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60% of all cancer treatment requires radiation therapy and the fundamental goal is to deposit sufficient energy in a tumor to irreparably damage its DNA. Unlike high energy photons which pass through the body, charged particles such as protons deposit the majority of their energy at a specific depth, the location of the Bragg peak. In theory, this allows high radiation doses to be delivered in submm proximity to critical organs such as the spinal cord (see Fig. 34.1.1). Unfortunately, mm-scale prediction of the in vivo Bragg peak location is challenging because particle energy loss and scattering are highly path-dependent and patient-specific, largely due to anatomical heterogeneity which drifts from day to day. Given this, clinicians usually reduce therapeutic doses or adjust treatment margins to avoid harming healthy, vital tissue in proximity to the tumor site. To alleviate this constraint, in vivo dosimeters (IVD) have been developed to report in situ radiation doses to clinicians, enabling more effective and safer closed-loop treatment. However, conventional IVDs can only measure the total accumulated energy deposited, i.e. the total dose [1]–[5]. Given this, conventional IVDs cannot distinguish between single high energy particle depositions and those due to the combined contributions from several lower energy depositions, which have significantly different biological effect on tissue [6]. Furthermore, these conventional lVDs employ a single large detector and thus suffers from low spatial resolution. Here, we present a mm-scale real-time single-particle dosimeter array for charged particle cancer therapy that solves these problems.