Lung inflammation from pembrolizumab immunotherapy returned months after stopping treatment

What happened, in summary

An 84-year-old woman presented with a painful lump in her left breast. Mammography and ultrasound showed several masses in the breast, with the largest measuring 27 x 19 x 48 mm, along with enlarged lymph nodes in the left armpit. Biopsies confirmed grade 3 triple-negative invasive ductal carcinoma involving an axillary node. PET-CT found no distant metastases.

She received 4 cycles of pembrolizumab with carboplatin and paclitaxel, followed by 4 cycles of pembrolizumab with doxorubicin and cyclophosphamide. In February 2023, she underwent a left simple mastectomy and sentinel-node biopsy. Pathology found no remaining cancer, showing a complete response to treatment. After radiation, she completed 9 additional cycles of pembrolizumab in December 2023.

Four months after pembrolizumab ended, she developed a new cough and shortness of breath. CT showed inflammatory changes in both lungs. Infection and other possible causes were investigated and not supported, leading to a diagnosis of grade 2 immune checkpoint inhibitor pneumonitis.

A 4-week course of methylprednisolone improved her symptoms and lung findings, followed by a slow dose reduction. One month after the steroid had been stopped, new lung infiltrates appeared. A second 4-week steroid course again produced marked improvement, and the final taper ended in October 2024. She remained in breast-cancer remission during routine follow-up.

Cultures and viral testing did not support infection. She improved enough to leave hospital after 3 days, but the later recurrence showed that lung inflammation could return after an apparently successful steroid taper.