Purpose: Proteomic profiling of patients undergoing intensity-modulated radiotherapy (IMRT) for prostate cancer can identify unique biomarkers that reflect acute toxicity in normal tissues. Our objectives were to measure inflammatory cytokine proteins during IMRT and assess the variability of individual proteomic signatures. Experimental Design: Forty-two patients with intermediate-risk prostate cancer were recruited as follows: group 1, definitive IMRT (78 Gy in 39 fractions, n = 22), and group 2, IMRT postprostatectomy (66 Gy in 33 fractions, n = 20). Blood/urine samples were collected at baseline and weekly during IMRT. Acute toxicity was graded weekly during radiotherapy using CTC-AE v3.0 criteria. Multiplexed immunoassays were used to quantify cytokines including granulocyte macrophage colony-stimulating factor, IFN-γ, tumor necrosis factor-α, interleukin (IL)-1α, IL-2, IL6, IL-8, IL-10, and IL-12p70. Results: We observed positive correlations between cytokine expression between serum and plasma, but not between serum/plasma and urine. The Mann-Whitney test showed a significant increase in IFN-γ and IL-6 during IMRT (P = 0.0077, 0.0035). Increasing IL-2 and IL-1 expression were associated with increased probability of acute gastrointestinal and genitourinary toxicity, respectively. Conclusions: Determination of radiation-response signatures is feasible using multiplexed immunoassays and is a promising predictive early biomarker of toxicity outcomes. (Clin Cancer Res 2009;15(17):5576-83) Cellular damage caused by ionizing radiation induces specific proteins involved in DNA repair, cell death, inflammation, and other pathophysiologic responses (1). The majority of biomarker studies in radiation oncology have focused on predicting tumor response and survival (2). Clinically, the acute toxicity of prostate cancer radiotherapy manifest as gastrointestinal and genitourinary symptoms based on validated scoring criteria, [e.g., Common Terminology Criteria for Adverse Events (CTC-AE); ref. 3]. Radiotherapy dose escalation using intensity-modulated radiotherapy (IMRT) is associated with improved biochemical tumor control, yet still has radiation-induced toxicity. Dose-dependent markers of acute normal toxicity could help predict individuals at increased risk of radiotherapy-related injury and help to maximize the therapeutic ratio for individual patients.Rubin and colleagues were among the first to describe the role of cytokines (small glycoproteins involved in intercellular signaling) in mediating radiation toxicity (4). They showed in preclinical and clinical lung studies that levels of interleukin (IL)-1, transforming growth factor (TGF)-β, and tumor necrosis factor (TNF)-α were increased immediately after radiation exposure, and that chronically elevated TGF-β levels were associated with increased risk of pulmonary fibrosis. The link between radiation toxicity and cytokine expression is supported by studies showing that prolonged cytokine expression postradiotherapy is correlated to specific lung radi...