Radiotherapy and oncology : journal of the European Society for Therapeutic Radiology and Oncology
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Multicenter Study
Relationships between DVHs and late rectal bleeding after radiotherapy for prostate cancer: analysis of a large group of patients pooled from three institutions.
Accurate modeling of late rectal reactions needs the collection of individual 3D dose-volume data (i.e. DVH) as well as clinical information of large cohorts of patients. The possibility of collecting a large number of patients with many different dose-volume combinations is very suitable for this purpose. ⋯ Our data confirm the role of the rectal DVH in separating groups of patients having prostate radiotherapy in low and high risk of developing late bleeding. Based on these results, V50 below 60-65% and V60 below 50-55% seem to be the robust cut-off values to keep the risk of developing late rectal bleeding reasonably low. However, due to the 'heterogeneity' of the considered population, the results found should be applied with caution in 'more homogeneous' groups of patients. The association of adjuvant hormone deprivation seems to be associated with an increased risk of rectal toxicity; the mechanism for this effect should be a focus of further research.
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In this paper, we elaborate on the proposals in the ICRU-62 report concerning planning target volume (PTV) margins for geometrical uncertainties during radiotherapy, such as variations in patient set-up and internal organ motion. According to the ICRU, these margins should be such that the planned dose in the PTV is representative of the real dose in the 'moving' clinical target volume (CTV). We demonstrate that the dosimetrical consequences of systematic and random geometrical uncertainties are fundamentally different, which should be reflected in margin calculations. ⋯ If, for both random and systematic uncertainties, the internal and external errors are uncorrelated and quantified by the standard deviations sigma(int), sigma(ext), Sigma(int), Sigma(ext), then Sigma(tot) = square root (Sigma(int)(2) + Sigma(ext)(2)) and sigma(tot) = square root (sigma(int)(2) + sigma(ext)(2)). If the PTV margin thus acquired is deliberately reduced to spare normal tissues, the planned PTV dose is not representative of the CTV anymore. Therefore, we recommend to also report the minimum dose in the volume originally defined by the recipe (designated RTV, i.e. representative target volume).