Abstract Introduction/Objective Immunotherapy provides an efficacious and durable therapeutic option for patients with advanced-stage neoplastic disease. Related adverse events are several. Sarcoid-like granulomatous reaction is a rare, not severe complication, more frequent in skin, lung and lymph nodes. It should not be misdiagnosed as tumor recurrence or progression. Methods/Case Report Case Report. Results (if a Case Study enter NA) Female 50 years old. Smoker for 40 years until 2016. Reumathoid arthritis at age 50. Palmo-plantar psoriasiform dermatitis since 2016. Sporadic corticoids use and topic therapy. Lung adenocarcinoma in june 2021, multifocal, acinar-papillary, pT4-pN1- pM1a. TPS PD-L1 70%. In treatment with pembrolizumab since july 2021, without toxicities. In november 2021, after 6 doses, PET/CT shows mild growth of bilateral nodules. Stereotaxic breast biopsy for microcalcifications of recent appereance in january 2022. Pathology report: “Sarcoid-like reaction. Microcalcifications, present in biopsied material, not visualized after cuts of all tissue (mecanically displaced). PAS, Grocott and Ziehl-Neelsen negatives”. Currently, after 16 cycles of pembrolizumab, the patient is in good condition. Conclusion Sarcoid-like reaction (epithelioid non necrotizing granulomas), during immunotherapy is rare, can be seen especially in skin, lung, mediastinal lymph nodes. For some authors it would be a marker for therapy response and correlated with improve overall survival. It is very important to differentiate this reaction with progression or recurrence of tumor disease. Our case is the first immunotherapy induced sarcoid-like reaction described in the mammary gland.