Comparative analysis of Pareto surfaces in multi-criteria IMRT planning.

In the multi-criteria optimization approach to IMRT planning, a given dose distribution is evaluated by a number of convex objective functions that measure tumor coverage and sparing of the different organs at risk. Within this context optimizing the intensity profiles for any fixed set of beams yields a convex Pareto set in the objective space. However, if the number of beam directions and irradiation angles are included as free parameters in the formulation of the optimization problem, the resulting Pareto set becomes more intricate. In this work, a method is presented that allows for the comparison of two convex Pareto sets emerging from two distinct beam configuration choices. For the two competing beam settings, the non-dominated and the dominated points of the corresponding Pareto sets are identified and the distance between the two sets in the objective space is calculated and subsequently plotted. The obtained information enables the planner to decide if, for a given compromise, the current beam setup is optimal. He may then re-adjust his choice accordingly during navigation. The method is applied to an artificial case and two clinical head neck cases. In all cases no configuration is dominating its competitor over the whole Pareto set. For example, in one of the head neck cases a seven-beam configuration turns out to be superior to a nine-beam configuration if the highest priority is the sparing of the spinal cord. The presented method of comparing Pareto sets is not restricted to comparing different beam angle configurations, but will allow for more comprehensive comparisons of competing treatment techniques (e.g., photons versus protons) than with the classical method of comparing single treatment plans.

[Intra- and intersubject variability of acoustically evoked otoacoustic emissions. II. Distortion product otoacoustic emissions]. / Intra- und interindividuelle Variabilität akustisch evozierter otoakustischer Emissionen. II. Distorsionsprodukt-Emissionen.

BACKGROUND: Distortion product otoacoustic emissions (DPOAEs) are evoked by simultaneously stimulating the cochlea with two tones. The DPOAE with the highest amplitude (at the frequency 2f1-f2) is usually used for routine audiological evaluation. Any interpretation of DPOAEs in a clinical setting must consider their intra- and intersubject variability. METHODS: DPOAE measurements were performed in 36 normally hearing adults in three weekly test sessions. Each ear was tested twice per session, and the results were statistically analyzed. RESULTS: All test candidates had measurable DPOAEs. Statistically significant differences in absolute DPOAE amplitudes were neither found between two measurements at the same day, nor at weekly intervals, nor between right and left ear of the same candidate. Absolute DPOAE amplitudes in women were significantly higher in the 2-6 kHz range whereas they presented with significantly lower amplitudes below 2 kHz when compared to men. CONCLUSIONS: The present results indicate that monitoring of the inner ear status can be reliably performed using DPOAE measurements. Change in DPOAE amplitudes effectively indicate changes in the cochlear function with high sensitivity. Compared to TEOAE measurements, DPOAEs offer a higher frequency resolution in a broader frequency band and thus more detailed information about the cochlear condition.