Olaparib and Radiotherapy Increase Grade 3 Toxicities in S1706 Trial

The pursuit of better locoregional control in inflammatory breast cancer has led researchers to test whether PARP inhibitors can sensitize tumors to radiation. However, a phase 2 trial conducted between May 2019 and June 2024 suggests the cost of this synergy may be too high for many patients. The study, led by Reshma Jagsi, MD, PhD, of the University of Michigan, found that adding olaparib to standard radiotherapy regimens created a notable spike in severe side effects.

S1706 Trial: Toxicity Rates and Olaparib Side Effects

The S1706 trial randomized patients with inflammatory non-metastatic breast cancer into two groups after they had undergone neoadjuvant systemic therapy and modified radical mastectomy. Both the control cohort (n=73) and the intervention cohort (n=73) received 50 Gy of chest wall and nodal radiotherapy, including bolus, plus a 10 Gy boost. The intervention group received an additional 25 mg of olaparib twice daily during the radiation process.

While no grade 4 or 5 treatment-related toxicities occurred, the gap in grade 3 adverse events was stark. This same 24.7% figure applied to all acute chest-region adverse events in the intervention group, while the control group saw a rate of 6.8%.

The Theory of Radiosensitization and Prior Findings

The push for this combinatorial approach stems from the fact that locoregional recurrence in inflammatory breast cancer remains substantial, with control rates often at 80% or less at five years. As Onclive reported, the hypothesis is that PARP inhibitors can improve these rates by leveraging the way radiation causes single-strand breaks in DNA.

Earlier research provided a more optimistic outlook on safety.

Broader Shifts Toward Tailored Radiotherapy

The S1706 findings arrive as the broader oncology community moves toward de-escalating treatment for lower-risk patients. Recent evidence suggests that for some, the standard “blanket” approach to radiation is no longer necessary. The SUPREMO trial, led by the University of Edinburgh, examined 1,607 patients with intermediate-risk breast cancer who had undergone mastectomy and modern anti-cancer therapy.

The results indicated that radiotherapy had minimal impact on overall survival after ten years.

“The SUPREMO trial provides no evidence to support the continued use of radiotherapy to the area of the chest wall in most patients with intermediate-risk breast cancer who have undergone a mastectomy if they are also treated with modern anti-cancer drug treatment.”

Professor Ian Kunkler, University of Edinburgh’s Institute of Genetics and Cancer

This trend toward precision is further supported by data presented at the 15th European Breast Cancer Conference (EBCC15) in Barcelona on March 25, 2026. Dr. Fleur Mauritz, a radiation oncologist in training at Maastro, presented a 10-year study of 848 patients where radiotherapy was tailored based on the presence of cancer cells in lymph nodes after chemotherapy.

Balancing Efficacy and Treatment Burden

The tension in current breast cancer care is the balance between aggressive local control and the long-term toxicity of treatment. While the S1706 trial sought to escalate the intensity of treatment to stop recurrence, the results highlight a significant increase in treatment burden.

Avoiding unnecessary irradiation is seen as a way to protect breast reconstruction outcomes and reduce the overall burden on the patient.

For clinicians, the takeaway from the S1706 data is cautious.