[HER2-low breast cancer - A new entity that could expand possibilities of getting treatment]

Breast cancer is the most common form of cancer among women in Sweden. Several decades ago it was recognized that the Human epidermal growth factor receptor 2 (HER2) is involved in a critical growth system for breast cancer cells. Overexpression of HER2 (immunohistochemistry [IHC] 2+/3+, in situ hybridization [ISH] positive) is present in 15 percent of all breast cancers. HER2-low breast cancer has been discovered as a separate entity; the most commonly used definition so far is IHC 1+/2+ and ISH negative, but general consensus is still lacking. Clinical studies with the HER2 antibody drug conjugate trastuzumab deruxtecan (Enhertu) have shown impressive antitumor activity among women with advanced HER2-low breast cancer and this is expected to become part of routine treatment in the near future. Research is needed to establish refined ways to define HER2-low breast cancer, and a possible role lies in new imaging methods such as HER2 positron emission tomography (PET) with a [68Ga]Ga-ABY-025 tracer.