What happened, in summary

This 38-year-old woman found a palpable mass in the lower outer part of her left breast at 39 weeks of pregnancy. An early ultrasound looked benign, but repeat biopsy confirmed grade 3 invasive ductal carcinoma, triple-negative subtype with ER-negative, PR-negative, and HER2-negative results. Staging CT of the chest, abdomen, and pelvis, including a bone scan, showed a 6.3 cm breast mass without distant metastases.

After induction of labour and delivery at 39 weeks, she started neoadjuvant therapy based on the KEYNOTE-522 approach: weekly paclitaxel and carboplatin followed by doxorubicin and cyclophosphamide, combined with pembrolizumab. The tumor softened clinically and had a partial radiologic response. She then underwent wide local excision and sentinel lymph-node biopsy. Surgery showed residual grade 3 invasive ductal carcinoma measuring 5.9 cm, ypT3 ypN0, with extension into the dermis, no lymphovascular invasion, and a residual cancer burden score of 3.

She continued adjuvant pembrolizumab every 3 weeks for 1 year and received radiotherapy to the left breast and regional lymph nodes, followed by a tumor-bed boost. Three months after radiation, she developed progressive numbness, tingling, and weakness in the left hand. PET-CT and MRI did not show recurrence or metastasis; testing supported immune-related brachial plexopathy. High-dose IV methylprednisolone and an oral taper gave little benefit, IVIG produced partial recovery, and plasmapheresis led to further functional improvement with EMG recovery. She later had marked improvement in grip and dexterity, mild residual tingling, and near-complete strength and sensory recovery. She remains in remission 30 months after surgery.