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Level of Breast Cancer Stem Cell Correlated with Tumor Radioresistence: An Indication for Individualized Breast Cancer Therapy Adapted to Cancer Stem Cell Fractions

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S Qi

S Qi*, F Pajonk , S McCloskey , D Low , P Kupelian , M Steinberg , K Sheng , UCLA, Los Angeles, CA


WE-E-BRE-10 Wednesday 1:45PM - 3:45PM Room: Ballroom E

Purposes: The presence of cancer stem cells (CSCs) in a solid tumor could result in poor tumor control probability. The purposes are to study CSC radiosensitivity parameters α and β and their correlation to CSC levels to understand the underlying radioresistance mechanisms and enable individualized treatment design.

Methods: Four established breast cancer cell lines (MCF-7, T47D, MDA-MB-231, and SUM159PT) were irradiated in vitro using single radiation doses of 0, 2, 4, 6, 8 or 10 Gy. The fractions of CSCs in each cell lines were determined using cancer stem cell markers. Mammosphere assays were also performed to better estimate the number of CSCs and represent the CSC repopulation in a human solid tumor. The measured cell surviving fractions were fitted using the Linear-quadratic (LQ) model with independent fitting parameters: α_TC, β_TC (TCs), α_CSC, β_CSC (CSCs), and fs (the percentage of CSCs in each sample).

Results: The measured fs increased following the irradiation by MCF-7 (0.1%), T47D (0.9%), MDA-MB-231 (1.18%) and SUM159T (2.46%), while decreasing surviving curve slopes were observed, indicating greater radioresistance, in the opposite order. The fitting yielded the radiosensitive parameters for the MCF-7: α_TC=0.1±0.2Gy⁻¹, β_TC= 0.08 ±0.14Gy⁻², α_CSC=0.04±0.07Gy⁻¹, β_CSC =0.02±0.3Gy⁻²; for the SUM159PT, α_TC=0.08±0.25 Gy⁻¹, β_TC=0.02±0.02Gy⁻², α_CSC=0.04±0.18Gy⁻¹, β_CSC =0.004±0.24Gy⁻². In the mammosphere assay, where fs were higher than the corresponding cell line assays, there was almost no shoulder found in the surviving curves (more radioresistant in mammosphere assays) yielding β_CSC of approximately 0.

Conclusion: Breast cancer stem cells were more radioresistant characterized by smaller α and β values compared to differentiated breast cancer cells. Percentage of breast cancer stem cells strongly correlated to overall tumor radioresistance. This observation suggested the feasibility of individualized radiotherapy prescription based on the fractions of cancer stem cells found in biopsy.

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