Abstract P6-10-24: High stromal platelet derived growth factor receptor beta predicts absence of benefit of radiotherapy after breast conserving surgery in breast ductal carcinoma in situ within the SweDCIS randomized trial
Authors: Strell Carina , Folkvaljon Dick , Holmberg Erik , Karlsson Per , Bergh Jonas , Bremer Troy , Akslen Lars A. , Wärnberg Fredrik , Östman Arne 
Published: March 2020

Background: With the introduction of mammography the detection of breast ductal carcinoma in situ (DCIS) has increased four- to sevenfold. Local recurrences occur only in a minority of patients. Radiotherapy (RT) is presently used without biomarker guidance. Therefore, novel biomarkers predicting benefit of radiotherapy are warranted to prevent overtreatment.

Studies on RT response-predictive biomarkers in DCIS have focused on tumor cell features and often neglected the tumor-associated stroma, despite growing evidence of its active influence on treatment response including RT. The platelet derived growth factor receptors alpha and beta (PDGFRa and -b) signaling pathways are major regulators of stroma cells. Recent studies have indicated a loss of PDGFRa in the tumor-associated stroma of DCIS while PDGFRb remains high and patients with PDGFRa(low)/PDGFRb(high) expression had a significant increased risk for any tumor recurrence. A limitation of this study was though the usage of a population-based cohort, thus not allowing firm conclusions on the impact of the marker on natural course of the disease or benefit to treatment.

Purpose: To explore if stromal expression of PDGFRb serves as prognostic as well as response-predictive biomarker for RT benefit with regard to ipsilateral breast tumor recurrence (IBTR) and overall survival (OS) for DCIS patients within a large and clinically well-annotated randomized trial.

Method: Immunohistochemical staining of PDGFRb was performed on full tissue sections of 705 patients from the SwedDCIS cohort, a randomized RT trial after breast-conserving surgery. Stromal PDGFRb expression was evaluated by two independent raters in up to ten high power fields within the tumor associated stroma region and scored as 0/negative, 1/low average stroma intensity, 2/moderate average stroma intensity or 3/high average stroma intensity. An average PDGFRb score was calculated for each tumor. The final data was split at median defining PDGFRb low and high patient groups. Outcomes were analyzed at 10 years of follow-up.

Results: High stromal PDGFRb was correlated with young age (p=0.003 Spearman’s Rank correlation). In multivariable Cox regression including RT, age, size, screen detection and tumor margin status, the PDGFRb high patient group showed a trend to worse prognosis with regard to IBTR (HR 1.40, CI95% 0.96-2.04, p=0.08) and OS (HR 1.76, CI95% 1.00-3.09, p=0.05).

Benefit of radiotherapy on IBTR (in situ and invasive) was absent in the PDGFRb high patient group (HR 0.76, CI95% 0.47-1.22, p=0.25)), but clearly detected in the PDGFRb low group (HR 0.29, CI95% 0.15-0.55, p&lt;0.001) in a multivariable analysis including age, size, screen detection and tumor margin status. Interaction between RT and PDGFRb was significant (p=0.020). No differences in OS after RT were observed within the PDGFR-defined patient groups.

RT benefit was also restricted to the PDGFRb low patient group in separate analyses for in situ IBTR (PDGFRb low group HR 0.27, CI95% 0.12-0.60, p=0.001, versus PDGFRb high group HR 0.66, CI95% 0.34-1.29, p=0.22) or invasive IBTR (PDGFRb low group HR 0.33, CI95% 0.11-0.98, p=0.045, versus PDGFRb high group HR 0.88, CI95% 0.45-1.73, p=0.71). Significant interactions between PDGFRb group and RT were not detected in these sub-analyses (p=0.11 for in situ and p=0.15 for invasive IBTR).

Conclusion: These data suggests high stromal PDGFRb as a marker of potential clinical utility for RT resistance regarding IBTR in DCIS patients.

Furthermore, the study supports the hypothesis of a fibroblast-mediated modulation of RT-sensitivity in DCIS cells. Experimental model studies, investigating the biological mechanisms and potential therapeutic relevance are therefore highly motivated.

Citation Format: Carina Strell, Dick Folkvaljon, Erik Holmberg, Per Karlsson, Jonas Bergh, Troy Bremer, Lars A. Akslen, Fredrik Wärnberg, Arne Östman. High stromal platelet derived growth factor receptor beta predicts absence of benefit of radiotherapy after breast conserving surgery in breast ductal carcinoma in situ within the SweDCIS randomized 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 P6-10-24.