Po‐Poster ‐ 29: Radiation‐induced apoptosis of lymphocytes to predict late toxicity from radiotherapy

An important challenge arises in radiotherapy when certain individuals present with abnormal radiosensitivities. Radiosensitive individuals may not tolerate normal therapy and encounter complications whereas radioresistant patients may not respond adequately to standard treatments regimes because of intrinsic cellular resistance. It has been shown (Crompton et al. IJROBP 2003; 45:707–714 and IJROBP 2001; 49:547–554) that late normal tissue toxicity was correlated to low level apoptosis in CD4 and CD8 lymphocytes. The assay developed by this group used a simple flow cytometry based assay. Our group has researched the sensitivity of alternative apoptosis assays (DiOC 6 , Caspase‐3, Annexin‐V, PI, 7AAD and Comet), and have shown that both the Annexin V‐FITC and the DiOC 6 assay have the greatest suitability in clinical use with respect to speed, simplicity and sensitivity. In our present study, 80 prostate cancer patients will be tested for radiosensitivity using lymphocyte apoptosis assays. These patients were formerly enrolled in a randomized dose‐escalation study (Sathya et al. JCO 2005; 45:1192–1199). Toxicity was recorded prospectively. The aim of this study is to determine a correlation between the proportion of T‐lymphocytes undergoing apoptosis when irradiated (2, 4 and 8Gy) and the occurrence of late toxicity (Grade II and above). We will present our current results showing a large inter‐individual variation within our patient cohort. Data obtained using the Annexin Assay measuring CD4 lymphocyte apoptosis receiving 8Gy show a wide range of responses (mean = 34% apoptosis, σ = 8.2) with z‐scores ranging from −1.5 to 2.4.