We present a comparative analysis of photodynamic-treatment induced changes in optical parameters of cancer cells obtained from individual patients with three solid tumor localizations. Accumulation of photosensitizer inside living cells was validated using far-field fluorescence microscopy. Measurements of their optical characteristics were performed by means of digital holographic microscopy. The quantitative analysis of cell death dynamics performed by digital holographic microscopy was shown to be promising for investigation of cells resistivity to treatment. It was shown that both the photosensitizer accumulation and post-treatment dynamics of average phase shift may differ significantly in cell cultures obtained from different tumor localizations and different patients. Some of the cell cultures demonstrated very low or even no response to treatment.