnostic weight it should carry. Everything is a story. No single sign makes or breaks a diagnosis. You’re assembling a picture. And that picture will help you to provide the owner with the most accurate information for them to make their decision as to how to proceed with treatment.

The Number That Impressed Me Most

Clinical assessment, based on the documented signs evaluated in this study, yielded a correct diagnosis in approximately 85% of cats. 

Invasive iris biopsy? An 81.6% average agreement rate across eight pathologists (Beetz et al., 2025), cited in this paper’s discussion. Clinical assessment outperformed biopsy—no anesthesia, no invasive procedure; just a thorough ophthalmic examination. Of note, this study does not negate the benefit of iris biopsy, and this diagnostic endeavor should still be discussed as a potential path forward.

What This Means in Practice

This study doesn’t reinvent the owner’s conversation. But if a client has financial constraints that makes referral or biopsy difficult, it gives you more confidence to say: here’s what I’m seeing, here’s why I’m concerned, here’s what I’d recommend. You’re not guessing. You’re grounding your clinical impression in objective, published data.

My approach when I see a new iris lesion in a cat: 

• Document it in the EMR with a photo on day one
• Set a two to four-month monitoring cadence with owner photos between visits
• Track for changes in size and surface character of the lesion, abnormalities within the anterior chamber and/or anterior lens capsule and IOP
• Have the enucleation conversation early—not as a decision, but as a possibility to prepare owners for.
  Be cautious when quoting a metastatic rate as there is a wide range that is of limited benefit to the owner as they navigate their decision

The decision to enucleate remains collaborative. Some owners will elect to remove the eye at any mention of metastatic risk; others will want to monitor. The wide metastatic range (19–63%) means reasonable clinicians will land in different places, and that’s okay—one-eyed cats do great! It is just important to stress the importance of subsequent histopathologic evaluation of the removed globe and to acknowledge that the results of said evaluation may reveal that we are dealing with a benign process.

The Clinical Markers Worth Prioritizing 

Based on this study and our Journal Club discussion:

• Iris thickening: Your strongest clinical predictor; know how to recognize it
  Pigment dispersion: Especially on the anterior lens capsule
• Elevated IOP and secondary glaucoma: Treat glaucoma in a cat with iris changes as presumptively malignant until proven otherwise
• Older female cats: Higher-risk demographic; age increased risk by a factor of 1.3 per year
• Dyscoria: This study suggests it may be less predictive than previously believed but, previous studies were strong as well—dyscoria should remain a clinical sign that is considered as these cases are managed.

Ready to champion evidence-based medicine wherever you work?

Journal Club is just one of the ways IndeVets doctors come together to learn, share insights, and keep advancing patient care. Join our next IndeVets webinar or connect with us to learn how you can be part of a community that’s transforming veterinary careers, one vet—and one case—at a time.

References

Spornberger, J., Soukup, P., & Allgoewer, I. (2026). Assessment of the diagnostic accuracy of clinical signs in feline diffuse iris melanoma. Frontiers in Veterinary Science, 13, 1795617. https://doi.org/10.3389/fvets.2026.1795617

Beetz, S., Klopfleisch, R., Naranjo, C., Scurrell, E., Bushe, J., Lehmbecker, A., Loesenbeck, G., Merz, S. E., Pischon, H., Spitzbarth, I., Müller, B., Thieme, K., Wellen, S., & Eule, J. C. (2025). Diagnostic value of iris biopsies for the assessment of iris pigment changes and diffuse iris melanoma (DIM) in cats: A descriptive histopathological study. Veterinary Pathology, 63(4), 3009858251403170. https://doi.org/10.1177/03009858251403170

IndeVets Journal Club, Spring 2026.