Predictive models of toxicity in external radiotherapy : dosimetric issues

Dose-volume modeling of late and acute toxicity in radiotherapy for prostate cancer is a rapidly evolving field of investigation. The availability of individual, 3-dimensional dose distribution and dose-volume histograms (DVHs) permits the quantitative assessment of dose-volume relations for specific endpoints by investigating the correlation between individual dose-volume data and clinical outcomes. These studies often entail a huge effort in collecting data from large populations that have been followed properly for long time. The rectum is the most investigated organ, especially concerning late bleeding, and a good consensus regarding serial-like behavior for this endpoint comes from various investigations. Concerning the bladder, the existence of a clear dose effect when the organ is wholly or partially irradiated is well known. Concerning erectile dysfunctions, the wide use of hormone therapy and drugs against impotence suggests that large, prospectively scored populations will be necessary to definitively assess dose-volume relations. Bowel irradiation during prostate cancer radiotherapy occurs during pelvic lymph node irradiation, and severe acute toxicity has yet to be modeled clearly. Evidence of a correlation between the DVH of the intestinal cavity and bowel toxicity recently was reported, providing important information about optimal dose-volume constraints to be used during whole-pelvis irradiation with intensity-modulated radiotherapy.