Background: The prevalence of multiple synchronous primary cancers has increased in recent decades. The development of simultaneous multiple primary cancers in a patient may depend on hereditary, environmental, iatrogenic factors, or unknown factors. The detailed clinical history of these patients, reviewing the primary pathology reports, and the impact of immunohistochemical diagnostic methods on the planning and treatment process for these patients are the main objectives of our report.

Case presentation: in this article, we present two cases of dual primary cancers. The first case is a 46-year-old healthy woman with no significant past medical or family history of cancer and no history of using immunosuppressive drugs, who presented with advanced metastatic breast and lung cancers with sudden-onset neurological deficits. The second case is a 52-year-old healthy woman with no history of intervening factors and simultaneous locally advanced breast cancer and early-stage thyroid cancer, presenting with a large mass in her right breast and axilla.

Conclusion:  Sampling from simultaneous masses in different tissues in a patient with a cancer diagnosis can play an important role in identifying multiple primary cancers. Moreover, newer techniques such as IHC have definitively proven the diagnosis of multiple primary cancers in individuals and serve as a reliable method for selecting the best and most optimal therapeutic approach for these patients s