Boron neutron capture therapy (BNCT) is an extensively studied radiotherapeutic strategy for cancer treatment. BNCT is based on irradiation of malignant tumour cells with neutrons after uptake of a 10B containing molecule. Alpha particles, locally produced by neutron irradiation kill the cancer cells. Important for ongoing research regarding cellular uptake and cytotoxicity of a large variety of 10B containing molecules is the accurate determination of boron concentrations in cell cultures. In this work, the sample preparation for quantitative inductively coupled plasma mass spectrometry (ICP-MS) analysis on cell cultures was optimized. By making use of acid digestion combined with UV digestion, low detection limits (0.4 mg L1) and full recoveries of boron could be achieved while measurements were free of spectral and non-spectral interferences. Finally, cell-associated boron in the form of 4-borono-Lphenylalanine (L-BPA) in vascular endothelial cell cultures could be determined with ICP-MS as (1.26 + 0.10) x 109 boron atoms per cell. The developed method can prove its importance for further BNCT research and elemental analysis of cell cultures.