Aetna wrongfully denied coverage of cancer patients’ proton therapy after the insurer tried to prove that the treatment was experimental and investigational, a federal judge ruled.

Aetna’s ambiguous definition of “medically necessary” failed to justify its exclusion of proton therapy for treating non-metastatic breast or prostate cancer in adults, Judge Kenneth Marra ruled in a summary judgment from the US District Court for the Southern District of Florida on Thursday. The lawsuit will now proceed to class certification that could expose Aetna to liability for a group of patients with similar complaints.

The lead plaintiffs are breast and prostate cancer patients Sharon Prolow and Mark Lemmerman, who won part of their lawsuit seeking coverage for proton beam radiation therapy. Marra granted the plaintiffs’ partial summary judgment and said it was unnecessary to take up the claim that Aetna’s denial violated the Employee Retirement Income Security Act.

“Given the extensive medical literature, detailed medical records, treatment plans and compelling evidence of individual risk factors supplied by Ms. Prolow’s and Mr. Lemmerman’s treating physicians, the court concludes that the recommendations of the plaintiffs’ physicians reflected prudent clinical judgment,” the ruling says.

Aetna, a CVS Health subsidiary, did not respond to a request for comment.

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More health systems are offering proton therapywhich uses targeted forms of radiation to fight cancer while limiting harmful exposure, as the technology becomes less expensive and more efficient. But not all insurers pay for its more expensive applications, citing limited evidence that the treatment is more effective than traditional radiation.

Proton therapy is more widely accepted for children with cancer and for advanced cancers. But use cases and the clinical evidence are mounting. UnitedHealthcare and others have faced lawsuits regarding their coverage policies.

Aetna wrote in its denial letters to Prolow that “clinical studies have not proven that the procedure is effective for treatment of the member’s condition,” which is non-metastatic breast cancer. But that reasoning was not linked to any of the five bullet points Aetna uses to guide its coverage policies and those unnamed “clinical studies” were not sufficient to justify denials, the court ruled.