Clinical analysis of the approximate, 3-dimensional, biological effective dose equation in multiphase treatment plans

A multiphase, approximate biological effective dose (BEDA) equation was introduced because most treatment planning systems (TPS) are incapable of calculating the true BED (BEDT). This work investigates the accuracy and precision of the multiphase BEDA relative to the BEDT in clinical cases. Ten patients with head and neck cancer and 10 patients with prostate cancer were studied using their treatment plans from Pinnacle3 9.2 (Philips Medical, Fitchburg, WI). The organs at risk (OARs) that were studied are the normal brain, left and right optic nerves, optic chiasm, spinal cord, brainstem, bladder, and rectum. BEDA and BEDT distributions were calculated using MATLAB 2010b (MathWorks, Natick, MA) and analyzed on a voxel basis for percent error, percent error volume histograms (PEVHs), Pearson correlation coefficient, and Bland-Altman analysis. The maximum BED values that were calculated using the BEDA and BEDT methods were also analyzed. BEDA was found to always underestimate BEDT. The accuracy and precision of BEDA distributions varied between the organs: for optic chiasm and brainstem, 50% of the patients had an overall BEDA percent error of <1%; for left and right optic nerves, rectum, and bladder, 60% to 70% of the patients had an overall BEDA percent error of <1%; and for normal brain and spinal cord, 80% of the patients had an overall BEDA percent error of <1%. BEDA distributions had maximum errors ranging from 2% to 11%, with the 11% error occurring for bladder. BEDA produced much more accurate maximum BED values with adjacent organs such as normal brain, bladder, and rectum. This study has shown that BEDA can calculate BED distributions with acceptable accuracy under certain circumstances. However, its consistency and accuracy strongly depend on the dose distributions of the different treatment phases. One should be cautious when using BEDA.