Breast adenocarcinoma recurring as small cell carcinoma in a patient with a germline BRCA2 mutation: Clonal evolution unchecked

Background: Because up to 30% of breast cancer cases may relapse, understanding the biology of recurrent breast cancer is imperative in preventing these poor outcomes. Thus, we present this unusual case of a BRCA2 carrier who presented seven years after her initial diagnosis of breast adenocarcinoma with a new lump in the left axillary tail, which proved to be small cell carcinoma. The second cancer bore no morphologic or immunohistochemical resemblance to the first. However, we aimed to understand whether the two cancers could have been related. Methods: We performed targeted Next Generation Sequencing on both cancer specimens in order to determine whether there was a genomic relationship between the two cancers. Results: We found that the initial breast adenocarcinoma was positive for a heterozygous mutation in PIK3CA (c. 1624, p.E42K) and a heterozygous 13-basepair deletion in TP53 (c.639-651del, p.H214fs). The small cell cancer was positive for the same mutation in PIK3CA, but negative for the mutation in TP53. Conclusion: We concluded that the small cell cancer may have arisen from a clone within the initial cancer, since they carried an identical genetic mutation. Furthermore, we postulate that the unusual morphology of the second cancer may be due, in part, to the patient's germline BRCA mutation.