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The radiation sensitivity index (RSI) holds promise of identifying which tumors, even within the same primary tumor location and histology, are most likely to respond well to radiation. This analysis dichotomized >10K prospectively obtained tumor tissue samples across a wide array of primary cancers samples into RSI-high (resistant) or -low (sensitive) according to its genetic signature falling on one side or the other of the median RSI for that tumor subtype. In the same way, each sample was dichotomized into immune cell infiltrate-high or -low, analyzing the presence of CD8+ T cells, activated natural killer cells, and M1-macrophages. As seen before, RSI varied greatly within tumor subtypes. What’s novel here is a finding that most tumor types with high immune cell presence are also characterized as RSI-low (sensitive), possibly signaling tumors most likely to respond to the addition of immune checkpoint inhibitors. In other words, the RSI “links two assumed distinct tumor attributes, radiosensitivity and immune activity, into one coordinated biologic readout.” | Grass, Int J Radiat Oncol Biol Phys 2022