What happened, in summary

A woman in her 40s attended a breast clinic after 2 months of yellow discharge from both nipples. Examination found a palpable left-breast mass with skin retraction. During the diagnostic and staging process, CT and PET/CT also identified an anterior mediastinal mass. Imaging could not reliably distinguish a thymoma from another condition, so the mediastinal lesion was removed for tissue diagnosis. Pathology showed that it was breast cancer metastatic to the thymus, an exceptionally uncommon site of spread.

Her breast-cancer treatment was reviewed by a multidisciplinary team. She underwent a simple mastectomy with sentinel lymph-node biopsy. The breast specimen contained an 80 mm grade 2 invasive lobular carcinoma together with high-grade ductal carcinoma in situ, making the total abnormal area approximately 100 mm. The cancer was oestrogen receptor-positive, progesterone receptor-positive and HER2-negative. The sentinel node contained cancer, and completion axillary clearance found involvement in 1 of 13 lymph nodes.

Because the thymic lesion represented metastatic disease, treatment also required systemic therapy. She received 4 cycles of abemaciclib, anastrozole and goserelin, also known as Zoladex. The combination provided CDK4/6-targeted therapy, endocrine therapy and ovarian suppression, and she had a good response. This approach was intended to control metastatic disease while supporting a longer-term endocrine treatment plan.

She continued endocrine treatment, with a plan for 10 years of therapy and surveillance mammography of the right breast. Her experience shows why an unusual mediastinal mass in someone with newly diagnosed breast cancer may need biopsy rather than being assumed to be an unrelated thymic tumour.