Clonogenic assays are routinely employed to measure the radiation sensitivity of cultured human and mouse cancer cells, and they are considered the gold standard for assessing reproductive cell death induced by radiation, which is more relevant to tumor response to radiation therapy than other measures of cell death quantified using short-term techniques. Clonogenic assays were designed for cells that grew in two-dimensional cultures, which do not recapitulate cell-cell interactions and interactions with the cellular matrix as they occur in the tumor. Patient-derived tumor organoids have emerged as tumor models that at least in part circumvent such limitation.

Conventional clonogenic assays cannot be employed as such to assess the radiosensitivity of patient-derived tumor organoids. Here, we detail the technique that harnesses live imaging of breast cancer patient-derived tumor organoids to monitor the organoid forming efficacy and growth rate after exposure to ionizing irradiation versus non-irradiated treatment. In the era of personalized medicine, assessing the sensitivity of patient-derived models is of the utmost importance as it provides the information that is required to design and personalize treatment strategies.

In the future, we will be addressing the role of the immune cells that are present in the tumor in modulating the response of PDTO to radiations therapy.