Abstract PO5-19-05: A Phase II Trial of Stereotactic Body Radiation Therapy and Fluoroestradiol Positron Emission Tomography in Patients with Oligoprogressive Estrogen Receptor Positive Metastatic Breast Cancer
Authors: Bazan Jose , Mortimer Joanne , Li Yun-Rose 
Published: May 2024

Background: The term oligoprogression (OP) refers to a clinical scenario in which patients with diffuse metastatic disease on systemic therapy have a limited number of metastases that have progressed or are new whereas the majority of metastases are stable or improved. OP disease is increasingly being encountered in clinical practice due to improvements in systemic therapy. Treating OP disease with local ablative therapies may therefore prolong the time to more diffuse progression necessitating a change in systemic therapy and may therefore lead to improved overall survival in these patients. Only one study has evaluated the role of SBRT in patients with OP MBC. This trial accrued 47 patients with OP MBC, with 2/3 having triple negative disease. There was no difference in PFS between patients that received SBRT versus those that did not (4.2 months vs. 4.4 months, p=0.2). Whether ablative therapies are beneficial in subtypes of breast cancer that have many effective systemic therapies available, such as ER+ breast cancer, remains an open question and an unmet clinical need. Design: Phase II study for patients with OP ER+ MBC (1-4 new and/or progressing metastatic lesions). Eligible patients will receive stereotactic body radiation therapy (SBRT) to the OP lesions. SBRT will be delivered to each lesion in 3-5 fractions. Each patient’s systemic therapy regimen will be held during study therapy and will resume upon completion of study therapy. Patients will then be restaged at 12 weeks post-SBSRT. Patients that have at least stable disease at that time point will continue on their systemic therapy and then be re-staged 12 weeks later (24 weeks after SBRT). This study will also study the role of ER-targeted positron emission tomography (PET) imaging with 16α-18F-Fluoro-17β-Fluroestradiol (FES) in the OP ER+ patient population with FES PET scans obtained at baseline, and at each of the 2 follow-up imaging timepoints. A key secondary hypothesis is that the use of FES PET in addition to standard imaging at baseline and in follow-up will help confirm patients have OP disease and will help assess for new lesions on subsequent restaging. Eligibility: Key inclusion criteria: age≥18 yo; histologically confirmed ER+/HER2- metastatic breast cancer; the presence of metastatic breast cancer at the time of study entry with progression in 1-4 lesions (including new lesions); SBRT must be feasible for all progressing lesions. Key Exclusion Criteria: &gt;2 lines of systemic therapy for metastatic disease; intracranial disease progression Objective: To determine whether using SBRT to treat OP lesions allows ER+ breast cancer patients to continue on their current systemic therapy for at least 24 weeks post SBRT. Objective: To assess whether FES-PET increases the number of lesions found prior to SBRT; To determine the impact of SBRT on patient quality of life; time to next line systemic therapy; PFS Statistical Methods Simon 2 stage optimal design with α=0.05 and 1-β=0.80. The null hypothesis is that the proportion of patients that remain on their original systemic therapy ≥24 weeks (2 restaging scans) post-treatment is 20%. The alternative hypothesis is that the proportion of patients that remain on their original systemic therapy after 2 restaging scans is 50%. In stage 1, 8 patients that proceed to SBRT will be enrolled. The study will be terminated if only 0 or 1 subjects remain on original systemic therapy after 2 restaging scans. However, if ≥2 patients remain on original systemic therapy after 2 restaging, then an additional 10 patients would be enrolled for a total of 18 patients. The null hypothesis will be rejected if ≥6 patients remain on their original systemic therapy after 2 restaging scans. Contact Information: Jose G. Bazan (jbazan@coh.org) Funding Source: City of Hope Comprehensive Cancer Center

Citation Format: Jose Bazan, Joanne Mortimer, Yun-Rose Li. A Phase II Trial of Stereotactic Body Radiation Therapy and Fluoroestradiol Positron Emission Tomography in Patients with Oligoprogressive Estrogen Receptor Positive Metastatic Breast Cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO5-19-05.