The impact of intensity-modulated radiation therapy plan normalization in the postprostatectomy setting-does it matter?

The International Commission on Radiation Units & Measurements -83 recommends prescribing intensity-modulated radiation therapy (IMRT) in a dose-volume manner. Despite this, clinical variation still exists in how prostate IMRT plans are prescribed. This study aims to investigate the impact of different plan normalization methods for postprostatectomy IMRT. IMRT treatment plans were created retrospectively for 20 postprostatectomy patients. These were normalized such that the dose received by 98% (D98) of the planning target volume (PTV) was equal to 100% of the prescribed dose. All plans were individually optimized to achieve target coverage and organ at risk (OAR) dose constraints. Each patient's plan was then copied and normalized such that the mean dose (Dmean) received by the PTV was equal to 100% of the prescribed dose. The International Commission on Radiation Units & Measurements -83 recommended dosimetric end points were extracted for targets and Quantitative Analyses of Normal Tissue Effects in the Clinic or Radiation Therapy Oncology Group 0534 end points extracted for OARs. Statistical analysis using the Wilcoxon signed-rank test to measure the difference between data from plans normalized to D98 and Dmean was conducted. Extracted dosimetric end points of the targets and OARs were significantly higher in plans normalized to D98 than Dmean (p < 0.05) with the exceptions of D2 of the rectum and right femoral head. Normalization impacts on dosimetric end points of a plan. Hence, reporting the normalization method used is necessary to allow for meaningful interpretation of IMRT dosimetric studies.