BACKGROUND AND PURPOSE: Predictive assays of the response of tumor and normal tissues in individual patients offer the possibility of individualized prognosis and treatment decisions. For this purpose a variety of assays are currently being explored. The impact of tumor volume on radiotherapy outcome has long been recognized and in this paper its predictive potential is investigated. METHODS: Re-evaluation of clinical data from the literature. RESULTS: Tumor volume significantly influences radiotherapy outcome and in many sites it is likely a superior prognostic indicator to tumor stage, which reflects tumor size only partially and is mainly correlated to operability. Tumors even of identical stage may vary by factors of more than 100 in volume and neglect of this heterogeneity clearly reduces the power of a study considerably. The precision requirements for the measurement of tumor volume are small; +/-50% is sufficient for reasonable results. CONCLUSION: The data evaluated here suggest that tumor volume is the most precise and most relevant predictor of radiotherapy outcome. Its determination is achievable with sufficient accuracy in most radiotherapy departments. Individual tumor volume should always be reported in clinical studies and considered in data analyses.
