Marin Cancer Care Data Breach 2026: 501 California Oncology Patients Exposed. Names, Medical, and Insurance Data. What To Do

What happened

Marin Cancer Care is a privately-held oncology and hematology practice at 1350 South Eliseo Drive, Suite 200, Greenbrae, California — a long-standing community oncology center founded in 1952. Services include medical oncology, radiation oncology, hematology, clinical trials, and integrative wellness / infusion. The practice serves Marin County (north of San Francisco).

Between November 22 and December 6, 2025, an unauthorized actor accessed Marin Cancer Care’s network. On approximately December 8, 2025, the intrusion was discovered and files were identified as potentially copied / exfiltrated. Marin Cancer Care filed with HHS OCR and the California AG on February 6, 2026 and posted a notice of cyber event — confirming 501 affected individuals.

On February 24, 2026, multiple plaintiffs’ firms announced class action investigations.

No ransomware group has publicly claimed responsibility. No leak-site listing has been observed.

Note: A separate June 2025 “Medical Center of Marin” attack was claimed by INC RANSOM — that is a different entity and should not be conflated with this Marin Cancer Care incident.

Why oncology breaches are uniquely sensitive

Marin Cancer Care’s exposed records are likely to contain cancer diagnoses, chemotherapy and radiation therapy regimens, and possibly genetic / molecular test results. The official entity notice does not enumerate these oncology-specific data elements, but the practice’s clinical scope makes them plausibly in scope.

Oncology records carry weight beyond standard PHI:

• Employment and life insurance underwriting consequences if cancer history is exposed
• Genetic information (BRCA, Lynch syndrome, hereditary cancer panels) implicates blood relatives who may not consent
• Mental health overlay — cancer diagnoses are intertwined with depression, anxiety, and end-of-life care

What was stolen

Per Marin Cancer Care’s entity notice (officially confirmed):

• Full name
• Medical information
• Health insurance information

Per plaintiff-firm characterizations (not entity-confirmed):

• Date of birth, address
• Social Security number
• Diagnosis codes, treatment histories
• Medication records
• Financial account information

Read your specific notification letter for the confirmed data elements applicable to your case.

What Marin Cancer Care is offering

• Complimentary identity monitoring through Cyberscout (inferred from CA AG filing mailer reference)
• Vendor and duration not explicitly disclosed in plain-text public sources
• Forensic investigators engaged

What to do

1. Read your specific notification letter and enroll in the offered monitoring.
2. Place free credit freezes at Equifax, Experian, TransUnion.
3. File IRS Form 14039 if SSN exposed.
4. If your cancer treatment is ongoing, separately notify your employer, life insurer, and family of the breach so that any future “leaked diagnosis” reference doesn’t surface as a surprise.
5. If your genetic test results were in scope, recognize the implications for blood relatives — they may have a privacy interest in your records even though they are technically yours.
6. Watch your insurance Explanation of Benefits for unfamiliar oncology, infusion, or radiation claims.
7. Stop the ongoing flow of your oncology records. HealthConsent files HIPAA restriction requests so the diagnostic, chemotherapy, and clinical-trial data exposed in this breach is not continuously re-shared.

Continue reading

• Your eight HIPAA rights, in plain English
• California CMIA: stronger protections than federal HIPAA
• Other 2026 healthcare data breaches