Objectives: Cancer morbidity rates have been increasing steadily. A longer lifespan and easier access to modern diagnostic and therapeutic methods are the main reasons for the growing number of cancer survivors. Additionally, some types of oncological treatment, such as radiotherapy or immunosuppression, may also increase the risk of secondary tumors. These factors have resulted in an increased incidence of primary multiple cancers. Multiple primary cancers are generally under­stood as either synchronous, in which the cancers occur at the same time, or metachronous, in which the cancers follow in sequence (for instance, more than 2 months apart).The results published in other studies show that between 2% and 15.8% of all cancer patients have more primary multiple cancers. Within this group with multiple primary cancers, some have bilateral breast cancer, and our study focuses on patients from this group.

Material and methods: Our study describes 10 patients who were treated for bilateral synchronous breast cancer at the Cracow Branch of the Maria Skłodowska-Curie Memorial Cancer Center and Institute of Oncology during the years 1992–2014 and who developed another primary tumor after their treatment bilateral synchronous breast cancer.

Results: In our discussion we present detailed data on the incidence of metachronous cancers in the 10 patients, including breast cancer, following the treatment of their other primary tumors.

Conclusion: The 10 cases of our study, and clinical experiences and publications in general show how important it is for patients to continue medical follow-up after treatment of primary tumors, not only to detect recurrences as early as pos­sible, but also to diagnose any other malignancies occurring in other sites, including secondary, treatment-related tumors.