Current diagnostic guidelines for a rare type of lymphoma miss a subset of patients with the disease, according to a Northwestern Medicine study published in the journal Blood.

Cutaneous T-cell lymphomas are a group of diverse and rare cancers which mostly affect the skin, but can sometimes spread from the skin to the blood or a lymph node. It is the most common type of lymphoma, and roughly 12 people per million are diagnosed with the disease each year, said Joan Guitart, MD, chief of Dermatopathology in the Department of Dermatology and senior author of the study.

"At Northwestern, we have one of the largest cohorts of patients with cutaneous T-cell lymphomas," said Guitart, who also leads the cutaneous lymphoma clinic at Northwestern Memorial Hospital.

"We noted a few years ago that some patients who had the leukemic variant of cutaneous T-cell lymphoma were being underdiagnosed using recently updated diagnostic criteria. We suspected that it may have something to do with the modified criteria being used for diagnosis."

The diagnostic guidelines used to identify two distinct subsets of T-cell lymphomas—the leukemic variant Sézary syndrome and the primarily cutaneous mycosis fungoides—rely on a flow cytometry blood count of atypical lymphocytes to determine the disease subtype and stage.

In the study, Guitart and his colleagues compared diagnostic guidelines from 2007 against updated guidelines created in 2022 by analyzing blood samples from 99 patients with cutaneous T-cell lymphomas.

The new criteria—which counts the total value of abnormal cells, as opposed to the previous methodology using percentage—underdiagnosed close to 10 per cent of study participants, according to the findings.

"About 10% of the population were underdiagnosed or misdiagnosed with the new proposed criterion," said Guitart, who is also a member of the Robert H. Lurie Comprehensive Cancer Center of Northwestern University.

"That's an important issue because we want to act quickly to treat our patients. When looking into the characteristics of those patients, we found that all were characterized by what's called lymphopenia, a small number of cancer cells at presentation, yet an equally poor prognosis."

The new criteria also did not accurately distinguish between stages of the disease, an important distinction clinicians use to predict survival.

"With this article, we are calling to the attention of clinicians that there is a flaw with the new criteria," Guitart said. "We are proposing a revision of blood staging (B0, B1 and B2) criteria that more accurately identifies patients who are at risk of progression, often leading to a poor outcome."

To address these concerns, Guitart is in talks with the International Society of Cutaneous Lymphomas and other advocacy groups to modify the diagnostic criteria.

"Our findings provide an impetus to all the involved national and international societies to improve the staging criteria to find a way that does not miss the subset of patients with low tumor burden at presentation, which is what's happening with the present updated criteria," Guitart said.