Rare Case of Collecting Duct Carcinoma With Complete Response to Nivolumab.

Collecting duct carcinoma (CDC) is a rare, aggressive subtype of renal cell carcinoma originating in the renal medulla. We report a unique case of metastatic CDC in a patient with prior breast ductal carcinoma in situ. Genomic profiling revealed a homozygous deletion of CDKN2A (encoding p16). After progression on systemic therapies, the patient received stereotactic body radiotherapy (SBRT) to metastatic lesions concurrently with nivolumab (anti-PD-1). This regimen achieved a rapid complete radiographic remission of all lesions. We present a 63-year-old Sudanese woman with metastatic CDC who achieved a complete remission of over 5 years following nivolumab therapy. The patient initially presented with right flank pain and hematuria. Imaging revealed an exophytic renal mass, and she underwent radical nephrectomy in June 2019. Pathology confirmed high-grade CDC (pT3aN1) with clear margins; immunohistochemistry was notable for positive vimentin, PAX8, CK19, and patchy AMACR, with loss of CDKN2A. Postoperative PET/CT was clear, but by October 2019, three intra-abdominal metastases were seen (liver and retroperitoneum). She was first treated with palbociclib and letrozole, but progression occurred after 3 months. Given reports of immunotherapy efficacy in CDC, she began nivolumab in May 2020. Imaging in October 2020 showed marked tumor regression, sustained on repeat scans. In September 2021, isolated para-aortic lymph node recurrence was treated with stereotactic radiation (20-25 Gy/5 fractions) while continuing nivolumab. Subsequent PET/CT scans (Feb 2022, Feb 2023, June 2023, March 2024, and January 2025) demonstrated continued complete metabolic response. This combined modality approach achieved an unprecedented durable response, underscoring that personalized multimodal therapy-linking radiotherapy, immunotherapy and targeted cell-cycle inhibition-can yield long-term control in CDC. For a disease as lethal as CDC, this outcome demonstrates that genomically informed therapy can achieve extraordinary benefit.