Abstract P3-08-08: Prognostic and predictive impact of stroma cells defined by platelet derived growth factor receptor beta in early breast cancer: Results from the randomized SweBGR91RT trial (2020)

Abstract Background: Although preclinical studies have indicated that stroma cells can modulate the radiosensitivity of tumor cells, non-leukocytic stroma cells have not yet been explored as potential predictive markers for radiotherapy (RT) through systematic analyses of clinical samples. Platelet-derived growth factor receptor beta (PDGFRb) is a key regulator of fibroblasts, pericytes and smooth muscle cells. A high expression of PDGFRb in tumor-associated stroma has previously been associated with worse recurrence free and breast cancer specific survival as well as reduced tamoxifen sensitivity in invasive breast cancer. Purpose: To analyze the prognostic and predictive impact of stromal PDGFRb regarding ipsilateral breast tumor recurrence (IBTR) and any recurrence in a large randomized RT trial. Methods: Stromal PDGFRb expression was assessed by immunohistochemical staining of tissue microarrays (TMAs) with 958 tumors from the SweBCG91RT trial (Swedish Breast Cancer Group 91 Radiotherapy). The trial enrolled node-negative, stage I or IIA breast cancer patients, who were randomized to postoperative RT or not after breast conserving surgery. Stained TMAs were evaluated by two independent raters for PDGFRb staining intensity (0/negative; 1/low; 2/moderate; 3/high) and positive stroma fraction (0/0%; 1/1-10%; 2/11-50; 3/51-75%; 4/76-100%). The evaluations of both raters were averaged and the product between the two scoring metrics was formed. For final analysis the data was split in tertiles, as predefined, and referred to as PDGFRb low, medium or high score group. Outcomes were analyzed at 10 years of follow-up. Results: A high PDGFRb score was correlated with ER negativity (p=0.003,), young age (p&amp;lt;0.001), large tumor size (p=0.058) and high histological grade (p=0.067) in Spearman’s Rank test. With regard to prognosis, a significantly increased risk for any recurrence was detected in univariable analysis for patients with a medium (HR 1.57, CI95% 1.11-2.23, p=0.011) or high PDGFRb score (HR 1.49, CI95% 1.06-2.10, p=0.021) compared to the PDGFRb low score group. The significance remained for the PDGFRb medium (HR 1.47, CI95% 1.03-2.11, p=0.034) but not the PDGFRb high score group (HR 1.32, CI95% 0.93-1.88, p=0.125) in a multivariable analysis including grade, age and RT. The benefit of RT regarding the risk of IBTR was significant in the PDGFRb low (HR 0.28, CI95% 0.12-0.65, p=0.003) and medium (HR 0.32, CI95% 0.16-0.61, p=0.001) score groups but not in the PDGFRb high (HR 0.61, CI95% 0.35-1.07, p=0.086) score group in a multivariable analysis including grade and age. The risk reduction from RT regarding any recurrence was also less pronounced in the PDGFRb high (HR 0.73, CI95% 0.48-1.12, p=0.149) score group compared to the PDGFRb low (HR 0.57, CI95 0.32-1.03, p=0.065) and medium (HR 0.46, CI95% 0.28-0.75, p=0.002) score groups in multivariable analysis. No significant interaction between RT and PDGFRb expression could however be detected for IBTR (p=0.14) or any recurrence (p=0.33). Conclusion: A higher PDGFRb score conferred a negative prognosis with increased risk of any recurrence in univariable analyses, which in part can be explained by its association with ER negativity, young age and grade. A predictive effect of RT benefit from PDGFRb could not be confirmed although a tendency toward less benefit was observed in the patient group with the highest PDGFRb score. Citation Format: Carina Strell, Reidunn J Edelmann, Axel Stenmark Tullberg, Per Malmström, Mårten Fernö, Arne Östman, Erik Holmberg, Per Karlsson. Prognostic and predictive impact of stroma cells defined by platelet derived growth factor receptor beta in early breast cancer: Results from the randomized SweBGR91RT trial [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P3-08-08.