Sterile, abscess-like cerebral lesion during trastuzumab therapy after HER2 status switch in a triple negative breast cancer patient: a case report and literature review.

Background: Breast cancer is a global health problem - it is the most common malignancy among women. Triple negative breast cancers (TNBC) account for 10-20% of female breast cancer. Most TNBC cases confer poor prognosis. Brain metastasis appears in more than 15% in the triple negative breast cancer population, which causes serious decrease in survival. Changes of immunophenotype are not uncommon in breast cancer, offering new therapeutic options in cases where targetable proteins or pathways are being identified.
Case Presentation: After five lines of chemotherapy and 82 months following the first diagnosis, our patient with brain metastatic triple negative breast cancer had human epidermal growth factor receptor 2 (HER2) genetic heterogeneity in the metastatic tissue sample interpreted as HER2 status conversion. After the removal of the metastasis, we started first line therapy for metastatic HER2 positive cancer with trastuzumab and paclitaxel. After the first cycle of trastuzumab, on day 8, she had a seizure, and neurosurgical examination showed an abscess-like lesion. The punctate proved to be sterile by microbiological and pathological examination, so we continued cytostatic therapy without the anti-HER2 antibody. 3 months later, we could not identify the previous abscess-like lesion in the control computer tomography (CT) scan, and our patient had no neurological deficits.
Conclusion: We emphasize the importance of regular tissue confirmation of predictive markers in progressive tumorous disease even if our presented case is not unequivocally a "conversion case". Tumor subtype is determined according to algorithms and definitions published in guidelines, nevertheless, use of different guidelines may lead to controversial interpretation in cases where HER2 genetic heterogeneity is present. Furthermore, we suggest that seronegative, aseptic intracranial fluid effusion after the removal of a brain metastasis may possibly be a side effect of trastuzumab.